Speed control using VFD of three phase motor

Question

what happens to Torque, HP, current and voltage when we increase Speed of three phase induction motor using a VFD?

Answer 1

The characteristic curves of a VFD-controlled induction motor are as shown below.

Up to the rated speed:

The ratio of voltage to frequency is a constant (V / f = k), with the power increasing linearly and the torque remaining constant at its rated value.

The lowered cooling effect of the in-built fan, at lower than rated speed, is not an issue as the power and consequent heating is lower.

Above the rated speed:

The voltage, and hence power, is constant at the rated value and the torque decreases.

Answer 2

If you have a 1HP motor and use a VFD to double the speed, it does not become a 2HP motor! Thus the torque has to halve. The VFD may decrease the voltage for the higher speed, so the current will increase (assuming a constant wattage load). Remember, HP or W is torque times speed. Similarly with W = V times A.

Answer 3

$$Torque \propto Current\propto (Load + Aceleration)$$ $$Voltage \propto Speed \propto frequency$$

There are various techniques in VFDs:

• V/f control: is the most common for entry levels VFD. The voltage rises with rising frequency. Current is not controlled, rather some overcurrent protection is available
• Vector control /w encoder feedback: Torque, Speed is controlled in a such way that the motor is capable to output a nominal torque even at speed=0.
• Sensorless vector control: The speed feedback is estimated from measuring the BEMF of the rotating motor. It's the most complex VFD. Since the voltage at low speed is very low, the correct motor speed can't be estimated correctly, so the motor stalls at low speed.

Answer 4

What happens when you increase the speed of a three-phase motor using a VFD depends on the type of motor, how the VFD is configured and the operating point prior to the speed increase. It also depends on the characteristics of the load.

With a squirrel-cage induction motor operating at rated frequency and voltage, you have two options.

1. Increase only the frequency while maintaining the rated voltage. In that case, the torque capability decreases approximately proportionally to V/f squared. If the torque required to operate the load remains constant or increases, the current will increase and the VFD, if properly protected, will either shut down quickly or refuse to increase the frequency and shut down within a short time. If the torque required to operate the load declines allowing the required power to remain constant, The current will remain constant until the required torque approaches the breakdown torque on the motor curve. At that point, the current will increase and the VFD protection will ast as described above.

2. Increase the frequency and voltage proportionally, maintaining the rated value of V/f. In that case, the torque capability will remain constant. If the load does not require more torque to operate at higher speed, the current will not increase and the motor will deliver more power in proportion to the increased speed and voltage. There will be a point at which the VFD can not produce a higher voltage or the increased speed or voltage causes problems for the motor.

If the VFD is operating at the motor's rated frequency and voltage, then increasing the speed by increasing the frequency with the rated value of V/f causes the available torque to remain constant. With a load that does not require more torque at increased speed, the current and torque will remain constant with the power increasing in proportion to speed. That operation can be maintained over as range of speed up to the limit of the VFD tor produce increased voltage as described above. As speed is reduced from rated speed, the V/Hz will need to increase somewhat to maintain optimum motor performance. At lower speeds, the motor loses self-ventilating air flow and may overheat if auxiliary cooling is not provided.

Answer 5

I know you think you asked a clear question. But this is all complicated. There are a lot of details.

Commonly, or in the simplest case, when you speed up a motor using a VFD, two things happen at the same time: The drive frequency goes up and the motor voltage goes up. This is how the drive increases the motor speed by altering these two variables.

What happens to the torque? Well, the drive controls the speed of the motor. The load decides on the torque that is required to make it move at that speed. So the torque is set by the load. It is important that this torque remains within the capabilities of the motor and drive or a fault might occur and the drive might shut down. But the drive will at least TRY to speed up the motor.

What happens to the current? The motor current goes with the torque. If the torque increases, the motor current increases. If the torque stays the same, then the motor current stays the same.

You didn't ask but what happens to the current going into the VFD, the input current? This is NOT the same as the motor current. To a first approximation, the input current depends on motor speed AND motor torque. If either one goes up, then the input current goes up. If they both go up then the input current goes up even more.