ECN Forum Forums General Topics of the Trade General Discussion Area Split Phase Motor

A Capacitor Statr / Run type motor, when the motor reaches near full operating speed apx. 75% of rated speed, the centrifugal switch opens the circuit start circuit. OK.

What happen to the motor if centrifugal switch did not opens???????

The starting windings are usually a much smaller wire gauge. They will eventually burn out (sooner rather than later) if the centrifugal switch fails to open.

Ok starting winding is in series with centrifugal switch, so if centrifugal switch fails to open motor already reaches about 75% of full speed. Will motor keep running or motor stops as soon as the start winding burned. Thanks for your help Mr. Tom

Maybe it will keep running,. When the windings fail, they may short turn to turn or they just might burn open. If they short turn to turn, this may trip a breaker or blow a fuse.

The thermal 0/Ls will trip continually eventually get week and will start to nuisance trip if not discovered straight away.The start winding may also burn. This sounds like the Canadian 442a exam to me.

[This message has been edited by frank (edited 12-04-2003).]

Most likely, the Motor will never achieve full rated speed, but remain around 85% of that speed.

If the shaft has no load on it, the Motor might survive as long as 2,3,4 or 5 Hours before flames take over and fry the start circuit.

Loaded shaft will result in rapid OCPD trip, or loss of smoke, or BOTH!

All depends on the Motor its self:
* can heat be sinked rapidly,
* is the start winding tweeked for heavy/hard starts or simple starts,
* is the R/XC of the start winding going to result in higher current draw when motor achieves full speed, or will it taper off to a fixed value,
* and some other techee stuff!

Normally, a 1Ø Split Phase Induction Motor will have a "Fixed Load" on the shaft - so the typical result will be smoke production within 5 minutes or less.
Motor will be restricted to < 90% of Rated Speed while under load (speed will slip behind Synchronous speed depending on load, and with start circuit locked in, speed will never exceed < 90% of the slip speed).

I REALLY hope this is not an exam question!!!

Scott35

p.s. edited summ speleeng air-ers!

[This message has been edited by Scott35 (edited 12-04-2003).]

So you guys don't think it would ride the thermals before failing?

Well, what say we get ahold of a motor & try it out? I really don't think the starting windings will take much abuse & it is just a SWAG that they will burn out before the built in thermal protector does its thing.

I run across this problem with motors on garage door operators. the run time is really short, so the starting windings take a lot of abuse & the thermal overload almost never trips out.

Scott, it's hard to refrain isn't.

Roger

There really is not enough info given in the question. I have seen this happen often with rooftop Exhaust fans and it's usually an overload condition (variable speed pulley). If the motor does not reach 75% rated speed it will ride the O/Ls until they are weak. I didn't think they would use a motor with a centrifugal switch on intermittent duty applications such as a garage door openers. I’ve only seen them used on equipment rated for constant duty like fans and icemakers so this is my only application experience with them. Where else do you see them used? The question doesn't give info on loading so the only other reason I can think of is a worn or broken switch and I've never seen it. I do remember this question on the exam (predictive Maintenance) There Are three exams and you won't get the same one twice. The questions are flipped so the next one you write it will probably ask what will happen if the centrifugal switch is stuck open at startup .Know your three way switches ,Boolean logic and RLC circuits as well .Always factor personal safety into the answer. Remember usually two of the choices are correct but one will be more correct. They are looking for a certain state of mind as much as trade knowledge.



[This message has been edited by frank (edited 12-07-2003).]