Member postings for colin hamilton

Just thought you might like to see this on ebay, Colin

**LINK** https://www.ebay.co.uk/itm/394734119321

They also have a metric one

MichaelG.

As long as the centrifugal switch is working and the motor is not getting hot the bigger capacitor may not be a problem. It is mainly purpose is to provide a phase shift. The current depends on both the capacitor and the winding.
The motor manufacturer will fitt the smallest capacitor possoble to reduce costs. I'd be more concerned if it was a run capacitor.

Robert.

It should not be in circuit for long as a ‘starting’ capacitor?

Solution of the circuit for Vc depends on the upstream resistance of the supply as well as all the other stuff. Inrush current of a single phase motor on start-up is about 9 times the Full Load Current, which is why single phase motors are a beast to start on a generator. With an upstream source resistance of typically a few ohms you can very soon see a significant voltage droop at the motor L-N terminals.

So the answer is always going to be an approximation. To a first guess, the squirrel cage rotor is a short circuit when stationary. This is magnetically coupled to the start and run windings, so they also look like a near short circuit until the rotor starts to turn. IF you've got enough oomph in the supply to get the rotor rolling.

If the start winding is effectively short circuited something approaching the full supply voltage appears at the terminals of the start capacitor, which is why it should be rated for the full supply voltage,

Incoming supply is nominally 230 volts RMS, so peak instantaneous supply voltage as experienced by the capacitor is 1.414 times 230 volts = 325 volts as near as the model needs.

However, as a related train of thought I found myself repairing a tumble drier recently, which resolved itself into replacing the series capacitor in a little pcb which controlled the power button latch function. Essentially the capacitor (0.22uF) was in series with the 24volts DC coil of a miniature relay and a bridge rectifier (no smoothing) with 230 volt mains applied to the circuit. So the capacitor was the dropper resistor. Measuring the capacitor revealed it was no longer a 0.22 uF one it had become 0.1 uF and the coil of the latch relay now had about 8 volts dc on it. I can't remember the RMS to Peak conversion factor for a full wave rectified waveform but any which way this wasn't enough to pull in the relay. The capacitor was marked 250 V and one would expect this to be the peak dc voltage rating. I tried a higher voltage capacitor, but it wouldn't fit the pcb, so against my better judgement I fitted an exact replacement. It's famous last words but it hasn't gone bang yet.

Which I offer as anecdotal evidence that the voltage ratings on these capacitors are conservative, though I guess one ought to consider the effects of the circulating currents the capacitor sees, and therefore the effects of heat generated by these currents and the internal resistance of the capacitor, So any old capacitor won't do, it needs to be a motor start capacitor designed for the duty.

Rgds to all

Simon

For a 1/2 hp motor the generic start capacitor recommendations from several sources range from 150 to 250 µF. That said exact value doesn't usually matter too much unless you are starting a significant load.

I'd try 200 µF.

Give that large numbers of aged single phase motors are still starting just fine 50 or more years after they were made you hafta wonder how close the current values of capacitance are to the brand new figures.

Clive

IF your luck is in and you look VERY carefully you should find the value of the one you have stampted on the tin tube, on the end and painted over ! Note that the voltage may well be 400V. It is possible with the right meter to check the value it currently is ! Or has the centrifugal switch failed ? Good Luck Noel.

Hi Colin,

Nice find, why not try one of the larger counterbores and see how it works as a face-mill.

Thor