3 phase motor compressor, switching to a single phase motor question

[Jgguinness]

Hi all
Bought a compressor today, decent but cheap enough. Trouble is its a 3 phase motor.

Plate on the motor says 2.2kw output (so just under 3hp), RPM 2850.
Plate on the compressor says working pressure Lb/in sq 171, piston displacement 13cfm, motor output 3hp, max speed 1115rpm

Anyone know what single phase motor would be suitable? I've seen a 3hp motor in Machine Mart, 2 pole 3000rpm. Starter amps required 10-16. Shaft dia is 24mm which I think is slightly larger than the existing motor shaft. Would this be any good?

Kind regards
Jonny

 

[brightspark]

best getting a slightly smaller motor pulley to suit motor to keep the revs the same and u would have to alter the wiring . new feed to the pressure switch then to the motor using 3 core flex preferably 2.5 mm

 

[Jgguinness]

Cheers Brightspark for your swift reply. Motor output size wise though, maybe an obvious question, but is it best to try and match as close as possible the motor output?

 

[brightspark]

Cheers Brightspark for your swift reply. Motor output size wise though, maybe an obvious question, but is it best to try and match as close as possible the motor output?

a 3hp motor will be fine just try and keep the speed of pump the same or near enough

 

[piman]

3Hello Jgguiness,

your original motor will have a frame size on the motor plate, match that to the single phase motor and it will fit straight away. The speed you quote, 3000 rpm, is wrong it will be more like the 2850 of your original motor.

Alec

 

[Cobs]

I've recently been through this exercise. I bought a TEC single phase motor from Signet Industrial in Doncaster. TEC are Chinese made motors but with a UK distributor. I'm told they are better quality than the Clarke motors but not in the same league as say EMG or Brook Crompton. I bought it off ebay but when it arrived there was a hairline crack in one of the feet. I phoned Signet and 2 days later a new set of feet arrived direct from the importers, absolutely no quibble. If the motor is half as good as the back up it should be fine. Here's a link:

http://www.ebay.co.uk/itm/TEC-Singl...t=LH_DefaultDomain_3&var=&hash=item19e668e19e

Specs here:

http://www.tecmotors.co.uk/sites/default/files/tcc dime.pdf

http://www.tecmotors.co.uk/sites/default/files/tcc elec.pdf

When you start delving into single phase motors it's a bit of a minefield however there are two basic sorts; a) permanent capacitor which are low torque typically used for fans, blowers etc where there is little/no load on startup and b) capacitor start/capacitor run which are capable of higher loads on startup e.g. compressors. The speed of the motor depends on the windings/internals however the most common are 2 pole which typically run at around 2800 rpm and 4 pole which run at 1400 rpm - actual speeds vary according to model/manufacturer. Sometimes you'll see a generic 3000/1500 as in the ebay link but if you look at the actual spec in the pdf for 2.2kw 2 pole it's actually 2810 rpm.

To work out the size of the pulley use the following:

Motor speed = pump pulley diameter
------------- ------------------------
Pump speed motor pulley diameter

eg if motor speed 2810, pump speed 1115 pump pulley diameter say 260mm then motor pulley diameter would be 103 (or nearest equivalent)

You can get pulleys from Machine Mart though plenty of places on the web sell them - things to check are spindle diameter and keyway size

There are a wide variety of mountings but you'll need foot mounted (B3). The foot frame sizes depend on the motor but can be different for different manufacturers. The TEC 2.2 has a 90 frame size - details in the pdf

Finally when you mount it you need to wire it so it's turning in the right direction. You may find an arrow on the compressor pump pulley or if not have a look at how the three phase motor is wired at the moment - there should be a diagram in the junction box lid
.
HTH

**re last bit see normspanners post below **

 

[Jgguinness]

Thats great thanks for the detail Cobs. How have you found the TEC motor? Mine wont be getting a lot of use as its in my private garage and will only be used as and when required.

Kind regards
Jonny

 

[Cobs]

I've only used it for a short time so far so not really long enough to give a valid opinion however it runs fine so far and seems well made.

 

[fenman]

simple and useful post cobs

 

[normspanners]

Finally when you mount it you need to wire it so it's turning in the right direction. You may find an arrow on the compressor pump pulley or if not have a look at how the three phase motor is wired at the moment - there should be a diagram in the junction box lid

An excellent post apart from the last little part single phase motors are a little harder to swap direction of rotation than just putting the input wiring the correct way, you have to swap the wiring which goes within the motor itself, and also you cannot tell with the 3ph motor which way it was running because it all depends how it was wired in the junction box, because swapping any two inputs changes direction.
Usually the compressor flywheel is combined with a fan and has blades cast into it, to blow air over the compressor cylinders it does make a difference because a fan blows with better direction than it sucks

 

[Cobs]

also you cannot tell with the 3ph motor which way it was running

Edited post to refer to your comments re three phase motors. On the single phase motors I've seen there have been metal jumpers which need changing in the junction box - it's a fairly simple process though.

 

[normspanners]

Cheers Cobs it is just that IMO not ALL single phase are that easy, a lot are being made, making it easier to change, with the mentioned jumpers but first thing is knowing if it is an induction motor or wound rotor motor.

As you rightly say, if an induction motor there are several types, split phase, capacitor start, capacitor start with capacitor run, all require reversing two leads to change polarity between main and aux windings, and the leads are not allways obvious also if the manufacturer does not bring all the leads out to the termination box you may not be able to reverse it. I have had to delve quite deep to reverse some. Shaded pole motors need to have the rotor physically removed and turned around to reverse rotation - this can depend on the design of the motor shaft if that is even possible to do.

 

[Cobs]

Thanks for the clarification normspanners. I know electric motors are a vast topic - I'm still learning!