How to size motor start cap?

I would contact a motor rebuild shop and see what they recommend. They are the pros in this situation...

John

Well, I tried a smaller value, about 1/5 of what was connected before, and the motor tries to start but just can't. So I know I need more than 200 MFD. Eric

What's the model#/part# on the nameplate of the motor? Also, many nameplates specify the required capacitors for the motor. Have you looked there yet?

Dave M

I looked up the motor number and Grainger, which sells Dayton Motors, no longer has data on the motor. Neither did an internet search yield anything. And the motor nameplate doesn't specify the caps. I though that maybe a newer Daytom motor would have the info but Grainger does not list the caps required for the motors on their website or on the motor nameplate. I can't understand why. Eric

Not sure which model you have but the caps are shown as in series on this model ( Cap start - run )

Any reason for not disclosing the motor model number? Yes, I know it's 40 years old and Granger can't find it.

There are two caps in most such motors. One is the "starting capacitor". The other is the "run capacitor". I'm fairly sure they should NOT be wired in parallel.

For 220VAC, the starting cap should be about 30 to 50uF/kW.

The run cap is usually about 5 - 20uF.

For the starting capacitor calculations, see: More:

I couldn't find anything on calculating the run capacitor.

The model number is 90653-N. The motor has only two wires coming out to connect to the caps, so there is no starting cap. I tried a known good cap that is about 200 MFD and it won't start the motor. The motor tries to turn and I bet if I took the belts off it would start spinning. The motor does spin easily enough but with the compressor load it takes so long to spin up with the 200 MFD cap I'm afraid the motor will overheat or the breaker will pop and so I turned it off after a few seconds. The compressor is a two stage air compressor that came with the motor as a set. The compressor is unloading properly so the motor is not trying to spin up a huge load and the motor has been spinning the compressor just fine for the last 40 or 50 years. I find it odd that there is no running cap but there just plain aren't connections for one. Eric

Looking again I see that there are TWO model numbers on the motor nameplate. The top one is 5K483-D, which is a good number as I found it on the Grianger website. The other model number is on the bottom of the ID plate and it says Motor Model Number whereas the top model number just says Model. I have never seen this before. Anyway, I called Grainger because the cap(s) weren't listed. The guy at Grainger had to pull a file but he did find the caps and there are two. Both caps are 485-582 MFD and are wired in parallel. They are both start caps and the motor has no run cap. I have two cap coming tomorrow but they are 540-685 MFD. Will them maybe be a problem? In the meantime I will try to get the proper caps. Thanks, Eric

OK - keep in mind that the tolerance in electrolytic caps is typically +100%/-20% unless noted otherwise.

Your larger caps will be just fine.

Peter Wieck Melrose Park, PA

There are two things wrong here.

First this is an AC motor with AC capacitors. Electrolytics will not work on AC as a general rule.

Newer capacitors, especially motor capacitors are much closer now in tollorence.

I have one here that is rated +- 5% I keep as a spare for my heat pump. Many are rated for a range of capacitance of about +- 10 % now.

Thanks Peter. Do you know why the MFD value of motor starting caps is such a wide range? The specified caps are 485-582 MFD. Is that the min and max of the cap? Do they spec them that way instead of using a tolerance? Eric

From Wiki:

olytic capacitors with non solid electrolyte and therefore they are only ap plicable for the short motor starting time. ... If a motor does not start, the capacitor is far more likely the problem than the switch.

A non-electrolytic cap, AC or DC of that rating would be as big as a footba ll, or larger.

The motor is 40+ years old. That is not 'newer'.

That covers both *wrong* things.

Peter Wieck Melrose Park, PA

Winki lets anyone post anything.

My 50 uF capacitor is not electrolytic and rated for AC. It is no where near footbal size. Not even beer can size. Even the 500 uF or so capacitors for motor starters are no larger than a beer can if that large.

People are quoting very old data. Maybe in 1950 the capacitors were larger, but no today.

ectrolytic capacitors with non solid electrolyte and therefore they are onl y applicable for the short motor starting time. ... If a motor does not sta rt, the capacitor is far more likely the problem than the switch.

otball, or larger.

Go to any site, any manufacturer - and you will find motor-start caps to be electrolytics. Really. Better yet, just open one up. And, exactly, how do you think they are made?

Peter Wieck Melrose Park, PA

This will be my last post on this thread. There is no use in me trying to educate a person that will not be educated . Unless you can point me to a page that specifies an electrolytic capacitor for a run and/or start capacitor for an AC motor. And I do not mean the capacitors used in a varitabble speed drive, just an ordinary AC motor.

If you go to the last sentence or two it will tell you that they are not suited for use on AC lines.

Here is a quote from your belovied Wiki

Reverse voltage

An exploded aluminum electrolytic capacitor on a PCB Standard electrolytic capacitors, and aluminum as well as tantalum and niobium electrolytic capacitors are polarized and generally require the anode electrode voltage to be positive relative to the cathode voltage.

Nevertheless, electrolytic capacitors can withstand for short instants a reverse voltage for a limited number of cycles. In detail, aluminum electrolytic capacitors with non-solid electrolyte can withstand a reverse voltage of about 1 V to 1.5 V. This reverse voltage should never be used to determine the maximum reverse voltage under which a capacitor can be used permanently.[51][52][53]

Solid tantalum capacitors can also withstand reverse voltages for short periods. The most common guidelines for tantalum reverse voltage are:

These guidelines apply for short excursion and should never be used to determine the maximum reverse voltage under which a capacitor can be used permanently.[54][55]

But in no case, for aluminum as well as for tantalum and niobium electrolytic capacitors, may a reverse voltage be used for a permanent AC application.

To minimize the likelihood of a polarized electrolytic being incorrectly inserted into a circuit, polarity has to be very clearly indicated on the case, see the section on "Polarity marking" below.

Special bipolar aluminum electrolytic capacitors designed for bipolar operation are available, and usually referred to as "non-polarized" or "bipolar" types. In these, the capacitors have two anode foils with full-thickness oxide layers connected in reverse polarity. On the alternate halves of the AC cycles, one of the oxides on the foil acts as a blocking dielectric, preventing reverse current from damaging the electrolyte of the other one. But these bipolar electrolytic capacitors are not adaptable for main AC applications instead of power capacitors with metallized polymer film or paper dielectric.

Bingo. More: There are TWO wiring diagrms for suffix BB and BA with different capacitor wiring schemes. I guess you have the BB suffix since it has

I couldn't find a reference with the exact capacitor value(s). The parts list should have the Dayton capacitor part numbers on it somewhere. Possible source to double check the values:

I've never seen anything like that but I guess it's possible, especially since Grainger was able to find documentation on the motor. The capacitance range of the new caps overlaps about 1/3 of the range of the originals. Not great, but it will probably at least start. Start caps are only used for increasing starting torque until the motor is up to speed. The motor might complain a little starting with too much capacitance, but once it starts, it should be ok. I've also never seen a 3HP motor without a run capacitor. Weird.

To cover some of the other comments:

Start caps are always non-polarized electrolytics. For the non-believers see this video where the mad bomber puts 220VAC across a start capacitor to easily disassemble it. Notice the electrolyte oozing out of the capacitor before it blows out. Looks like the guy has done this more than a few times in the past. Run capacitors are also non-polarized, but beside electroltyic, can also be oil filled. Not the best document on the topic but at least covers some of the details: "DIFFERENCE BETWEEN RUN AND START CAPACITORS"

According to various web piles, the listed tolerance on most motor starting caps is usually +/-6%. When a range of capacitance is specified, that includes this tolerance. However that doesn't seem to be the case here as the 540-685 cap is a much wide tolerance range. If the capacitor is a nominal 612uF, then that range would be the same as

Oh, be nice, or would trial by combat be a better way to settle the matter?

Are these sufficient?

Note the lack of polarity markings. Keltron Aluminium Electrolytic Motor Start capacitors are manufactured...

From the Gainger catalog under Dayton: These electrolytic, nonplarized capacitors are designed for normal intermittent service on single-phase AC motor starting circuits.

Greetings Jeff, Thanks for the help. As it turns out the motor I have is suffix D. The guy at Grainger found that exact motor and he too was surprised that it did not not have a run cap. The power factor of the motor, according to the label, is 1.15. Wouldn't a run cap improve that a bit? Maybe because the motor was paired with a compressor with a large flywheel the pulsing torque from a single phase motor without a run cap was not considered a problem because the flywheel would smooth things out. My best bench and pedestal grinders are Baldor motors with run caps and they run very smooth. Virtually no vibration. This makes a difference when sharpening drills and grinding tools. In any case the motor never runs hot and the compressor seems fine. Since there are two caps connected in parallel I can connect caps of different values as long as they add up to the correct value needed, right? I ask because I was told that if a smaller and larger value cap are connected in parallel the lower value cap will work harder and so fail sooner. Changing the subject, this is the second time I have called Grainger where they have had to look in a filing cabinet for documentation not in the computer. The previous time was for a compressor and I was given the number for some guy in a warehouse in the Midwest. I told him what I needed and he riffled through some files, found what I needed, and then scanned and emailed me the documentation on the compressor. I think that's pretty good service. Especially since Grainger wasn't gonna be able to sell me new parts to fix the compressor. A part was missing from the unloader assembly and the documentation enabled me to make a new part and wind a new spring and now the compressor works properly. Eric

Yep. That's the purpose of the run capacitor. Start on Page 19: The way it works is when stopped both the start and run caps are in parallel. When the motor gets up to speed, a centrifugal switch opens and disconnects the start cap leaving the run cap in the circuit. That means if you try to add a run cap, you'll need to decrease the value of the start cap by the same amount. Since you're already have too high a capacitance on the two parallel start caps, this will mean that you'll be replacing at least one of the two new capacitors.

My guess(tm) is the start winding was intentionally undersized (which is why the start cap is so large). Exactly why, I don't know.

Nope. Two caps in parallel will distribute the current going through them by the ratio of the capacitive reactance (Xl - 1/(2Pi*f*c). The larger capacitor will have the smaller series reactance and therefore draw the most current. However, in the motor, we have a resonant situation, where the run capacitor has a capacitive reactance equal to the inductive reactance of the motor run winding, and therefore cancel each other leaving just the winding resistance to dissipate any power. That's also the ideal power factor point. So, if you parallel a mess of capacitors to make a run capacitor with the correct value for PF=1, then the capacitors will dissipate zero power.

Impressive but what will they do when the warehouse guy retires?