Thermostat N/O and N/C is N/C ever used?

[ban-all-sheds]

I may have misunderstood, but I thought you were saying that there could not have been an change in impedance via the mechanism described for an impedance protected motor because you had not observed a change in current.

 

[JohnW2]

Many possibilities have been discussed in this thread, but my most recent comments were in response to bernard's suggestion about "a solid state self resetting fuse that disconnects one of the windings".

Had anything like that occurred, I would have expected to have seen the current increase appreciably (immediately stalling happened) before that 'disconnection of one winding' occurred, and I would also have expected PF to have changed as a result of that disconnection - neither of which I observed. As for 'warming up' theories in general, there was no noticeable change in DC resistance until the motor had been in the stalled state for over 30 mins - throughout which period current remained unchanged (as compared with the non-stalled state).

It seems to me fairly clear that something is preventing the current increasing immediately the motor stalls, rather than that such an increase does occur, but is subsequently 'reversed' by some mechanism (e.g. what bernard suggestion) dependent, inter alia, on 'warming up' (which would take a finite amount of time).

 

[ban-all-sheds]

Many possibilities have been discussed in this thread, but my most recent comments were in response to bernard's suggestion about "a solid state self resetting fuse that disconnects one of the windings".

My apologies - I can only assume I allowed myself to be misled by the 9 posts you made concerning current prior to that one of Bernard's.

 

[SimonH2]

My guess ...
The motor is designed such that the back-emf when running is minimal - fairly weak field and rotor interaction - since the motor is expressly designed to stall. Thus explaining why there's naff all change when it does stall.
As to why it draws more current than the measured resistance indicates - don't forget that there's a shorted turn (shaded pole) which will dissipate power when AC is applied, but not show up with a DC measurement.

 

[JohnW2]

My guess ... The motor is designed such that the back-emf when running is minimal - fairly weak field and rotor interaction - since the motor is expressly designed to stall. Thus explaining why there's naff all change when it does stall.

That sounds credible - although I would be surprised that it could generate appreciable torque if there were only a 'minimal' counter-emf.

As to why it draws more current than the measured resistance indicates - don't forget that there's a shorted turn (shaded pole) which will dissipate power when AC is applied, but not show up with a DC measurement.

That might make sense, were it based on a correct premise, but ... the motor draws about 4 times less current than would be the case if the DC resistance were the only load. At first sight, that suggests that the majority of the impedance is reactive, but that explanation goes rather wrong when one notes the near-unity PF!

Kind Regards, John

 

[ban-all-sheds]

Maybe you should register and discuss it here: https://cr4.globalspec.com/thread/119338/Small-Synchronous-Motors

 

[JohnW2]

Who, I wonder, is this "Codemaster" (of Stoke-on-Trent) who quoted from a post from me in this thread in that forum?

Edit ... answer discovered! ....

If anybody's still interested, there's a forum CR4 I get on, with an electrical section, so I posted the question there. ....

I commented on the responses in that other forum in msg #114 above.

 

[ban-all-sheds]

I think you'd probably get a larger number of people with expertise engaged, and more direct engagement, if you DIY rather than act through fixit.

 

[JohnW2]

I could (and may) try, but I don't see why my direct participation would result in engagement of any more people with expertise than were involved in discussion following fixit's post there - and those people with expertise have not yet added anything that we have not already discussed and/or dismissed here. It seems to be a pretty basic question, which I would haver thought that any competent electrical engineer ought to be able to answer, so maybe I/we are looking in the wrong places for such expertise.