I am rebuilding an industrial pedestal grinder. The original motor was 1.5 HP three phase. I replaced it with a modern 1.5 HP motor. The local supply house sold me a 1 HP drive that has a higher current capability than the motor nameplate current. The in house electrician states that this is not safe because the motor is larger than the drive rating. I disagree.

The grinder is wired as follows. Fused disconnect, appropriate size motor starter with overloads set by the motor nameplate data, VFD configured to start as soon as it receives power, and finally the motor. The grinding spindle is connected via a flat belt to the motor output shaft.

The system was modified because they wanted to be able to change the wheel speed based on the operation. The VFD is set up to operate between 43 and 86 Hz.

Am I correct in thinking it should be treated like any other fixed in place tool? The tool builder is responsible for picking the appropriate sized components?

Thanks.

I'm sort of using an undersized vfd as a phase converter. It's a 20 hp vfd driving a 15 hp motor, so the input rating isn't high enough for the single phase amps needed to drive a 15 hp, 3 phase motor. I adjusted the output fla of the vfd to the calculated maximum input amps (allowing for efficiency of the vfd) and it's fine. The supplier approved the set up.

The vfd will look after itself and the motor.

I'm not okay with a vfd on a contactor. When you power down a vfd you need to give it time so the capacitors can discharge. If someone cycles the vfd, there could be problems. As least, that's what the instructions on the vfd say.

You don't need overloads if you have a vfd.

If you need the contactor in the circuit, put it on the load side of the vfd and program the vfd to restart when the motor is connected.

Using an undersized vfd should raise some red flags but I wouldn't call it dangerous. vfds are cheap and the worst that can happen is that you need to buy another one. As an experiment, I would definitely do it.

A number of people are using 3 HP VFD's as a phase convertor with 5 HP motors on table saws, they claim it works, I am not comfortable with it, but doubt it is unsafe.....


I suppose there are a few NEC articles to prohibit it though.

Thanks for the replies.



I am under the impression that VFD's get upset when you run them with no load attached. That is one of the reasons I did not put the motor starter downstream of the VFD.



So I do not need the overload relays? Just the start/stop circuit and VFD.

When the motor disappears from the vfd circuit, the vfd sees it as a fault. You can set the parameters of the vfd to restart after that fault clears. I work mainly with one make but they usually have the same capabilities.

I set one up as a phase converter on a bench grinder. The switch for the grinder was on the front of the grinder and still worked. The downside was that on initial power-up the run button on the vfd had to be pressed. After that, it ran off the switch on the grinder. It isn't a preferred method, but the switch on the grinder was important.

When you set the full load current of the motor in the vfd, the vfd will shut down on overload.

The drives that I use adjust the overload level for the speed. If you try to start a motor with a frozen load, it shuts down on overload for that speed.

I put a vfd on a pressure testing system for oil wells. The operator would ramp up the pressure and stop before it reached the required pressure and jog it up a few times. Sometimes the motor couldn't start under the load and the drive would shut down for a few minutes. Under locked rotor conditions, the drive never got anywhere near rated full load current.

When I add a vfd for phase conversion, the power goes directly to the machine so it can have line voltage for the control circuit. Then, instead of the line wires going to the top of the contactor, they to the line side of the drive. The load side of the drive goes to the motor leads. With the contactor not being used for the motor, a set of contacts can be used to control the drive. It leaves the machine control circuit with no modifications.

I have a Rockwell ( the nicest running grinders once made) 200V 3Ø bench grinder that uses a VFD to run, the existing contactor remains, pressing the start button, powers the drive & a control transformer to power the 120V eye shield lights, a toggle switch starts the grinder, regret not configuring the drive for 3-wire control, & powering the control X-former as long as the tool is plugged in, using the VFD made it simple to run a tool intended for 208V where only 240V is avail.