Motor capacitor value

Maybe SOT, but I think I've seen discussions here before on motors and capacitors. Basically, how critical is the capacitance to an electric motor (probably start rather than run)?

Background... I have a lawnmower with a 1600w motor. A couple of years ago it failed to start, just producing a loud hum and getting very hot. The repair (fortunately still within the guarantee period) stated "capacitor replaced".

Last week the same fault appeared - no rotation, just a loud hum. As it was out-of-guarantee, I took it apart and found the capacitor - 30uF

450v. I ordered a new one and fitted it. Unfortunately the motor still just hummed. I dismantled the motor and cleaned it, and replaced it and the old capacitor. It now works, but I found some mechanical damage on dismantling. I might have to replace the motor some time.

On the motor itself I found a label which, amongst rotation speed, power, and other things, had "25uF 450v" on it. So was the original repair replacement capacitor at 30uF slightly too high? I see these capacitors have a 5% tolerance. Unfortunately I can't find a spec page for the motor, but the current replacement (GJC-1600) at refers to a 16ufF capacitor!

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Jeff
Reply to
Jeff Layman
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I'd say within 10%. Don't think there's any particular reason for it to be more accurate than that.

Does it still run if you give it a spin, then plug it in? (It needs to coast down long enough for you to get your hand out of there, obviously.. do this carefully..)

Tim

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Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
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Reply to
Tim Williams

use a flimsy stick not your hand. Plug in and poke it.

NT

Reply to
tabbypurr

Yes. Before I dismantled the motor I pushed the blade around with a long stick. It soon picked up and rotated at full speed. Now it works without "assistance", but there are some clonking noises coming from the motor when it starts.

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Jeff
Reply to
Jeff Layman

They're not hugely critical, and there's no guarantee that as the manufacturer optimized the thing for price and profit that they may have inadvertently optimized it for longevity, efficiency, an good performance as well.

If it's a run cap it might be more critical -- and I could see how a lawnmower might want a run cap and not just a start cap.

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Tim Wescott 
Wescott Design Services 
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Reply to
Tim Wescott

I'm sure you are right that it's a run cap. I couldn't see anything externally or internally which would disconnect it after the motor started. FYI, the motor label shows: Ningbo Racing/Hujiang Electric 1116000

230v/50Hz IP00 SI I:c:F 25uF 450v 7.65A P1=1600W 2800 r/min cos ?: 0.95

The capacitor is only a couple of cm from the motor and gets very hot after an hour of mowing. It is rated at 85 deg C. It needs to be!

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Jeff
Reply to
Jeff Layman

This has the smell of insufficient AC current rating for the capacitor.

Using a starter capacitor for run cap?

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-TV
Reply to
Tauno Voipio

Usually start caps have a high uF and lower voltage rating due to the fact they are switched out of circuit after speed has been achieved. From the voltage rating alone that is a run cap and as stated there is no switching gear in the motor.

Reply to
Rheilly Phoull

Does the motor bearing need lubrication?

I found out, *after* I had junked an oscillating fan (but kept the motor pieces!) that all it had needed was some lube on the bearings.

Michael

Reply to
mrdarrett

It may not have been designed for being used that long -- if you have room, get some air circulation in there, or at least mount the cap away from the motor. That'll keep things cooler and make things live longer.

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Tim Wescott 
Wescott Design Services 
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Reply to
Tim Wescott

It wasn't the motor bearing which needed lubrication, but (apparently) the motor safety brake. See photo at

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. The damage to the end of the rotor (see red arrow) appeared to be caused by the part marked with the blue arrow. There are three of these on the dome-shaped piece which is supposed to slide up and down the shaft, and, I guess, being spring loaded stops the rotation when the power button is released. But it didn't slide; it stayed in contact with the rotor, so it couldn't start. I lubricated the dome and the motor now works ok, although it makes a noise when it starts.

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Jeff
Reply to
Jeff Layman

I've been considering that. The motor and capacitor are covered by an unventilated thick plastic shroud - the only air circulation is that forced by the motor fan. A small hole - maybe 5mm or so - in the shroud above the capacitor might help. But I've also got to consider this is a lawnmower and I've found a few bits of grass inside the shroud anyway. A hole might allow even more in, and wet grass and 230V connectors don't make good company!

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Jeff
Reply to
Jeff Layman

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