Rotator motor capacitor

[Niall]

I have acquired a small Stolle aerial rotator without the controller. I found a diagram for a controller which shows that it requires a 120 uF capacitor to feed one motor winding depending on the direction of rotation.
I have built the controller using 3 Maplin non polar 100 volt capacitors in parallel and it works, but the capacitors are quite physically small and as I suspected, they get warm quite quickly.

What sort of capacitor would be suitable? Mains voltage motor run caps are physically much larger than the cap in the other rotator controller I have and the motor is only 20V.

My rotator doesn't have the position pot so I have not attempted to reproduce the positioning circuitry, direction lamps etc. which is partly electromechanical anyway.

[Refugee]

The capacitor looks right to me. The motor is 20 volts and the currant will be a little over 10 times that of a mains motor. The phase shift capacitor will have to be increased in value accordingly. It will only need to be 50V AC working though. It should not be physically too large. Avoid those reversible electrolytic types as they are not reliable.

[Niall]

It's a two phase cap run motor, direction is reversed by swapping the capacitor to the other winding. Standard practice with small mains induction motors but the cap is usually wired to give the correct rotation on installation when reversing is not required.

The only capacitors I can find of that value are the 100 volt crossover caps I am using or mains motor caps. The motor runs but the caps are clearly not up to dealing with the current for long as they heat up.

Mains rated motor run caps are not available much more than 80 uF. I could use 2 60 uF in parallel but they would be huge.
I'm wondering if I could use a mains rated motor start cap, whether the reduced voltage would allow it to be used for more than the rated period, given that it won't be used continuously.

Basically I'm looking for a non polarised capacitor about 50 volts working, 120 uF, which can pass a couple of amps for a minute or two without going bang.

[GlowingAnode]

Hi Niall, the aerial rotators I've worked on used non-polarised electrolytics.
Do you know for certain the capacitor needs to be 120uF? As I understand the diagram is from a different unit. The value of the capacitor should be adjusted to give approximately the same voltage across each winding with the motor running.
A motor start cap should be ok for brief operation. Alternatively you could make your own non-polarised electrolytic by wiring two 240uF caps in series with both -ve terminals connected together.

Cheers,
Rob.

[Niall]

Hi Rob,

The diagram is from a different unit but the rotator looks physically the same so I'm assuming they made a simple version without position feedback. I can't find any info on the simple one. It's the gold coloured casing with the drive tube passing right through it so there are no end stops.

The other complete unit I have got is a Kenpro / Yaesu KR600 which has a 26 volt transformer and a 100 uF capacitor so it's about right.

Another complication is I'm using a 24 volt control transformer in place of the specified 20 volt because I have some around. Getting about 24v on the direct winding, about 18v on the capacitor fed winding when running.

[Niall]

Found a couple of motor start caps and took some readings. I also inserted 2.3 ohms into the 24 v feed to drop the voltage a bit.

Capacitor marked 80 uF -0 +25% Direct winding 21V cap winding 14V
Capacitor marked 100 - 125 uF Direct 21V cap 16V

Measured with Isotech IDM19 DMM

I have to go out now, think I will measure the voltages on the Kenpro to see how close they are this evening.

[GlowingAnode]

Hi Niall, those voltages sound ok so the cap must be about the right value.
Just got to watch you don't form a resonant series tuned circuit.

Cheers, Rob.

[Niall]

The Kenpro is much closer at 29v and 28.5v.
I am now wondering if its capacitor is leaky as I think it is original.

[GlowingAnode]

I have built the controller using 3 Maplin non polar 100 volt capacitors in parallel and it works, but the capacitors are quite physically small and as I suspected, they get warm quite quickly.

Are you using these ones; http://www.maplin.co.uk/p/47f-100v-none ... itor-ja72p ?
They look way too small, the ones I had in mind are about 1" diam x 3" long.
You might like to consider using two of these in parallel; http://www.ebay.co.uk/itm/72-F-72uf-72- ... 4d25b16b9d

Rob.

[Niall]

The Maplin ones are what I had originally, 100u in parallel with 2 x 10u. The 10u ones are very small and get noticeably warm after only a few seconds operation.
I think I will go with the motor start caps, the 80u one is marked "3 min" which I assume is the maximum use duration. These are salvaged from scrapped pump control panels where they would be pulling quite a bit more current than this application.