Thermal switch in Crompton Parkinson motor

One of my 0.5 bhp, single phase, Crompton Parkinson motors, developed classic centrifugal switch, dirty contact syndrome.

On stripping the motor down, I was surprised to see that the thermal safety switch was in the start winding circuit. This is the type of thermal switch which has two or three turns activating a bimetal switch. So it monitors current, rather than motor temperature.

In past servicing of electric motors, I seem to remember that these thermal switches were in the main winding. Was I wrong about this? Seems to me that the run winding needs overload protection more than the start winding?

Do I have a motor that has been got at by someone, or is my memory playing me false?

Andrew.

Edited By Andrew Tinsley on 02/08/2020 11:12:40

I've always understood that a significant weakness of single phase motors is over-heating of the start winding if the motor is started too frequently. A thermal switch in the start circuit would counter this.

Hello Clive,

Not so! This "thermal switch" operates on current NOT on motor temperature, so it would be ineffective as a device to keep the start winding temperature down,. As a current trip, surely it would be far more effective in the main run winding?

In addition to this, the motor is of the capacitor start variety. The overheating of the start winding when doing lots of stop / starting is a bad characteristic of split phase motors . Capacitor start motors are much kinder to the start winding in this respect, I always wondered why Myford persisted in using split phase motors for their lathes. The cost difference between capacitor start and split phase motors is very little.

Andrew.

If it is a bi-metal switch it opens when it gets hot. If you apply an external source of heat to such a switch it will open, even if there is no current flowing through it at all.

Whether the heating effect of the current flowing through the switch is of more immediate importance that that ever likely to be produced in the windings before they self destruct does not mean that the action of a switch of this type is not thermal.

Hello Nick, the switching action is thermal. However the heat is produced by a winding which takes either the start winding current or Run winding current. It does exactly the same as a "thermal" trip in a DOL starter So it has nothing to do with the motor temperature or winding temperature. It purely trips on over current for exactly the same reason as the over current trip in a DOL starter..This switch is stood off the case by 1/2" or so. If it were supposed to detect motor temperature it would not be mounted on standoffs.

It is not an effective use of time to debate if it is a current trip or temperature trip and is pretty irrelevant to the main question, which is, should it be in the start winding or run winding? I am pretty convinced that it should be in the run winding. So I suspect someone has altered the internal wiring of the motor.

Come on, is there not a practical sparkie in the house? Steviegtr where are you!!

Andrew.

Edited By Andrew Tinsley on 02/08/2020 13:58:24

Andrew

You say the thermal switch is in the start circuit, is that in series with the winding or embedded in the winding ?

If it is in series with the winding it will only protect the start winding, thus allowing the motor to keep running, only preventing further starting until it is either reset manually or cools down and closes contacts.

The only times I experienced thermal cutouts as described were on Gryphon motors that included a red reset button on the terminal box, I never checked if they were in the start winding or main winding.

Emgee

On some European motors it is possible to find a thermal switch that can be connected into the starter stop circuit, these can be found on some multi function woodworking machines.

Edited By Emgee on 02/08/2020 14:10:27

Isn't it detecting indirectly via temperature that current has been flowing for too long? The bimetallic strip is gradually warmed by the current until it clicks out, as when the start winding overheats due to a failed centrifugal switch. The motor's normal operating temperature isn't anything like enough to operate the cut-out, but it will if start winding is hot and passing current at the same time.

I admire and dislike single-phase motors electric motor. Clever design gives us a self-starting single-phase AC electric motor which is a major advantage in many circumstances. Apart from that they're complicated & unreliable with low starting torque plus they vibrate. A mildly nasty compromise. Outperformed by almost every electric motor ever invented apart from the Universal type, which is also useful up to a point.

My crystal ball predicts the gradual demise of single-phase motors. Brushless motors walk all over them but until recently reliable inexpensive electronics were a problem. Cracked I think - not impossible in 2020 to make a brushless motor controller or VFD cheaper than the high-voltage components needed to build a conventional single-phase motor.

Dave

Hello Emgee,

The switch is in series with the start winding, not buried in the winding, If for some reason the start switch doesn't drop out on getting up to speed, the start winding will not be protected by the "thermal" switch as the current will, if anything, drop as the start winding overheats.

This is why i think the switch should be in the run winding, where it will act as a DOL thermal over current trip.

Andrew..

All I can add is a General Electric 3/4hp that I had stopped the motor running after 1hour in a 10degC garage (it was drawing 10A) the switch was on the rear end cover.

A thermal trip is what it states and will work on temperature to protect the motor if this oveheats the motor will stop The start winding will try to restart the motor when it has cooled down

Regards Bee M

Brian, the thermal trip is in the start winding. Once the motor starts, then there is no current flowing in the start winding. The motor can fry and the thermal trip may open, but the current is flowing in the run winning. Hence the trip offers NO motor protection in this scenario.. It will NOT shut the motor down, It will run to burnout.

This is why I feel that the trip should be in the run winding. I have done a little research and apparently in some Crompton Parkinson motors the trip is in the run winding, but in others it is the start winding.

Andrew.

Brian, the thermal trip is in the start winding. Once the motor starts, then there is no current flowing in the start winding. The motor can fry and the thermal trip may open, but the current is flowing in the run winning. Hence the trip offers NO motor protection in this scenario.. It will NOT shut the motor down, It will run to burnout.

This is why I feel that the trip should be in the run winding. I have done a little research and apparently in some Crompton Parkinson motors the trip is in the run winding, but in others it is the start winding.

Andrew.

Andrew

The live feed tto start winding is removed when the centrifugal switch changes to run winding .

I have had this problem with Commercial Tumbler Dryers overheating because the surrounding temperature was to high causeing the motor to overheat and then trip out and also Comercial Fridge Motor Overheating because they were to hot .If your motor has not been rewound then the trip is in the correct place?

If your trip operates the motor must stop Rather Like switching your main switch of or removeing the fuse it stops.

Further Investigation is required .

Good Luck look forward to the answer

Bee M