Switching power source causes motor to stop 1

[surf40]

A friend of mine is an electrician that has never worked with motors before. He asked me to help him out. He has a customer that has a 120v 60 hz AC electric motor that has some medical applications. It’s power source is a wall socket. They are trying to connect it to an alternate power source (a generator). They have a switch that will change the power from the wall to the generator. The generator is providing the same 120v 60hz. But when they throw the switch to change the power source, the motor stops. During the switch change, the motor is without power for about 100-200 ms. He can disconnect the generator after the motor stops, and when he connects it back up, the motor will start and run just fine.
So what is he missing?
Thanks

Surf40

 

[itsmoked]

Is there a relay between the ON button and the motor that is dropping out during the 200ms. Get it down to 30ms and it might work.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[electricpete]

The motor retains a residual voltage.
Reclosing residual voltage out of sync with incoming voltage can cause large torques and currents.
If power supplies are not sync'd and no attempt is made to check phase, who knows what you get.
The quicker you reclose, the worse the potential transient (more time allows the residual voltage to decay).

Certainly fast reclosing without any attempt to sync or check is bad news for a large machine.
I don't know about small machines. Erring on side of caution, a hobbyist might want to steer cleer unless it has been carefully thought out.

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(2B)+(2B)' ?

 

[electricpete]

I'd be more inclined to try a 1 or 2 second dropout.
That should allow time for residual voltage to decay.

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(2B)+(2B)' ?

 

[electricpete]

Actually 1 - 2 seconds would be for small three phase motor.
For single phase with cap, I don't know if cap has an effect.

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(2B)+(2B)' ?

 

[waross]

Best to let the small motor stop (if it has a centrifugal switch). I have seen a lot of small motors perform well with a fast transfer, but if it doesn't work- don't do it.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[hooverdale]

Changing the power source manually while the motor is still running will cause the motor to stop and should accelerate again once the generator power kicks in. Why the motor didn't accelerate again might be caused by the centrifugal switch. (Just a guess please don't bash me)

What is the HP rating of the motor? Basing on your voltage and application this should be a small motor. Tell him to use a UPS if his main concern is power reliability. Plug the motor directly to the UPS and the manual transfer switch (fed by wall and generator power) supplying the UPS. This will guarantee a smooth transition even if you manually switch power.

Sorry i cant explain it very well.

 

[waross]

Good idea hooverdale.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[blacksea]

Except UPS using, if you use frequency inverter you can add capacitors to DC bus line for keep its voltage during AC sources switching.

 

[OperaHouse]

I had a customer that used a 3oA linear power supply just to puwer one relay just to prevent dropout. Seemed like a lot of expense just to get a large electrolytic cap. He had a machine in a foriegn country that suffered from short power line dropout. I would imagine this problem has nothing to do with the motor, but the control system.

 

[DaveMeharg]

A UPS could be a good solution. However, be warned that you will need to size the UPS to handle the startup current of the motor. Generally UPS units don't handle motor loads very well unless they are overized to compensate for them. Many UPS units have a dynamic bypass that will momentary bypass the UPS during an overload condition such as the motor starting. However, if you are going to frequently start the unit this is not the best solution. I would recommend contacting the UPS manufacturer you are considering and present to them the motor size information in order to properly match the UPS to your load. If it is properly matched I think will work well with your switching sitaution.

 

[davidbeach]

unless they are overized to compensate

Make that

unless they are GROSSLY overized to compensate

 

[Compositepro]

From the wording of the original question my guess is that this is not simply a bare motor we are talking about, but a "medical device" that contains a motor and plugs into a wall outlet. Like Keith said, there is probably a relay or other controls that prevent the motor from automatically restarting after a power failure or when you plug it in. This is a very common safety feature.

 

[electricpete]

I agree that is the likely cause... a so-called "three wire control" where the load stays off when power is lost (three wire control usually has a push-button or spring-return switch instead of a 2-position stay-where-you-put-it switch). Maybe op can confirm a little bit:

He can disconnect the generator after the motor stops, and when he connects it back up, the motor will start and run just fine.

Connecting it back up involves operating the motor push-button or switch?

Are you guys sure fast bus transfer of small motors is ok without careful review of timing?
I don't work with small motors, but it seems a little dicey to me.

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(2B)+(2B)' ?

 

[electricpete]

Are you guys sure fast bus transfer of small motors is ok without careful review of timing?
I don't work with small motors, but it seems a little dicey to me.

An important thing to understand would be whether the two power sources are in-phase. We make assumptions, but perhaps op can confirm.

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(2B)+(2B)' ?

 

[OperaHouse]

Don't hold your breath. The OP has only logged in twice and that was the day he posted.

 

[MikeHalloran]

I'm guessing there's a no-restart relay inside the box somewhere.

As for small motors producing big transients, I have it on good authority that a 1/15 HP Bodine PSC, when 'plugged' with a relay to reverse it, suffered no noticeable damage, possibly because the 15A breaker in the wall box launched itself out of said box, instantly clearing what it clearly perceived as a fault. The breaker shattered when it hit the floor, so I do not know if the breaker could have protected the motor again.



Mike Halloran
Pembroke Pines, FL, USA

 

[sibeen]

So, Mike, when you replaced the CB with a 20 amp module, how did things work out?

 

[OperaHouse]

Well, that particular breaker was probably just going to fly out of the panel anyway. Isn't that why they are supposed to be rated for 10,000A interupting. There is always more to the story. The general public may think technology is like Voyage To the Bottom Of The Sea where sparks are flying out of control panels all the time, but not here. Not saying that there isn't a surge or that doesn't happen but properly rated devices are supposed to handle that. OP says this is a medical device and it sounds like a bad design. At home we have numerous momemtary power drop outs where the TV keeps running but the clocks all start flashing. I'd say that was about 100-200ms and wouldn't want to be on life support hoping that someone comes around to restart a motor

 

[surf40]

Sorry to all that I have not been more proactive in answering you questions about my original post. This really is for a friend who english is not his native language, Also, the "generator" as I called it is a proprietary power source that he is developing, so he has been less than forthcoming about the details of that, how it generates the AC, etc. So that probably explains a lot about why it is not a seamless transfer when he switches the sources.

He likes the idea of using the UPS in-between the two sources and the motor. I cautioned him about start up loads while looking for the right UPS

Also, I'm not sure about the relay on the motor, and if that might be the cause of the problem. I'll have my friend check on the exact way the "on/off" switch on the motor works.

I know this all sounds fishy with "my friend", but it really is another person, and he is grateful for the advise I've passed on to him from this forum, after I translate it to another language!

Thanks again,

Danny