Capacitor selection

I picked up a motor at a local auction. Its a single phase 0.5hp induction motor. It doesn't self start but runs fine if I give the output a spin. I'm assuming the starter capacitor is faulty. Could someone push me in the direction of a suitable replacement (size and type).

Thanks

Colin

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.

I had a good look but couldn't see any trace of markings. The plate is stamped 230V and it runs well on the domestic mains once started. What makes you think it could be 400V? Thanks Colin

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

The plate refers to an RMS voltage, the capacitor needs to be rated to withstand a peak voltage.

Andrew

It depends on the windings. As far as I know there isn't a simple way of working out what the capacitor should be by looking at the plate. To be certain the motor's dataheet is needed or the capacitor's value and working voltage might be marked inside the protective outer tube.

If it were mine, I'd try 100uF 260vac. Make sure it's a Motor Start Capacitor. Many sources, this example is Farnell. Until reliability is proven by not going bang after several stop starts, assume the capacitor might go pop! Although far from spectacular exploding capacitors sometimes spray hot chemicals about. Eye protection, gloves and old clothes.

I had to leave in a hurry after a big capacitor in an old radio blew up and ruined a friend's new carpet. It was clear his house-proud wife wasn't going to take any prisoners...

Dave

Dave

Sounds good thanks

If you have access to an old RS components catalogue they used to have a table in it of cap values. I have a book with some value in and it says 80/ 140 pF, they are not critical at all, and are special AC electrolytic type, 400 volt or more rating. But be very careful with that string starting method. When you have run the motor did you hear a click sound as the motor slowed down,before stopping as that would be the centrifugal switch closing, it doesn't say its OK though, the contact could be burnt, its an old motor. Can you borrow a capacitor from a friend, before wasting your money. Finally you could dismantle the motor then you will have access to the start winding terminals, then, one way or another. you will be certain of its continuity. John

Colin, my reference to 400v was the capacitor rating as mentioned by Andrew J, NOT the motor voltage. Noel

Hi Colin, I also agree that 400V is the lowest you should go, for the same reason that Andrew has given.

Regards Nick.

Thanks everyone. I'm going to have a look at the centrifugal switch first and if that looks OK I'm going to go for a 400v 200uf and see what happens!!

Agreed. A 400V capacitor is only ~10% above the peak mains voltage from the 230V mains. Better to go minimum 450V.

Edited By not done it yet on 10/09/2023 20:56:07

Got some nervous players on the forum I see. Well, can't disagree with better safe than sorry, but here's the case for the defence: a 230Vac capacitor rating takes the peak voltage into account. Not necessary for the customer to calculate anything. Anyone buy 400V light-bulbs from their supermarket, or are we all happy with the ordinary 230Vac types?

Hard question, in the circuit below, when V=230vac and the centrifugal switch is closed whilst starting the motor, how many volts are across C?

Dave

Depends on:

Frequency of supply

Value of L2

Value of C

Only thing we can say for certain is VC + VL =230V

Robert.

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

So any suggestions to what I should buy? Thanks Colin

The capacitor in your tumble dryer is a safety critical component. It is, or should be, a X rated mains capacitor. The 250V rating for a X capacitor is the maximum nominal supply voltage it may be used on. The actual test and breakdown voltage is significantly higher to allow for the spikes and surges you see on the mains supply. For a X1 (highest specification) the peak voltage rating is 4000V for an X3 it's 1200V.

Sorry Colin, I can't give much more insight to the value of capacitor you need than has been suggested by others. Using the minimum required to start the motor is optimum. This also depends on the load. If the motor is starting with no load at all dur to a clutch or loose belt then a much smaller capacitor will work. If it has a fixed load with a lot of inertia it will need a larger capacitor.

Robert.

A simple answer would be either take the cap to a motor rewind company, ask them to test and then sell you a new one IF faulty or below spec, or get a capacitor of between 100 - 160 uf 400v motor start capacitor and see what happenes ! Don't be alarmed at the fact it may be 10% tolerance. They are not expensive. You have not indicated what use this motor is being put to, if starting on a light/no load then go for a lower value if starting on load then the higher value. I still say the rating should be on the casing. One clue as to it being not the right value will be that the motor will run hot or draw excessive current. As your O/P states your working on an assumption and there are many reasons why a motor may not spin up to speed. Good luck Noel.

A 100uF motor start capacitor. One of these. If brave, 260Vac working. If nervous 450Vac working.

Dave

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.

There we go again, not everyone who gives advice presents the full picture. It may be them trying to keep it simple or not actually knowing. The grid voltage is not fixed at 230V. As stated, that is the nominal voltage. It can legally be as high as 253V - which at peak voltage is close to 360V, not 325V.

Indeed, mains voltage in the UK can fall as low as 216V and still be classed as acceptable. Your real, practical supply must, by law, remain within the range of 216.2-253V. Your local transformer (particularly if you are near the end of the transmission line) supplying your grid power will have several tappings for the output voltage expected from the potential loads on that line.

Capacitors from some sources are doubtless close to the minimum specification. Just calling the the capacitor ‘230VAC’, like Dave suggests, is not necessarily the whole story. A far better metric is the maximum DC voltage that can be applied across the given capacitor - and unless the user knows that actual value, they could be purchasing an item with little or no safety factor.

Buy a non-polarity-conscious capacitor with a 450VDC rating. Buy from a reliable source. Buy cheap, buy twice.

As far as I am aware, the “start” capacitor does not make any contribution to the motor beyond that first few milliseconds at initial starting (until the switch in the motor disconnects the winding from the circuit).

I’m not an electrician, but know enough about mains voltages to be reasonably safe. Anyone who is not, would be well advised to take their issues to a specialist in the field - in this case a motor rewind business would be a good port of call.