Thread: Simple phase conversion

I have an opportunity to buy a 4kW 3 phase motor, and thought it might be good to make a rotary phase converter out of it. I have a 2.2kW 3ph motor on a lathe that I'd like to power up from single phase 240V house power. My question is: Is 4kW too much to expect to be able to be powered by a 15 amp single phase house circuit? I'm looking to avoid using VFDs or capacitors, and have had encouraging results with smaller 3 phase motors. I'm just not sure if a 4kW motor will trip my circuit breakers. Only one leg of the 3 ph motor will be powered up from 240V. Here's a video of my bench test of .56kW (3/4 hp) 3 ph motor:[ame="http://www.youtube.com/watch?v=pDNLkdyFiL0"]YouTube - 1 2 3phase[/ame]

4kW is too great a load for a 15 amp circuit.. 2.2KW is the limit

Is the lathe motor 415V????

Thanks for the confirmation - I thought as much but wasn't sure, given I'm only going to use one leg of the 3 phases for the "master" motor.
Yes it's 415V, which means star connection if I got that right. I gather VFDs need to drive delta-connected motors to work properly, so doing it with a simple rotary phase converter instead also does away with that need.
I'll get a 2.2kW motor and see what happens.

Jordan

Originally Posted by .RC.

4kW is too great a load for a 15 amp circuit.. 2.2KW is the limit

Is the lathe motor 415V????

Not trying to sound smart here, but how do you get 2.2kw as the max load on 15 amps? 15 x 240 = 3600 so 3.6 kw is max for 15 amps, or am I missing something (maybe in the 3ph bit which I don't get)?

Not really my field (pardon the pun) but should that not be worked out at peak not RMS?

Good questions. Anyway, I missed out on the 4 kW motor.
Also posed my questions elsewhere, and the response was that for 240V 15A, the largest 3ph motor that could be used is about 2.2kW or 3HP, but didn't get the rationale.

Jordan

Also, be aware that without a step-up transformer, a simple rpc will generate 240v three phase. You will need to connect your load motor in delta (240v three phase) for it to work.

There is a way to fetch the delta windings out to the motor terminal connections if they are not there already.

Yes, that makes sense. My tests were with star connection and they did spin up, but I guess with less power? I got a 3hp (2.2kW) motor at a garage sale a couple of days ago, and happily is configurable for both star or delta. It's an old, heavy beast - nice!
Jordan

I'm glad you weren't able to go ahead with your plan. Those types of converters are very poor machines.

To those who may read this, don't be tricked into thinking that this kind of rotary converter is ok to use with your somewhat expensive & perhaps sentimental, tooling equipment.
If you don't like to needlessly throw money away, don't ever use one of these types of coverters. Phase imbalance (due to voltage imbalance) is one of it's biggest problems. It is this problem alone that will cause serious damage to electric motors.

The next problem with this type of converter is "protection & isolation", which must be done according to ASNZ3000:2007. The protection alone (for the two motors used in the converter) will be expensive...& this is even before you protect your lathe motor (load).

I can only think of two instances in which such a low quality converter can be used;
1] resistive loads.
2] heating (resistive or inductive).

These awful converters must never be used to supply power to another motor of any reasonable quality.

Thanks for the info. I think most of your objections can be answered though:

Phase/voltage imbalance - Isn't that fixable with tuning capacitors? I'm interested to know though, what is it that harms the motor, if it's not balanced? Overheating? What causes that?

2 motors? - I'm only planning on using one for the converter itself, another cap will provide the phase shift for starting. At least that's what I've read can be done!

Protection/isolation - What's the aim? Is there something inherently dangerous in doing this? I'd like to know if there is. There must be many thousands of these simple converters in use, and I haven't heard any horror stories. Got any?

Your first objection is cost, but after seeing how much is being asked for commercial RPC's, and even VFD's, it convinced me I've gotta look into alternatives. Even if I burnt out the lathe's motor, I could replace it with a second hand one for far less than a "cheap" no-name VFD.

Anyway, it's also a bit of fun.

Jordan

It may be that he's speaking of a pony-motor driven rpc without capacitors. I have been running an rpc for four years on my high quality Seimens 3 kW load motors without difficulty or adverse effects, as have many thousands of professionals and hobbyists worldwide.

There are many expensive 40 hp CNC milling and turning machines being run on rpc-generated three phase. In all cases it pays to fine tune the voltages by changing capacitor values. (My rpc has a max voltage difference of 6 volts as measured with two Fluke meters. Less under load.)

As far as isolation: So, big deal. Buy the appropriate stuff on ebay for cheap. I think my Clipsal brand switched disconnects, plugs and Allen Bradley motor contactors totalled under $120. (vice approx 800 retail)

another note: there are solid state converters around that are garbage. I wonder if he's thinking of that?

Greg

Hi Gents. My comments in blue.

(nadroj). I have an opportunity to buy a 4kW 3 phase motor, and thought it might be good to make a rotary phase converter out of it. I have a 2.2kW 3ph motor on a lathe that I'd like to power up from single phase 240V house power. My question is: Is 4kW too much to expect to be able to be powered by a 15 amp single phase house circuit? I'm looking to avoid using VFDs or capacitors, and have had encouraging results with smaller 3 phase motors. I'm just not sure if a 4kW motor will trip my circuit breakers. Only one leg of the 3 ph motor will be powered up from 240V.

Your 3 phase lathe is most likely to be a typical "single voltage" induction motor. If so, it will be designed for 400v to 415v, Delta connected.

(gregoryq). Also, be aware that without a step-up transformer, a simple rpc will generate 240v three phase. You will need to connect your load motor in delta (240v three phase) for it to work.

There is a way to fetch the delta windings out to the motor terminal connections if they are not there already.

Greg, this could be a big mistake. If Jordan's lathe motor is a typical 415v Delta connected induction motor (which I suspect it is), connecting it in Star at an even lower voltage will not allow the motor to produce enough torque.

(nadroj). Thanks for the info. I think most of your objections can be answered though:

Phase/voltage imbalance - Isn't that fixable with tuning capacitors? I'm interested to know though, what is it that harms the motor, if it's not balanced? Overheating? What causes that?

Yes. This problem can be overcome with capacitors but you said "I'm looking to avoid using VFDs or capacitors".
If each phase voltage across each motor winding is not the same, the winding with the reduced voltage will use more current. This directly affects the "magnetic circuits" in the motor & will cause vibration plus excessive heating...all of which will slowly destroy the motor.

2 motors? - I'm only planning on using one for the converter itself, another cap will provide the phase shift for starting. At least that's what I've read can be done!

Most high quality rotary phase converters use 2 machines.(e.g. motor/generator).

Protection/isolation - What's the aim? Is there something inherently dangerous in doing this? I'd like to know if there is. There must be many thousands of these simple converters in use, and I haven't heard any horror stories. Got any?

The fact that you are working with electricity is inherently dangerous. If you want your motors to have a happy & long life, you must employ protection.
ASNZ3000:2007 2.3.4.5 states that Appliances and accessories, including motors, shall be provided with devices for isolation and switching, in accordance with relevant clauses of Sections 4 and 7.

4.13.2 Protection against overload.
Each electric motor having a rating exceeding 370 W shall be provided with control equipment incorporating means of protection against overload of the motor. Exception: This Clause does not apply to motors incorporated in an item of electrical equipment that complies with an appropriate Standard.

4.13.3.1 General.
Electric motors shall be provided with overtemperature protective devices complying with Clause 4.13.3.3 where they—
(a) may be required to run unattended (see Note) and have a rating greater than—
(i) for shaded-pole type motors ....... 480 VA; or
(ii) for other unattended motors ....... 240 VA; or
(b) have a rating greater than 2250 W.

Your first objection is cost, but after seeing how much is being asked for commercial RPC's, and even VFD's, it convinced me I've gotta look into alternatives. Even if I burnt out the lathe's motor, I could replace it with a second hand one for far less than a "cheap" no-name VFD.

Anyway, it's also a bit of fun.
Jordan

(gregoryq). It may be that he's speaking of a pony-motor driven rpc without capacitors. I have been running an rpc for four years on my high quality Seimens 3 kW load motors without difficulty or adverse effects, as have many thousands of professionals and hobbyists worldwide.

Generally speaking, a good quality electric motor should last for at least 20 years provided it is maintained correctly, is used suitably & has a good quality supply.
It is not uncommon to see well maintained motors still running perfectly after 40 years of use.

There are many expensive 40 hp CNC milling and turning machines being run on rpc-generated three phase. In all cases it pays to fine tune the voltages by changing capacitor values. (My rpc has a max voltage difference of 6 volts as measured with two Fluke meters. Less under load.)

It sounds like you have done your homework.

As far as isolation: So, big deal. Buy the appropriate stuff on ebay for cheap. I think my Clipsal brand switched disconnects, plugs and Allen Bradley motor contactors totalled under $120. (vice approx 800 retail)

another note: there are solid state converters around that are garbage. I wonder if he's thinking of that?

Greg

I have been looking at buying a eurotech kits to make a phase converter.

Details available at http://www.coastelectrical.com.au/1%...onverters.html

These run a controller which varies capacitance and so adjusts for changing load and comes with all the breakers and stuff... Price seem very reasonable, less than $2k for a 4kW and less the $2.5k for 8 kW.

Anyone see any problem with something like this?

elkangorito said >

> Your 3 phase lathe is most likely to be a typical "single voltage" induction motor. If so, it will be designed for 400v to 415v, Delta connected.

Here's where I'm a little misinformed it seems. I thought for 3-phase, 240V = delta, 415V = star. All the 400-ish volt only motors I have are marked star.

> This problem can be overcome with capacitors but you said "I'm looking to avoid using VFDs or capacitors".

Oh yes, I forgot I said that at the beginning! More accurately, I want to see if I can do it without caps, but will use them if necessary.

> Most high quality rotary phase converters use 2 machines.(e.g. motor/generator).

That'd be great - just use a single phase motor mechanically coupled to drive a 3 ph generator. I haven't checked, but the gen is probably an expensive thing, compared to using a recycled motor? Or, could a driven 3ph motor indeed be used to act as the genny? Way of getting nice, balanced output?

> The fact that you are working with electricity is inherently dangerous.

One can't be reminded of it too often. Thanks heaps for the comments. I take it all in for consideration.

Jordan

My comments in blue.

Originally Posted by nadroj

elkangorito said >

> Your 3 phase lathe is most likely to be a typical "single voltage" induction motor. If so, it will be designed for 400v to 415v, Delta connected.

Here's where I'm a little misinformed it seems. I thought for 3-phase, 240V = delta, 415V = star. All the 400-ish volt only motors I have are marked star.

There are many varieties of 3 ph motors.
Single voltage (6 connection terminals) are usually Star/Delta - full line-to-line voltage (400v Delta) & line-to-neutral voltage (230v star). Usually, star connected motors of this type do not require a neutral wire (balanced load).
True Dual Voltage motors have 12 connections & can be connected in star or delta, according to the supply voltage.

I find it strange that you have 400v motors that are star connected. This is not the usual type of motor since the indicated voltage represents the winding voltage. If they are "single voltage" (6 terminals) & star connected for 400v , it means that they will be delta connected at 230 volts (which is as you said).

The "standard" single voltage 3 phase induction motor is usually 400v delta connected.
Jordan