Different speed / power motor

[el_stig]

Hi again all...following on from my thread trying to figure out my slow fill speed I have noticed that I am running a 1300rpm motor (2.2kw) on my slow filling compressor, while my smaller one runs a 2640 or so rpm (again 2.2kw).

The motor itself looks as though it has been changed as the wiring is different, the capaciter has been changed and the motor now sits on a mounting plate instead of direct to the frame.

What I am wondering is whether there is any way of finding out what speed motor should be on it? I have spoken to a supplier who thought it was around 2400. How hard is changing a motor? And could I potentially upgrade to a 4hp motor instead?

Once again, any and all help is greatly appreciated! Stu

 

[rtbcomp]

Can you remind us what make/model of compressor it is, especially whether belt or direct drive.

 

[el_stig]

Sorry totally forgot!

It's a Rand N3, belt drive, 3hp, 150 litre tank. I can get images if that helps? I've been in the dark with most things about it as it's second hand

http://www.excelcompressors.co.uk/c.compressors/rand-equipment/nimrod-reciprocating-compressors

Can be seen on the right on that link....it's the nimrod portable 150 litre 3hp. That company do not reply to correspondence though....!

 

[el_stig]

Any ideas on this one then guys?

 

[rtbcomp]

Generally speaking belt driven compressors use slower motors which develop more torque for given power.

This means you need a large motor pulley, which in turn reduces the risk of belt slippage.

For a particular model it all depends on the pulley sizes and bore/stroke of the pump.

Have you tried emailing Excel compressors and asking them?

 

[el_stig]

I have emailed them about a number of queries and recieved no response. I have had responses from Associated Compressor Engineers who also do Rand spares. I spoke to their service department after I had changed the valve plates, gaskets and oil. I explained my problem and he suggested that the motor was too slow, he thought it should have a 2400rpm or so.

The motor on mine has definatly been changed at some point during its life. The whole thing sounds very slow when running next to my other 3hp compressor.

Would putting a faster motor on damage the pump if it was too quick? I'm sticking to 3hp which was what the compressor started (is still) as, due to not having to change pulleys and get new belts made up.

 

[fenman]

why not change the motors from one to the other and see what happens

 

[el_stig]

I have thought about that....but I'm one of those people who are likely to break both! Ham fisted would be a pretty apt term!

Think you are right tho, try that before forking out £120 odd for a new motor. So what potential damage could this cause?

 

[fenman]

my comp is 3hp 1ph and runs at 2800 so should be ok to try

 

[eil]

Have you tried phoning excel`s technical dept and asking them direct. Is this compressor supposed to run on a three pin plug ?. If it is it would be probably a 2pole motor 2800 rpm.

 

[el_stig]

I spoke to both excel and ACE recently and both believe it should run on a 2800 ish motor...but there are no spec sheets anyone can find to confirm this.

Rand equipment are bust and I don't know anyone who has one!!!!

It is single phase and 13amp...

 

[eil]

Yes rand have gone bust as I understand, It most probably had a 2800 rpm 2 pole on it to run on a three pin plug, you need a bigger pulley on your existing motor, a pulley should be cheaper than another motor.

 

[el_stig]

Ok....this is an interesting angle I hadn't thought of! Change the pulley not the whole motor!

Where can I buy pulleys? And is there a formula to work out what size I need? Do you need a gear puller to get them off the motor?

Thanks for everyone's on going input!

 

[penfold]

Bearing boys do pulleys; there are loads of internet suppliers for pulleys and stuff of that ilk.

 

[rtbcomp]

If you're looking at doubling (1300 to 2600 rpm) the speed then double the diameter of the motor pulley.

If the pulley is an aluminium one heat it and pull with a puller. Don't forget to loosen any grubscrew holding a key in place. Be very careful with the puller, aluminium pullies are easily damaged. Don't ask how I know