Motor capacitor for antenna rotor  - Page 1
Forum section: electronics / beginners

author: DonRon
author_rank: Regular Contributor
timestamp: February 24, 2012, 08:02:57 pm
content: Hi guys and girls,I have an old antenna rotor which I will use to install a ham radio antenna. Cleaned the unit and also build a new control box and everything is fine now except the motor capacitor. This 100 Volts 100 microF motor capacitor is defective. In Germany I did not find any electronics shop where I can get it. They always sell these very large > 200 Volt motor capacitors.So i think about using a usual bipolar electrolytic capacitor with 100 microF and 100 Volts but i am not sure. Anyone can tell me if this is okay?? (The motor is a 24 Volt AC type and it will run only a few seconds when the antenna is turned. Current will be around 1 to max 2 Amps).ThanksRonald

author: sorin
author_rank: Frequent Contributor
timestamp: February 24, 2012, 11:27:25 pm
content: NoYou cant use electrolytic capacitor, with AC motoryou can use > 200 Volt motor capacitors without any problem, but with the same capacity (100uf)*sorry for my bad english

author: DonRon
author_rank: Regular Contributor
timestamp: February 25, 2012, 12:22:18 am
content: Hi Sorin,I knew that. A Standard electrolytic cap cannot be used because of polarity.Therefore I want to use a BIPOLAR electrolytic cap which can be used for AC. These caps are sometimes sold as "audio capacitor". (Also motor caps are bipolar electrolytic caps as far as i know.)I do not want to use these > 200 V motor caps because they are too large to fit inside the rotor control box. (The original 100 V motor cap has a small size).Greet'sRon

author: Psi
author_rank: Super Contributor
timestamp: February 25, 2012, 02:30:07 am
content: There's a difference between a bipolar electrolytic cap, which is effectually two polarized electrolytic caps in one package, and a true non-polarized capacitor.As i understand them, with NP electrolytics each cap is separate, so they don't effect each other. Unlike a true non-polarized cap

author: vk6zgo
author_rank: Super Contributor
timestamp: February 25, 2012, 05:02:02 am
content: My suggestion for what it's worth, is to go toQRZ.com--join if you are not a member already,& ask the same question.The guys on that forum are Hams & will know more about the rotator than we do.They probably know where you can find the original type of capacitor,or a similar type.In general,motor start (& run) caps are special types,designed for the rugged service environment they are used in.Although I am a Ham myself,I haven't had anything to do with the capacitors in rotators.

author: SeanB
author_rank: Super Contributor
timestamp: February 25, 2012, 05:48:13 am
content: You can take 2 220uF 63V capacitors and connect the positive leads together, then connect the negative leads in the circuit. As it is only used intemittently you should get a reasonable life on the capacitors. Not the best solution, but will work.If you want a real 100uF 100V capacitor look for a motor switchgear supplier and get a 100uf motor START capacitor. These are smaller, as they are only rated for a few starts per hour, and are not expected to be powered continuously. Thus thinner foil, thinner dielectric and a much smaller case and shorter life designed in. They will still last years.

author: sorin
author_rank: Frequent Contributor
timestamp: February 25, 2012, 01:40:58 pm
content: hello, the current that is consumed by a AC motor when it start is 10 time bigger that normal working currentso..., I think that the BIPOLAR electrolytic capacitor will degraded very rapidly

author: SeanB
author_rank: Super Contributor
timestamp: February 25, 2012, 02:57:51 pm
content: This is a antenna rotator. If operated once an hour that will be a lot. It is not being used as a RADAR antenna. If you have to change the capacitors every 2 years or so that will not be an issue, you will have to clean and oil the rotator anyway, and clean the switches anyhow. That is why I suggested 63V capacitors, they will have a longer life ( thicker oxide) than 50v items. I have a whole lot of 470uF 50V units in my stock, so could use 4 in series if needed, just make sure there are 2 in each direction. They will probably last as long as the original.

author: sorin
author_rank: Frequent Contributor
timestamp: February 25, 2012, 03:20:58 pm
content: Quote from: SeanB on February 25, 2012, 02:57:51 pmthe capacitor is used just to START the motor, won't matter how many hours the motor will run, the problem is how many cycles(start-stop) that the motor will do.so, I don't understand the comparison with the RADAR...

author: SeanB
author_rank: Super Contributor
timestamp: February 25, 2012, 06:30:13 pm
content: The motors in the rotator is a 24VAC unit, with a permanent split capacitor winding. The capacitor is powered all the time when the motor is energised. There is a momentary action switch that applies power to one side or the other of the capacitor to run the motor in the desired direction. The motor does not turn all the time like in a Radar dish, but only when movement is desired. Thus the rating being fine to use a motor start capacitor as it only will operate every so often. I have an old TV camera rotator that turns from stop to stop, which has a similar motor, with a 50uF 400V capacitor in the box, due to the long duty cycle needed. It does have a very low torque though, and can be stalled by a fingertip.

author: Zero999
author_rank: Super Contributor
timestamp: February 26, 2012, 08:07:33 pm
content: Quote from: SeanB on February 25, 2012, 05:48:13 amFor optimum capacitor life, use low ESR types with a voltage rating of more than 50V and put a diode in reverse parallel with each capacitor, .

author: DonRon
author_rank: Regular Contributor
timestamp: February 27, 2012, 01:24:25 pm
content: Thank's a lot guys for all the information.I will use the version with 2 capacitors first and again will try to find a motor capacitor (100 V) which is not too large. The caps are inside the conrol box and not inside the rotor. So if they die one day or I find the right one its easy to replace them.Thank's again guys and cheers from Germany,Ronald

author: SeanB
author_rank: Super Contributor
timestamp: March 04, 2012, 04:24:30 pm
content: Am I right in guessing the original capacitor is a round aluminium unit, around 20mm diameter by 40mm long, with a white rubber bung on each end, with a rivet and a wire attached providing the leads. Made by FRAKO sometime in the 1970's.

author: Zed
author_rank: Contributor
timestamp: March 05, 2012, 03:41:25 pm
content: I have had difficulty sourcing capacitors for single phase induction motors in the past from the usual suppliers like Maplin, RS, Farnell, Rapid etc. and have had to find a motor specialist company who supplies spares. They are often the best bet as they usually have the ''odd ball'' capacitors in stock.

author: DonRon
author_rank: Regular Contributor
timestamp: March 06, 2012, 04:16:31 pm
content: Quote from: SeanB on March 04, 2012, 04:24:30 pmWill have a look when I am back home (living at another place during the week).As far as I remember - the size might be okay - but the cap is brown and does not look like those "standard" aluminium unit motor capacitors.I also tried some motor specialist company, but in germany the always offer you capacitors for a voltage of 230 Volt and up.And I don't want to use them, because they are very large and they won't fit in my rotor control box.

author: SeanB
author_rank: Super Contributor
timestamp: March 07, 2012, 08:05:50 pm
content: The capacitors I have seen and am describing are not motor caps, but were bipolar electrolytics that were made in the last century, and which lasted for decades, only being killed by eventually going open circuit.