Starting capacitor sizing

A friend came by with a non functioning olde 1 hp motor the other day. A quick check showed no starting circuit (bad cap or bad centrifugal switch). Pulled the old cap out, it is the old style aluminum case, NO MARKINGS.

So what size cap to put in?? I just picked a ~200mfd and called it good but I don't think that is the right answer. So what/how is the cap sized? Too small a cap and it won't do anything useful. But is bigger better? Or is the cap matched to the motor in a resonant circuit?

Motor in question is a Westinghouse 1hp 120/240 motor. Judging from the grease caps, case style, paint color, etc, I'd guess 1940's, maybe early

Reply to
Loading thread data ...

If you can read the nameplate, look closely - normally there's a call-out for size and voltage ratings.

And if not, call Westinghouse with the model and serial number and ask - sometimes they still have the old records available.

There's a LOT of old equipment out there still in daily service, and sometimes people decide to rebuild rather than replace. And sometimes you can't GET a new replacement without spending megabucks on something custom, so you have to rebuild the old one.


Reply to
Bruce L. Bergman

When I asked about starting capacitor values at the local motor repair shop, I was told that 500uF per HP is the correct size/value.

Many of the fractional HP motors that I've serviced all seem to follow this same guideline. The marked values of about 110uF for 1/4 HP, 140uF for 1/3 HP, etc.

FWIW, the actual measured value will vary considerably from the value marked on the capacitor case, as the capacitance value tolerance can be 20%.

Reply to

Hey, I'm not THAT slow on the draw! I looked, the label was quite clear, no capacitor data.

This application does not rise to the need for calling 'Circle W' I was more looking for some sort of rule of thumb.

Generally referred to as 'between a rock and a hard place'. BTDT.

Reply to

I headed to the shop, pulled apart 3 motors:

-2hp cap start/cap run TEFC running on 240 volts: 189-227mfd/165 volts

-1hp cap start compressor running on 120 volts: 534mfd/125 volts

-1hp cap start saw duty running on 120 volts: 216-259mfd/110 volts

Now I'm really c> When I asked about starting capacitor values at the local motor repair

Reply to

The purpose of the capacitor in the motor's starting circuit is to increase torque during startup (thus reaching the run switch speed sooner), so a manufacturer could utilize a more common or cheaper capacitor if the OEM is specific about the motor application.

There are a lot of split-phase induction motors that aren't built with a starting capacitor, and they work fine in applications where there is none, or a very low load at startup.

When I've used general purpose utility motors for applications where the starting load torque was more significant than say, a fan application for example, I've added a starting capacitor to increase the starting torque.

When dealing with used motors, if I see signs of previous repairs (stripped screw slots, etc), I'll review my motor notes to see if the start capacitor value is within reason for similar motors, or comparable to the 500uf/HP guideline.

In my experience, not many motors are marked with start or run capacitor values, with the exception of PSC permanent split capacitor types, and not many motors can be looked up since most manufacturers usually don't provide capacitor values for split-phase models.

In the HVAC servicing trade/industry, there are add-on booster capacitors for increasing a motor's starting torque which may be reduced by voltage drops or other issues. These capacitor-booster products typically utilize thermistors to limit the application time duration of the boost.

Reply to

aaah, this is partially correct, but the way to look at it is that the capacitor, along with the start winding provides a phase shift to act "kind of" like a 3 phase motor - so you can get the rotating magnetic field rather than just a 180 deg back/forth magnetic field which will just make the motor buzz -

so, you want whatever value gets you about 60 deg phase shift so you can get the rotation of the field

Reply to
Bill Noble

RoyJ wrote in news:0d2dndt_gJ7gds3XnZ2dnUVZ

Start windings on dual voltage rated motors are center tapped by the jumpers so that the winding always sees the same (lower) voltage regardless of which supply voltage it is connected for.

Compressors are typically hard starting loads so it makes sense that the motor is chosen to have high initial torque, and a high value capacitor.

Saws have very low starting torque requirements, seems reasonable to have a smaller capacitor.

There's way too many motor construction and starting torque variables for the 500uF/Hp rule of thumb to always fit. Voltage ratings on the capacitors you listed are somewhat normal. The start winding is so low duty cycle they use lower lated capacitors. Run capacitors are usually 1.4 x rms voltage or peak ac voltage rated since they are constantly in circuit.

Reply to
Charles U Farley

I trust you are correct, Bill. Perhaps a better choice for a word to replace (the) *purpose*, would be (the) advantage ... of the capacitor in the motor's starting circuit is to increase torque during startup

Reply to

close, but still not quite right - for example, take an old time clock motor (really old time - like 1930) - many of these will run either way and there is a little knob to twist to start them in the right way - if you do nothing, they sit there and buzzzzzzz. spin the knob and off they go. So, the start windings on your larger motor are like this little knob - they produce a rotating magnetic field, like a 3 phase motor. To do this, you need to get the phase angles, and that is where the capacitor comes in. It forms an LC (inductance/capacitance) circuit with the coil and that creates the phase shift.

So, yes, the start winding increases torque, and the capacitor gives the phase shift so it will turn and in the right direction

Reply to
Bill Noble

I don't think anyone would expect to be able to get the suggested clock motor to do any real work, Bill.

As I mentioned earlier, there are many split-phase motors that are built/manufactured without start capacitors, which operate properly in their intended applications (low or no starting load).

Simply put, capacitor start split-phase motors have greater starting torque than an equivalent non-capacitor start, split-phase motor, for applications where a load is present at startup.

When most of us want to power a project machine, the main considerations are voltage, speed and motor output. For many, assuming that bigger is better, one could use 5HP motors for almost all of their shop machines.

I've encountered situations (powering small machines with spare motors) where an adequately sized motor wouldn't start occasionally, and by adding a suitably sized start capacitor to a non-capacitor start, split-phase motor would increase the starting torque to the point where the machine would start perfectly. That, to me, is an advantage, in that I didn't need to go find/buy a higher output motor. I suspect that many others would have assumed that the motor was inadequate, and gone looking for one with higher output.

This same example would be advantageous to a manufacturer, when specifying motors, to select capacitor start, split-phase motors for more starting torque instead of choosing a higher output motor (higher cost and weight, greater energy consumption etc).

I don't understand what you were trying to say with your last sentance.

The start winding dictates which direction a split-phase motor will spin up. I'm fairly certain that all of the split-phase motors I've enountered will

*run* in either direction, except that some motors are only provided with a wiring tap for a single direction, but if one chose to start their split-phase motors manually (eliminating the start winding circuit altogether), the motors would operate equally well in either direction.

The capacitor(s) (start or run) in split-phase motors have nothing to do with rotational direction, according to anything I've read in motor books.

Reply to

Good save, Roy. I have a lot of confidence in older motors, and they just seem to keep running, and probably cooler internally than newer models.

As long as the mechanical parts, and the contacts of the centrifugal switch remain in good condition, the old ones still remain almost trouble-free with a small amount of maintenance. When the mechanical weight/spring parts start to beome unreliable, thay can be replaced with another mechanism with similar charcteristics.

Although I haven't needed to try it, there have numerous reports of new contacts being installed on the switch leaves of older motors.

Reply to

I have, on numerous occasions, touched them up with emery cloth. Gerry :-)} London, Canada

Reply to
Gerald Miller

For nearly all electrical points/contacts, I try to avoid any abrading/cutting actions that will create clearly visible scratches. Files and sandpaper-type abrasives tend to leave fairly rough surfaces. I'm aware that there are numerous makers of tools called "points files", but they're kind of counterprodutive. Very fine emery likely exists, but I haven't encountered any that I would consider to be of powder consistency.

To me, using something that results in a rough surface on switch contacts, evokes an image of two combs as the contact surfaces with high peaks between valleys. Getting something to run briefly, or for evaluation purposes is one thing.. but using abrasives on contact points shouldn't be a regular service practice, IMO.

FWIW, when contacts have become pitted, they should be replaced, as the gradual crown shape which helps suppress arcing has been compromised. Generally, they will continue to erode at an advanced rate, as the surface plating/coating has likely been burned away. For a motor that only sees infrequent use, the erosion is likely to go unnoticed until it eventually fails to start properly.

If I can burnish the surface with a hardwood stick dipped in deoxider, that's about as cautious/safe as one can be with the precious metal surface coating.

I can hardly resist buying a decent looking used motor, but my habit has always been to disassemble used equipment before using it. A little cleaning and oil can go a long way toward having a reliable motor when a future need arises.

Lately, I've been trying an ultra-fine crocus cloth for various cleaning or polishing tasks with good results. The coating on the cloth is a very fine, powdered material. In contrast to common emery grades and other abrasives which use granulated materials, noticable scratches aren't created by the very fine coating.

I've read comments regarding replacement of start switch contacts over the years, with high silver content pieces, and I think there may have been a mention of a source of appropriately-sized new contacts not long ago, but I don't remember if it was in RCM or the SER group. There are lots of sources for heavy duty, high current relay contacts as commonly available repair parts, but I suppose that motor switch contact replacement has often involved a bit of improvising.

Reply to

If I had a nickel for every contact I have filed through the years...

It depends on the contacts. If they are gold plated, special in someway, you need to be careful and try not to remove the plating. For that type of contact I usually just settle for cleaning them with some thin cardboard/folded over paper maybe soaked some in rubbing alcohol. Dry everthing thoroughly afterwards.

For anything else I have had the best luck using anywhere from 80 to 320 grit sandpaper folded over. Most of the time I would fold a thin piece of sandpaper over my burnishing tools blade. I found the burnishing tools alone to be pretty much worthless, but they work well to support a thin piece of sandpaper. Use a pair of hemostats to clip down on the sandpaper-burnishing metal and clean/sand away.

When you get done with the sandpaper replace it with a piece of folded over paper and clean some more. When you can run the paper through and it comes out clean your done. On smaller contacts/relays you may need to tweak/adjust the contacts a little. There should be a little bit of wiping action. This can be tricky and if you have a working or new relay around, it would be worth studying it first to see how much is enough.

For the most part this would give like new performance, often times lasting as long as a new replacement.

The problem I saw most often was that people weren't aggressive enough and didn't take the burnt area down to new metal. A bit of pitting doesn't bother much, don't take off more contact than necessary either.

Reply to
Leon Fisk

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.