single phase motors.

[DavidA]

Dave,

I burnt a bit of midnight oil an and managed to work out how the switch should be wired.  So now it works correctly.
The secret seems to be that the run winding needn't be switched. It stays the same in both forward and reverse.
So, switching the start winding becomes  a simple change-over switching operation.
As it's wired at the moment I don't have a centre off position on the switch.  Two forwards and a reverse. I know why this is but as I will be turning the motor on and off with the zero-voltage stop-start switch it doesn't matter.  Most of the time I will be in forward for normal turning. I do know how to fix it,  but have had enough of switches for the time being.

Dave

[Bluechip]

Dave
 
That's right, one winding is always 'On'. ie. Not switched.
Just 'end for end' the other one. Doesn't matter which, but usually the start winding.
 
But if the motor is running and you just change over the start winding the 'run' usually wins and the motor now has the run winding going one way and the start trying to go the other.
 
As that winding is in series with a capacitor it's current is restricted by the reactance of the cap, so it doesn't have the guts to do much. ( If it's 10uF the Xc is some 318 ohms @ 50 Hz).
 
Did a cap. run motor 3-4 years ago 330W for a mate ( drove an extractor fan ) someone wired it up just with a DPDT switch and it didn't reverse. Just went slower and growled a bit .. so I cobbled up a box with 2 switches and told him to switch it off, change direction, switch it back on. A bit clumsy but it did the job.
He was then able to have a blow or suck as circumstances required. 
 
So, if you have a 'NVR O/L Stop Start' wotsit you're in business ...
 
Dave BC

[DavidA]

Dave BC,

Way back in this post you remarked...Why not ask Machine Mart ???..

Today,  I was messing about with the switch again and just out of curiosity I emailed Machine Mart.

They replied with two diagrams within three hours.

Credit where it is due.

Dave.

[DavidA]

Yesterday I had reason to go to Bradford and decided to call in Machine Mart and see if they had one of the electric motors that I am using in stock.  My aim being to ask to look in the contact box and see how the thing was wired up.

I still had the receipt for the original purchase back in July 2008.

They didn't have one there, but had a similar motor which they allowed be to look at.

But more interestingly,  when the assistant typed in the code for the motor he said that they were marked down for discontinued line. And that they would be replaced by a different one.

Now,  the good bit.

The motor was still available for £23 +vat. (£27.60). Would I like one ?

Now , the original cost me £76.36 inc vat. The new version is going to cost £95 +vat.

Sure I would like one. It seems they are held at Some place in the midlands,  and it will arrive at Bradford next Monday. I will have to collect it as from there. No sweat.

So,  If you want or will be needing a single phrase 1 HP motor then ring MM and tell them you would like to know if motor code number 010210080 description motor 1 - 4 - 1 BOF is still available.

For £27.60 you can't go wrong.  Just don't order the new expensive one by mistake.

You will have to collect from you nearest MM.

Dave.

[hermetic]

Just to clarify a few points here chaps, to reverse any single phase motor, you reverse the polarity of the start winding, this is often marked as Z1 and Z2, but with motors coming in from all over the globe, your experience will probably vary! It doesnt matter if the start winding is just a winding, a winding and a capacitor, or a winding a capacitor and a centrifugal switch, if you treat the whole circuit as "the start winding" swap the wires over where it connects to the live and neutral, and the motor will reverse. As correctly stated above, a single phase motor on the run winding will operate in either direction, if started without the starting circuit and spun by hand, it is the start winding that gives a directional kick to the motor, reverse the kick, and you reverse the motor. As the OP has already discovered, the clever bit is linking the reversing switch so that it does this, and they are all different. You should always allow the motor to stop before reversing it, or you risk damaging the start winding.
Phil

[Arbalist]

Many years ago someone gave me a motor of around 1/2 HP for a small Lathe I had and no matter which way I wired it, it would only run one way. I asked the guy who gave it to me about this and he said some motors can't be reversed. Is this correct? It wasn't a problem for me as I was able to physically mount the motor the other way round!

[Manxmodder]

I had a similar situation a few months back,a mate gave me a brushless ac motor to put on a small bench drill.

It had the wrong rotational direction and to correct it I had to press the rotor of the shaft and press it back on the other way round. Just reversing the whole rotor assembly would have worked but the keyway in the shaft would have ended up on the wrong end of the motor......OZ.

[hermetic]

Hi Arbalist, yes it is correct that some motors cannot be reversed, because the manufacturers "hide" the wires or connections you need to do the reversing. It also happens with motors like the old fashioned shunt universal (AC/DC) motors where you have to reverse the brush holder connections.........but the wires arent long enough!! If you are willing to tear the motor to bits and extend/alter wiring, you can usually do it, but it is more sensible to get another motor.
Phil

[DavidA]

Hi,  doing a bit of dead thread resurrection.

I happened to be wondering why the motor (subject of above posts) gets so hot when running.

Could it be because the because the start winding and it's associated capacitor are always in circuit ? And thus are across the run winding.

I was thinking that the phase difference between the two windings may be causing the heating.

Perhaps a modification to cut the start winding out when the motor is running would be beneficial, what are your thoughts ?

[vintageandclassicrepairs]

Hi David,
In my experience (35+yrs) I have never seen the run winding with a higher resistance than the start
so I think the capacitor is connected to the wrong winding ????
Have you measured the current when the motor is running and compared it to the data plate?
No load amps will be much lower than full load, but incorrectly connected amp readings will be higher
Of course cheap imported motors will not have top quality lamination's and these will run hotter
There maybe a temp rise figure on the data plate ?


John

[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]

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

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.

[vintageandclassicrepairs]

Hi Muzzerboy and All,

It has been a very long time since I was lectured at college on motor theory 
so I got the theory incorrect
However
Practical lessons and experience do not always agree with the books !!

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.

I have found the same issues as quoted on several occasions throughout the last 40 odd years
An example is a 1/2Hp motor I have used on a pillar drill without issues or overheating despite lots of  hard use
I was given this motor for free as on bench testing (without load) it would get very very hot
It does not even get warm when driving the drill for long periods

John