Speed Control of Induction Motors

Speed Control of Induction Motors

Recall the equation for synchronous speed:

Ns = 120 f / p

Therefore, to control the speed of an induction

Motor, we can control the frequency of the

Supply. Changing the frequency changes the

Nominal speed of the machine.

However, we also want to keep the flux (Ö) in the

Machine at the design value. Recall the flux

Linking equation:

V = 4.44 Ö N f

·  Speed Control

· Pole Changing

Early machines were designing with multiple poles to facilitate speed control by pole changing. By switching in different numbers or combinations of poles, a limited number of fixed speeds could be obtaining.

· Variable Rotor Resistance

The speed of induction motors can however be varied over a limited range by varying the rotor resistance as noted in the section on slip but only by using wound rotor designs negating many of the advantages of the induction motor.

· Variable Frequency

Since motor speed depends on the speed of the rotating field, speed control can be effecting by changing the frequency of the AC power supplied to the motor.

As in most machines, the induction motor is designed to work with the flux density just below the saturation point over most of its operating range to achieve optimum efficiency.

The flux density B is given by:

B = K₂ (V/f)

Where V is the applied voltage, f is the supply frequency and k₂ is a constant depending on the shape and configuration of the stator poles.

In other words if the flux density is constant, the Volts per Hertz is also a constant. This is an important relationship and it has the following consequences.

• For speed control, the supply voltage must increase in step with the frequency; otherwise, the flux in the machine will deviate from the desired optimum operating point. Practical motor controllers based on frequency control must therefore have a means of simultaneously controlling the motor supply voltage. This is known as Volts/Hertz control.
• Increasing the frequency without increasing the voltage will cause a reduction of the flux in the magnetic circuit thus reducing the motor's output torque. The reduced motor torque will tend to increase the slip with respect to the new supply frequency. This in turn causes a greater current to flow in the stator, increasing the IR volt drop across the windings as well as the I²R copper losses in the windings. The result is a major drop in the motor efficiency. Increasing the frequency still further will ultimately cause the motor to stall.
• Increasing the voltage without increasing the frequency will cause the material in the magnetic circuit to saturate. Excessive current will flow giving rise to high heat dissipation due to I²R losses in the windings and high eddy current losses in the magnetic circuit and ultimately failure of the motor due to overheating. Increasing the voltage will not force the motor to exceed the synchronous speed because as it approaches the synchronous speed the torque drops to zero.
  
  The variable frequency is normally providing by an . inverter.See more about motor control

Note also that since the induced current in the rotor is proportional to the flux density and the flux density in turn is proportional to the line voltage, the torque, which depends on the product of the flux density and the rotor current, is proportional to the square of the line voltage V.

V/f CONTROL THEORY

As we can see in the speed-torque characteristics, the

Induction motor draws the rated current and delivers

The rated torque at the base speed. When the load is

Increased (over-rated load), while running at base

Speed, the speed drops and the slip increases. As we

Have seen in the earlier section, the motor can take up

To 2.5 times the rated torque with around 20% drop in

the speed. Any further increase of load on the shaft

Can stall the motor.

The torque developed by the motor is directly

Proportional to the magnetic field produced by the stator.

Therefore, the voltage applied to the stator is directly

Proportional to the product of stator flux and angular velocity.

This makes the flux produced by the stator

Proportional to the ratio of applied voltage and frequency of supply.

By varying the frequency, the speed of the motor can

Be varied. Therefore, by varying the voltage and

Frequency by the same ratio, flux and hence, the

Torque can be kept constant throughout the speed range.

Clearly, Ö is proportional to V / f. Therefore, as we

vary the frequency, we must also vary the

voltage in proportion. (Volts per Hertz Rule)

The effect of VVVF speed control is to retain the

shape of the torque-speed curve, but shift it

along the speed axis.

 

*For VVVF control, because the shape of the

Torque-speed curve is the same at all

frequencies, it follows that the torque of an

induction motor is the same whenever the

slip speed (rpm) is the same.

Slip speed = Synchronous Speed – Actual Speed

• With VVVF control, the speed range possible is from

about 10% to 150% of rated speed.

• Below 10% of rated speed, the volts per hertz ratio

has to be progressively increased to compensate for

the IR drop in the stator. This is because at very low

frequencies the stator resistance dominates the

magnetizing reluctance (= 2 ð f L).

• Above rated the speed is limited by centrifugal

. forces on the rotor

• To implement VVVF control we need a

VVVF AC supply

• A supply of this nature is realized with

power electronics.

  

Typical torque curves for different line frequencies. By varying the line frequency with an inverter, induction motors can be kept on the stable part of the torque curve above the peak over a wide range of rotation speeds. However, the inverters can be expensive, and fixed line frequencies .