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mush
08-01-2008, 11:50 AM
Hi,

I have a compressor in a Taylors ice cream machine (panel sticker included in post) RLA of 10.5amps and a LRA of 69amps it runs fine on mains power but when I try and use it with a generator it has problems when the refrigeration compressor starts, I used a amp meter with peak/hold and get min 85amps to max 120amps depending on how long the compressor has been running for, the machine is only 1 yr old and I have always had this problem. At first I was told the generators weren’t big enough but when the machine is running the compressor only draws 7-8amps and I have problems with some generators that are up to 15kva,

Sorry I am only just learning about LRA and compressor but is it normal for the start-up load to be so far above the documented LRA?

I'm beginning to think there might be a problem with the compressor , I was first told that because the compressor was for a freezer unit it would take more to start, but instead of getting bigger generators do I need to start looking at the compressor or is the nameplate LRA 69 just a guide.

thanks in advance for your help
hamish

monkey spanners
08-01-2008, 06:08 PM
I just had a quick look with google on sizing a generator for a 69 amp start current and to keep within a 15% volts drop it needed to be about 20KVA:eek:
Basically the generator needs to be able to supply 220V to 240V as the compressor is starting with no other loads placed on the genny from other apliances.

What voltage do you have as the compressor is trying to start? It needs to be within what is stated on the lable for it to start cleanly.

A motor is basically a rotating magnetic field, the size of the field depends on the motor construction and the voltage applied to it. This magnetic field resists the flow of current effectivly limiting it. This is measured in 'henrys'. I'm guessing here that if your genny is too small it is not suppling enough voltage to build up a big enough magnetic field an thus the motor is pulling more current than on the lable, but is just a guess, its 17 years since i did motor theory:rolleyes:
What sort of current does it pull on a mains supply start?

If it has worked for a year ok on mains i doubt its the compressor thats at fault.

Your best bet would be to talk to some generator technical people to see what size you should have and if it turns out yours is big enough then look to see if you freezer is faulty.

Jon

Grizzly
08-01-2008, 07:57 PM
Mush.
Without the technical jargon. I leave that to those that are much better at it than me.
The basic answer is YES. The Start up load can be huge. It is not uncommon to see a 3 phase Recip Compressor normally running at say 70a Pull 200a on startup.
Screw Comps can have a scary start-up load. On some Marine applications the second largest load is the screw comps on start-up.
I will now pass in the hope that one of the more eloquent guys continues.
Grizzlyhttp://www.refrigeration-engineer.com/forums/images/icons/icon14.gif

Josip
08-01-2008, 10:14 PM
Hi, Mush :)


Hi,

I have a compressor in a Taylors ice cream machine (panel sticker included in post) RLA of 10.5amps and a LRA of 69amps it runs fine on mains power but when I try and use it with a generator it has problems when the refrigeration compressor starts, I used a amp meter with peak/hold and get min 85amps to max 120amps depending on how long the compressor has been running for, the machine is only 1 yr old and I have always had this problem. At first I was told the generators weren’t big enough but when the machine is running the compressor only draws 7-8amps and I have problems with some generators that are up to 15kva,

Sorry I am only just learning about LRA and compressor but is it normal for the start-up load to be so far above the documented LRA?

I'm beginning to think there might be a problem with the compressor , I was first told that because the compressor was for a freezer unit it would take more to start, but instead of getting bigger generators do I need to start looking at the compressor or is the nameplate LRA 69 just a guide.

thanks in advance for your help
hamish


You answer all your questions....why you started to think there is some problem with compressor:confused:

I'm not an electrician, only trying to use common sense...

Best regards, Josip :)

Brian_UK
09-01-2008, 01:16 AM
Generator is too small and unable to cope with the start up current.

mush
09-01-2008, 06:01 AM
thanks for your reply's

I wanted to confirm that the start current could exceed the LRA , as my sparkies couldn't understand how a compressor could possibly draw 120amps when it was only a 10amp compressor , that said will work on up sizing the gennerator, thanks again for your help