Larger Starting Capacitor?

Hello, I've got a device that needs just a tiny bit more start up torque-- maybe 15% more, I'm guessing. As it is now it starts (albeit reluctantly) one out of three times when things are lined up just so. Other times I need to give it a little nudge to get things going. It is a fractional HP motor with a 4mfd cap on it. The cap checks out OK on my multimeter...

Will adding more capacitance improve starting torque? Should I just jump up to 8mfd or more? What are the trade offs? Does that make it have less running torque or make it run hotter or?

Thanks,

--Max

Reply to
Max Krippler
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sounds like you may need a larger motor...

i
Reply to
Ignoramus12714

Hi Max

Does the motor switch the capacitor out after the motor gets up to speed? If the start cap gets switched out after start-up, there shouldnt be a "heating" problem caused by the capacitor. I once did an experiment with start capacitors and found that by resonating the start cap with the start winding gave max start torque.

Jerry

Reply to
Jerry Martes

Capacitors age and dry out, just like the rest of us. Does the motor tag indicate the value of the capacitor? I would try a replacement capacitor of the same size, first.

Paul

Reply to
co_farmer

Thanks for the response. Yep, I've done that part already. I guess the next step is to sub some different value caps and see what happens...

--Max

Reply to
Max Krippler

Thanks for the response.

How does one go about "resonating" the cap to the winding? I imagine I could substitute several different values of cap and see which one works best, but I wanted to avoid the tedium...

Thanks,

--Max

Reply to
Max Krippler

4mfd is definately a run capacitor, not a starting capacitor. a Starting cap will be closer to 190 mfd. Based on that I will jump to the conclusion that either the wrong cap is installed in there, or it is a capacitor run motor, with no starting switch.

Based on the symptoms, I would try a larger capacitor.

Reply to
Half-Nutz

In my case, I isolated the start winding and measured its inductance. The fact is, I didnt actually ever get an exact *resonance* with the start capacitor. But, I was able to see that the torque went up as the capacitor value got closer to "ideal". It would demand alot of fine tuning to get the exact capacitance My conclusion was that you might benefit from instaling a bigger capacitor to test the effect. And, since the capacitor gets switched out of the circuit as soon as the motor gets up to speed, the "overheating" should be no problem.

Jerry

Reply to
Jerry Martes

Since it is only a 4mfd cap, I REally don't think it is just for startup. Most likely a split phase motor, and the capacitor is in continuously. If it was closer to a few hundred mfd, it would likely be a start only capacitor.

Reply to
Half-Nutz

Motor and HVAC shops have "hard start" units that contain an extra start capacitor and a relay or other device to cut the unit out after a short time. They are commonly used on air conditioning and refrigeration compressors and I do not know if there is a unit suitable for your size motor.

Don Young

Reply to
Don Young

Remember the ohmmeter is for resistors and can be used as an simple test on other parts.

Caps are the hardest since the meter shows if it is open or shorted or very leaky.

It doesn't show if the cap has capacitance at all. If it is leaky at voltage or when hot.

I'd first get a new 4mfd and try that.

The caps are used to electronically reduce the inductance of the coil and allow more current to flow. If you get the effective reactance near zero - the Inductance is polarized 180 electrical degrees from each other. The current limit is the copper value of the wire and the reactance resultant. Making the cap to large can burn the winding up!

Buy a new cap. Replace the bearings. Reduce the impulse load at startup.

Martin

Mart> Hello,

Reply to
Martin H. Eastburn

I have been using ohm meters fro measuring caps for many years. You can get a good idea if the cap is good or not by doing a couple simple tests. First understand that you are using the ohmmeter as a charging instrument with the ohmmeter indicating the rate of current flow into the cap. The first thing you do is short out the cap. Then hook up the leads to the cap and watch the incication on the meter. The meter will go up and then back to infinite. Its the timing or length of the time it takes to charge the cap. that is proportional to the value of the cap. Measure a good one and compare it to the unknown one. The second test is to hold the leads on the cap and charge it to full capacity. Then take the leads off and wait a half minute and then put the leads back on the terminals. If the meter goes up and then back down the cap is leaky. Granted these tests are not like using a precision capacity meter but they work for a general indication of good and bad caps.

John

Reply to
John

John sez: "> I have been using ohm meters fro measuring caps for many years. You can

Reply to
Robert Swinney

Sorry about the double post. What I intended to say is while John's suggestion good in general terms it may be misleading when checking larger value capacitors. Electrolytics have a way of "healing themselves", so to speak. They can be totally ruined in so far as working at their rated voltage and still appear to be ok on a simple ohmmeter test. A formal capacity tester shows leakage, or the lack thereof, at some working voltage.

Reply to
Robert Swinney

Well, I "was" going to correct the top posting, but, since it has gone on for a while...

You can often "heal" an electrolytic by powering up the cap at low voltage and gradually increase it to its full rated voltage. How long it will last is anyone's guess...

But I somehow doubt that a 4 uF AC capacitor is necessarily an electrolytic...

And I'm not quite sure what the function of this capacitor is... 4 uF at 60 Hz has a reactance of around 660 ohms. One would expect a capacitor to have a reactance of, maybe, 20 ohms to resonate a start winding. Of course, it is not necessarily a good thing to go all the way to resonance. Maybe the idea is just to shift the phase a bit. Then, one might expect a reactance of, guessing wildly, maybe 100 ohms.

I would tend to suspect a 4 uF cap is for noise suppression or maybe power factor improvement. Recall that the OP described the cap as being "on it." It didn't say, as most of the respondents have assumed, that it is in series with a winding. 4 uF in series with anything isn't going to allow for much current. It may well simply be in parallel with the entire motor...

Jerry

Reply to
Jerry Foster

You also need to know the ESR of any electrolytic when you suspect a problem. The internal resistance rises with heat and age, and lowers the "Q" of the circuit, reducing torque.

Reply to
Michael A. Terrell

You have to use a high impedance range and the modern meters just measure and auto range. Analog meters will do it not real measure but pretend t=R*C and if the Resistor range is high enough then the C will then give a time that the needle to climb to the full value.

Modern meters just don't cut it with the current source they use.

Martin

Mart> "Mart>> Remember the ohmmeter is for resistors and can be used as an simple test

Reply to
Martin H. Eastburn

According to Martin H. Eastburn :

A good one can be locked into a high resistance range, and some (like the Flukes) have a bargraph to replace a needle, so it is easy to see the charging trend.

Enjoy, DoN.

Reply to
DoN. Nichols

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