# VFD (Variable Frequency Driver) for 3-phase motors.

#### Y2KEDDIE

Sep 23, 2012
259
A lot of good information be shared here by all. I can sometimes be vague or use wrong terminology, for which I apolgize. When posting about the Rotary Phase Converter, the term Pony motor was used in place of what the video calls the idler. This ”idler” must Be in operation continously. And due to losses , needs to be bigger than the motor your trying to drive.
The windings within the idler motor interact with the driven motor to produce the end current phases.

My Capacitor Phase Converter post . Correct Terminology : Static Converter, Uses 3 large capacitors and a small relay board.It is called an AutoGen, model CD14, made by Smith Electric Motorworks GWM, Marceleine , Mo. For “moderately. Loaded 3-7hp 3ph motors. These work great. For small lathes. Physical size: 7”x 4 1/2” x 9 1/2”. No “idler required”
Smith Electric Motorworks’web site has nice training and application link, describing their diffent type converters.

When I posted about using a several 100Hp motor to start a gas turbine; that starting motor is called a pony motor. (pony motor: a small auxiliary motor used to get a prime mover , moving.) in the case of your automobile: the electric starter is a pony motor.

The VFD basic function is to control speed and torque. Speed determined by the frequency of applied current (and physical number of poles.) Other parameters are in the process, thus the wave shape changes.
PWM can part of the scheme. I think PWM control applies more to. DC motor control.

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#### Robo_Pi

Oct 5, 2015
45
I'll take responsibility for the misuse of the term "Pony Motor". I just looked back (see my post #20 on the first page of this thread). @ about 1:55 in that video the man refers to his idler motor as a "pony motor". That's where I first picked up the term. Apparently he's using that term incorrectly. But I'm glad I rewatched that video. I see that his idler motor is 7.5 HP. So I'm starting to get a feel for what people are using. Probably the bigger the better, but a 5 hp might serve me since I'm expecting all my machines to being using 3 hp or less. Most of the machines I've been looking at have 2 hp motors.

Anyway, that's where I picked up the term "pony motor". But I see now that the correct terminology is idler motor.

#### Minder

Apr 24, 2015
3,335
PWM can part of the scheme. I think PWM control applies more to. DC motor control.
PWM is typically used to vary the power content of a VFD 3 phase waveform.
M.

#### Robo_Pi

Oct 5, 2015
45
PWM is typically used to vary the power content of a VFD 3 phase waveform.
M.

That's the way it appeared to me on the waveforms I saw. I can't recall where I saw them but somewhere during my searches I saw the output of a VFD 3 phase variable speed control and it appeared to be three sine waves that were broken into pieces just as you would expect to see on a DC PWM output.

Apparently all they are doing is turning the sine wave outputs on and off to control how much of the power is applied. They call it "variable frequency control'. But I don't think they are changing the frequency of the sine waves. They are just changing the frequency of the PWM.

At least that's what it looked like to me. I really don't know. But I do know what DC PWM looks like, and that's exactly what those sine waves appeared to be. They were just broken up into dashes outlining the sine wave. Like the sine wave itself was being turned on and off at a much higher frequency.

#### Minder

Apr 24, 2015
3,335
The frequency/rpm is set or derived from the 3phases, the energy content of each phase is decided by the PWM switching rate.
M.

#### Robo_Pi

Oct 5, 2015
45
I just found another video of a guy who made his own rotary phase converter. Strangely he doesn't appear to use capacitors at all. Until he pulls out this Phase-A-Matic static converter box. Which he doesn't explain very well at all. I take it this Phase-A-Matic converter box has the capacitors in it.

He said these are cheap on eBay. I looked them up and I wouldn't call $200 cheap. Although based on the other videos I'll probably end up investing$200 just in the capacitors I'll need to build the ones we've already seen. Still, I'd rather build it using discrete capacitors. That way if one cap blows I can just replace that cap. If the Phase-A-Matic box blows, you need to replace the whole box.

#### Bluejets

Oct 5, 2014
6,067
just found another video of a guy who made his own rotary phase converter.

Watch and believe videos like this will just get you into more strife than Flash Gordon.
All they are doing here is making a "crude" cap start/run motor.
Nothing to do with VFD.

#### Robo_Pi

Oct 5, 2015
45
I'm no longer looking for a VFD. I've decided to go for a Rotary Phase Converter instead.

#### Bluejets

Oct 5, 2014
6,067
Good luck with power output and torque.

The phrase "wouldn't pull the skin off a rice pudding" sort of comes to mind.

#### Y2KEDDIE

Sep 23, 2012
259
Watch and believe videos like this will just get you into more strife than Flash Gordon.
All they are doing here is making a "crude" cap start/run motor.
Nothing to do with VFD.

It's a question of semantic's. Like the other post indicated: many VFD's use PWM to "control energy".
A Rotary Phase Converter isn't a VFD per definition.

In the original post the goal was to operate a 3 phase motor from single phase line. This can be done with a VFD, Static Phase Converter, and a Rotary Phase Converter. (To name a few)

The rotary phase converter does produce motor action. Its is an Electrical/ mechanical device. The capacitors create a phase shift and this helps create a rotating field by motor action. Once the field is rotating it can self induce and actually be tapped for the energy produced, as an alternator. (very crude description)..

The Rotary Phase Converter in the video is real, and works well. These have been manufactured and used commercially many years.
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Another devise I have worked with are "Lima Generators". They were called generators (slang), but were actually alternators. They were v-belt attached to the crankshaft on 1000hp Clark stationary 2 cycle gas engines. Then wired directly to a large remote 3 phase motor (440VAC) for fin fan (radiator) cooling .
As the engine speed changed from stopped to 330 rpm, the alternator made the fan motor follow. speed in step, providing cooling based on engine speed. These Lima devices provided up to 480 V 3 phase, with a 40 HP parasitic load to the engine.

I guess you can technically call this a variable speed drive, and there is no waveform modification other than frequency or voltage..

#### Robo_Pi

Oct 5, 2015
45
Unfortunately I started this thread thinking that I would be interested in a VFD, only to later discover that a Rotary Phase Converter is more suitable to my actual needs and purpose. So unfortunately this thread is going to become confusing at this point.

Perhaps I should start a new thread on Rotary Phase Converters, since I understand now that this is what I'm actually interested in.

I do have more questions about Rotary Phase Converters, but I'm hesitant to ask about them in this thread due to the confusion the original subject title of the thread suggesting that it's about VFDs.

I really don't want to get bogged down with the constant distraction of VFDs at this point. Or getting into which is better, etc. I'm not interested in those arguments. I'm sold on the Rotary Phase Converter for my application. I don't need speed control. And I'd prefer to stay away from expensive semiconductor switching electronics when I don't really need it. So I've already decided that I'm going for the Rotary Phase Converter. But I do have a few more questions on those.

Thanks.

#### Y2KEDDIE

Sep 23, 2012
259
Go to Smith Electric Motorworks web site.
This will give you a good education on Rotary phase Coverters.

#### Robo_Pi

Oct 5, 2015
45
They are only interested in answering questions about their products. They aren't going to like my questions.

For example, when you build a rotary phase converter you already have a 3-phase motor running (i.e. the idler motor). My question is, why can't that motor run a machine directly? You already have a motor running.

Obviously the idea with a rotary phase converter is to produce a 3-phase output to run other 3-phase motors. I already understand how that works. What I'm not understanding is why the idler motor can't be the actual motor on the machine you want to run in the first place. You obviously have a motor running already.

I mean, if you only have one machine, why not just use its motor as the idler motor? Then there's no need to buy a second motor.

#### Minder

Apr 24, 2015
3,335
Ah and therein lies the rub, as Will would say.
Now you are going to place a mechanical load on the motor as well as one carrying an electrical load.
Don't forget, this is not a synchronous 50hz/60hz generator.
Just close to it.
Max,.
.

#### Robo_Pi

Oct 5, 2015
45
Now you are going to place a mechanical load on the motor as well as one carrying an electrical load.

But where is there any electrical load if it's not driving any other 3-phase motors?

Perhaps I'm not understanding these converters as much as I think. But here's how things appear to me:

The idler motor is only being driven by the 220 volt legs. Which are really two 110v legs 2-phase right?

It's missing the 3rd phase. But once it is running, due to the start-up capacitor that temporarily gave it the 3rd phase kick, it is now running on 220 at 2/3 it's original power. In other words, just to keep numbers simple let's assume it's a 6hp motor. Then running on just two phase instead of three it only has 4hp instead of 6. This is because the 3rd phase isn't there to push its third of the work.

Now even as an idler motor in a rotary phase converter it can only drive up to 4hp. So this is why an idler motor needs to be at least 1/3 again larger than the motors it drives.

So my thinking is that if you just used the idler motor directly as the mechanical power to the machine it should drive it. You just aren't going to get the total rated hp out of it. You'll only get 4hp or less from a 6hp motor. Because you're missing the 3rd phase. It also may not run as smoothly as one might hope.

I guess when it's used as an idler motor its own inertia "pushes" some 3-phase power out the line to the next 3-phase motor. So when used as an idler it actually does provide the full 3 phase to the motors it feeds. But clearly at a loss of power. At least in terms of what the idler motor is rated at. In other words, a 6 hp idler motor can only drive a 4 hp 3-phase motor at best. And even that presumes 100% efficiency.

In short, (or it seems that in theory), an idler motor should be usable to drive a mechanical device. It's just not going to drive it at its original rated hp because that rating assumes all three phases are pushing the motor.

It seems to me that the idler motor should be able to drive at least some mechanical load. Especially if it's not also being used to push out a 3-phase line to other motors too.

Anyway, I was just thinking about this the other night. I'm thinking once you build the rotary phase converter you have a 3 phase motor running. It may only be running on 2 phases, but it's still running none the less. It could surely be used to drive some amount of mechanical load at that point. After all, it's going to be expected to be pushing a 3rd phase out to other motors, so it's going to need to do some work somewhere along the way.

#### Y2KEDDIE

Sep 23, 2012
259
Did you ask technical service at Smith that specific question?

My guess is you probably can drive a load directly with a rotary converter but it would have to greatly oversized and It has to be Started with no load.

The Static converter doesn’t require an idler,and can start under load< but at a cost of efficieny, hence only light loads.

Price wise , I really think the Static converter on each machine is the way you should go. Provided you can tolerate the reduced horsepower. You won’t have to change motors,electrically or mechanically, and you can always revert back to 3 phase operation if you want to in the future.

Now you are realizing why there’s such deals on surplus machines powered by 3 phase motors.

#### BobK

Jan 5, 2010
7,682
Just an off the wall question, because I have no idea. Is it possible to get 3-phase power to a residence? And if so, how much would it cost to install?

Bob

#### Robo_Pi

Oct 5, 2015
45
Just an off the wall question, because I have no idea. Is it possible to get 3-phase power to a residence? And if so, how much would it cost to install?

Bob

You'd need to call your utility company for that. I imagine they are all different. It would also depend on whether 3-phase is available on the lines near your house. I know that 3-phase runs right through my property. So it's definitely right there. I haven't called to see what it would cost to have it hooked up. I hear that sometimes they don't want to do it for a residence. I guess that will depend on the utility company you're dealing with.

I might call them today myself just to ask. It might be cheaper than I think. If that's the case I'd be better off just having 3-phase installed directly. Then I'd be done with the problem forever. It certainly doesn't cost anything to call and ask. I'll do that when I get back. I'm going out to buy a new car right now.

#### Minder

Apr 24, 2015
3,335
In N.A. it is virtually impossible to get 3ph added, They did it for me once in the UK to supply a bank of storage heaters.
As to using the Idler motor for motive power, the capacitors are tuned for optimum P.F. and for correct phase angle, so using the motor for mechanical power also, is detrimental to tuning..
BTW N.A. residential supply is 1ph not 2 phase, it is 240v with a centre tap.
M.

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