Capacitors for Single Phase Motors

If the information is not marked on the nameplate on the side of the motor or inside the terminal cover, and the labels on the capacitors are not legible (or are missing) then rather than us taking a guess you would be better off looking on the internet at the motor manufacturer's website for exact wiring details and capacitor sizes for your specific motors.

Also have a look at previous CR4 threads and posts on the subject.

If they are capacitor start motors, there should already be a capacitor mounted on each. Are you sure these are not three phase motors?

If that information is not given on the name plates, then send the motors back and buy some from a reputable dealer!

'Are you sure these are not three phase motors?'

Lol!!! this is a damn good question coz what if they are?

As they only have two field windings, how can they be 3 phase?????

By failing to install the third phase (called; manufacturing defect).

C'mon Andy! Lets just have some fun today. In my part of the world it's still sunday all day and will remain until midnight. So, no need for you to come along nd spoil it. You can get more serious from tomorrow, yeh?

Meanwhile let the OP have some fun by himself, he has more dough on his hands to play with than the rest of us combined u comprendo?

OKay

Since he did mention only two resistances, you're probably right, but 4 wires could be three phases and neutral for a wye connected motor.

It's hardly clear, since first he says 6.3Ω, than says one motor has 6.3 and 4.8Ω... He did not specifically say there was no connection between the two pairs.

I would assume (dangerous?) that he did not get a connection between one pair and the other pair......then it has to be single phase.

There is no neutral connection in a Wye connected motor, check this web site, you will notice that Neutral is never mentioned in conjunction with any 3 phase motor...

Believe me, neutral is never needed when 3 phases are used.....unless single phase equipment must be connected.....

I hope this helps your understanding further....

By the way, for reasons of maintenance, especially on larger 3 phase motors, a fourth connection (the connection of the "other ends" of the field coils) is sometimes brought out to allow resistance checks of each field coil individually.

It could be that both ends of all 3 field coils are brought out and connected to each other, allowing each field to be disconnected for checking of insulation values for example......

This connection is a "neutral point", but must NEVER be connected to neutral.....that is something else!!!

AH! You're right, as usual!

I have only worked on one wye-connected motor (actually a wye-start, delta-run motor, 10 or 15hp, on a Romanian horizontal boring mill of 30 years ago), and did not remember that there was no connection to neutral. I took a look at the diagram, and saw that I had placed a note on it that the only neutral connection on the entire machine is through the bolts to the floor (which hardly counts).

I was asked to look at this motor because the wye start does not work - it hums but does not spin until it switches to delta, which of course then has a large surge of start-up current. I checked the wye contactor, and all three pairs of contacts measure at the lead resistance of my DVM when closed. Any suggestions?

Ok, now what would happen if that "neutral point" were connected to supply neutral?

As this is normally never done, nor is there a need either, I will have to guess a little (a lot!), I would think that if the motor is built reasonably accurately, no current will flow.....in practise, I am pretty sure that some heavy currents may flow back through the neutral wire.....

I am not confident though.....

I have no idea what power system you have as you do not have a location (5th dimension ) They sound like single phase, only two field windings. So only connect between phase and neutral. NOT PHASE TO PHASE!!!

You will need a clamp AC current meter as test equipment. Attach motor so that you can check current to each winding individually.

The question is do they have the centrifugal switch internally to switch out the start winding, or are they rated for continuous usage (this implies a far better quality capacitor is required!) (No start switch).

Also, I did not understand correctly, do you know what value of cap is needed?

If yes, to be safe, by ONE cap for continuous start winding. Attach to one field, start and see what happens. Check to see if current is running through BOTH fields or only one (Cent switch activated). If only one, this is the "run" winding, attach cap to the "start" winding only. Test again.

Make sure supply is fused.....have a good supply of fuses available.....be careful!

thanks alot for ur info.am in kenya and am using 240vts single phase.(phase to neutral).the value of the capacitor is not known.thro ur info have managed to run one of the motors with a 10 microfarad cap(it has a cent switch).the other doesnt have am working on it .i will keep u informed otherwise thanks. we are in a very remote area where internate is not readly available therefore sorry for my delay.

I believe from your test results from point of view; high current starting would have a lower resistance winding, from the formula itself they are inversely proportional and the higher resistance would be the running winding, try to open the internal structure of the motor and look the starting capacitor rating but dont alter the connection inside. If you are indoubt, but you are only after which wiring is starting or running. that's all.