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Posted By Topic: Single phase motor issues

Oct 04 2018 17:07

I have just installed a 1 phase capacitor start motor in place of the previous 5.5 hp 3 phase motor that was on my compressor. The motor has no info plate also second hand, but the seller claims that it is 3 hp.

The issue is however when I put my belt from my compressor on it it usually just humms or moves it very slowly. I managed to get it to pump air for about 5 seconds once then I turned it off. However now it either humms then trips the switch or it just moves the belt very slowly. It works fine when there is no belt on it.

Would a larger start capacitor help at all?

It also runs in reverse instead of forward, can I swap the input of 2 wires to fix that?

Thanks in advance

Oct 04 2018 17:57

5.5 down to 3 HP could be an issue. Cap could be stuffed. Is it cap start only or start/ run? Usually there is a centrifical switch if cap start only. To reverse direction you need to reverse either the start or run winding. Not both. Also I’ve seen people playing with single phase motors and mix up the windings and use the start winding to run and run as the start. That’s generally releases the smoke in the motor. May be better finding a motor remainder to check it out for you.

Oct 04 2018 18:23

There are 2 capacitors so must be cap run aswell, ones white ones black with the white being the larger of the 2. it only has 2 wheels so is on an angle I\'ve got it closer to level by putting timber under the front and it seems to be abit more keen now it\'s closer to level, maybe I also tighten the belt a bit and see.

Oct 04 2018 18:27

Perhaps I should try a larger capacitor

Oct 04 2018 19:26

How on earth would you think a 3HP 1Ph motor would provide more torque that a 5.5HP 3Ph motor?

Am I missing some basic physical fundamental here?

Oct 04 2018 20:29

Perhaps I wasn\'t clear enough. I\'m using it in a belt driven air compressor.

Everyone told me that if I wanted to run it in my garage then downgrading to a single phase motor would be better, cheaper and easier than trying to convert the 3ph power to 1ph.

Most of the single phase motors I found were 2hp or less and I have seen afew examples of 3 phase set ups running a 1 phase motor.

So when I came across this 3hp motor I thought, yea cool (it might lose some power but I\'m not using it in an industrial environment, just to paint the odd car here and there).

So if I can get it to work then why not. It\'s been done before.

Oct 04 2018 21:00

Is this compressor what the VSD + motor was about in your earlier question? (see link below)

If so, I\'m gonna assume they come with the compressor. And likely were appropriately spec\'d to run the compressor. So, all you needed to do was sort out connecting the supply.

Oct 04 2018 21:10

They did not come with the compressor they were purchased separately, I decided that I can not run that vsd of a 10a house supply so I have decided to run a single phase motor on it. Safer and easier.

Oct 04 2018 21:24

The VSD, nor the 3-phase motor, wasn\'t what made it unsafe.

It was you trying to do the work without anyone experienced to help you.


Oct 04 2018 21:27

Hence why I asked on here before attempting to hook it up to power, and as a result of the feedback I got on here I never did.

Oct 04 2018 22:00

Lol, that\'s far from safe.


Oct 04 2018 22:06

Lol what\'s unsafe about seeking advice here before even considering hooking something up? You clearly have been too much time on your hands or have somehow taken offense to something I\'ve said. I suggest you post constructive replies or get a hobby outside of this forum.

Oct 04 2018 22:07

The advise is you need direct hands on help.

Where did the 5.5 hp figure come from, btw, if it wasn\'t off the supplied motor?


Oct 04 2018 22:08

err, integrated compressor motor, rather.

Oct 04 2018 22:14

The 5.5 figure is on a label on the compressor, the original 3 phase motor was 5.5 hp

Oct 04 2018 22:14

I hope you realise this forum is run by and for electrical industry persons.

You\'ll note I\'m about the only person here to extend as much as I have to you.


Oct 04 2018 22:22

Ok I understand that now but forgive me for thinking this would be a better source for advice than Dr Google

Oct 04 2018 22:43

I would say your problem is that a 3 hp single phas motor has bugger all starting torque cared to a .5 hp have motor. Compressors need starting torque unless they have an unloaded fitted.

Oct 04 2018 23:29

What might work better is to change the gearing ratio of the belt drive, so a 3HP motor is sufficient. It\'ll mean the compressor turns more slowly, and thus there are less CFM, but you\'ll get air, as opposed to no air.

Having said that, I\'d have thought a 3HP would be sufficient grunt to turn the compressor over onto an empty tank.

Oct 05 2018 00:19

Depends what kind of compressor it is.

Reckon its a talk issue.

Just chuck a couple more phases into it mate.

Oct 05 2018 00:21

Or it could actually be a source problem. Chuck out the HP and get some Watties for f***s sake!


Oct 05 2018 03:21

Unless you need 5hp why not just get a 2.5hp compressor for $200 from supercheap or similar. Comes with a 10 A plug.