Classification of Electrical MotorsIntroduction

We all know that without electricity, we cannot do any of our job. If we look into the world we live in, the major development in technology and civilization took place only after the introduction of electricity and electrical devices. Can we imagine a life with out air conditioners/ceiling fans, lights, computers and communication devices and many more.

So it is quiet evident that electricity and electrical equipments play a vital role in every inch of our life. One such equipment which created a giant leap to the mankind in both domestic & industrial sectors is the "Motor". The usage of AC motors is much more prevalent than DC

motors due to several practical reasons which we shall learn later on.

A.C. Motors are playing a very vital role in everyday life, right from pumping water to overhead tank to modern robot's manoeuvring arm. The main factor which lead to the adoption & wide usage in various fields is its flexibility and its huge variety which can be matched with almost any kind of demand. To know what are the different types of A.C.Motors available, to match it perfectly with the demand, it is highly essential to know about the different classifications of A.C.Motors.

Types of AC Motors

Classification Based On Principle Of Operation:

(a) Synchronous Motors.

1. Plain

2. Super

(b) Asynchronous Motors.

1. Induction Motors:

(a) Squirrel Cage

(b) Slip-Ring (external resistance).

2. Commutator Motors:

(a) Series

(b) Compensated

(c) Shunt

(d) Repulsion

(e) Repulsion-start induction

(f) Repulsion induction

Classification Based On Type Of Current:

1. Single Phase

2. Three Phase

Classification Based On Speed Of Operation:

1. Constant Speed.

2. Variable Speed.

3. Adjustable Speed.

Classification Based On Structural Features:

1. Open

2. Enclosed

3. Semi-enclosed

4. Ventilated

5. Pipe-ventilated

6. Riveted frame-eye etc..

Overview of A.C.Motors

1. Synchronous Motors & it's Uses: These motors have the rotor(which is connected to the load) rotating at the same speed as the speed of rotation of the stator current. In other words, we can say these motors don't have slip with respect to the stator current. They are sometimes used no to drive the load but instead act as "synchronous condenser", to improve the power factor of the local grid to which it is connected to. These kind of motors are used even in high precision positioning devices like modern robots.They can also act as stepper motors.

2. Asynchronous Motors & it's Uses: The most common form of motor which is used in everyday life from pumping water up the overhead tank to power plant boiler feed pumps, these kind of motors rule. These motors are very flexible to use and matches the load demand almost

for everything. The most widely used Induction Motors are very important for many industries due to their load bearing capacity and flexibility. These motors,unlike synchronous motors, slip when compared to the stator current field. They are generally used for various types of pumps, compressors and acts as prime movers for many machineries.

3. Single & Three Phase Motors and their Uses: The A.C.Motors can find their usage in 2 forms based on their power supply. The single phase motors are generally found their use in low power requirements/domestic appliances like ceiling fans, mixer grinders, portable power tools etc. The three phase motors are generally found for high power requirements like power drives

for compressors, hydraulic pumps, air conditioning compressors, irrigation pumps and many more.

4. Constant, Variable & Adjustable Speed Motors: As already said, A.C.Motors are highly flexible in many ways including their speed control. There are motors which should be run at a constant speed for air compressors. Certain cooling water pumps driven by a.c.motors can be run at two or three speeds by just switching the number of poles used. If the number of poles are changed then the speed also changes. These serve best for sea water cooling pumps in marine engine room applications & many power plants. The speed of the motors can also be varied continuously by some electronic arrangements thus this can be suited for certain applications like a ship's cargo pump, whose discharge rate has to lowered as per the terminals requirement.

5. Varied Structure Motors: These types of motors have different outer cage arrangements, depending upon the usage or any special industrial requirement. For motors used in gas and oil terminals, the casing must be of intrinsically safe, thus it may either have a enclosed casing or a pipe ventilated arrangement such that the sparks produced inside the motor does not cause a fire outside it. Also many motors are totally enclosed as it may be open to weather like those used in hydro-electric power plants.

STATE the applications of the following types of AC motors:

a. Induction b. Single-phase c. Synchronou

b. AC Motors AC MOTOR TYPES AC MOTOR TYPES Various types of AC motors are used for specific applications.   By matching the type  of  motor  to  the  appropriate  application, increased  equipment  performance can be obtained. EO 1.5 DESCRIBE  how  torque  is produced  in  a  single-phase AC motor. EO 1.6 EXPLAIN why an AC synchronous motor does not have starting torque. EO 1.7 DESCRIBE how an AC synchronous motor is started. EO 1.8 DESCRIBE the effects of over and under-exciting an AC synchronous motor. EO 1.9 STATE  the  applications  of  the  following  types  of  AC motors: a. Induction b. Single-phase c. Synchronous Induction Motor Previous  explanations  of  the  operation  of  an  AC motor  dealt  with  induction  motors.    The induction motor is the most commonly used AC motor in industrial applications because of its simplicity,  rugged  construction,  and relatively  low  manufacturing  costs.    The  reason  that  the induction  motor  has  these characteristics  is  because  the  rotor  is  a  self-contained  unit,  with  no external connections.   This  type  of  motor  derives  its  name  from  the  fact  that  AC  currents  are induced into the rotor by a rotating magnetic field. The induction motor rotor (Figure 5) is made of a laminated cylinder with slots in its surface. The windings in the slots are one of

two types.  The most commonly used is the "squirrel-cage" rotor.   This rotor is made of heavy copper bars that are connected at each end by a metal ring made of copper or brass. No insulation is required between the core and the bars because of the low  voltages  induced into  the  rotor  bars.    The  size  of  the  air  gap  between  the  rotor  bars  and stator windings necessary to obtain the maximum field strength is small.

Single-Phase AC Induction Motors If two stator windings of unequal impedance are spaced 90 electrical degrees apart and connected in parallel to a single-phase source, the field produced will appear to rotate. This is called phase splitting. In a split-phase motor, a starting winding is utilized. This winding has a higher resistance and lower reactance than the main winding (Figure 6). When the same voltage VT is applied to the starting and main windings, the current in the main winding (IM) lags behind the current of the starting winding IS (Figure 6). The angle between the two windings is enough phase difference to provide a rotating magnetic field to produce a starting torque. When the motor reaches 70 to 80% of synchronous speed, a centrifugal switch on the

motor shaft opens and disconnects the starting winding. Single-phase motors are used for very small commercial applications such as household appliances and buffers.

Synchronous Motors Synchronous motors are like induction motors in that they both have stator windings that produce a rotating magnetic field. Unlike an induction motor, the synchronous motor is excited by an external DC source and, therefore, requires slip rings and brushes to provide current to the rotor. In the synchronous motor, the rotor locks into step with the rotating magnetic field and rotates at synchronous speed. If the synchronous motor is loaded to the point where the rotor is pulled out of step with the rotating magnetic field, no torque is developed, and the motor will stop. A synchronous motor is not a self-starting motor because torque is only developed when running at synchronous speed; therefore, the motor needs some type of device to bring the rotor to synchronous speed. Synchronous motors use a wound rotor. This type of rotor contains coils of wire placed in the rotor slots. Slip rings and brushes are used to supply current to the rotor. (Figure 7).

Types of Motors

Direct Current (DC) Motors

DC motor—Click image to enlarge and view reference

Direct-current (DC) motors are often used in variable speed applications. The DC motor can be designed to run at any speed within the limits imposed by centrifugal forces and commutation considerations. Many machine tools also use DC motors because of the ease with which speed can be adjusted.

All DC motors, other than the relatively small brushless types, use a commutator assembly on the rotor. This requires periodic maintenance and is partly responsible for the added cost of a DC motor when compared to an alternate-current (AC) squirrel-cage induction motor of the same power. The speed adjustment flexibility often justifies the extra cost.

Alternate Current (AC) Motors

AC motorClick image to enlarge and view reference

Alternate-current (AC) motors - As in the DC motor case, an AC motor has a current passed through the coil, generating a torque on the coil. The design of an AC motor is considerably more involved than the design of a DC motor. The magnetic field is produced by an electromagnet powered by the same AC voltage as the motor coil. The coils that produce the magnetic field are traditionally called the "field coils" while the coils and the solid core that rotates is called the "armature."

Two basic types of AC motors:

Induction motor—The induction motor is a three-phase AC motor and is the most widely used machine. Its characteristic features are:

Simple and rugged construction. Low cost and minimum maintenance.

High reliability and sufficiently high efficiency.

Needs no extra starting motor and does not need to be synchronized.

Two basic assemblies make up an induction motor: the stator assembly and the rotor assembly. The motor "induces" current into the rotor by the rotating magnetic flux produced in the stator. Motor torque is developed from interaction of currents flowing in the rotor bars and the stator's rotating magnetic field. When a three-phase voltage is applied to the stator winding, a rotating magnetic field of constant magnitude is produced. This rotating field is produced by the contributions of space-displaced phase windings carrying appropriate time displaced currents. The rotating field induces an electromotive force (emf).

Synchronous motor—The most obvious characteristic of a synchronous motor is its strict synchronism with the power line frequency. The reason the industrial user is likely to prefer a synchronous motor is its higher efficiency and the opportunity for the user to adjust the motor's power factor.

A specially designed motor controller performs these operations in the proper sequence and at the proper times during the starting process.

AC Motor Types Summary In a split-phase motor, a starting winding is utilized. This winding has a higher resistance and lower reactance than the main winding. When the same voltage (VT) is applied to the starting and main windings, the current in the main winding lags behind the current of the starting winding. The angle between the two windings is enough phase difference to provide a rotating magnetic field to produce a starting torque. A synchronous motor is not a self-starting motor because torque is only developed when running at synchronous speed. A synchronous motor may be started by a DC motor on a common shaft or by a squirrel-cage winding imbedded in the face of the rotor poles. Keeping the same load, when the field excitation is increased on a synchronous motor, the motor operates at a leading power factor. If we reduce field excitation, the motor will operate at a lagging power factor. The induction motor is the most commonly used AC motor in industrial applications because of its simplicity, rugged construction, and relatively low manufacturing costs. Single-phase motors are used for very small commercial applications such as household appliances and buffers. Synchronous motors are used to accommodate large loads and to improve the power factor of transformers in large industrial complexes.

(a) Synchronous Motors.1. Plain2. Super(b) Asynchronous Motors.1. Induction Motors:(a) Squirrel Cage(b) Slip-Ring (external resistance).2. Commutator Motors: (a) Series(b) Compensated(c) Shunt(d) Repulsion(e) Repulsion-start induction(f) Repulsion induction

Classification Based On Type Of Current:1. Single Phase2. Three Phase

Classification Based On Speed Of Operation:1. Constant Speed.2. Variable Speed.3. Adjustable Speed.

Classification Based On Structural Features:1. Open2. Enclosed3. Semi-enclosed4. Ventilated5. Pipe-ventilated6. Riveted frame-eye etc..