Consequent Pole Motor Trips MCP 1

[gobigorgohome]

Has anyone experienced tripping of a magnetic overload protector in a 2 speed motor starter when switching from low to high speed. The units in question have a 3 pole contactor for low speed and a 5 pole contactor for high speed.

This is a new installation with new contactors and motors.

Before everyone gets hung up on contactors closing simultaneously, wiring connections done incorrectly, allow me to ensure that this miserable thing is wired correctly.

Wiring of this unit is consistent with Allen Bradley bulletin 520, variable torque consequent pole. In the literature is states "Motor can be started in High or Low speed. The change from low to high or from high to low can be made WITHOUT pressing the STOP button.

The most interesting phenomenon is that the FLA high speed is 186A, the FLA low speed is 86 amps. The locked rotor current for high speed is 1250A

The MCP that trips is a 250A unit set to the highest instantaneous trip setting of 2500A. We have measured current pulsed through the MCP between 3000 and 4000A!

There are more than one motor and more than one starter exhibing this issue.

Things to note are that:

We have a very low impedance power supply.
Tripping is worse on closer motors (ie shorter field cables)

Are there any motor winders out there that could comment on this being a "normal" phenomenon or could the motors be wound funny?

They run fine at either speed. The pulse occurs with a quick change between contactors or a 2 second both open change between contactors.

There are a number of pretty bright people working on this, just looking for some type of unusual story where someone may have encountered this before.

Motors are 4 / 8 pole, 200 / 50 HP, 575V
Application is cooling tower fan.

 

[jraef]

Assuming they are new motors and thus most likely "Energy Efficient" designs, there is a known issue with the instantaneous (magnetizing) inrush now being much higher than it used to be. So in days past you may have used an MCP that was allowed to be adjustable up to 11x the FLC of the motor, now you are permitted to go up to 17X as a result of this issue. 186 x 17 = 3182, no coincidence. You may need to use a larger breaker. If that's not possible, you could try adding a load reactor (even though you may not need it for it's primary purpose). I've never done that but I've been told it works.

Might have been a good idea for someone to have considered a VFD for that.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[gobigorgohome]

Attached is the motor vendor data sheet. 94 and 91 percent efficient 4pole / 8pole.

The 17x figure, do you recall the lead-up, background and source of this new rating?

Thanks for you help so far.

 

[crthompson]

This may help...

 

[electricpete]

fwiw, based on currents measured several times higher than locked rotor currents, one would expect that the phase of the incoming voltage to differ from phase of motor residual voltage for some reason. The situation of transitioning between winding configurations also occurs during wye-delta starting and can produce similar very high currents and instantaneous trips. This can depend on whether there is closed transition, open transition, time delay of open transition. Search the forum for previous threads on wye-delta trips.

=====================================
(2B)+(2B)' ?

 

[jraef]

Actually, you can see in in the NEC, table 430.52 where you select OCPDs. Read the exceptions below the table, that's where it says that for "energy efficient" Design B motors, you can increase the trip value to 1700% if it has been demonstrated (as in your case) to not work otherwise.

There is a White Paper available out on the web from the DOE from an old program that was called something like "Motor Matters", which got republished more recently. The original paper had much more detailed engineering explanations and examples of studies they did on various manufacturers, but that probably was removed because it made some of them look really bad (it exposed the total lack of design standards and how people were marketing certain motors as EE when they really were not). Here is the updated but weaker paper.

http://www1.eere.energy.gov/industry/bestpractices/pdfs/avoid_nuisance_motorsys_ts6.pdf

Here is another one that does show some of the deeper issues behind this issue.

http://samplecode.rockwellautomation.com/idc/groups/literature/documents/wp/2100-wp005_-en-p.pdf

You have to notice and understand the subtle difference between Locked Rotor Current, which is frequently mislabeled as "motor inrush", and the true meaning of Inrush Current.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[zlatkodo]

Probably it is about the "Energy Efficient" motors (as mentioned Jraef) and I think it should be a time delay between changing the speed (as mentioned Electricpete), particularly in the case of changing from higher to lower speed.
Here is what the PAM-motor manufacturer (TECO) recommends :

"The PAM motor should be started on its low-speed
winding to limit the inrush current. This prolongs motor
life by keeping rotor and core temperatures to a
minimum. Starting on the low speed is also more
desirable for driven equipment considerations.
When starting the motor with the speed changing switch
at the low-speed setting, the main breaker is closed. To
change speed once the motor is operating, the main
breaker must be opened, the switch transferred to the
other three leads, and the main breaker closed again. It is
important, however, to allow the magnetic flux in the air
gap to decay before finally closing the main breaker. This
pause will usually take about one to two seconds,
depending on motor size."
Zlatkodo