single phase motors

Hello capblanc

You state: <"....The capacitor data cannot be read....">

You also state: <"....We can read RPM, Volts, Amps, phase shift between V-V, V-A or A-A and capacitance.....">

Without any calculations whatsoever, if you can "read capacitance", then measure the capacitance of the damaged capacitor.

Then fit one of the same capacitance to the motor.

The actual capacitance of Capacitors for this type of fan motor are normally rated at +40% to -20% of the actual marked value on the case of the Capacitor.(They are not precision capacitors)

Without your advising further details in that's about the best offer you are going to get.

Kind Regards....

Hi Sparky,

Replace "we can read" with "we have measuring equipment which will read"

The caps used on most motors are +/- 5%

The value of the cap could not be measured or read as the can had abraded through where the specification was written and the cap was now short circuit.

The motor designers must have a formula to decide the value of the cap winding and the capacitor and, I suppose that this formula can be transformed to allow us to fit a cap, measure the results and adjust the value to achieve a "good fit" into the parameters.

We have textbooks on this sort of stuff but they seem to gloss over the single-phase motors as being unimportant. They tend to work with the bigger 3 phase motors 25hp and upwards even though the 1ph machines outnumber the 3ph units many times.

regards Chas

the AC voltage rating of the capacitor will have to be higher than the peak to peak AC voltage. The number of microfarads will be a function of the frequency and the horsepower of the motor, and a search of various makers will give you this

3 Phase motors don't use caps

Hello ckartson

You are correct - the 3-phase motor will run normally without any capacitors.

However, for the sole reason of correcting the Power Factor of a 3-phase motor, often a capacitor is directly fitted to the motor terminals, where the PF correction capacitor is of best effect.

Alternatively, in a large installation, an "automatic Power Factor" Capacitor unit is installed at the Main Switchboard.

This switches in and out, varied amounts of Capacitance to the desired level, to correct the Power Factor of the entire electrical installation, as motors, welders and the like, switch on/off or vary their individual amperage requirements, according to loading and speeds.

Re Power Factor, refer: http://en.wikipedia.org/wiki/Power_factor

Trust that assists you.

Kind Regards....

I have to make a few guesses here as you did not supply enough information in your post. It sounds like you are talking about a fractional HP motor of the PSC design. That being said, the run capacitor will be sized at about 50 mfd per HP. For a 1/2 HP motor, start with a 25 mfd cap. Run the motor with its load and measure the current in both the main winding and the winding fed by the cap. They should be approx equal. Adjust the cap size to achieve this or a few tenths of an amp less in the cap fed leg. Let the motor run for awhile and check the temperature of the motor to assure that no overheating is occurring. There you go.

If you have a cap start/cap run or cap start with internal switch, or any other type of motor than a PSC (permanent split capacitor), the above method is not valid. Give us more details.

Good luck...

If you know something about the load the fan motor is driving, that would have been helpful. Get this information if you don't have it already. Is this load stable/static, or does it change?

You will cause more problems by replacing the old unit with too much capacitance, as the motor won't like a leading power factor.

When working on PF correction for an industrial site, I was told to limit your capacitor size at a motor to 20% of load KVA, hence the importance of knowing the load and load profile! If you can read the current draw of the motor when it is running under the lightest load, then grab the line to line voltage (if multi-phased, or line to N if single phase) and multiply by the appropriate factor (which is the square root of 3, or 1.732, for 3 phase, or 1 for single phase) to learn what KVA is being required of your motor. Multiply this product by the 20% for your starting point, then check the case size to see if it is similar enough to be your solution. Could be the capacitor was oversized for the load the motor was applied to in the first place, if the motor was greatly oversized.

Hope this helps.

Don

Thanks FKIA, that's exactly the type of advice I'm looking for.

This problem, which we face fairly regularly, is indeed with motors from a few hundred watts through to 2.5kW. We don't see 1ph motors much bigger than this.

We come across all types of motors, (ac &dc) but the most common, and to which this post refers, is with two windings, one of which is in series with a capacitor which is permanently connected. I suppose this is what you refer to as a PSC motor. Would you care to hazard a guess as to a percentage figure minus for the cap winding current? I am a bit surprised at the figure of 50μF /hp. From memory I would have guessed it at about half that.

Tucson Don, you've missed the point. In the marine service business, where the client arrives with equipment from all over the world with no info in many cases, we need to work with things that can be deduced rather than referred back, as the trail back is often too long and has gone cold. None the less thanks for the input.

thanks

Chas

Don't have a calculation but maybe this criteria will help.

1. Take your best guess. If this is a motor with a starter winding and centrifigal switch, the capacitor will be taken out of the circuit once the motor starts so as to not be a factor during continued running. If the capacitor is for running then this may not be a good option.

2. Try different values until you get a sensible current draw and reliable starting. Use less capacitance than you think you need and work your way up.

3. The rating plate figures for current would only help you if you are talking about a run capacitor as opposed to a start capacitor. Do you have 2 capacitors on this motor? And by the way, the solid start start switch is not a capacitor. They replace centrifigal start switches on newer motors. At any rate, the rated load should not be exceeded by the actual load unless you want to seriously diminish the life expectancy of the motor.

4. Use your measuring instruments to try to determine running amperage within safe limits. Use your senses to try to determine if the motor is performing as it should. Checking for overheating is a good thing but it may be difficult to tell unless you see or smell smoke. Motors are designed to run hot sometimes (usually in my experience) so you may not be able to hold your hand on the motor while it is running under normal conditions.

5. If in doubt, wait it out. If at all possible, wait till the factory can advise you. You will probably save yourself the price of a new motor and a lot of heartache.

Good Luck,

Keywalker