Induction Pump Motor Back Spin Protection

[maintee]

We had a situation recently where a 1000 hp 11.5 kV induction motor (full voltage start) was burned up because someone accidently tried starting the pump while it was spinning backwards. The operator was switching from one pump to another and was trying to perform a backflush of the pump intake by leaving the outlet valve open and using system pressure to force debris from the river intake screen. The motor started spinning backwards and making noise, so he panicked and tried restarting the motor.

The motor protection relays are of the old mechanical type and I would like to replace them with newer electronic types. Does anyone have any reccomendations for motor protection relays that could incorporate back spin protection. I see a GE Multilin 369 relay that advertises back spin protection, anyone have any experience with this unit or other ideas. I plan on interlocking the oulet valve to the motor starter, but want a backup protection on backspin.

Thanks for your time!

 

[MikeHalloran]

A big ol' mechanical ratchet or sprag clutch?



Mike Halloran
Pembroke Pines, FL, USA

 

[davidbeach]

A good drive that can catch a spinning motor (either direction) and take it to the desired operating condition?

 

[TommyOO7]

I would say that good electronic motor protection relay is enough. Basically when this happens the motor is in short circuit mode and (well tuned) motor starter should open the circuit before any damage to the motor.
Check GE Multilin and Switzer ...

TommyOO7

 

[powerjunx]

try Areva motor management relay,

 

[jraef]

Coast-down / Back-spin protection consists of a simple Off Delay timer circuit. Once the motor is de-energized, the Back-spin Timer diables the Start command so that the motor cannot be re-started again until the timer finishes its cycle. You can do this with any Off Delay timer, but the new solid state Motor Protection Relays offer so much more extra protection that it is worth the investment IMHO.

A minor drawback to this simple timer function is that the user must determine the back-spin time and program the timer (or function) to that much time or a little more. No big deal, it just takes some observation at set-up by someone qualified to understand the issue (I have seen many people who were NOT smart enough to figure that one out!). Another potential drawback is if the motor is spinning backwards from some external force not related to its having just been running. An example would be a leaky check valve allowing head pressure to back-spin the motor even though it has not been running for a long time. If this is a possibility, you can back up the MPR with a zero speed switch. The 469 comes with that capability built-in so all you need is the sensor, with the 369 you could still do it but the trip mechanism would need to be self-contained in an external device.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[MikeHalloran]

The zero-speed switch would prevent one from STARTING a motor that was spinning backwards, or forwards.

It would not prevent the motor from actually spinning backwards, e.g. while the attached pump was being intentionally backflushed, as in the extant case.



Mike Halloran
Pembroke Pines, FL, USA

 

[jraef]

MikeHalloran,
That's right, although the zero-speed switch would have saved him in this case because he would not have been able to start the motor, which is when the damage occurred. We don't know if the motor spinning backwards when off-line was a bad thing or not. Many times it's not a problem as long as the motor is not energized at that time.

davidbeach,
Your idea has merit if the VFD was justified for other reasons, but an 11kV VFD (upwards of US$300,000.00) to solve this problem is a tad bit of overkill, no?

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[mc5w]

If you energize a motor when it is spinning backwards it will draw more than locked rotor current, which is what leads to damage. You could try connecting a brake to the motor, either direct electrical operation or electric control of compressed air. An example is the kind of brake that is used on a bridge crane.

If you are deliberately backspinning a pump such as the flushing operation that you describe, you need some kind of limit switch on the valve for backflushing such that the motor cannot be started until a cetain amount of time after the valve closes. You could also use a compressed air jet refitted to the pump to stop it once the backflushing has been done. The compressed air jet would be a lot cheaper than trying to start the motor using an extra heavy autotransformer starter and for what you are doing would be cheaper than an external brake.

Also, you could attach a shaft position encoder to the pump shaft. You would then take the A and B transistor transistor logic signals a run them through a circuit that tells you if the motor is running backwards. The encoder generates a 2-phase waveform that is 50% on for each of the A and B channels and the B channel is 90 degrees out of phase with the A channel. The circuit would just simply analyze the on-off sequence to figure out the direction.

I have also seen a 20 step per revolution encoder that was built using two 5 spoke gear spiders and 2 inductive limit switches!

Mike Cole

 

[maintee]

Thanks to all for your fine suggestions!

 

[SteveKW]

You should consider the stresses to the shaft when instantly reversing it. Breaking it can get expensive too!