3-phase motor as generator?

[iXod]

My electronics workbench needs 3-phase power. I have a spare 1 HP (0.745 kW), 240V, 3-phase motor.

Can I use this to generate 3-phase voltage? If I use another motor to drive the armature at rated RPM will it give rated voltage (240)? How to determine the current capability?

What haven't I thought of?

Thanks.

[ahbushnell]

I assume it is an induction motor which wont work.

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[BrianHG]

You may use a permanent magnet 3 phase BLDC motor if you want a generator.  However, these motors usually have special drive electronics with them and they usually cost more than a 3 phase induction motor.

[iXod]

There is success with induction motors as generators:

http://www.redrok.com/cimtext.pdf

[NiHaoMike]

How much power do you need at 3 phase? (I'm assuming not much since you're considering using a 1HP motor to supply it.)

Using the motor as a phase converter looks pretty easy:
http://www.homemetalshopclub.org/projects/phconv/phconv.html

[Circlotron]

You may use a permanent magnet 3 phase BLDC motor if you want a generator.  However, these motors usually have special drive electronics with them and they usually cost more than a 3 phase induction motor.

You could use one of those motors just fine without any electronics. I have one right in front of me about the size of a drink can, used for operating a commercial sliding door. It is a 6 pole delta motor and at 50Hz it makes 14vrms. I’d try something like that with 3 single phase transformers to step up the voltage to whatever you need. Assuming you don’t need a great deal of power.

[BradC]

I assume it is an induction motor which wont work.

Yes it does. You sometimes need to poke some voltage into the windings to get it "excited", but you can absolutely use a 3 phase induction motor as a generator. It's far from ideal and very difficult (read nearly impossible) to regulate, but it will work.

[duak]

I needed 0-500 Hz three phase so I used an induction motor driven from a VFD to turn a BLDC servo motor as a generator.  One should be able to get the servo motor's rated power out of it providing the induction motor is big enough to provide enough mechanical power to overcome the losses.

Also, an induction motor can be wired to provide three phase power from single phase if a phase shift capacitor is connected between one of the input lines and the third phase.  Look for "rotary phase converter".  There are a number of considerations and limitations but the technique is definitely used in smaller workshops to provide three phase power if only single phase is available.

Cheers,

[james_s]

I remember when it was very common to use a rotary phase converter made from a surplus 3 phase motor in home machine shops. These days it's usually much easier to just use a VFD on the machine tool but rotary converters are still easy to make and effective. No need for multiple motors.

[woodchips]

It is called an induction generator. Will work only if connected to a 3 phase supply to provide the magnetising current, even then it will only provide a leading current, not lagging.

A static converter, capacitor to create the third phase, will only work well if the motor is run at 400V, star, in delta at 230V you get horrible circulating currents.

Surely the easiest is to simply use a VFD with the motor acting as a filter, frequency, voltage etc filter. Run the three VFD phases through three transformers, or a 3ph one if you have it, to provide usable 3ph supply.

[Zero999]

I assume it is an induction motor which wont work.

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That's not true. An induction motor can be used as a generator, as long as it's given a small voltage to magnetise it. Simply connecting a three phase induction motor to the mains and spinning its shaft faster, than its synchronous speed will cause it to back-feed power into the grid.

[Cyberdragon]

I assume it is an induction motor which wont work.

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That's not true. An induction motor can be used as a generator, as long as it's given a small voltage to magnetise it. Simply connecting a three phase induction motor to the mains and spinning its shaft faster, than its synchronous speed will cause it to back-feed power into the grid.

Exactly, an induction generator just needs a starting voltage to magnetize the stater. Stand-alone induction generators have a much smaller PM generator on the same shaft as the "field generator". I think sometimes it's in the same case but on horizontal shaft stationary generators it can literally be a tiny generator stuck to the end of the shaft coming out of the main generator.

[Raj]

There's a guy known to the internet by the name of 'Ave'.
He has a tutorial on vfd on youtube.
He has a lathe,a drill press, a milling machine,an air compressor, all connected to a vfd.

[james_s]

VFDs are cheap enough these days that it usually makes sense to have one unit hardwired to each machine. In addition to running off single phase, you can then have full range variable speed and reversing without messing with any complex mechanical belt/gear arrangements.

[rbm]

I've used a 3-phase "Y" configured induction motor as a phase converter, to change single phase 220V into 3 phase 220V to drive woodworking machinery.  I've never tried to use the same motor as a stand alone generator though.

[iXod]

OP here.

I have no 220v available at my bench so phase converter won't work for me.

Thanks.

[Doctorandus_P]

Induction motors can and are often used as generators, but as said before, they may something extra to start the magnetic flux in the beginning. Can be some external voltage, or a permanent magnet embedded in the motor.

Induction motors do have somewhere between 2 and 5% slip, and that is why these motors are often rated for 2800rpm on 50Hz, while a synchronous motor would run at 3000rpm on 50Hz.

VFD's are indeed cheap nowaday's, cheaper than the motor they connect to.
VFD's can often generate a 3-phase output up to 400Hz, but if you run a 50Hz motor above 50Hz, the maimum available torque diminishes. At low RPM you need a fan for forced cooling of the motor.

VFD's are cheap, but relays are still cheaper.
I share a VFD between a metal lathe and mill, since I am unable to operate them at the same time. Especially for the Mill it is wonderfull. It has a 12 speed V-belt setup, but I only have to change V-belts between low and high speed.
I've also programmed the VFD in such a way that the motor has a lot of slip at low RPM.
It is a 1.5kW (2hp) motor, but I can tap M5 threads in blind holes with the motor, and have the motor stop at the bottom because of the torque limit.
Just start with 0 rpm, slowly increment rpm untill it starts tapping, any change in needed torque tranlates to a rpm change.

[GeorgeOfTheJungle]

OP here.

I have no 220v available at my bench so phase converter won't work for me.

Nor 110 volts? Most VFDs can even run off DC.

[NiHaoMike]

OP here.

I have no 220v available at my bench so phase converter won't work for me.

Thanks.

Run a VFD off a voltage doubler. Or look up "Quick 220" for a gadget to use two different phases as a 240V outlet, including how to build a DIY version complete with interlock relays.

[Zero999]

Or use a transformer to convert the 110V to 230V.

[Seekonk]

VFD's have a V-F curve so you might not get the voltage you want.  They will also need a lot of filtering of the output.  I picked up some single phase in 110V .1KW 3 phase out on ebay for about $10.  Who knew they made such a thing.  I used it on an aircraft radar.