VFD Basics

[mkgrady]

I have wired a few vfd's and left the startup to others so I know very little about them.

Can any ac motor be run from a vfd?

Are they limited to three phase?

Besides the obvious speed control benefit what are some other benefits to using a vfd over a no vfd motor?

I learned here that one drawback is that multiple vfd outputs should not be installed in a single conduit. Are there any other drawbacks?

 

[GoldDigger]

I have wired a few vfd's and left the startup to others so I know very little about them.

Can any ac motor be run from a vfd?

Are they limited to three phase?

Besides the obvious speed control benefit what are some other benefits to using a vfd over a no vfd motor?

I learned here that one drawback is that multiple vfd outputs should not be installed in a single conduit. Are there any other drawbacks?

Any use of a VFD with a single phase motor could still provide control over voltage and frequency, but would have no way to provide starting torque to the motor initially.
A custom frequency and voltage control that connects separately to both the start and the run windings of capacitor start motor might be buildable if anybody wanted it, but a standard VFD could not handle that.

One other advantage of a VFD would be controlled starting current, especially for motors that have a low torque load when starting.


Where a VFD can provide some adaptation is by taking single phase as its input and providing three phase to a motor.

 

[mkgrady]

Any use of a VFD with a single phase motor could still provide control over voltage and frequency, but would have no way to provide starting torque to the motor initially.
A custom frequency and voltage control that connects separately to both the start and the run windings of capacitor start motor might be buildable if anybody wanted it, but a standard VFD could not handle that.

One other advantage of a VFD would be controlled starting current, especially for motors that have a low torque load when starting.


Where a VFD can provide some adaptation is by taking single phase as its input and providing three phase to a motor.

Is the three phase motor anything special? Does it have to be made to run off a vfd?

 

[Besoeker]

I have wired a few vfd's and left the startup to others so I know very little about them.

Can any ac motor be run from a vfd?

Are they limited to three phase?

Besides the obvious speed control benefit what are some other benefits to using a vfd over a no vfd motor?

I learned here that one drawback is that multiple vfd outputs should not be installed in a single conduit. Are there any other drawbacks?

In my experience VFDs are commonly used pumps for energy savings.

 

[Besoeker]

Is the three phase motor anything special? Does it have to be made to run off a vfd?

Generally, it does. A VFD output waveform is usually a series of pulses with very sharp rising edges. These can break down the winding insulation if the motor is not designed to take them. VFD output filters can be added to mitigate this problem but it's extra cost and space.

 

[Sahib]

VFD output filters are not required if cable length is short and motor is VFD rated. Isn't it?

 

[Besoeker]

VFD output filters are not required if cable length is short and motor is VFD rated. Isn't it?

Yes, that's generally the case.

 

[kwired]

Any use of a VFD with a single phase motor could still provide control over voltage and frequency, but would have no way to provide starting torque to the motor initially.
A custom frequency and voltage control that connects separately to both the start and the run windings of capacitor start motor might be buildable if anybody wanted it, but a standard VFD could not handle that.

Submersible wells are one application where a VFD is specifically designed to run a single phase motor. I have one on my water system.

 

[Jraef]

Submersible wells are one application where a VFD is specifically designed to run a single phase motor. I have one on my water system.

It's possible, there are a couple of companies that make VFDs for single phase motors, but they are very expensive (relative to 3 phase output) and can only be used on SPECIFIC TYPES of single phase motors; PSC and Shaded Pole, but since Shaded Pole motors can be just as easily controlled with a dimmer, it's really boils down to PSC motors. A lot of times in submersibles however, you may not notice it but the motor is actually 3 phase and they are using the VFD to convert a single phase supply to 3 phase for the motor (although I'll grant that you would know the difference, I'm saying this for the benefit of others).

 

[Jraef]

Is the three phase motor anything special? Does it have to be made to run off a vfd?

There are several very good reasons to use motors that are specifically designed for running from a VFD, the generic term for them is "Inverter Duty", although that can be interpreted in several ways. Beside the insulation risk already mentioned, many motors are air cooled with fans and if the motor is slowed down so is the cooling fan, so the motor can over heat. Also there is a risk of damaging the motor bearings under some circumstances as well. Most of these issues can be mitigated individually if you deeply understand and investigate them, but if not, an Inverter Duty motor is a safer way to go. The things to look for in that (because again, lots of ways to interpret);
Spike resistant magnet wire, 1600V insulation minimum (if used at 480V), higher is better.
Insulated bearings or shaft grounding bushings
TENV cooling or separately powered cooling fans
Copper rotor bars (just a good idea, but not as critical)
High efficiency (otherwise you may need to get a bigger drive).

Also, understand that motor mfrs will tell you that even if a motor says it has a 1.15 Service Factor, if used on a VFD, it becomes 1.0. That's because the output harmonics of the VFD cause some added heating inside of the motor. So that boils down to "no fudge factor on the motor sizing".

 

[kwired]

It's possible, there are a couple of companies that make VFDs for single phase motors, but they are very expensive (relative to 3 phase output) and can only be used on SPECIFIC TYPES of single phase motors; PSC and Shaded Pole, but since Shaded Pole motors can be just as easily controlled with a dimmer, it's really boils down to PSC motors. A lot of times in submersibles however, you may not notice it but the motor is actually 3 phase and they are using the VFD to convert a single phase supply to 3 phase for the motor (although I'll grant that you would know the difference, I'm saying this for the benefit of others).

Mine drives a PSC motor, actually it is cap start, cap run motor with remote capacitors and a potential relay to switch the start cap, but you don't use the caps and the relay, you wire the pump leads directly to the drive. The drive is capable of running a single or three phase motor - you have to set a parameter for which you are using.

Yes they are expensive compared to a general use drive of similar power capacity. It does come with pressure transducer and is already set up to operate for a fairly specific purpose where a general use drive would need additional accessories and programming, but even after considering that is probably still more expensive.

I got mine because I seem to go through a lot of pumps. What usually failed was the splines on the motor shaft in the motor-pump coupling. I got the drive for soft starting purposes and don't have to replace pumps as often now. Have had motor bearing and seal failures since then though and not sure if the drive contributes to that or not, but they are not as frequent as shearing splines was.