2 Speed Motor Control with Breakers

[PwrEngng]

Hi everyone,

I recently commissioned a 13.8kV 2-speed (Variable Torque type) motor starter gear with Vacuum Breaker control. The gear has three breakers, one for Low speed operation and the other two for High speed operation. The breaker operations are controlled electrically using selector switch. The control design is as follows;

* For low speed application, the selector switch contacts trips the two High speed breakers and allows closing of Low speed breaker with the manual operation of the Start push button.
* For high speed application, the selector switch contacts trips the Low speed breaker and allows closing of two High speed breaker with the manual operation of the Start push button.

So, every time the speed needs to be changed the motor will be offline for the time required to hit the Start button.

My question is, do I have to consider a minimum time between a stop and start of the motor for the purpose of changing speed without damaging the motor.

Thanks

 

[jraef]

Yes, you must allow time for the magnetic field in the motor to collapse before re-connecting, but a couple of seconds is all it takes.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[Zogzog]

That will put a beating on a breaker, better to use a contactor or a magnetic breaker.

 

[waross]

You may consider monitoring the back EMF (voltage at the motor terminals) when the motor is disconnected from the line to see how fast the magnetic field and the resulting back EMF decays. That will give you an accurate indication of the time required. It varies.

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

 

[PwrEngng]

Thanks guys, it was helpful.

There isn't a safe way to monitor the back emf at the motor terminals, there is no meter provision and it is also a 13.2kV motor.

Initially I was leaning towards using contactors as motor starter but the client designed the gear with breakers.