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Electric Motors for Industry

• satyendra
• March 28, 2024
• 26 Comments
• AC, AC motor, Asynchronous motor, conductor, DC, DC motor, Electrical motors, Hysteresis motor, Induction motor, Magnetic flux, NEMA, Rotor, Squirrel cage, Stator, Synchronous motor,

Electric Motors for Industry

Electrical motor is an electro-mechanical energy conversion device which convert electrical energy to mechanical energy through the action of a magnetic field. It is always advantageous to utilize electrical energy since it is cheaper, can be easily transmitted, easy to control, and very efficient. The electrical energy is normally generated from natural resources such as water, coal, furnace oil, diesel, wind, sunlight, and atomic energy etc. From these sources, first mechanical energy is produced by one way or the other and then that mechanical energy is converted into electrical energy by suitable generating machines. For the utilization of electrical energy, it is again converted into other forms of energy such as mechanical, heat, or light etc. It is a well-known fact that the electric drives have been universally adopted by the industry because of their inherent advantages.

Electric motors are either made to National Electrical Manufacturers Asso­ciation (NEMA) standards or are made to International Electrotechnical Commission (IEC) standards. Motor standards for these motors are MG 1 for NEMA and the 60034 series and 60071 for IEC which define the mechanical, electrical, and performance specifications. The most obvious mechanical difference is that NEMA motors are defined in inch units and IEC are defined in metric units.

The NEMA standards cover a wide range of electric motor frame sizes used in North America. However, there are some electric motors which are larger than the largest NEMA frame sizes. These motors are typically used in heavy industrial applications, and their frame sizes are often designated by different standards. The IEC standard defines frame sizes for AC motors used in Europe and other parts of the world. The standard also defines large electric motor frame sizes. These frame sizes range from 315 to 1,000, with corresponding shaft heights ranging from 400 mm to 1,600 mm. Another standard for larger electric motor frame sizes is the IEEE (Institute of Electrical and Electronics Engineers) standard. The IEEE standard defines frame sizes for large AC motors and DC motors used in industrial applications. These frame sizes range from 315 to 1,000, with corresponding shaft heights ranging from 400 mm to 1,600 mm.

IEC motors are normally made with either finned cast aluminum or iron frames, while NEMA motors are made with the finned cast frames and rolled steel frame housings. There are only slight differences in frame sizes based on shaft height. The shaft diameters and lengths are different between NEMA and IEC as are the mounting base dimensions and mounting hole spacing.

In the electrical motors, conversion of energy from electrical to mechanical form results from the two electro-magnetic phenomena namely (i) when a conductor moves in a magnetic field, voltage is induced in the conductor, and (ii) when a current-carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force.

The above two effects occur simultaneously whenever energy conversion takes place from electrical to mechanical or vice versa. In motoring action, the electrical system makes current flow through conductors which are placed in the magnetic field. A force is produced on each conductor. If the conductors are placed on a structure free to rotate, an electro-magnetic torque is produced, tending to make the rotating structure rotate at some speed. If the conductors rotate in a magnetic field, a voltage is also induced in each conductor. In generating action, the process is reversed. In this case, the rotating structure, the rotor, is driven by a prime mover (such as a steam turbine or a diesel engine). A voltage is induced in the conductors which are rotating with the rotor. If an electrical load is connected to the winding formed by these conductors, a current ‘I’ flow, delivering electrical power to the load. Moreover, the current flowing through the conductor interacts with the magnetic field to produce a reaction torque, which tends to oppose the torque applied by the prime mover.

In the electrical motors, the total magnetic flux can be divided into two components namely (i) main flux (air gap flux), and (ii) leakage flux. The main flux enables electro-magnetic energy conversion, but a proportion of the total flux does not participate in energy conversion, and this part is called leakage flux. The main flux is required to cross the air gap of rotating motor and its function is electro-magnetically connected to both stator and rotor windings. The leakage flux is linked only with this winding in which it has been created.

The three basic and normally used electric motors are DC (direct current) motors, induction motors, and synchronous motors. The operation of these motors such rely upon their magnetic circuits. The closed path followed by the magnetic lines of force is called a magnetic circuit. The operation of all the electrical motors depends upon the magnetism produced by their magnetic circuits. Hence, to get the needed characteristics of the motors, their magnetic circuits are to be designed carefully.

Basic structure of electric motors – The basic structure of an electric motor has two major components namely stator and rotor, separated by an air gap as shown in Fig 1. Stator does not move and normally is the outer frame of the electric motor. Rotor is free to move and normally is the inner part of the electric motor. Both stator and rotor are made of ferro-magnetic materials.

Fig 1 Basic structure of an electric motor

In the majority of the electric motors, slots are cut on the inner periphery of the stator and outer periphery of the rotor structure, as shown in Fig 1. Conductors are placed in these slots. The iron core is used to maximize the coupling between the coils (formed by conductors) placed on the stator and rotor, to increase the flux density in the motor, and to decrease the size of the motor. If the stator or rotor (or both) is subjected to a time-varying magnetic flux, the iron core is laminated to reduce eddy current losses. The thin laminations of the iron core with provisions for slots are shown in Fig 1.

The conductors placed in the slots of the stator or rotor are inter-connected to form windings. The winding in which voltage is induced is called the armature winding. The winding through which a current is passed to produce the primary source of flux in the motor is called the field winding. Permanent magnets are used in some motors to provide the major source of flux in the motor.

Types of electric motors – There are several types of electrical motors which are used extensively for electro-mechanical energy conversion. Use of synchronous motors and DC motors for heavy duty, and precision drives etc. are also common. With the introduction of variable frequency drives (VFD) for speed and torque control, the 3-phase induction motors are finding increasingly acceptance for applications where DC drives have been earlier used.

An asynchronous motor is an AC (alternating current) motor in which the rotor does not turn at a synchronous speed. In industry, most commonly used motors are 3-phase squirrel cage induction type.

A permanent magnet motor is a type of electric motor which uses permanent magnets for the field excitation and a wound armature. The permanent magnets can either be stationary or rotating, interior or exterior to the armature for a radial flux motor or layered with the armature for an axial flux topology.

A DC motor is an electrical motor which uses DC to produce mechanical force. The most common types rely on magnetic forces produced by currents in the coils. Nearly all types of DC motors have some internal mechanism, either electro-mechanical or electronic, to periodically change the direction of current in part of the motor. The DC motors are very useful where wide range of speeds and perfect speed regulation is needed such as electric traction.

In DC motors, the process of reversing current is known as commutation. The function served by the commutator of a DC motor is to convert the induced AC in conductors into a DC output. Before applying the DC current to the motor’s coils, commutation is utilized in DC motors to switch the direction of the current. There are five major types of DC motors in general use. These are (i) separately excited DC motor, (ii) shunt DC motor, (iii) permanent-magnet DC motor, (iv) series DC motor, and (v) compounded DC motor.

DC motors are used in rolling mills, in traction, and in overhead cranes. These motors are also used in several control applications as actuators, and as speed or position sensors. With AC being universally adopted for generation, transmission and distribution, there are almost no practical uses now of the DC motors as a power generator. Its use as a motor-generator (AC motor-DC generator) for feeding DC drives has also been replaced in modern practice by rectifier units. In certain applications DC motors act as generators for brief time periods in the ‘regenerative’ or ‘dynamic braking’ mode, especially in electric traction systems.

An AC motor is an electric motor driven by an alternating current. The AC motor normally consists of two basic parts, an outside stator having coils supplied with AC to produce a rotating magnetic field, and an inside rotor attached to the output shaft producing a second rotating magnetic field. The rotor magnetic field can be produced by permanent magnets, reluctance saliency, or DC or AC electrical windings. AC motors are normally of three general types namely induction, synchronous, and series-wound motors.

Induction motors are electric motors which use AC, propelled by a magnetic field which rotates. They are made up of a rotor, a stator, and coils which convert electrical energy into mechanical energy using electro-magnetic induction.

The squirrel cage induction motor has a cylindrical shaped cage which fits around the shaft with bars extending between its two ends. In it, the secondary circuit (squirrel cage windings) consists of a number of conducting bars having their extremities connected by metal rings or plates at each end. The attached end rings create a short-circuit for induced current to flow through.

Synchronous motors are those which have a constant rotational speed. They operate at the supply’s synchronous speed. They are normally utilized to improve the power factor by operating at a constant speed under no-load conditions. At a given rating, synchronous motors lose less energy than induction motors.

In poly-phase motors, the stator contains multiple distinct windings per motor pole, driven by corresponding time-shifted sine waves. In practice, these are two or three phases. Majority of large (higher than 1 kilowatt industrial motors are poly-phase induction motors. Large industrial motors are 3-phase.

A hysteresis motor is a single-phase synchronous motor whose operating principle is based on the effect of magnetic hysteresis. It produces output torque through the hysteresis effect of magnetic materials. It has the advantages of a simple structure, high-speed operation, high temperature resistance, low noise and self-starting capability. It can be applied to some special occasions needing high speed and high stationarity. However, its disadvantage is low torque density, low efficiency, and low power factor. The permanent magnet hysteresis motor is a compromise of the characteristics of permanent magnet motor and hysteresis motor, and it can be self-starting in the case of having a torque density comparable to that of a permanent magnet motor. In addition, there are some new structures of hysteresis motors, which open up the direction for innovative applications.

A reluctance motor is a type of electric motor which induces non-permanent magnetic poles on the ferro-magnetic rotor. The rotor does not have any windings. It generates torque through magnetic reluctance. Reluctance motor sub-types include synchronous, variable, switched, and variable stepping.

A stepper motor, also known as step motor or stepping motor, is an electric motor which rotates in a series of small angular steps, instead of continuously. Stepper motors are a type of digital actuator. Like other electro-magnetic actuators, these motors convert electric energy into mechanical energy to perform work. A stepper motor is a brushless DC electric motor which divides a full rotation into a number of equal steps. The motor’s position can be commanded to move and hold at one of these steps without any position sensor for feedback (an open-loop controller), as long as the motor is correctly sized to the application in respect to torque and speed.

A linear motor is an electric motor which has its stator and rotor ‘unrolled’, hence, instead of producing a torque (rotation), it produces a linear force along its length. However, linear motors are not necessarily straight. Characteristically, a linear motor’s active section has ends, whereas more conventional motors are arranged as a continuous loop.

The universal motor is a type of electric motor which can operate on either AC or DC and uses an electro-magnet as its stator to create its magnetic field. It is a commutated series-wound motor where the stator’s field coils are connected in series with the rotor windings through commutator. It is frequently referred to as an AC series motor. The universal motor is very similar to a DC series motor in construction, but is modified slightly to allow the motor to operate properly on AC. This type of electric motor can operate well on AC since the current in both the field coils and the armature (and the resultant magnetic fields) alternates (reverse polarity) synchronously with the supply. Hence the resulting mechanical force occurs in a consistent direction of rotation, independent of the direction of applied voltage, but determined by the commutator and polarity of the field coils

There are seven common types of NEMA motors as given below.

Open drip proof (ODP) motors – These motors allow air to circulate through the windings for cooling, but prevent drops of liquid from falling into motor within a 15-degree angle from vertical. Typically these motors are used for indoor applications in relatively clean, and dry locations.

Totally enclosed fan cooled (TEFC) motors – These motors prevent the free exchange of air between the inside and outside of the frame, but does not make the frame completely air tight. A fan is attached to the shaft and pushes air over the frame during its operation to help in the cooling process. In these motors, the ribbed frame is designed to increase the surface area for cooling purposes. The TEFC style enclosure is the most versatile of all. It is used on pumps, fans, compressors, general industrial belt drive, and direct connected equipment.

Totally enclosed non-ventilated (TENV) motors – These motors are similar to a TEFC, but these motors have no cooling fan and relies on conventional cooling. The motors have no vent openings, are tightly enclosed to prevent the free exchange of air, but the motors are not airtight. These motors are suitable for uses which are exposed to dirt or dampness, but not very moist or hazardous (explosive) locations.

Totally enclosed air over (TEAO) motors – These are dust-tight fan and blower duty motors which are designed for shaft mounted fans or belt driven fans. The motors are to be mounted within the airflow of the fan.

Totally enclosed wash down (TEWD) motors – These motors are designed to withstand high pressure wash-downs or other high humidity or wet environments. Available on TEAO, TEFC, and ENV enclosures, these motors are totally enclosed, hostile, and severe environment motors. These are designed for use in extremely moist or chemical environments, but not for hazardous locations.

Explosion-proof enclosures (EXPL) motors – The explosion proof motor is a totally enclosed motor and is designed to withstand an explosion of specified gas or vapour inside the motor casing and prevent the ignition outside the motor by sparks, flashing, or explosion. These motors are designed for specific hazardous purposes, such as atmospheres containing gases or hazardous dusts. For safe operation, the maximum motor operating temperature is to be below the ignition temperature of the surrounding gases or vapours. Explosion proof motors are designed, manufactured, and tested under the rigid requirements of the Underwriters Laboratories.

Hazardous Location (HAZ) motors – Hazardous location motor applications are classified by the type of hazardous environment present, the characteristics of the specific material creating the hazard, the probability of exposure to the environment, and the maximum temperature level which is considered safe for the substance creating the hazard.

Motor ratings – The ‘name plate’ of a motor serves to identify the motor. It gives rated voltage, current, power, speed, and possibly other data useful to the user. The motor ratings are the values of the different parameters for which the motor run continuously without over-heating or other damage. In practice, the ratings can be exceeded for short periods. If, however, the ratings are exceeded for considerable periods of time, there can be permanent damage, particularly to insulation.

Motors are normally manufactured in standard frame sizes. They also come in different enclosures to suit different environmental conditions and duties (e.g., drip‐proof, splash‐proof, or submersible). All materials are subject to physical limitations, i.e., limits beyond which they no longer retain their desired physical characteristics. Proper design and use of motors ensure that none of the constituent materials exceeds its physical limitations under normal operating conditions. For selecting an appropriate motor for the system, the users are to know clearly the needs of the application (e.g., speed, voltage, and power etc.) and the conditions under which the motor is going to be running. The users are then able to select the most economical motor which meets the requirements and the conditions.

The DC motor, the synchronous motor, and the induction motor are the major motors used in industry. The merits of the squirrel cage induction motor are lightness, simplicity, ruggedness, robustness, less initial cost, higher torque-inertia ratio, capability of much higher speeds, and ease of maintenance etc. The most important feature which declares it as a tough competitor to DC motors in the drives field is that its cost per kVA (kilo volt ampere) is around one fifty of its counter-part and it possesses higher suitability in hostile environment.

Unfortunately, induction motors suffer from the draw-back that, in contrast to DC motors, their speed cannot be easily and effectively adjusted continuously over a wide range of operating conditions. On the other hand, the synchronous motor has the merit of being operated under a wide range of power factors, both lagging and leading, and are much better suited for bulk power generation. In the induction motor, AC is applied to the stator and alternating currents are induced in the rotor by transformer action.

In the synchronous motors, DC is supplied to the rotor and AC flows in the stator. On the other hand, a DC motor is a motor which is excited from DC source. It is a normal practice in industry to use AC motors whenever they are inherently suitable or can be given appropriate characteristics by means of power electronics devices. Yet, the increasing complexity of industrial processes demands higher flexibility from electrical motors in terms of special characteristics and speed control. It is in this field that the DC motors, fed from the AC supply through rectifiers, are making their mark.

Classification of electrical motors – Electrical motors take several forms and are known by several names. The three basic and common ones are DC motors, induction motors, and synchronous motors. There are other motors, such as permanent magnet motors, hysteresis motors, and stepper motor etc.

Traditionally, electrical motors are classified into DC commutator (brushed) motors, induction (asynchronous) motors, and synchronous motors. These three types of electrical motors are still regarded as fundamental types, despite that DC brushed motors (except small motors) have been gradually abandoned and PM (permanent magnet) brushless motors (PMBM) and switched reluctance motors (SRM) have been in mass production and use for at least two decades.

Recently, new topologies of high torque density motors, high speed motors, integrated motor drives, and special motors have been developed. Progress in electric motors technology is stimulated by new materials, new areas of applications, impact of power electronics, need for energy saving, and new technological challenges. The development of electric motors these days are mostly stimulated by computer hardware, residential and public applications, and transportation systems (land, sea and air). There are other motors, such as permanent magnet motors, hysteresis motors, and stepper motors.

There are several methods of classifying electric motors as given below. The very common is by the electric power supply. Electric motors are classified as DC motors and AC motors and also as per their stator and rotor constructions as shown in Fig 2.

Fig 2 Classification of electrical motors

NEMA standards – NEMA standards are voluntary standards of the National Electric Manufacturers Association and represent general practice in the industry. They define a product, process, or procedure with reference to nomenclature composition, construction, dimensions, tolerances, operating characteristics, performance, quality, rating, and testing.

NEMA has standardized frame-size motor dimensions, including bolt-hole sizes, mounting-base dimensions, shaft height, shaft diameter, and shaft length. Existing motors can be replaced without reworking the mounting arrangement. New installations are easier to design since the dimensions are known. Letters are used to indicate where a dimension is taken. For example, the letter ‘C’ indicates the overall length of the motor, and ‘E’ signifies the distance from the centre of the shaft to the centre of the mounting holes in the feet. Motor manufacturers provide tables in the motor-data sheet that reference the letter to find the desired dimension.

NEMA categorizes standard frame sizes as either fractional or integral. Fractional frame sizes are designated as 45 and 56, and mainly include horsepower ratings of less than 1. Integral (or medium) horsepower motors are designated by frame sizes that range from 143T to 445T. A ‘T’ in the motor frame size designation of integral horsepower motors indicates that the motor is built to current NEMA frame standards.

The frame-size designation is a code to help identify key dimensions. For example, the first two digits are used to determine the shaft height. The shaft height is the distance from the centre of the shaft to the mounting surface, given in inches. For calculating the shaft height, the first two digits of the frame size is divided by four. For example, a 143T frame size motor has a shaft height of 3.5 inch (14/4). The third digit in the integral ‘T’ frame-size number is the NEMA code for the distance between the centre lines of the mounting bolt-holes. The dimension is determined by matching the third digit in the frame number with a table in NEMA MG-1 standard.

Motors which are larger than the NEMA frame sizes are referred to as above-NEMA motors. These motors typically range in size from 150 kW (kilowatt) to 7,500 kW. There are no standardized frame sizes or dimensions for above-NEMA motors since above-NEMA motors typically are constructed to meet the specific requirements of an application. NEMA classifications of electric motors are summarized in Tab 1.

NEMA classes categorize electric motors based on a motor’s starting-torque and its accelerating load. The four standard NEMA design classes are given below.

NEMA class-A motors have a maximum 5 % slip with a high to medium starting current, normal locked rotor torque, and normal break-down torque. These characteristics make them well suited for a wide variety of applications and they are frequently found powering fans and pumps.

NEMA class-B motors have a maximum 5 % slip and a normal break-down torque but include a low starting current, and a high locked rotor torque. These motors are well suited for different applications which need normal starting torques and are normally found in HVAC (heating, ventilation, and air-conditioning) systems where they are used with pumps, blowers, and fans.

NEMA class-C motors have a maximum 5 % slip, low starting current, high locked rotor torque and a normal breakdown torque. These motors work well with conveyors and positive displacement pumps which need a high starting torque and high inertia at start-up.

NEMA class-D motors show a maximum 5 % to 13 % slip with a low starting current and a very high locked rotor torque. Applications which need very high inertia starts, like hoists and cranes frequently use class-D motors.

Class-E motors are outside the NEMA classes and are fairly mid-range in locked rotor torque (75 % to 190 %), pull-up torque (60 % to 140 %), and break-down torque (160 % to 200 %). These motors have the highest locked rotor current (800 % to 1,000 %). Their slip is also somewhat smaller than the other designs (0.5 % to 3 %). These aspects give the class-E motors the highest efficiency out of the NEMA ratings. They can be used in similar applications to NEMA-A and NEMA-B motors like fans, pumps, and blowers with low starting torque.

Basic features of electric motors – The basic structural features of a DC motor are given here. The stator carries the field winding. The stator together with the rotor constitutes the magnetic circuit or core of the motor. It is a hollow cylinder. The rotor carries the armature winding. The armature is the load carrying member. The rotor is cylindrical in shape. The armature winding rotates in the magnetic field set up at the stationary winding. It is the load carrying member mounted on the rotor. An armature winding is a continuous winding. i.e., it has no beginning or end. It is composed of a number of coils in series. Depending on the manner in which the coil ends are connected to the commutator bars, armature windings can be grouped into two namely lap windings, and wave windings. Wave winding gives higher voltage and smaller current ratings while the lap winding supplies higher current and smaller voltage ratings.

Field winding is an exciting system which can be an electrical winding or a permanent magnet and which is located on the stator. The coils on the armature are terminated and inter-connected through the commutator which comprised of a number of bars or commutator segments which are insulated from each other. The commutator rotates with the rotor and serves to rectify the induced voltage and the current in the armature both of which are AC.  Brushes are conducting carbon graphite spring loaded to ride on the commutator and act as interface between the external circuit and the armature winding. The field winding is placed in poles, the number of which is determined by the voltage and current ratings of the motor. Slot / teeth is for mechanical support, protection from abrasion, and further electrical insulation, and non-conducting slot liners are frequently wedged between the coils and the slot walls. The magnetic material between the slots is called teeth.

On the other hand, the basic constructional features of an AC motor (e.g., induction motor) are given here. In case of rigid frame, the whole construction ensures compact and adaptable design at low weight and low vibration level in all operating conditions and through-out the whole speed range. The stator core is a stack of thin electrical sheet steel laminations insulated by a heat resistant inorganic resin. The radial cooling ducts ensure uniform and efficient cooling. The stator package forms a solid block which retains its rigidity through-out its long life-time of the motor.

The rotor of AC motors can be of wound type or squirrel cage type. Depending on the number of poles and whether the shaft is of the spider or cylindrical type, the rotor core is shrunk onto the shaft and the conductor bars are tightly caulked into the slots to prevent bar vibration.

The bearings are designed for reliable continuous operation and ease of maintenance. Depending on the rated power, either spherical seated self-aligning sleeve bearings or anti-friction bearings with a life time of over 100,000 hours are available.

Basic principles of operation – Electric motors are a group of devices used to convert electrical energy into mechanical energy, by electro-magnetic means. Two related physical principles cause the operation of the motors. The first is the principle of electro-magnetic induction which is that if a conductor is moved through a magnetic field, or if the strength a stationary loop is made to vary, a current is set up or induced in the conductor. The opposite of this principle is that of electro-magnetic reaction which is that if a current is passed through a conductor located in a magnetic field, the field exerts a mechanical force on it.

Motors consist of two basic units, namely the field, which is the electro-magnet with its coils, and the armature which is the structure that supports the conductors which cut the magnetic field and carry the exciting current in a motor. The armature is normally a laminated soft-iron core around which conducting wires are wound in coils.

Determination of motor parameters – The name-plate gives sufficient information on the rated current, power, frequency, voltage, winding temperature, and stator winding connection. However, it can be necessary to determine the winding resistances and reactances as well as the mechanical properties of the motor performance under both steady and dynamic conditions. For example, the following tests are normally carried out to determine the parameters of an asynchronous motor.

The first is the no-load test. The aim of the no-load test is to determine (i) stator ohmic / copper losses, (ii) stator core losses because of hysteresis and eddy currents, (iii) rotational losses because of friction and windage, and (iv) magnetizing inductance. The test is carried out at the rated frequency and with balanced poly-phase voltages applied to the stator terminals. Readings are taken at rated voltage, after the motor has been running for a considerable period of time necessary for the bearings to be properly lubricated. At, no load, the motor slip and the rotor current are very small and hence resulting to a negligible no-load rotor loss.

The second is the blocked-rotor test. The blocked-rotor test provides the information necessary to determine the winding resistances, and the leakage reactances. In this test, the rotor is blocked by external means to prevent rotation. In the blocked-rotor test, the slip is unity (s = 1) and the mechanical load stance, RM is zero, hence resulting in a very low input impedance of the equivalent circuit.

The third is the retardation test. The retardation test is carried out to determine the test motor moment of inertia. In this test, a no load is carried out with and without additional standard induction motors which can be obtained from manufacturer’s data as well as from the finite-element analysis (FEA) calculation results. Fig 3 shows such standard curves for efficiency and power factor of induction motors.

Fig 3 Standard curves for efficiency and power factor of induction motors

Electric motors protection and maintenance – Electrical and mechanical faults can impose unacceptable conditions and protective devices are hence provided to quickly disconnect the motor from grid. In order to ensure that the electrical motors receive adequate protection, extensive testing is performed to verify the high quality of assembly. After a motor of a particular type has been type tested for electrical characteristics, all subsequent motors of the type undergo a routine test programme.

Routine and type test programmes can take several forms namely (i) bearing control, (ii) control of the insulation, (iii) ohmic resistance measurement, (iv) vibration measurement, (v) short circuit test, (vi) no-load test, and (vii) high voltage test, also type of programmes such as (i) routine test, (ii) no-load curve, (iii) load point, and (iv) heat run test.

After the type test programme, the electric motor is identified with a protective symbol. Degrees of protection by enclosures for electric motors are quoted with the letters IP (ingress protection) and two characteristic numerals. The first numeral designates the degree of protection for persons against contact with live or moving parts inside the enclosure and of motors against ingress of solid foreign bodies. The second numeral designates the degree of protection against harmful ingress of water. Some of the degrees of protection are IP23, IP54, IP55, and IP56. For example, in IP23, the first numeral ‘2’ means operation against contact by a finger with live or moving parts inside the enclosure while the second numeral ‘3’ denotes protection against water.

Normally, when deciding on a particular type of motor protection, it is to be done with actual operating conditions in mind. Motors are protected by current–dependent motor protection circuit breakers and / or over current relays. These are particularly effective in cases like locked rotor or interrupted run-up, and are hence indispensable in large motors with thermally critical rotor.

The temperature-dependent devices serve to protect the motor against the effects of excessive winding heating because of the overload, increased ambient temperature, impaired cooling, intermittent operation, high switching frequency, and phase failure.

Unscheduled downtime and resultant high repair cost reduce profits. There is a need to set objectives to manage maintenance, schedule repair, adjustments, and control cost. When carrying out servicing or repairing electric motors, the things which are to be done are (i) to make sure the unit is off-line, (ii) to make sure the motor is stopped, (iii) to make sure all equipments are disconnected, and (iv) to make sure all capacitors are discharged.

The motor is a component of the motor set and is to be tested with the entire system. The service manual provides tests to determine if the motor is the cause of a motor set malfunction.

The procedure which is to be used to help identify and define the problem, consists of (i) to perform visual checks to help identify the problem, (ii) if previous tests have been performed from the service manual, the test results can be used to help identify the problem, (iii) to check trouble-shooting knowledge-bases to help identify the problem, and (iv) to perform the functional test to help identify the problem. After the work has been carried out on the motor, the motor is to be marked by an additional name-plate with such data as (i) date, (ii) operative organization, (iii) if necessary, mode of repair, and (iv) if necessary, signature of the specialist.

There are three major motor types, although they differ in physical construction and appear to be quite different from each other, are in fact governed by the same basic laws. Their behaviour can be explained by considering the same fundamental principles of voltage and torque production. Different analytical techniques can be used for the motors, and different forms of torque or voltage equations can be derived for them, but the forms of the equations differ merely to reflect the difference in construction of the motors. For example, analysis shows that in DC motors, the stator and rotor flux distributions are fixed in space, and a torque is produced because of the tendency of these two fluxes to align. The three basic and normally used motors namely DC motor, induction motor, and synchronous motor as described below.

DC motors – In the DC motors, the field winding is placed on the stator and the armature winding on the rotor. A direct current is passed through the field winding to produce flux in the machine. Voltage induced in the armature winding is alternating. A mechanical commutator and a brush assembly function as a rectifier or inverter, making the armature terminal voltage unidirectional.

DC motors are built the same way as generators are. Hence, a DC machine can operate either as a motor or as a generator. To illustrate, consider a DC motor in which the armature, initially at rest, is connected to a DC source by means of a switch. The armature has a resistance, and the magnetic field is created by a set of permanent magnets. As soon as the switch is closed, a large current flow in the armature since its resistance is very low. The individual armature conductors are immediately subjected to a force since they are immersed in the magnetic field created by the permanent magnets. These forces add up to produce a powerful torque, causing the armature to rotate.

DC motors drive devices such as hoists, fans, pumps, calendars, punch-presses, and automobiles. These motors can have a definite torque-speed characteristic (such as a pump or fan), or a highly variable one (such as a hoist or an automobile). The torque-speed characteristic of the motor is to be adapted to the type of the load it has to drive, and this need has given rise to three basic types of DC motors namely (i) shunt motors, (ii) series motors, and (iii) compound motors as shown in Fig 4.

Fig 4 Basic types of DC motors

DC motors are seldom used in ordinary industrial applications since all electric utility systems furnish AC. However, for special applications such as in steel plants, mines, and electric trains, it is sometimes advantageous to transform the AC into DC in order to use DC motors. The reason is that the torque-speed characteristics of DC motors can be varied over a wide range while retaining high efficiency.

The DC motors are versatile and extensively used in industry. A wide variety of volt-ampere or torque-speed characteristics can be got from different connections of the field windings. Their speed can be controlled over a wide range with relative ease. Large DC motors (in tens or hundreds of kilowatts power rating) are used in machine tools, printing presses, conveyors, fans, pumps, hoists, cranes, paper mills, textile mills, rolling mills, and so forth. Additionally, DC motors still dominate as traction motors used in transit cars and locomotives.

Small DC motors (in fractional kilowatt power rating) are used primarily as control devices, such as tacho-generators for speed sensing, and servo-motors for positioning and tracking. The DC motors definitely plays an important role in industry. The construction of DC motor is described below

In a DC motor, the armature winding is placed on the rotor and the field windings are placed on the stator. The stator has salient poles which are excited by one or more field windings, called shunt field windings and series field windings. The field windings produce an air gap flux distribution which is symmetrical about the pole axis (also called the field axis, direct axis, or d-axis).

The voltage induced in the turns of the armature winding is alternating. A commutator-brush combination is used as a mechanical rectifier to make the armature terminal voltage uni-directional and also to make the mmf (magneto-motive force) wave, because of the armature current fixed in space. The brushes are so placed that when the sides of an armature turn (or coil) pass through the middle of the region between field poles, the current through it changes direction. This makes all the conductors under one pole carry current in one direction. As a result, the mmf because of the armature current is along the axis midway between the two adjacent poles, called the quadrature (or q) axis.

Because of the commutator and brush assembly, the armature mmf (along the q-axis) is in quadrature with the field mmf (d-axis). This positioning of the mmfs maximize torque production. The armature mmf axis can be changed by changing the position of the brush assembly. For improved performance, inter-poles (in between two main field poles) and compensating windings (on the face of the main field poles) are needed.

Induction motor – The induction motor is an AC motor which differs in several ways from the DC motor, but works on the same principle. Analysis indicates that the stator flux and the rotor flux rotate in synchronism in the air gap, and the two flux distributions are displaced from each other by a torque-producing displacement angle. The torque is produced because of the tendency of the two flux distributions to align with each other. It is highlighted at the outset that AC motors are not fundamentally different from DC motors. Their construction details are different, but the same fundamental principles underlie their operation.

The induction motor is the most rugged and the most widely used motor in the industry. Like the DC motor, the induction motor has a stator and a rotor mounted on bearings and separated from the stator by an air gap. However, in the induction motor, both stator winding and rotor winding carry AC. The AC is supplied to the stator winding directly and to the rotor winding by induction, hence the name induction motor.

In induction motor, the stator windings serve as both armature windings and field windings. When the stator windings are connected to an AC supply, flux is produced in the air gap and revolves at a fixed speed known as synchronous speed. This revolving flux induces voltage in the stator windings as well as in the rotor windings. If the rotor circuit is closed, current flows in the rotor winding and reacts with the revolving flux to produce torque. The steady-state speed of the rotor is very close to the synchronous speed. The rotor can have a winding similar to the stator or a cage-type winding. The latter is formed by placing aluminum or copper bars in the rotor slots and shorting them at the ends by means of rings.

The induction motor is undoubtedly a very useful electrical motor. It is extensively used in several applications. It is used in different sizes. Small single-phase induction motors (in fractional kilowatt power rating) are used in several domestic appliances, such as blenders, lawn mowers, juice mixers, washing machines, and refrigerators etc. Two-phase induction motors are used mainly as servo-motors in a control system. Three-phase induction motors are the most important ones, and are most widely used in industry. Large three-phase induction motors (in tens or hundreds of kilowatt power rating) are used in pumps, fans, compressors, steel plants, paper mills, and textile mills etc. The linear version of the induction motor has been developed mainly for use in transportation systems.

Unlike DC motors, induction motors have a uniform air gap. The stator is composed of laminations of high-grade sheet steel. A three-phase winding is put in slots cut on the inner surface of the stator frame. The rotor also consists of laminated ferro-magnetic material, with slots cut on the outer surface. The rotor winding can be either of two types, the squirrel-cage type or the wound-rotor type. The squirrel-cage winding consists of aluminum or copper bars embedded in the rotor slots and shorted at both ends by aluminum or copper end rings. The wound-rotor winding has the same form as the stator winding. The terminals of the rotor winding are connected to three slip rings. Using stationary brushes pressing against the slip rings, the rotor terminals can be connected to an external circuit. In fact, an external three-phase resistor can hence be connected for the purpose of speed control of the induction motor. It is obvious that the squirrel-cage induction motor (Fig 5a) is simpler, more economical, and more rugged than the wound-rotor induction motor.

Fig 5 Three-phase squirrel-cage induction motor

The three-phase winding on the stator and on the rotor (in the wound-rotor type) is a distributed winding. Such windings make better use of iron and copper and also improve the mmf wave-form and smooth out the torque developed by the motor. The winding of each phase is distributed over several slots. When current flows through a distributed winding, it produces an essentially sinusoidal space distribution of mmf.

The three-phase stator winding, which in practice is a distributed winding, is represented by three concentrated coils for simplicity. The axes of these coils are 120 electrical degrees apart. Coil aa’ represents all the distributed coils assigned to the phase-a winding for one pair of poles. Similarly, coil bb’ represents the phase-b distributed winding, and coil cc’ represents the phase-c distributed winding. The ends of these phase windings can be connected in a wye (Fig 5b) or a delta (Fig 5c) to form the three-phase connection. If balanced three-phase currents flow through these three-phase distributed windings, a rotating magnetic field of constant amplitude and speed is produced in the air gap and induce current in the rotor circuit to produce torque.

Synchronous motor – A synchronous motor rotates at a constant speed in the steady state. Unlike in induction motor, the rotating air gap field and the rotor in the synchronous motor rotate at the same speed, called the synchronous speed. Like most rotating machines, a synchronous machine can also operate as both a generator and a motor. In large sizes (several hundred or thousand kilowatts power rating) synchronous motors are used for pumps, and in small sizes (fractional kilowatt power rating) they are used in electric clocks, timers, record turntables, and so forth where constant speed is needed.

Majority of the industrial drives run at variable speeds. In industry, synchronous motors are used mainly where a constant speed is needed. In industrial drives, hence, synchronous motors are not as widely used as induction or DC motors. A linear version of the synchronous motor (LSM) is being considered for high-speed transportation systems of the future.

An important feature of a synchronous motor is that it can draw either lagging or leading reactive current from the AC supply system. A synchronous motor is a doubly excited motor. Its rotor poles are excited by a direct current and its stator windings are connected to the AC supply (Fig 5). The air gap flux is hence the resultant of the fluxes because of both rotor current and stator current.

In induction motors, the only source of excitation is the stator current, since rotor currents are induced currents. Hence, induction motors always operate at a lagging power factor, since lagging reactive current is needed to establish flux in the motor. On the other hand, in a synchronous motor, if the rotor field winding provides just the necessary excitation, the stator draws no reactive current, i.e., the motor operates at a unity power factor.

If the rotor excitation current is decreased, lagging reactive current is drawn from the AC source to aid magnetization by the rotor field current, and the motor operates at a lagging power factor. If the rotor field current is increased, leading reactive current is drawn from the AC source to oppose magnetization by the rotor field current, and the motor operates at a leading power factor. Hence, by changing the field current, the power factor of the synchronous motor can be controlled. If the motor is not loaded, but is simply floating on the AC supply system, it hence behaves as a variable inductor or capacitor as its rotor field current is changed. A synchronous motor with no load is called a synchronous condenser. It can be used in power transmission systems to regulate line voltage.

In industry, synchronous motors are sometimes used with other induction motors and operated in an over-excited mode so that they draw leading current to compensate the lagging current drawn by the induction motors, thereby improving the overall plant power factor.

The stator of the three-phase synchronous motor has a three-phase distributed winding similar to that of the three-phase induction motor. Unlike the DC motor, the stator winding, which is connected to the AC supply system, is sometimes called the armature winding. It is designed for high voltage and current.

The rotor has a winding called the field winding, which carries DC. The field winding on the rotating structure is normally fed from an external DC source through slip rings and brushes. The basic structure of the synchronous motor is shown in Fig 6.

Fig 6 Basic structure of the three-phase synchronous motor

Synchronous motors can be broadly divided into two groups namely (i) high-speed motors with cylindrical (or non-salient pole) rotors, and (ii) low-speed motors with salient pole rotors. The cylindrical or non-salient pole rotor has one distributed winding and an essentially uniform air gap. On the other hand, salient pole rotors have concentrated windings on the poles and a non-uniform air gap. Salient pole motors have a large number of poles, sometimes as many as 50, and operate at lower speeds. Smaller salient pole synchronous motors in the range of 50 kW to 5 MW are also used. Salient pole synchronous motors are used to drive pumps, cement mixers, and some other industrial drives.

Single phase motors – Single-phase motors are small motors, mostly built in the fractional kilowatt power rating range. These motors are used for several types of equipment in homes, offices, shops, and factories. They provide motive power for washing machines, fans, refrigerators, lawn mowers, hand tools, record players, blenders, juice makers, and so on. In fact, the number of single-phase (fractional kilowatt power rating) motors today far exceeds the number of integral kilowatt power motors of all types. Single-phase motors are relatively simple in construction. However, they are not always easy to analyze. Because of the large demand, the market is competitive, and designers use tricks to save production costs. Fig 7 shows single phase motor.

Fig 7 Single phase motor

Single-phase motors are of three types as given below.

The first is single-phase induction motors. These motors have cage rotors and a single-phase distributed stator winding. Such a motor inherently does not develop any starting torque and hence does not start to rotate if the stator winding is connected to an AC supply. However, if the rotor is given a spin or started by auxiliary means, it continues to run.

The majority of fractional kilowatt power rating motors are of the induction type. They are classified as per the methods used to start them and are referred to by names descriptive of these methods. Some common types are resistance-start (split-phase), capacitor-start, capacitor-run, and shaded-pole.

The second is single-phase synchronous motors. Motors of the synchronous type run at constant speed and are used in applications such as clocks and turntables where a constant speed is needed. Two types of single-phase synchronous motors which are in common use are (i) the reluctance type, and (ii) the hysteresis type.

The third is single-phase series (or universal) motors. Motors of the series type can be used with either a DC supply or a single-phase AC supply. These motors provide high starting torque and can operate at high speed. They are widely used in kitchen equipment, portable tools, and vacuum cleaners, where high-speed operation permits high kilowatt power rating per-unit motor size.

In synchronous motors, the rotor carries the field winding and the stator carries the armature winding. The field winding is excited by direct current to produce flux in the air gap. When the rotor rotates, voltage is induced in the armature winding placed on the stator. The armature current produces a revolving flux in the air gap whose speed is the same as the speed of the rotor, hence the name synchronous motor.