Selecting Power Cables for Induction Motors

There is no need to change either the cable or the motor. It is the connection arrangements at both ends of the cable that will require attention

A motor with "Class B" insulation should not be driven by a VFD, unless you are happy to buy a new motor in the very near future.

"Class F" is the minimum recommended rating for VFD driven motors.

can u tell me why a class B insulated induction motor should not used as vfd

thank you

Thank you very much for your quick response .

I have some queries regarding selection of VFD..

Our application of the said motor is to drive blower fan motor.

The Centrifugal ablast type LL blower is 1070 RPM, 25000cfm, class 2,static pressure

145mm of water gauge and the mechanical power transmission is belt driven.

Motor Capacity: 40 HP, 3-Ph, 50 Hz, RPM 1440.

Would You please tell me detail about the factors to be taken care of while

selecting VFD for the above mentioned application

There are only a couple of extra factors that need to be considered;

1] the distance that the VFD will be from the motor. The longer the distance, the more possible problems (capacitive).

2] the prospective fault current that will be presented to the VFD. A higher fault current may mean the need for reactors.

3] the minimum RPM of the motor. If the motor is operated at lower than 25% of its rated RPM for extended periods, additional motor colling will be required.

Again, the motor insulation should be minimum "class F". If you install a VFD that drives an old motor that has less than "class F" insulation, expect problems in the near future.

class B insulation having 130*C...so what is the drawback for using the B class inslulation motors in VFD

Before I continue, I'll make a few assumptions.

1] the motor in question is not new. It is "some" years old.

2] the motor has not been subject to any analysis to ascertain it's present condition.

3] the installation conditions of the motor are that it is in an ambient temperature of no greater than 40 degrees Celsius & that it is not in any hazardous/polluted environment.

4] the motor has not been & is not currently subject to, more than 2 starts per hour.

5] the motor is fully enclosed (no ventilation slots).

How long does the operator wish to keep this motor? I would expect for as long a time as possible.

How will the life of the motor be reduced? By connecting it to a VFD, bearing in mind the above assumptions.

This scenario is worsened if the motor is unenclosed (has ventilation slots). It will be further worsened if the motor is to be operated at reduced RPM (less than 50% of rated RPM) if no additional cooling system is employed.

Please be aware that some additional heating to the motor windings will inevitably occur because of the inverter's "synthesized" AC wave form. Also, inverter voltage waveforms can be particularly stressfull to motor windings (spikes etc).

All of this means one thing...reduced motor life.

My advice is don't do it unless you've got a bucket full of money & you don't care about how long the motor lasts.