AC Motors (Single & Three phase AC motors)

Single Phase Induction Motor

The single phase induction machine is the most frequently used motor for refrigerators, washing machines, clocks, drills, compressors, pumps, and etc.
The single-phase motor stator has a laminated iron core with two windings arranged perpendicularly.

One is the main and
The other is the auxiliary winding or starting winding

The single-phase induction motor operation can be described by two methods:
Double revolving field theory; and
Cross-field theory.
Double revolving theory is perhaps the easier of the two explanations to understand.

Double revolving field theory:

A single-phase ac current supplies the main winding that produces a pulsating magnetic field.
Mathematically, the pulsating field could be divided into two fields, which are rotating in opposite directions.
The interaction between the fields and the current induced in the rotor bars generates opposing torque.
The rotor is invariably of the squirrel cage type.
In practice, in order to convert temporarily the single phase motor into two-phase motor, auxiliary conductors are placed in the upper layers of stator slots.
The auxiliary winding has a centrifugal switch in series with it.
The function of the switch is to cut off the starting winding, when the rotor has accelerated to about 75% of its rated speed.
In capacitor-start motors, an electrolytic capacitor of suitable capacitance value is also incorporated in the starting winding circuit.
The main stator winding and auxiliary (or starting) winding are joined in parallel, and there is an arrangement by which the polarity of only the starting winding can be reversed.
This is necessary for changing the direction of rotation of the rotor.

STARTING METHODS OF SINGLE-PHASE INDUCTION MOTORS:

A single-phase induction motor with main stator winding has no inherent starting torque, since main winding introduces only stationary, pulsating air gap flux wave.
For the development of starting torque, rotating air gap field at starting must be introduced.
Several methods which have been developed for the starting of single phase induction motors, may be classified as follows:
a) Split-phase starting.
b) Shaded-pole starting.
c) Repulsion-motor starting and
d) Reluctance starting.
A single phase induction motor is commonly known by the method employed for its starting.
The selection of a suitable induction motor and choice of its starting method, depend upon the following:
(i) Torque speed characteristic of load from standstill to the normal operating speed.
(ii) The duty cycle and
(iii)The starting and running line-current limitations as imposed by the supply authorities.

SPLIT-PHASE STARTING:

Single phase induction motors employing this method of starting are called Split phase motors.
All the split phase motors have two stator windings, a main (or running) winding and an auxiliary (or starting) winding.
Both these windings are connected in parallel but their magnetic axes are space displaced by 90 degree electrical.
It is known that when two windings spaced 90 degree apart on the stator, are excited by two alternating e.m.f. that are 90 degree displaced in time phase, a rotating magnetic field is produced.
If two windings so placed are connected in parallel to a single phase source, the field produced will alternate but will not revolve since the two windings are equivalent to one
single phase winding.
If impedance is connected in series with one of these windings, the currents may be made to differ in time phase, thereby producing a rotating field.
This is the principle of phase splitting.
Split phase motors are of following types.

Resistor split phase motors
Capacitor split-phase motors
Capacitor start and run motors
Capacitor run motors

3-phase INDUCTION MOTORS

Three phase Induction Motors are the most widely used ac motors.
This type of motor converts alternating current electrical energy into mechanical energy.

Construction:

It mainly consists of two parts i) Stator (ii) Rotor.
The rotor is the rotating part.
The stator is the stationary part.
They are separated by a small air gap.

Stator:

The Stator of an Induction Motor is very similar in construction to the stator of an Alternator as shown in Figure.
It is a hollow and cylindrical core having slots in its inner surface to house windings.
It consists of a set of silicon steel laminations attached to the yoke as shown in figure below.
In the slots of the laminations, stator conductors are placed with proper insulation.
These conductors are properly interconnected to form a balanced star or delta connected winding.

Rotor:

There are two types : (i) Squirrel cage rotor (ii) Phase wound rotor (Slip ring rotor) and are shown in Figure.
(i)Squirrel cage rotor:
The copper or aluminum heavy bars form the rotor conductors.
One bar is placed in each slot.
Slots are made of steel laminations.
All the bars are welded at both ends to two copper end rings thus short circuiting them at both ends.
Since they are short circuited on both ends, no external resistance can be connected to it.
This type of rotor has low starting torque.
To look at, it resembles a cage, hence the name.
The motor with this type of rotor is named as Squirrel cage Induction motor.

(ii)Phase wound rotor (Slip ring rotor):

This rotor is a laminated, cylindrical core having uniform slots on its outer periphery.
3 phase winding which is star connected is placed in these slots.
The open ends of the star windings are brought out and connected to three insulated slip rings, mounted on the shaft of this rotor with carbon brushes resting on them.
The rotor winding can be shorted through external variable resistance.
The motor with this type of rotor is termed as Slip ring Induction motor.

Rotating Magnetic Field:

When a three phase supply is given to the three windings of the stator, three fluxes are produced in the three windings.
From above analysis we can say that as the angle varies from 0 to 360, the resultant flux also rotates with the same angular velocity and it has a constant magnitude.
Thus when 3phase supply is given to the stator windings of induction motor, a rotating magnetic field of constant magnitude and rotating with synchronous speed is produced.
The synchronous speed is given by NS is 120f divided by p.

Principle of Operation:

When a three phase supply is given to the three phase stator winding, a magnetic field of constant magnitude i.e. one point five times of maximum flux and rotating with the synchronous speed NS is produced.
The same field links with the rotor conductors.
The rotor conductors cut this magnetic field and an emf is induced in these conductors in accordance with the faradays laws of electromagnetic induction.
The direction of the induced emf is to oppose the very cause of it i.e, the relative speed between the rotating magnetic field and the static rotor.
As the rotor conductors are short circuited, the induced emf sets up a current in the rotor conductors in such a direction as to produce a torque which rotates the rotor in the same direction as the magnetic field so that the relative speed decreases.
The speed of the rotor gradually increases and tries to catch up with the speed of the rotating magnetic field.
But it fails to reach the synchronous speed because if it catches up with the speed of the magnetic field, the relative speed becomes zero and hence no emf will be induced in the rotor conductors.
The torque becomes zero.
Hence the rotor will not be able to catch up with the speed of the magnetic field, but rotates at a speed slightly less than the synchronous speed.
The difference between the synchronous speed Ns of the magnetic field and the actual speed of the rotor N is called as the slip speed.
The slip(S) of an induction motor is defined as the ratio of slip speed to synchronous speed.
When slip becomes unity, rotor speed will be zero.

Frequency of Rotor Current:

When the motor is stationary, frequency of the rotor current is same as the supply frequency.
But when the rotor starts rotating, the frequency depends on relative speed or slip speed.

Starters in Induction Motor:

When a 3- phase motor of higher rating is switched on directly from the mains it draws a starting current of about 4 to 7 times the full load (depending upon on the design) current.
This will cause a drop in the voltage affecting the performance of other loads connected to the mains.
Hence starters are used to limit the initial current drawn by the 3 phase induction motors.
The starting current is limited:
a)by applying reduced voltage in case of squirrel cage type induction motor (by using star delta starter where initially stator winding is connected in star connection and later its changed to delta connection)
b)by increasing the impedance of the motor circuit in case of slip ring type induction motor (by changing the value of variable resistance)

Applications:

Squirrel cage induction motor:

Squirrel cage induction motors are simple and rugged in construction,
are relatively cheap
require little maintenance.
Hence, squirrel cage induction motors are preferred in most of the industrial applications such as in
i. Lathes
ii. Drilling machines
iii. Agricultural and industrial pumps
iv. Industrial drives.

Slip ring induction motors:

Slip ring induction motors when compared to squirrel cage motors
have high starting torque
smooth acceleration under heavy loads,
adjustable speed
good running characteristics.
They are used in:
i. Lifts
ii. Cranes
iii. Conveyors , etc.,

Control System Motors

Two forms of control system are used mostly a) the regulator and b) the servosystem, or remote position control (r.p.c.) system.

The regulator is concerned with varying operational speed, whereas servomechanisms are associated with varying position.

It has been observed that the speed of a d.c. motor can be adjusted accurately.

The field winding can be even better controlled if it is excited from a separate source such as an electronic amplifier.

The r.p.c. system most commonly uses a stepper motor although it is possible to use a mechanism such as a Geneva cam.

Motors for Regulators:

In regulators, we are seeking to control the speed of the motor generally with a high degree of accuracy.
The motor which has a good torque/speed characteristic for such applications is the d.c. shunt motor.
However, in the shunt motor it is assumed that the field winding is excited from the same source as the armature winding.
This is not necessarily the case and we can have separately excited motors, i.e. motors in which the field winding is excited from a completely separate source
The d.c. motor is a relatively expensive motor to manufacture and it is not so robust as the a.c. induction motor.
The speed of the induction motor is effectively set by the supply frequency.
If we could vary the supply frequency, it follows that we could control the speed.
There are power electronic devices which allow us to vary the supply frequency.
If these electronic devices are controlled by the control system amplifier, then these are robust speed-controlled motor.
There are other regulator motors such as the variable-frequency synchronous motor, the brushless d.c. motor and the switched reluctance motor to be found in control systems, but none of these carry the general acceptance like the two considered above.

Remote position control:

The r.p.c. motor requires to move its load to a position determined by the control system.
The system experiences two limitations:

Limit of mechanical ability of a motor to move its load due to too small angle of required rotation
When the load is almost aligned to the input objective, the error is so small that the amplifier can no longer drive the motor.

By using ADC & DAC this problem of converting small values into analog values for proper driving of motor can be achieved.
Yet even here we find limitations.
First of all, the reduction gear introduces an additional load and the greater the accuracy of positioning the load, the greater the additional load on the motor & also the greater the gearing the more slack in the gear train, introducing further misalignment.
Second, the more we reduce the gearing, the more difficult to stop the motor at exactly the required position.
This can be overcome by applying a mechanical brake, but such brakes experience wear.
These limitations have been avoided by two arrangements:

Mechanisms such as the Geneva cam.
The stepping motor

Remote Position Control

Geneva cam:

The Geneva cam permits the motor to move without moving the load.
However, during a quarter of every rotation, the motor engages with the cam and moves the load through a defined angle, which is the mechanical equivalent of a digital bit.
During the remainder of the motor rotation there is no engagement, so it does not matter that we cannot stop the motor in exactly any given position.
Rather we can stop the motor anywhere that it is not engaged.
If the striker is rotated through 60 degree, the system arrives at the position shown in (b), i.e. the cam also has rotated through 60 degree.
A further rotation of the striker by 60 degree brings the system to the position shown in (c).
Here the striker has disengaged, leaving the cam rotated through exactly 90 degree.
Further movement by the motor will provide no further cam rotation until the striker continues to position (a).
The system therefore ensures exact progressions of the load and, so long as we can accept such digital progressions, it is possible to obtain accurate positioning of the load.

The stepping (or stepper) motor:

In this, when the stator poles are excited by direct current, the rotor will align itself and stop in the exact position of alignment.
If the current in the windings is reversed, it is possible that the rotor will rotate through 180 degree to be aligned in the opposite direction, but equally nothing might happen because the rotor is confused as to the direction in which to rotate, both directions being equally attractive.

To avoid this difficulty, we can develop the motor to have four salient stator poles as shown in Fig.

In this arrangement, let us excite poles 1 and 3 so that the rotor aligns accordingly.
Once stability has been achieved, let us now seek to align the rotor to poles 3 and 1.
To do this, we first excite poles 2 and 4; this causes the rotor to rotate through 90 degree, a movement which is quite decisive and causes no confusion.
Subsequently let us excite poles 3 and 1 which causes a rotation of a further 90 degree, thus resulting in a total 180 degree rotation which is the objective we were seeking.
we could obtain continuous rotation with speed control finally stopping in any one of four desired positions.
Every pulse of excitation sent to a winding steps the rotor round by 90 degree (hence the name of the motor: the stepping motor).
By counting the pulses, we can determine the rotation achieved.
This basic control is illustrated in Fig.
It will be noted that the control has two elements: the number of pulses which determine the angle of rotation, and the direction data which determine the order in which the poles are excited.

Practical stepping motors which come in two forms:1. The variable-reluctance (VR) motor. 2. The hybrid motor.

The Variable Reluctance Motor:

It avoids the need to use a rotor which is permanently magnetized by the simple expedient of the rotor having four poles and the stator six, as shown in Fig.
If the No. 1 stator poles are excited, let the marked rotor pole align itself to the No. 1 pole marked N.
In this position, the rotor poles at right angles to the marked pair have no function whatsoever since they are effectively neutral.
Let us refer to these poles as the unmarked pair.
Now let us excite the next pair of stator poles.
In this instance, the next pair is selected by moving clockwise around the stator; these are the No. 2 poles.
The salient poles of the unmarked pair are nearest to the position of alignment and therefore the rotor moves to align the unmarked rotor poles with the No. 2 stator poles.
This causes a rotation of 30degree anticlockwise.
Again, let us move on to exciting the No. 3 stator poles moving round the stator in the clockwise direction.
Once more, the nearest rotor poles to alignment come from the marked pair which align themselves by rotating a further 30degree anticlockwise.
A complete rotation of the stator excitation only causes the rotor to rotate through 180degree, hence we are required to complete two cycles of stator excitation to rotate the rotor through one revolution.
This arrangement is not dependent on the direction of the excitation current.
Instead only the order in which the pole pairs are excited is important.
By varying the numbers of poles, the angle of motion (the step angle) can be varied from the 30degree demonstrated to values such as 15degree, 22.5degree, 45degree, etc.
But all of the values are quite large.
In order to obtain smaller angles of alignment, we have to turn to the hybrid stepping motor which typically has step angles of 1.8 degree and 2.5 degree.

The Hybrid Stepping Motor:

Instead of having just four, six or eight rotor salient poles, the hybrid motor has increased the number significantly.
In Fig. (a), the rotor has 36 saliencies which have become so small that they appear as rotor teeth.
The stator poles also have teeth which are set at the same pitch.
This is emphasized by the top pole in Fig. (b), where the rotor and stator teeth are seen to match at the point of alignment.
However, the teeth are not aligned at the adjacent pole.

Instead of having just one rotor with teeth, we introduce a second rotor mounted further along the same axis.
The two arrangements are identical in construction, but between them is mounted a single permanent magnet as shown.
The result is that all the teeth on the left hand rotor are effectively N poles while all the teeth on the right hand rotor are S poles.
To balance the double rotor, we require a double stator.
It follows that all the stator poles on the left hand stator will act as S poles while all those on the right hand stator will act as N poles.
This means that the magnetic circuit has to be completed between the two stators, hence there is an external yoke.