# Thread: VFD's? Frequency effect on motor power and torque

1. ## VFD's? Frequency effect on motor power and torque

If a standard 60Hz AC motor is ran with a VFD, will the motors torque and horsepower output vary at different frequencies.

Ex. Would the same motor running with 120Hz have 4 times as much power/torque than if it was running at 30Hz?

Could a standard 1750rpm 60Hz AC motor run safely at 30Hz with out over heating?

Would a standard 4 pole or a 2 pole AC motor handle low frequencies better?

2. There are methods to obtain constant torque right down to 0 rpm for some VFD's, this usually involves some kind of feedback device in order to know exactly the motor rpm.
It has been possible to simulate servo type control in some applications such as elevator and crane hoists.
The HP calculation includes the rpm so it is also dependant on this.
I generally use 4 pole motors and take them up to the normal rpm of a 2 pole, i.e. around 3600rpm.
If you repeatidly use low rpm at high load for long periods, you may have to fit an auxiliary fan.
Al.

3. Originally Posted by glengeniii
If a standard 60Hz AC motor is ran with a VFD, will the motors torque and horsepower output vary at different frequencies.

Ex. Would the same motor running with 120Hz have 4 times as much power/torque than if it was running at 30Hz?

Could a standard 1750rpm 60Hz AC motor run safely at 30Hz with out over heating?

Would a standard 4 pole or a 2 pole AC motor handle low frequencies better?
With a high performance VFD, which is with the ability of large torque at low frequency, the standard 4 pole or 2 pole AC motor can handle low frequencies with strong torque.

For standard 1750 rpm 60 hz AC motor can run safely at 30 hz without over heating too, if using VFD.

4. Most inverter duty motors are spec'ed for constant torque up to rated speed and constant HP above rated speed.

A 4 pole motor will have more torque at slower speeds.

For slow speed applications, motors can be purchased with blower cooling, which do not rely on motor sped to drive their fan.

Matt

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