Hi there, I have been given what looks like a very useful single phase motor. 1 1/8 inch shaft. It is capacitor start, seems to be OK, but it is taking 9 amps running light. It is also getting warm and does not sound "happy".
The capacitor is fine, but what is likely to be wrong, that's about 3 Hp, isn't it?
Any comments welcome. Thanks George.
I would say you have a failed winding, probably why you were given it.
If it is a true capacitor start, the fact it starts turning by itself suggest that side is OK.
9 Amps maybe reactive current, but should sound smooth with a slow pulsing 100Hz note. At most it should get luke warm with no load.In message , George writes
If fitted, centrifugal start switches can jam *on*.
Bearings? Does it rotate freely by hand? Shorted turns in the stator winding?
What did the *benefactor* say?
regards
If it turns out to be a shorted winding, you can always run it in series with a 3kW non-fan heater. Performance is affected a fair bit with a 2.2kw device, but it often makes it runnable.
Bearing trouble is easy to spot, if it wont turn easily or squeals.
NT
No there is no bearing problem, I think I shall have to take it apart, It makes a sort of buzzing noise when it is running at full speed, although it runs up to speed immediately, so the start capacitor is doing it's job, but if it has a speed switch then it could have shorted. It was just given to me by a neighbour who was leaving and turned out a lot of junk, some of it is good stuff, so I can hardly complain.
If it does have shorted windings, how much it is likely to cost to have repaired, I have a very nice compressor that is crying out for a
3 HP motor like this? I ran it via a device I still have called a Somar Powerboss, which saves power used by a motor by reducing the voltage untill the speed starts to drop and then saves power by keeping it running at the lowest voltage possible to maintain revs. It hunted between 200and about 170v but the current varied between 6 and about 4 amps when it shot up to 6 I don't really know what this indicates though as I cannot find the handbook for the Powerboss up to now.Cheers George.
This is a great way to kill your powerboss. Dont use it on the motor till the latter is fixed. 9A will be the average current, the current when it hits the partially shorted wind will be many times this. You're lucky it survived, typically the triac dies immedaitely.
NT
I take your point. I have probably been lucky! This morning I dismantled it and inside it is like new. No sign of overheating. No sign of any wire or centrifugal switch on the rotor either, it is a - squirrel cage, I think it is called and is a composite construction of alluminium and iron set on the shaft with nothing but the two bearings. There is only the rotor, the capacitor and the field stator. It was made by EFACEC of Portugal IEC 34-1 Model BF 5C
100L 44 50Hz 240v 2.2 Kw 14a 1400 /min. There does not look much can be wrong with it.The only thing that looks at all untoward is a very slight marking in the rotor and the stampings on the stator that indicates that a tiny bit of contact has occurred. I have cleaned the whole thing up inside and I think I shall take the body to my local friendly rewind shop for a professional opinion, he may have a growler, if such things still exist, maybe there is something better now. This will have to wait till Monday now, but I am tempted to actually try it on load, Is an indicated current of 9 amps actually excessive? It is obviously not getting very hot, if it is out of phase with the voltage, the actual wattage generated could be quite low? Why should the powerboss hunt between two voltages though, showing a much lower current when in circuit, too?
Regards George.
9A is almost what it would draw fully loaded, idling it should eat a fraction of that.
NT
check the bearings. Should be a mm or so of clearance all round. Even a bit of contact ups the idle current a lot.
I have cleaned the whole thing up inside
I cant remember the theory of squirrel cages, but my guess is that it will take a more or less constant current with the phase shifting as load is applied.
I am not so sure.
reading up on the darned things, off load essentially the thing is one soding great inductor slapped across the mains. As load is applied this essentially adds in phase current into the mix and power is drawn.
Wiki says this
"The idle current draw is often near the rated current, due to the copper and core losses which exist even at no load. In these conditions, this is usually more than half the power loss at rated load"
so it may be this is actually what its supposed to do.
He's just quoted the rating plate as saying 14 A full load current (FLC) and that sounds about right to me:
2.2 kW (3 HP) output, efficiency about 85% for that sort of motor and power factor about 0.8 at FL, soFLC = 2200 W / (230 V * 0.85 * 0.8) which is near as dammit to 14 amps. Efficiency and PF both fall with load and 9 A doesn't sound too far out for no load. What are the cooling arrangements - does it blow a gale? The fans built in to some motors use more power than you might think.
Now we are getting somewhere, The fan is just a centrifugal one on the other end of the main shaft, quite a blast of air flows over the outside casing which is equipped with cooling channels. I shall take a few photos of it all this morning. I am wondering if the thing had a blow when running to cause what looks like a bit of internal contact. It does not sound as if it is catching now, however there was what I described as an unhappy sound. I can post a short video on my website.
Regards George.
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