Switching power source causes motor to stop
Switching power source causes motor to stop
(OP)
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
So what is he missing?
Thanks
Surf40





RE: Switching power source causes motor to stop
Keith Cress
kcress - http://www.flaminsystems.com
RE: Switching power source causes motor to stop
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)' ?
RE: Switching power source causes motor to stop
That should allow time for residual voltage to decay.
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(2B)+(2B)' ?
RE: Switching power source causes motor to stop
For single phase with cap, I don't know if cap has an effect.
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(2B)+(2B)' ?
RE: Switching power source causes motor to stop
Bill
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"Why not the best?"
Jimmy Carter
RE: Switching power source causes motor to stop
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.
RE: Switching power source causes motor to stop
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Switching power source causes motor to stop
RE: Switching power source causes motor to stop
RE: Switching power source causes motor to stop
RE: Switching power source causes motor to stop
Make that
RE: Switching power source causes motor to stop
RE: Switching power source causes motor to stop
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)' ?
RE: Switching power source causes motor to stop
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(2B)+(2B)' ?
RE: Switching power source causes motor to stop
RE: Switching power source causes motor to stop
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
RE: Switching power source causes motor to stop
RE: Switching power source causes motor to stop
RE: Switching power source causes motor to stop
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
RE: Switching power source causes motor to stop