Tell Me About a Capacitor in a Golf Trolley

[SCEtoAUX]

I got given an electric golf trolley. It was pretty grubby so I took it all apart and cleaned it up.

The thing runs and works fine but within the electronic gubbins a capacitor had fallen off. It should have sat across the 12v DC coming from the battery.

Now I have a basic idea of what a capacitor does (blocks DC, lets AC through), so what might it be there for if the thing works anyway?

It's rated at 50v/2200uF

My guess is to stop some kind of voltage spikes but I really have no idea...

Thanks

[MJNewton]

It'll be to reduce electrical noise. It'd be worth putting back on even if just to avoid interference in nearby TV/radio reception.

[SCEtoAUX]

Ah thank you. It's got a leg broken so I'll buy a new one. Cheers.

[MJNewton]

When you said 'leg broken' I was thinking 'eh?'... then I realised I'd been picturing a golf *buggy*, not a *trolley*!

[48k]

MJNewton said:

When you said 'leg broken' I was thinking 'eh?'... then I realised I'd been picturing a golf *buggy*, not a *trolley*!

I think he means a leg of the capacitor is broken.

[guindilias]

That large a value capacitor is likely the start capacitor for the motor - it provides an extra high current surge when the motor needs to start from a dead stop.
Means the manufacturer can use a cheaper, lighter battery which can't provide enough oomph from a dead stop to get it going well, as the capacitor can discharge much faster than the battery can.
Mouser will sell you one for a couple of quid, if you can read a part number off it you will be able to get an identical one, otherwise just go by the pictures, and get a 2200uf capacitor rated at 50v or above.

[MJNewton]

A start capacitor is only required for an AC motor.

[SCEtoAUX]

48k said:

MJNewton said:

When you said 'leg broken' I was thinking 'eh?'... then I realised I'd been picturing a golf *buggy*, not a *trolley*!

I think he means a leg of the capacitor is broken.

Yes, that.

It works just fine but I will repair it nonetheless.

[guindilias]

MJNewton said:

A start capacitor is only required for an AC motor.

I have a few ex-MoD water pumps that run on 12vc DC, and they have beefy capacitors to get them going. If they are disconnected you have to spin the motor shaft by hand to get it to run up to speed.

[Liamjrhodes]

MJNewton said:

A start capacitor is only required for an AC motor.

Its a little bit more specific than that. Single phase AC motors require them but 3 phase do not. DC motors should not need them

[sospan]

It could be doing a similar function to the capacitors on car distributors from “the old days”.
A simple repair/replacement should be ok.

[guindilias]

These motors are from WW2 if that's any help!

[WindyCommon]

A flux capacitor would be a good choice for a golf trolley if you can source one.

[MJNewton]

guindilias said:

I have a few ex-MoD water pumps that run on 12vc DC, and they have beefy capacitors to get them going. If they are disconnected you have to spin the motor shaft by hand to get it to run up to speed.

A start capacitor temporarily delays the current flow through a secondary coil and leads to a much greater starting torque due to the phase difference of the two windings. There is no phase to differ in a DC supply/motor.

Edited by MJNewton on Wednesday 7th August 19:06

[guindilias]

I know how start capacitors work... I have built a few wood splitters with large AC motors with start or start and run caps - these capacitors are nearly the same size as the motors themselves, on the small ones - one is s "Stuart Turner No.1", about the size of a coke can!

[MJNewton]

With respect, I don't think you do otherwise you wouldn't be saying they are used on DC motors to aid starting. If anything, it'd inhibit starting whilst the capacitor is being charged.

Edited by MJNewton on Wednesday 7th August 19:11

[guindilias]

And if the capacitor is charged by the battery while the motor is "off"? Surely it would provide a boost of current when the motor is switched on, enough to kick it into spinning up?

[MJNewton]

The capacitor is connected in parrallel with the motor so with the motor 'off' (disconnected) so is the capacitor. Futhermore, when 'off' the capacitor will discharge through the motor windings so it'll need charging back up next time the power is applied.

It is a common (and understandable) misconception that start capacitors function by 'boosting' power directly by charging/discharging but its the phase delay property that is being exploited (and only in AC motors; specifically single phase ones as liamjrhodes rightly points out as 3-phase motors already have a phase difference to induce rotation).

Edited by MJNewton on Wednesday 7th August 23:19

[guindilias]

In the ones I have, the cap is between the motor switch and the battery - so it won't discharge with the motor off. In parallel with the battery, but before the motor switch. I would take a pic to explain better but the pumps in question are a 2 hour drive and a 20 minute boat trip away!
Will be up there at the end of the month though, so will get pics then.

[MJNewton]

There's no need for pics; I do believe you and can understand your description but they are still not going to operate in the way you think they do. The energy stored in the capacitor is tiny compared to that within the battery (you can confirm this by disconnecting the battery and seeing how long the motor continues to run for) and will not aid starting of the motor. Perhaps more to the point is the fact that a DC motor really doesn't need assistance with starting as its torque is maximum at standstill (and decreases the faster it rotates).

Edited by MJNewton on Wednesday 7th August 23:11