Constant Vs Variable Torque VFD Ratings

[slushin]

I'm having difficulty understanding why variable torque VFD's have a higher current rating than constant torque VFD's. Is it assumed that the variable torque VFD will be running below it's maximum speed all of the time?

 

[electricuwe]

Usually you will find the same hardware beeing offered with a higher variable torque rating than for the constant torque version (and of course different SW rsp. parameters).

A customer will benefit from this variable torque design , if he wants to operate variable torque loads like fans and most kind of pumps.

This higher rating for variable torque is possible because for constant torque the highest stress for the IGBTs inside the drive occurs when the full torque is required at low speed. With a variable torque load like described above this can't happen.

 

[ScottyUK]

Variable torque drives typically have a 110% / 30 second rating. Constant torque drives typically have a 150% / 30 second rating. It's a marketing ploy: manufacturers sell essentially the same drive in two guises: a larger rating and less overload margin, or a lower rating and larger overload margin. Ultimately it is down to keeping the heat in the semiconductor switches below an acceptable threshold.


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If we learn from our mistakes I'm getting a great education!

 

[LionelHutz]

What ScottyUK said.

Generally speaking, a fan or a pump will never require more than rated torque from the motor which means also never more than rated current from the motor. So, install a VFD that is rated to just match the motor rated current without any extra capability.

A so called constant torque drive has a higher current capability so it can drive the loads which require short duration torques and currents which are greater than the motor rated current and torque.

Most ratings really are somewhat off. Here is an example.

Lets take a 100A rated drive.

Variable Torque - listed as 100A continuous and 110%/30sec overload.

Constant Torque - listed as 73A continuous with 150%/30sec overload

All in all, both are the same components and both are 100A continous rated but that 100A continous only applies when above a certain frequency. As the frequency goes lower, each device is operating for a longer period to create half of the output waveform and this leads to extra heating of the devices. Maybe to be more correct, the device will heat to a higher peak temperature before cooling again while it is off and the opposite device is working. So, by de-rating the VFD as a "constant torque" model it can run with the de-rated current at a lower frequency without failing.

 

[jraef]

thread237-235513


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