Motor drawing high current when unloaded

[PhaseShift]

We were trying to start a 5hp motor today with a FLA of about 7A. The motor is a 480V six lead motor. As soon as the motor was started under load it would draw about 100A and then quickly trip the overload. This 100A seems extremely high for this motor

We then meggered the motor to find it was o.k. and uncoupled it from the load and found that we were able to spin the shaft by hand so that the motor wasn't binding. We then went ahead and started the motor uncoupled and saw the same results.

The motor nameplate was missing however we believe we found the correct datasheet and that the motor should be wired in Delta. However if the motor is supposed to be wired in wye, can having it wired in Delta cause this increased current that we are seeing?

What else could possibly be causing this if everything else mentioned above looks fine? We verified all motor connections and verified that the motor is not single phasing.

 

[gar]

100329-2256 EST

Ask the question what is the ratio of the line to line voltage in a balanced 3 phase system relative to line to neutral? This may be a key to your problem. Is it possibly a dual voltage motor? Are the leads connected properly?

Try the motor in a Y connection and see if steady state unloaded is maybe 5 A. Peak starting inrush is maybe 50 to 70 A.

It would be best if you could find data on the motor.

.

 

[Jraef]

The "supposed to be..." is meaningless without the correct connection data. There are many ways to build and wire a motor, if you do not know how yours is designed, hooking it up incorrectly can definitely result in extremely high current. Basically if t is a dual voltage motor and you have it strapped for 240V but apply 480V, that would be one of those scenarios.

 

[Smart $]

Not knowing the nameplate data or the connection configuration per manufacturer, the first thing you should do is check lead to lead resistance (and impedance if you are equiped to do so).

I know it is possible to determine HP of a motor missing its nameplate, but I am curious as to how you did it...???


ADD: There are three standard connections for a NEMA 6 lead motor... http://www.malloyelectric.com/pages/tech/connections.html

 

[Besoeker]

However if the motor is supposed to be wired in wye, can having it wired in Delta cause this increased current that we are seeing?

Yes, it certainly could. The excessive winding voltage would produce magnetic saturation resulting in the very high currents you saw.
With the motor uncoupled you could try running it wired as a WYE connection and then measure the current. If it's the correct connection, I would expect around 2A to 3A running uncoupled.

 

[PhaseShift]

I made the On-Site rep hunt down a motor sheet for this motor, and sure enough it said to connect it wye after he swore it was to be delta.

We connected it in wye and sure enough the motor started fine and was pulling only about 4-5A.

I didnt realize that this wrong connection could cause so much current increase. I guess the increased V/Hz really creates and increased flux density and saturates the core causing increased current for no additional work. Where does this saturation occur? Stator or rotor?

Thanks for the help.