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Topic Title: No load motor current
Topic Summary: Too high compared with full load current?
Created On: 14 November 2017 08:38 PM
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 14 November 2017 08:38 PM
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Dreckly

Posts: 45
Joined: 03 December 2014

Hello

I've just been looking at a grain auger motor which was "blowing fuses"
Single phase, capacitor start with centrifugal switch , capacitor run, with a 13A plug, and no motor starter or overload. 1441rpm, 8.9A full load current on rating plate.
I tested capacitors and both are fine. Insulation test, windings to frame 12M ohm. I took the drive belts off and measured 7.7A no -load current. This seems too high and only leaves 1.2A to produce useful power.
Am I missing something?
What could be the problem?
What could be the solution?
Any help will be appreciated.

Regards

Dave
 14 November 2017 09:22 PM
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Kenny79

Posts: 7
Joined: 12 May 2015

Sounds as though it could be the bearings on the motor.
See how it feels when you rotate the motor by hand. Also check the auger.

Kenny
 14 November 2017 09:49 PM
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Dreckly

Posts: 45
Joined: 03 December 2014

Hello Kenny

This was with the belts off so no auger connected. Initially it was quite sluggish to turn by hand. I took off the fan cowling cleaned out the dust and sprayed oil around the bearings. That improved things so that after a good flick it took a few seconds to run down to stop but still taking much the same no load current.

Dave
 14 November 2017 10:22 PM
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broadgage

Posts: 2435
Joined: 07 August 2007

The readings noted might in fact be fine.
Some types of motor draw a lot of current unloaded, but this is at a low power factor and does not translate to a lot of power absorbed.
 14 November 2017 10:32 PM
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potential

Posts: 1641
Joined: 01 February 2007

A stab in the dark, have the windings been swapped round at some time so the starter winding is now the running one?
 14 November 2017 11:13 PM
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mapj1

Posts: 9568
Joined: 22 July 2004

are these winding currents or line currents - are we including the current in the run capacitor ?
If it was really losing 2kW you might expect there to be a smell of burning and maybe blistering paint. If so, time to order a new one, if not it sounds like power factor is up the creek, are the capacitors the right way round in terms of start and run ?

-------------------------
regards Mike
 14 November 2017 11:57 PM
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Kenny79

Posts: 7
Joined: 12 May 2015

I would still check the Auger. The fuses are only blowing when the belts are on.
Make sure belts aren't too tight.
Have you tried running it yourself with the belts on to get a running current or is the fuse popping straight away?
 15 November 2017 07:37 AM
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Dreckly

Posts: 45
Joined: 03 December 2014

Thanks all for your advice
I'm back there later today and am going to start by fitting a motor starter with a correctly set overload.
Motor runs on a 13A BS1362 fuse with belts off. 7.7A measured on supply cable, not individual windings.
Fuse pops within a second if belts are on. NB Auger is 100mm diameter and about 6m long so will need a significant amount of power to turn even if empty.
Thanks for your comments about power factor, and correct connection of the windings and capacitors. I will check that out although the farmers says "It worked fine last time we used it"
There is no sign of overheating so no reason to think that the current is generating heat.

Regards

Dave
 15 November 2017 09:34 AM
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davezawadi

Posts: 3905
Joined: 26 June 2002

Probably the bearings are shot. It should be easy to spin and run on for some time. If not change the bearings first. It could be electrical but not likely. Does the motor have a centrifugal switch, it could be stuck closed? This then gives this symptom. You will need to take the motor apart with a puller or if you are not used to this a new one is not expensive from machine mart (4 pole).

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 16 November 2017 08:20 PM
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Dreckly

Posts: 45
Joined: 03 December 2014

Hello

I've been back to this job today and fitted a proper motor starter and overload. Then I ran the motor for 30 minutes or so with the belts off and sprayed oil around the bearings. If the bearings are sealed, this should not have helped(?), but it did. After a brisk manual flick of the motor shaft, it carried on turning for a few seconds. After the 30 minutes, the motor was slightly warm but not paint blistering hot.
Belts and guards back on. Auger runs without blowing fuses or tripping out overload (set at 7.9A).
It looks as if Kenny and DavidZ were right. Thank you. Advised customer, if problem recurs, change bearings or replace motor.
Thanks for the tips about capacitors and/or windings being wrongly connected. Not applicable this time but I wouldn't have thought of that and you never know next time.

Regards

Dave
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