Speed Control using VFD

[Muhammad Umer ansar]

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

 

[retirede]

Again, look at a speed/torque curve for the motor in question.

 

[Jraef]

UP TO the motor base rated speed, torque can remain constant (in theory) because the VFD maintains the ratio of V and Hz that the motor was designed to deliver. But if you increase the Hz above the base motor speed, and you can no longer increase the V because you are at maximum, your running torque (Full Load Torque or FLT) begins to decrease linearly with speed, and your PEAK torque capability of the motor, that which is used to respond to a step-change in load, is decreased at the SQUARE of the ratio decrease, so you motor becomes less stable when responding to changes in the load.

For example: if you have a motor designed for 400V 50Hz, the motor was designed to receive a V/Hz ratio of 8:1 (400/50). So at any frequency below 50Hz, the VFD maintains the V/Hz ratio to be the same as the motor was designed for, ergo at 25Hz the V is 200, at 10Hz the V is 80 etc. . So long as that ratio is maintained, the motor can deliver FLT at any speed.

But the VFD cannot create voltage that is not given to it, so your maximum voltage is 400V. So if you increase the speed to 100Hz, the V is still 400 and therefore the V/Hz ratio is now only 4:1. The FLT is thereby reduced to 1/2 of what the motor was designed to deliver and the PEAK torque (Break Down Torque) is reduced to 1/4 of what the motor was capable of. BDT is typically 200% of FLT, so now the BDT is reduced to 50% of the original FLT and since the FLT is reduced to 50% also, the BDT is essentially equal to the FLT, meaning any increase in load may be difficult to re-accelerate.

 

[Muhammad Umer ansar]

thanks mate