Speed Control of Three Phase Induction Motor

SPEED CONTROL OF THREE PHASE INDUCTION MOTOR

Induction Motor:

Constant speed motor is like a d.c. shunt motor

D.C. Shunt motor:

i. Speed can be varied smoothly just by using simple rheostats

ii. This maintains the speed regulation and efficiency of d.c. shunt motor.

Three Phase induction motors:

i. It is very difficult to achieve smooth speed control

ii. If speed control is achieved means the Induction motor power factor, efficiency etc. gets adversely affected.

Methods of speed control

1. From stator side.

2.From rotor side

Stator side

i. Supply frequency control to control Ns called v/f control

ii. Supply voltage control

iii. Controlling number of stator poles to control Ns

iv. Adding rheostats in stator circuit

Rotor side

i. Adding external resistance in the rotor circuit.

ii. Cascade control

iii. Injecting slip frequency voltage into the rotor circuit

a. Kramer system

b. Scherbius system

(a) Supply frequency control (or) V/f control

K₁ - Stator winding constant

T_ph1 - Stator tums per phase

V - Supply voltage

f- Supply frequency

f→ Varies that affects air gap flux also gets affected. Saturation of Stator and

rotor cores.

Hence it is necessary to maintain air gap flux constant. When supply frequency fs changed.

V/f→ Constant

Disadvantages

i. Supply obtained cannot be used to supply other devices which require constant voltage. hence an individual scheme for a separate motor is required which makes it costly.

(b) Supply voltage control

Low slip region

Disadvantages

i. Reduction in voltage, current drawn by the motor increases.

ii. Large change in voltage for small change in speed is required is the biggest disadvantage.

iii. Increase in current, the motor may set overheated, additional voltage changing equipment is necessary.

iv. Hence this method is rarely used in practice.

This method is used like motors driving fan type of loads.

(c)Controlling Number of poles :

It is possible to have one, two (or) four speeds control steps by changing the no. of stator poles.

Disadvantages

A continuous smooth speed control is not possible by this method.

1. Consequent poles method

2. Multiple stator winding method

3. Pole amplitude Modulation method

For, P = 8, f = 50 Hz, → N_S = 750 rpm

For, P = 4, f = 50 Hz, → N_S = 1500 rpm

Disadvantages

Speed change is in step and smooth speed control is not possible. Similarly the method can be used only for the squired cage type motor as squired cage rotor adjusts itself to same no. of poles as stator which is not the case in slipning Induction motor.

(c) Multiple Stator winding method

Limitations

1. Can be applied to only squirrel cage motor

2. Smooth speed control is not possible.

3. Two different stator windings are required to be wound which increases the cost of the motor.

4. Complicated from the design point of view

3. Pole amplitude modulator method

Basic disadvantage: Non availability of smooth, speed control is eliminated by this method.

Basic principle of this method is the modulation of two sinusoidally varying mmf waves with different number of poles.

Advantages

i. Reduces the size to a greater extent and hence cost of machine.

Limitations

i. It can be used only for squirrel cage motor.

(d) Adding Rheostats in stator circuit

Disadvantages

i. Large power

ii. This method is not efficient from speed control point of view hence used as a stator rather than as a speed control method.

Rotor Circuit

Advantages

i. R₂ ↑, T↓ = but when the load is same

T_st of motor ↑ α to rotor resistance

Disadvantages

i. Large speed control is not possible.

ii. Large speed → Need Large resistance → cause. Large rotor copper loss to reduce the η.

iii. This method can not be used for the Squirrel cage I.M.

iv. The speed above the normal values can not be obtained

v. Large power losses occur due to large I'R loss

vi. Sufficient cooling arrangements are required which make the external rheostats bulky and expensive

vii. Due to large power losses, n is low

This method is rarely used in practice.

(a) Rotor Rheostat Control

i. Applicable to slip-ring motors alone.

ii. Motor speed is reduced by introducing an external resistance in the rotor circuit.

iii. This method is in fact, similar to the armature rheostat control method of d.c. shunt motors.

T α S/R₂

For a given torque → Slip can be increased i.e. speed can decreased by increasing the rotor resistance R₂.

Serious Disadvantages:

i. With increase in rotor resistance, I²R losses also increase which decrease the operating η (efficiency) of the motor. Loss is a directly proportional to the reduction in the speed.

ii. Double dependence of speed - not only on R₂ but on load as well.

iii. Due to above disadvantages, it is used where speed changes are needed for short periods only.

(b) Cascade (or) Concatenation (or) Tandem operation

Two motors are ordinarily mounted on the same shaft (or) both run at the same speed.

Stator winding of the main motor 'A' is connected to the mains, while that of the auxiliary motor 'B' is fed from the rotor circuit of motor 'A'.

Main motor 'A' should be phase wound (a) slipring type with stator to rotor winding ratio of 1:1, so each motor may be run from the supply mains separately.

3 ways (some times four ways) to run the Induction motor.

1. Main motor 'A' may be run separately from the supply.

In this case N_sa =120 f / P_a, P_a = No. of stator poles of motor A

2. Auxiliary motor B may be run separately from the mains (with motor A being disconnected)

In this case

N_sb = 12f/P_b , P_b = No. of stator poles of motor B

3. The combination may be commulative, such a way that the phase rotation of the stator fields of both motors is in the same direction.

The synchronous speed of the cascaded set

N_sc = 120 f / (P_a+ P_b)

Differential cascade

In this method, the phase rotation of stator field of the motor B is opposite to 30 that of the stator of motor A. This reversal of phase rotation of stator of motor B is obtained by interchanging any of its two leads.

This method is rarely used.

The above expression for synchronous speed becomes meaningless for P_a = P_b

(c) Injecting an e.m.f. in the Rotor circuit

i. The speed of an induction motor is controlled by injecting a voltage in the rotor circuit.

ii. The injected voltage to have the same frequency as the slip frequency.

iii. When we insert a voltage which is in phase opposition to the induced rotor e.m.f. it amounts to increase the rotor resistance.

iv. Inserting a voltage which is in phase with the induced rotor emf is equivalent to decreasing its resistance.

v. Speed control using Kramer system, used in the case of large motors of 4000 kW (or) more. (steel rolling mills, large induction motors).

Rotary converter - Converts the low slip frequency a.c. power into d.c. power, which is used to drive a d.c. shunt motor 'D' mechanically coupled to the main motor 'M'.

Main motor is coupled to the shaft of the d.c. shunt motor D. The slip-rings of M are connected to those of the notary converter C. The d.c. output of C is used to drive D. Both C and D are excited from the d.c. bus-bars (or) from an exciter.

There is a field regulator which governs the back emf of E, of D and hence the d.c. potential at the commutator of C which further controls the slip-ring voltage and therefore, the speed of M.

Advantages:

i. Smooth speed control is possible.

ii. Wide range of speed control is possible.

iii. Any speed, within the working range can be obtained.

Rotary converter is over-excited, it will take a leading current which compensates for the lagging current draw by main motor M and hence improves the power factor of the system.

Design of a rotary converter is practically independent of the speed control.

(d) Scherbius system

i. Controlling the speed of large I.M.

ii. The slip energy is not converted into dc and then fed to a d.c. motor, rather it is fed directly to a special 3-phase (or 6-phase) a.c. commutator motor is called a scherbius machine.

iii. The poly Phase winding of machine C is supplied with the low-frequency output of machine M through a regulating Transformer (RT).

iv. The commutator motor C is a variable-speed motor and its speed (and hence that of M) is controlled by either varying the tappings on RT (or) by adjusting the position of brushes on C.

Disadvantages:

Used only for slipring induction motor.