Motor run capacitors

[stokes]

Anybody know a source for motor run capacitors for use in tape machines.All I see on the net are for heavy duty industrial types.Thanks.

[Enzo]

I usually think of W.W.Grainger for such things, but Allied lists motor run caps, so does MOuser. What do you need?

[R.G.]

Anybody know a source for motor run capacitors for use in tape machines.All I see on the net are for heavy duty industrial types.Thanks.

Sure. Take out your telephone yellow pages and look up HVAC (heating, ventilation, air conditioning) suppliers. They generally have a big selection of motor run caps at much cheaper prices. Grainger always has stuff, but they charge a price for keeping big stocks.

[stokes]

Thanks for the suggestions,I've been to Grainger,Mouser and Allied.The trouble is that these HVAC caps are huge.They wont fit in the unit.It has a 48uf/42v cap in it now.It is an inch and a quarter in diameter,I might be able to fit 1 1/2"but even that will be a squeeze.All the ones I've seen at those sites are about 2' in diameter.I have seen a couple of smaller squareish caps for Sony units on ebay,but they are 1.5uf and 2uf,way too little capacitance.Thought if I could find someone who stocks caps specifically for recorders,I could find a better fit.

[Enzo]

Stocks recorder caps? When was the last time someone made a tape deck motor using a run cap? Aren't most of them DC motors? I'd be guessing a good 20 years ago now.

So what you need is a small cap with AC ratings sufficient for your needs. A non-polar electrolytic.

[stokes]

Stocks recorder caps? When was the last time someone made a tape deck motor using a run cap? Aren't most of them DC motors? I'd be guessing a good 20 years ago now.

So what you need is a small cap with AC ratings sufficient for your needs. A non-polar electrolytic.

Yeah,like I ever work on anything less than 20 years old!I really just started delving into tape echo units about a year ago,did a couple of tube Echoplexes after being harassed into the first one by a guy I do work for.So I have no idea what they make them with today.So,Enzo,you think a regular non-polarized electro cap would suffice?That had crossed my mind,but I wasnt sure.Voltage rating isnt that important,its only a 42volt winding off the PT.Its a German made Dynacord from '62,sounds fantastic now,but the motor is running slow.When I disconnect the cap and measure it,it only measures 29uf,supposed to be 48uf,so I am assuming its the cap and not the motor.Thanks for the input.I'll hunt down a non-polarized and give it a shot.

[R.G.]

If it's a start cap, sure, an NP electro will work. If it's really a run cap, the high rms current will eat it up.

Look here: Mouser catalog, page 708.
Look for the 581-FFB44E0476K, 47uF, 60VrmsAC rated, $13.75. It should work and is only 1.23" square by 0.83" wide.

[stokes]

Thanks,R.G. that looks good.I made the mistake of searching Mouser for "motor run caps" all I saw was those big HVAC caps.

[Enzo]

I am over my head in this, but on the small 24v motors used on things like Conn strobe tuners, both caps were NP ecaps.

For high rms currents, what would be the difference in applications between something like a motor run cap and something like the output cap in a single supply audio power amp? other than the audio cap is not NP.

For reference example I have an Acoustic 370 bass head here. AMp runs 0n 90VDC with the output bus at 45v. Then a 3900uf cap from there to the speaker. COmpare the motor application to this amp producing a 60Hz sine out at a large percentage of it power rating. I am pretty light on motors.

[Steve Conner]

They are run capacitors, and you probably shouldn't use electrolytics, non-polar or otherwise. Check out power factor correction caps, they come in small sizes for building into fluorescent light fixtures. If these don't suit, try any other plastic film cap with the right voltage rating. I'm sure you could get 10 4.7uF 63V plastic film caps and parallel them up.

Enzo: A coupling cap just lets the AC through without developing a voltage across it. A motor run capacitor has lots of alternating voltage across it and has to develop reactive power.

[Enzo]

Steve, even in a speaker environment?

[Steve Conner]

Strictly speaking, yes. The use of non-polar electrolytics in speaker crossover networks is just one of those unpleasant facts of life. They really should be plastic film, except if they were, the crossover would probably end up bigger than the speaker box.

If you meant the output coupling cap in an AC-coupled power amp, then yes, it should be sized big enough that it appears as a short circuit at all audio frequencies, and practically no AC voltage appears across it. Otherwise it would be wasting bass.

[R.G.]

Correct, Steve.

Enzo: the whole idea of a motor cap (start or run) is to cause a phase shift between the current through it and the AC power line. These were once called "phase splitting" caps, as what they do is to give you a second "phase" of AC power to a second motor winding. This lets the motor start as a two-phase motor; once started, you can run an induction motor on a single phase; they just won't self start on a single phase.

So the start/run caps are deliberately set with the motor's impedance to cause a sizable chunk of phase shift, and hence there is a sizable voltage across it and significant power losses in the cap. Bad for electros. Speaker caps are, as Steve notes, run with the AC voltage across them as low as it can be made, so the power is low in the cap.

Single phase induction motors without caps are started by a resistively split phase. The second winding is a high resistance one and the inductive nature of the main winding shifts its AC current phase relative to the high-resistance winding.

On the other hand, Steve, the new generation of medium power film caps can give you film caps in remarkably small sizes. You can get upwards of 50-100uF at a couple hundred volts in only a few packages. I'm amazed at what's happened to caps recently.

[Enzo]

Thank you, Steve and RG, always happy for someone to fill in holes in my experience. Motors is one of those holes. Hopefully microwave will not show up in guitar amps.

And yes Steve, I meant the cap coupling the output of a power amp to a speaker.

[stokes]

Great info guys.I ordered the one R.G. pointed out,have every confidence it will work fine.Thanks much for finding that for me,R.G.Just one more question.I am assuming this cap is a "run" cap since it remains "in circuit" all the time,my limited knowledge on these things tells me a "start" cap has to be removed from the circuit after a start up,is that correct?Thanks again for all the help.