Greater than LRA on starting due to unbalance

[TestBeforeTouch]

Is it possible to draw greater than LRA on starting due to a voltage unbalance caused by a bad connection?

This weekend a 1000HP 4160volt motor with a normal 12 second acceleration time tripped two of its 3 mechanical overloads in 5 seconds during a start. The installation was checked and a bad connection was found on one of the R rated fuses causing it to be severally discolored. After the problem was fixed the motor started normally.

 

[jraef]

No, you will not draw greater than LRA, but unbalanced power will cause the motor to heat up much faster than it will with balanced power because negative sequence currents will create counter torque pulsations in the motor that cause it to fight itself as it tries to operate, therefore pulling higher amps. Some solid state OL relays will monitor imbalance percentages and bias their trip curve to compensate for that to provide better protection for the windings, but I have never heard of bimetal OLRs doing that. Maybe old dashpot oil OLRs might have because the excess current in two phases would heat the oil in those pots faster than the other. When you say "mechanical overloads" what type did you mean? And how did you know it was only 2 of them? Are they separate for each phase? that is unusual for newer OLRs.

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[waross]

Hi jraef;
Actually some motor overload relays had some ingeneous machanical arrangements that made them respond to unbalanced currents at less than rated balanced currents.
I can still vividly recall the mechanical arrangement of one type, but alas, I cannot recal the manufacturer.
Respectfully

 

[TestBeforeTouch]

The overload relays are a GE IC2824, unfortunately I do not have any information on them. The equipment was installed in the mid 80's. The coil on each of the overload relays is very large (about the size of a fist) and must have a high burden because there are two CT's per phase with their secondaries connected in series to drive the relay. Before the overloads and lockout were Reset I measured the contact resistance and found two of the overload relays had tripped. I have pictures of the relay's but I do not know how to upload a picture.

The problem I have is that I can't figure out how the overloads could trip in 5 seconds if they routinely see a 12 second inrush.

 

[dpc]

Is it possible the OLs were already hot from previous start attempts?

 

[TestBeforeTouch]

No, the motor had been down for about two hours before a restart was tried.

 

[dpc]

You can put a small clamp-on current sensor on the CT secondary wiring to the OL and check the actual current to OL during starting.

 

[aolalde]

I am not sure, but the negative sequence must induce extra voltage in the rotor bars that are short circuited, that generates a negative sequence current superposed to the positive sequence current and that combination be reflect in the primary ( stator ) as a reactance decrease and higher current in at least two lines.
The sad part is that the extra current (negative sequence) generates a braking torque.

 

[TestBeforeTouch]

After the overloads tripped I did put a monitor on each of the overloads and watched the motor current as the motor came up to speed and it was picture perfect.

If the blower was spinning slowly from process pressure before the motor was started could we draw more than LRA?

 

[davidbeach]

If the motor was spinning backward you would certainly get more than LRA when you energized it.

 

[jraef]

Even if not more than LRA, certainly LRA for longer than if the motor were at standstill. If your normal acceleration time to full speed was 12 seconds, you were probably not at LRA for more than a second during a start. If the motor was back spinning this time, it may have been at LRA for the entire time until your OL tripped. Still, 5 seconds is a short trip time. Unless of course you have a Class 5 OL relay, in which case that is exactly the trip time you would look for!

The other problem with a back spinning motor is that sometimes a little residual magnetism in the rotor can have the motor acting as a generator when you first energize it, but the field of that generator is not in synch with the line, so you get a massive spike of current as the line pulls that generator into itself.

 

[electricpete]

On the original question, an unbalance and/or reduced voltage on one phase reduces the torque capability and prolongs the starts. This can result in tripping of overloads. You don't need to go above locked rotor amps to trip an overload. If your instantaneous tripped, then you would know you had something over locked rotor amps.

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