# V/f speed control of 3 phase Induction motors

• makkena
In summary, the conversation discussed the relationship between torque and slip frequency in a 3-phase induction motor where the voltage and frequency are maintained constant. There is confusion about whether torque is proportional to slip only or slip and frequency. The question also involves finding the speed at 30 Hz with a constant load torque. The answer is 840 rpm, which is determined by the equation T proportional to slip X frequency. Some textbooks suggest an alternate equation involving slip, voltage, and frequency, but the answer given by GK publishers is based on the first equation.
makkena
When the voltage / frequency is maintained constant in 3 phase Induction motor i.e. flux is maintained constant by changing the voltage and frequency in same proportion, What is the torque proportionality equation.
Whether

Torque is proportional to Slip X Frequency

or

Torque is proportional to only Slip ?

Based on the above equation I need to solve a question given in GATE EE paper 2006.

The speed of a 4-pole induction motor is controlled by varying the supply frequency while maintaining the ratio of supply voltage to supply frequency (V/f) constant. At rated frequency of 50 Hz and rated voltage of 400 V its speed is 1440 rpm. Find the speed at 30 Hz, if the load torque is constant.
(A) 882rpm
(B) 864 rpm
(C) 840 rpm
(D) 828 rpm

GK publishers Gate previous papers Book is given the answer as 864 rpm. This answer will come if we use T proportional to Slip.

But I saw in some textbooks Torque proportional to slip X (square of Voltage) / f.
From this eauation when V/f is constant, T should be proportional to slip X Voltage i.e. T proportional to slip X frequency.

I am confusing on this which Torque equation to follow when V/f is maintained constant.

Thanks

If the flux is maintained at the same level then for the same torque you require the same slip frequency. So the answer is "C".

thanks uart.

## What is V/f speed control of 3 phase Induction motors?

V/f speed control is a method of controlling the speed of a three-phase induction motor by varying the voltage and frequency of the power supply. It is also known as voltage-frequency control.

## How does V/f speed control work?

V/f speed control works by changing the frequency of the power supply while keeping the ratio of voltage to frequency (V/f) constant. This results in a change in the speed of the motor, as the motor's speed is directly proportional to the frequency of the power supply.

## What are the advantages of V/f speed control?

V/f speed control offers a cost-effective and energy-efficient way to control the speed of three-phase induction motors. It also provides smooth acceleration and deceleration, reduces stress on the motor, and allows for precise speed control.

## What are the limitations of V/f speed control?

One of the limitations of V/f speed control is that it can only be used for controlling the speed of induction motors within a certain range. It is also not suitable for applications that require high starting torque or constant torque at low speeds.

## What are the common applications of V/f speed control?

V/f speed control is commonly used in industries such as HVAC, pumps, fans, and conveyor systems. It is also used in household appliances such as refrigerators and washing machines, as well as in electric vehicles.

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