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

#### Robo_Pi

Oct 5, 2015
45
Has anyone built a VFD, or know of any schematics or other information on how to build one?

I have an old 3-phase, 220volt, 1-1/2 HP, 4.2 AMP motor. I would like to build a VFD to run it.

I know that I can just run out and buy a VFD, but that's not the point. The whole idea here is to learn how to build a VFD.

So can anyone point me in the right direction for information on how to do this?

Thanks.

#### Robo_Pi

Oct 5, 2015
45
Ok, I found the following schematic. It seems fairly straight-forward.

What I'm not clear on is how to hook up the 230 VAC at the input of the bridge. This schematic appears to assume a single-line 230 volt 1 phase. But what I would like to do is use a standard residence 220 VAC. That comes in as two 110 VAC lines that are of opposite phase with respect to a common neutral. Is that right?

I don't know how to hook that up at the input of the bridge to get the 220 VDC at the output. Would I hook up a 110v line to the switch and the other 110v line to where it says 230 volts on the schematic? And if so then what about the neural line?

I guess what I'm really asking is how to build a 220 VDC power supply using a standard residence electric service. Once I get a suitable 220 VDC power supply the rest of the schematic is pretty simple. Most of the magic will be in the PWM programmed into the MCU. That part I know how to do.

#### Robo_Pi

Oct 5, 2015
45
Another question:

I would like to start out building a low-voltage prototype. Where would I find a 12 or 24 volt 3 phase motor to test with? Do they make such a thing? I can't seem to find one. Everything I search for in 3 phase seems to be 220 volts.

#### Bluejets

Oct 5, 2014
6,197
Think you will find the circuit shown would be a much simplified version drawn up for the purpose of understanding basically how it works.
There will be a lot more to it including the uC requirements and programming.
I would not imagine a standard pwm generator programming would be sufficient to duplicate sine waves or the phase timing.

Low voltage motors ( brushless) are essentially 3 phase motors and will work with mostly voltages from 2s lipo upwards perhaps to 50 volt.

#### Minder

Apr 24, 2015
3,366
I assumed you are asking for a induction motor VFD?
For N.A. 220/240 you ignore the centre taped neutral and use the L1-L2 legs for 240vac.
The typical induction motor VFD not only creates a 3 phase sine wave but modulates it with PWM for energy content of the 3phase.
M.

#### Robo_Pi

Oct 5, 2015
45
Low voltage motors ( brushless) are essentially 3 phase motors and will work with mostly voltages from 2s lipo upwards perhaps to 50 volt.

Do they actually have 3 wires for the three phases?

I'd definitely like to experiment building a low-voltage VFD before moving up to a full-blow 220 volt version.

#### Robo_Pi

Oct 5, 2015
45
I assumed you are asking for a induction motor VFD?
For N.A. 220/240 you ignore the centre taped neutral and use the L1-L2 legs for 240vac.
The typical induction motor VFD not only creates a 3 phase sine wave but modulates it with PWM for energy content of the 3phase.
M.

I can do that. I can create nice sine waves with something as simple as a 555 timer IC. And then PWM those using an Arduino. So I don't see where there should be a problem producing perfect sine waves. Using three sine wave generators I can phase those three sine waves relative to each other however I want. So I'm not real worried about that end of the circuit. There are probably even better chips to use for generating precise sine waves. I'm from the old school so a 555 IC is the first thing I grab.

In any case, here's the motor I ultimately would like to drive. Don't be discouraged by the appearance. It actually turns as smooth as a Swiss watch.

Here's the tag on it. It's 1-1/2 HP, 220 volts, 4.2 amps.

I just want to do this as a learning experience.

It doesn't matter if I burn this motor up, or blow up the VFD.

And I know better than to touch any high voltage parts. I have great respect for electricity. I do all my own house wiring by the way, so it's not like I haven't worked with 110 and 220. I work with them all the time.

I would just like to learn how to build a dependable VFD..

Just for the record (in case you might be curious), I am hoping to purchase a few machines this summer. A lathe, a milling machine, and a surface grinder. These are all old and rusty (kind of in the same shape as the motor in the photo above). But they will all come with 220v 3-phase motors on them.

I could swap the motors out for single phase motors. In fact, I was going to do that originally. But the guys on the machine shop forums tell me that the 3-phase motors are much better and smoother running with more control.

I could also run out and buy a brand new VFD for each machine, but hey, if I can build my own why not? Not only will I save money, but I'll also learn a lot about them, and I'll be able to repair or customize them as needed. I might also be buying more 3-phase machines in the future as well.

So anyway, that's the story.

I'm willing to take things slowly. As I've said, I would really like to experiment with some low-voltage 3-phase motors first to perfect the design. Like 12 or 24 volt 3-phase motors. Only after I get that down pat will I move up to trying the 220 volt real thing.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
Is this what you're after?

#### Robo_Pi

Oct 5, 2015
45
Is this what you're after?

That might work. It says it's for motors up to 2 HP including 3-phase motors.

I'll look into it. Thanks.

#### Bluejets

Oct 5, 2014
6,197
Do they actually have 3 wires for the three phases?

Last time I looked, yes.

#### Robo_Pi

Oct 5, 2015
45
Is this what you're after?

Yes, I just looked into it and was able to download the article. It's exactly what I was looking for. This article tells you precisely how to build a 220 volt VFD step-by-step, and it will handle motors up to 2 HP. They even offer kits that have the circuit board, components and enclosure. Strangely the kits cost more than you can by a VFD for. But I don't need the kit. All I need are the plans and schematic. And that's all in the article. I can build my own PC Board.

So thank you Steve! This is what I wanted. Can't beat an article that takes you though the entire build step-by-step.

This is perfect! And I would have never found that article on my own. Or at least if I did it would have been after searching through a jungle of junk I don't want. So thank you, thank you, thank you!

#### Minder

Apr 24, 2015
3,366
I'm willing to take things slowly. As I've said, I would really like to experiment with some low-voltage 3-phase motors first to perfect the design. Like 12 or 24 volt 3-phase motors. Only after I get that down pat will I move up to trying the 220 volt real thing.

You may have quite a hard time finding 3phase induction motors for 12v/24v .
There are many P.M. field 3ph motors but they require a different controller to a induction motor.
If experimenting with the old 220v motor, it would be wise to ensure it is totally free of moisture internally, before applying H.F. pulses.
M.

#### Robo_Pi

Oct 5, 2015
45
You may have quite a hard time finding 3phase induction motors for 12v/24v .
There are many P.M. field 3ph motors but they require a different controller to a induction motor.

I thought there would be a catch. Thanks for pointing that out.

If experimenting with the old 220v motor, it would be wise to ensure it is totally free of moisture internally, before applying H.F. pulses.

Yes, I won't be using the motor in the photo as-is. I'm going to completely disassemble it, give it a really good inspection and cleaning inside and out, paint the case, and then reassemble it. I'm getting into a new hobby of rebuilding old (often antique) metal working machines like lathe's, vertical mills, surface grinders, etc. The one's I'll be buying will all have old 3-phase motors on them in about a similar condition to the one in the photo. So restoring these old motors is just one aspect of this hobby. And I still haven't even made a solid commitment to go this route. I might end up just selling all the 3 phase motors and replacing them with single phase motors.

So this whole deal is just in the idea stage at this point in time. It will all depend on how easily and dependably I can make my own VFDs and how well they work. I'm a hobbyist, so it's all in fun. If I were in business I'd just either go with converting them all to single phase motors, or equipping them all with factory made VFDs, But as a hobby the idea is to experiment and learn new things.

There's also the profound joy of accomplishment when a goal like this has been conquered. Or the agony of defeat should I fail.

This is one reason I'd like to start with low-voltage 3-phase induction motors. Less agony if I fail.

But even failing at something like this still results in a lot of learning. So there's always some good that will come out of it.

#### Minder

Apr 24, 2015
3,366
The one's I'll be buying will all have old 3-phase motors on them in about a similar condition to the one in the photo. So restoring these old motors is just one aspect of this hobby. I might end up just selling all the 3 phase motors and replacing them with single phase motors.
.

There are not many of those old 3ph motors out there that are solely 220v, you may also find that some of that vintage are repulsion-induction motors and have a wound rotor and a commutator for the repulsion start. They look like the one posted but are 1ph however.
Good luck with the VFD.
M.

#### Y2KEDDIE

Sep 23, 2012
259
I applaud your interest in building a VFD. I've gone this route myself and found it not economically feasible, not to say you won't be successful. You may have better resources.

There are options.

The reason for my reply to your post is the fact I wanted a VFD for the same application. There are a great variety of Industrial surplus machinery available at low cost; with 3 phase motors.

The reason for 3 phase motors is typically because the motor size and weight is smaller than single phase motors. In your case, using a VFD allows 3 phase operation from a single phase power source.
Typically VFD's are mostly used with 3 phase input and have a variable output allowing variable speed & torque control.(with a whole lot more enhancements/ characteristics.) Speed,torque, and other enhancements are what make the circuitry complex /expensive. The schematic you provided is the basic power output stage of a VFD. Not shown is the signal input to that stage. A 555 IC will give you an input signal, but you will need more conditioning before and after the IC, and .of course feedback.circuitry, NOt impossible to build, good luck.

I've experimented/ worked with VFD's for fractional horsepower motors up tp 100-400 HP. (used as pony motor for starting large gas turbine engines.)

The smallest VFD I used (1/15- 1/2 hp) was made by Gleason , and I bought from Mc Master Carr (p/n 6488k21 $350),.worked great! It's about$100 more for each hp.increase..

If you only need to operate your machine without, speed and torque control, ( running the motor off of single Phase mains): a Capacitor Phase Converter can be used. It is much less expensive and easy to construct.

The Capacitor based Phase Converter use Capacitors and the windings of your three Phase motor to create the phase shift for rotation. A.commercial Capacitor Phase Converter for a 5 HP motors can run less than $200, and purchasing parts for such a device under$100. They are not as efficient (speed and torque as a VFD, but they work.. It has been my experience that efficiency is not a concern; and most machine tools have mechanical speed adjustment, the motors are usually much bigger than needed so there is plenty of margian.

There are small low voltage 3 phase motors available. I recently repaired a small leisure fishing boat lift that used a 3 phase motor and VFD combination. The replacement motor cost was around $150. Last edited: #### Robo_Pi Oct 5, 2015 45 If you only need to operate your machine without speed and torque control, (just run the motor off of single Phase mains), a Capacitor Phase converter can be used. It is much less expensive and easy to construct. Actually all I need to be able to do is get a 3 phase motor to run off regular single phase house current. I'm not worried about speed control. I'm rebuilding antique machine shop equipment, and they usually have mechanical speed controls. Either physically changing belt positions, or by using a mechanical cone pulley that changes the diameter of a belt pulley even while the machine is running. So the machines I'll be rebuilding already have mechanical speed controls built in. I'm on a machinist forum and those guys seem to be convinced that running 3 phase motors with VFD's that have speed control is the latest and greatest thing. And I guess it is for people who don't want to bother changing belts, etc. But I really don't mind changing belt positions I've been doing that all my life so I'm used to it. I'm thinking I might be better off just selling the 3 phase motors that come on these machines and buying single phase motors. That way I can just plug the motors in directly with a simple direction switch and not need to worry about any additional electronics at all. I only just realized this when someone on the machinist forum talked about buying a Bridgeport mill that had a VFD and saying how he would never need to change belt positions. If that's the only reason for a variable speed drive then I certainly don't need it. I have no problem changing belt positions. It's really not a big deal as far as I'm concerned. So now I'm thinking of just going with single phase motors and keep things simple. I'll use the built-in mechanical speed controls. One lathe I'm looking to rebuild actually has a dynamic cone pulley so you can change the speed while running the lathe. It's all mechanical. The motor just runs at one speed. That works just fine. In fact, I'm thinking that even in industry these 3 phase motors just ran at one speed. They still used belt changes for speed control. They didn't have variable speed VFD's. They just had 3 phase service. One speed using 3 phases. So I really don't need the speed controller. Also, the largest motors I expect to be dealing with will most likely only be about 2 HP. I don't get into the really big machines. Thanks for the info. #### Y2KEDDIE Sep 23, 2012 259 Your biggest issue you will have in replacing 3 phase motors with single phase motors is the motors footprint. A 1 hp single phase motor is huge: 3 times the physical size oft he 3 phase motor. If you can reduce your motor requirement to say' 1/2 hp,you might get by. You can reduce your tool speed through gearing which will give you more torque. Typically I run my home shop lathe and mill at lower speeds. I like to see my tooling at work and I don't need production speeds. I once used a VFD on a 10hp air compressor with a 3 phase motor. Huge cost savings on the motor. Where it was used , only had a heavily loaded Single phase 220V service. The VFD had a programmable ramp up providing a soft start. A couple issues with a VFD: 1) reducing speed of a FCM (fan cooled motor) reduces the cooling at lower speed, heat produced,a detriment 2) when using a VFD the motor needs to be Inverter rated. (Have Higher insulation voltage rating) because the irregular waveform of the current. ( full of spikes and pulses by nature/design). #### Minder Apr 24, 2015 3,366 2) when using a VFD the motor needs to be Inverter rated. (Have Higher insulation voltage rating) because the irregular waveform of the current. ( full of spikes and pulses by nature/design). Not strictly true, All the VFD's I have installed since the 80's have been on regular 3ph motors, two were on a machine I retro-fitted with motors from the 50's! If concerned, a 3ph choke/inductor can be fitted between VFD and motor. This helps both motor and VFD. M. #### Robo_Pi Oct 5, 2015 45 I just found this 3 part video series on how to build a rotary phase converter. This wouldn't have speed control. It's not a VFD, but it might be far easier to build and suit my purposes. I'm not sure how these work. I don't understand how the pony motor works or what size the pony motor needs to be, etc. So I'll have to study this up. But this kind of phase converter might be a lot easier and less expensive to build. Although I'm not sure what to do yet. I've also seen videos of a VFD that can be purchased from Bang-good for just under$100. It's far smaller, has speed control, and doesn't require a pony motor.

I just don't know which is the best solution for me yet.

If I can buy a suitable VFD for under $100 that might be the way to go. They are quite small, and will supposedly run up to a 2 hp 3 phase motor, complete with speed control. I don't really need the speed control, but if it's a simpler system for under$100 may as well go for it.

This rotary phase converter seems pretty simple too, but seems kind of awkward with the need of a pony motor. I'll need to find some actual applications that I can check out in person before I decide what might be best for my needs.

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