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Oversize VFD for smaller motor - 50HP drives 40HP

It has no issues using an oversized VFD. Assuming that's a voltage source drive (most common topology). This design manages the voltage and frequency to attain commanded speed. The current supplied to motor is only what is needed to magnetize the windings plus the torque producing current. In other words, the VFD is a 'current allower' and not a 'current pump'.

The key to your application is to properly program the variable frequency drive to protect this motor which is the MOL or motor overload protection parameter in all drives.

Other than your wallet being lighter, depending on the application it might be in your benefit. Depending on the machine frame size your losses may actually be less than the 40 HP. But as noted, make sure you have your protection and motor parameters set correctly or you could cook your motor.

Only problem you might see if the HP variance is too great is possibly your current measurement resolution may not be good as the scaling on the sensor may be too coarse for a small motor. 40-50 probably not, but if you tried it with a 150HP variable frequency drive, you may not be able to draw enough current to register on the larger machine accurately.

When setting up any VFD application use the motor nameplate data to provide an accurate look at what is happening with the load and providing suitable motor circuit protection. Drives are a current delivering device and are marked by horsepower for convenience only. Application, load, environment, derating for temperature, altitude or even stopping/braking may require additional capacity.

The motor data is not just amps but the electrical properties of the motor, motor constants. The VFD should have parameters where you let the VFD know the size of the motor, number of poles and nameplate voltage. The VFD will probably have a stored set of motor constants for each that it will load to get close to optimum efficiency.

If you have a 50 hp VFD running a 5 hp motor there may be some filtering issues but a 40 hp is no problem.

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