OT - Electric motors single / 3 phase
Turbo Mini Forums

I've got the chance of a 3phase two poster lift for my work shop. but I only haver a single phase supply, a single to three phase converter man enough for the job is around ?400, so I was thinking of changing the motors to single phase. I may be wrong here, but I think you can run a 3 phase motor in fwd and rev, but I think if you have a single phase ac motor, which ever way around you power it (normal or reversed connections) it will always run the same way. Is this correct?

Any ideas how I can get around this problem, I know you can buy single phase lifts, how do they get around this problem?

Edited by Jimster on 29th Apr, 2005.

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single phase motors to match the same hp as 3 phase motors are expensive and a lot larger so im lead to believe. Hence larger hobby compressors (3hp ish) use 3 phase convertors on the top.

also think your right on the reversing issue.

I'm getting a quote to get 3phase installed in my garage (which is next to my house in residential area) does anyone know if this will be silly money?

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a single phase ac motor, which ever way around you power it (normal or reversed connections) it will always run the same way. Is this correct?


this is correct

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I thought so, so how do single phase lifts work when you want to make them go down?

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Gravity.

Mind you an electric drill goes backwards,and thats by electric not gears.

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yea good point, maybe they have small ac dc transformers in them?

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An electric drill is really a d.c. (or universal motor). You reverse them by reversing the connections to the field or armature, not the supply as piontes out above.

I think an inverter is the easiest solution I'm afraid. There is apparently a way to do it using capacitors, but they would need to be a specific size and very big. Reversing the motor still wouldn't be easy either.

Wil

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A single phase motor will run in both directions, it just depends on which direction it starts. Unfortunately the starting mechanism is normally contained within the motos so it will only go one way.

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There is a way to convert a three phase motor to use single phase using capacitors, yes. You need to use the capacitors to make the single phase "lead" and "lag" in phase from the orignal supply to effectively give you three phase. However in doing this the motor effectively runs at less than a third of its output power on "proper" three phase. In some applications you can get away with this, for a car lift i think you would struggle.
If you went down this route it is a simple wiring job to reverse the direction using two contactors, one for "up" one for "down". Three phase motors are easy to swap direction, you just have to swap two of the phases over, so your "simulated" three phase will allow this easily.

It is worth asking about three phase to your house, but I think you might struggle. I would say that if you live near to any kind of factories or industrial units you have a better chance as there will be a three phase supply to them that could be tapped into and diverted.

I am actually close to wear there is 3 phase, I'll await my quote

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OK. If you want some ideas on how to wire a panel to control it if it hasn't got one let me know.

mate, i think you'll find its about 6k.....

ouch! think maybe I'll buy a new trolly jack instead!

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The other thing to do I suppose is get the shaft size and HP (kW) rating off the motor and price up a single phase item to do the same job......and then think up some ingenious way of lowering it off........

yeah i guess, much of 3hp and you'll be struggling i reckon. well for a sensible price

Use 2 single phase motors attatched to a common shaft
facing each other.One for up one for down.

Big diesel starter motor and a few batteries

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Hi

I'm pretty sure (maybe wrong) but inside a single phase motor where the wires are on the stator if you switch some of those round they will run backwards. We usde to do it on industrial sewing machines when they jammed up.

hi jimster
if you cannot find a solution i can give the lift a good home pm me if interested

my nieghbour has a 2post lift that is 3 phase, they bought a converter like you talking about that was around £400. however it isnt able to cope and you have to stop to let it cool down

however its ok with my car because of its light weight!!!!!

god knows how it works but i was picking up some landrover stuff from an electronic guy and he had made his own 3 phase converter using 2 motors that are driven off another motor. if your having problems i can find out how he did it

if you could find out some info nick, that woudl be great

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When you get the quote for 3 ph Jim can you let me know a rough price, just out of interest for future plans.....

look into invertors and single phase motors. i did a class on power engineering, and learned how to converto 3ph motors to 1ph, and al the asociated maths, but that was 2 years ago and iv forgot :(

if your only lifting the mini, you may get away with just 1 meaty 1ph motor, or the 3ph motor and a starting cap. or even a (relatively) small invertor.

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Denis O'Brien.

I was speaking to a mate on the weekend, who told me that he has many times reversed the polarity on a single phase motor and made it run in reverse. Are we wrong, or his he talking crap?

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