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single phase motors.

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DavidA:
John,

Just been looking through my notes, and I also see a reference to the start winding being higher resistance to the run winding.

At the moment I have the wires disconnected from the motor terminal block as I was doing some coil resistance measurements. But tomorrow I will re-connect and see what the current draw is.

My thinking was that as it appears the two windings are in parallel (with one winding having the capacitor in series with it), the winding with the capacitor would be 90% out of phase with the other.  OK for kick-starting the motor, but maybe not so good for continuous run.

I'll get back tomorrow and let you know what I find.

Dave.

DavidA:
Well, I ran a few checks, I have come to the conclusion I am worrying unnecessarily.

The motor plate states 230 Volt, 1 HP at 5.6 Amp.

I measured a mains supply of 247 Volt (checked that with another meter, it's correct) and a current of 4.87 Amp.

The above is no-load.

Motor sounds ok. I dismounted it from the lathe and ran it on the bench.

Also found out that it will start and run without the start-winding and capacitor in circuit. But it draws more current (6.64 Amp) and the direction of run is random.

So, I'll clip my thermocouple meter to one of the fins and run it for an hour on the bench to check the temperature does not get too high. Then bolt it all back on the lathe.

Dave.

vintageandclassicrepairs:
Hi David,
Some motors will get hot(ter) when run free
No mechanical load can mean the motor will run nearer synchronous speed so then they do not generate
a back EMF to reduce the current (well thats my theory)
I hope that makes sense, its not in any information I have read just something I have found out over the years

John

ZebraDriver:
Just to add a bit to this thread.

 Most small 240 volt AC motors are capacitor start. They normally have a single capacitor and a centrifugal switch that drops the capacitor and the start winding out of the circuit once the motor has reached running speed. The switch can usually be heard closing again (with a "click") once the motor has been switched off and runs down.  To reverse this type of motor all that requires to be done is to swap the live and neutral feed to the start winding/capacitor, whilst maintaining the live/neutral unchanged to the run winding. Some motors have a set of links within the terminal box (usually in a "L" shaped configuration) that allows you to select the required rotation direction. Some motors have a permanent connection but a bit of detective work will let you find the right winding ends to swap, but please be careful to make sure that you have the right type of motor (and there are several different types !) and the right winding ends before you attempt any reversal work.

If you have the type of motor mentioned (with a centrifugal switch) then a simple on/off/on two pole toggle switch can be used as a reversing switch. Simply feed live and neutral to the two centre contacts and then wire one end of the switch to feed the start winding /capacitor. Wire a pair of cross over wires between the four outer contacts, these will feed the motor/capacitor with a reversed supply for running in the opposite direction. The switch can be operated when the motor is running as it will have no effect (because the centrifugal switch will already have switched the winding and capacitor out of the circuit), it will only take effect when the motor is re-started.

The comments regarding motor heating are consistent with a single phase motor that we fitted to a customers machine recently. The motor would run quite hot (but within its design temperature) if it was working at full load or at no load. We were concerned and the supplier gave us a further two motors to test, both of them gave the same results. The motor manufacturer was given the two motors back and as yet have not explained the temperature rise, but we did note that the current drawn when running without load was only slightly less than the full load current.

Martin

Muzzerboy:

--- Quote from: vintageandclassicrepairs on October 02, 2019, 07:34:51 PM ---Hi David,
Some motors will get hot(ter) when run free. No mechanical load can mean the motor will run nearer synchronous speed so then they do not generate a back EMF to reduce the current (well thats my theory)

John

--- End quote ---

Sorry - quite wrong there. At no load, the motor is indeed running close to synchronous speed but the back emf equals the source voltage. Apart from the magnetisation current and some copper losses, there should be minimal load current and heating.

If there is a significant current flowing under no load conditions, there is something wrong with the motor or the way it's connected. One vague possibility is that it's a lower voltage machine operating above its rated voltage. This seems unlikely but would result in higher magnetisation currents and possibly saturation of the laminations.

PS - to be fair, it's not very helpful to simply consider back emf when understanding how AC motors work. In reality, the magnetic field in the air gap is rotating at a fixed speed (the applied frequency divided by the number of pole pairs) and the rotor is trying to follow it.

In the case of a synchronous (permanent magnet) motor, the angle between the rotating field and the rotor varies proportional to the applied torque (in both motoring and generating), whereas in the case of an asynchronous (induction) motor, the rotational speed difference varies proportional to the applied torque (in both motoring and generating). In the induction motor, this difference in frequency is applied to the windings in the rotor, which in turn create a magnetic field which results in the torque. Without any slip frequency, there can be no induced rotor current and field. In both machines though, torque is proportional to current if you ignore magnetisation current which should be fairly constant

With a single phase motor, there is no rotating field of constant field strength as such, just a sinusoidally pulsating field. At stall, there is no torque available unless some additional mechanism is brought to play. If the rotor can be brought up to near synchronous speed, it can then develop a decent torque. There are various ways to create a phase delayed magnetic field which will result in a starting torque. A capacitor is one means and a "shading" winding is another. With a starting capacitor, the direction of the starting torque can be reversed by reversing the phase of the start winding.

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