Starting Current of Freewheeling Fan Motor

[mellomain]

I have a 460V 75HP fan motor (85.9 FLA and 684 LRA) that tripped its feeder breaker on startup. Everything has checked out good. It is possible that this fan was freewheeling backwards when we tried to start its motor. About how many amps could that situation draw? Or is there a calculation - rule of thumb - etc... Thanks in advance!!

Man is troubled by what might be called the Dog Wish, a strange and involved compulsion to be as happy and carefree as a dog --- James Thurber

 

[waross]

Take a look at the Cowern papers, page 7;
You probably have a design "B" motor. Starting torque is about 150% rising to a pullout torque of about 200%
The torque of a back spinning motor is about 150%.
The current is closely proportional to the torque, so you can assume a plugging current roughly equal to starting current.
A "T" frame motor won't last very long at that current.
Of concern is the speed of the backspin. I have seen instances where the reverse speed was greater than the forward speed.
Your issue is not so much the current but more the length of time that it takes to slow the motor to a stop.
A number I remember from years ago in relation to "T" frame motors is 15 seconds to burnout at locked rotor. You will be drawing close to locked rotor current.
The 15 second value may be subject to correction but whatever the figure the point is you don't have much time to get a "T" frame motor up to speed.
In your position I would do some research to determine the allowable locked rotor time for this motor. Then I would try a time setting or inverse time setting that allows about 50% to about 70% of allowable locked rotor time before tripping.
Be careful. That trip may have saved you a motor.
If the feeder is tripping on instantaneous, you may be experiencing a severe current transient when the applied EMF hits the back EMF due to residual magnetism. You may be able to set the instantaneous trip slightly higher. Check your code for the maximum allowable setting of instantaneous trips.
I have assumed that this is a time over-current issue.
If you determine that the breaker is going out on instantaneous trip that is more likely a transient issue.If so, let us know and we can go from there.

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

 

[mellomain]

The rest of the story - yes it is a T frame, design B motor. These motors have a damper system but it is not air tight and back spinning does occur. This motor is fed from a molded case thermal magnetic breaker and thermal overload relay (this is a standard design we use because the thermal trip on the molded case breaker will typically not go low enough) in an MCC. The MCC is fed from an air circuit breaker in a load center. The MCC breaker and thermal overload did not actuate. The load center breaker tripped within 3 seconds of trying to start the motor. The MCC was very lightly loaded at the time. Indication on the load center breaker says it tripped on Short Time; this breaker uses electromechanical trip units; it has a Short Time and Long Time but no Instantaneous setting.

I do not have "great" coordination for the MCCs. The molded case breaker was set at 130A Thermal and 2000A Inst. The TOL was set at 100A. The load center breaker was set at 3000A Short Time and 720A Long Time. I have attached the drawing; it is the first two breakers from the left if you can see them ok. If I had 2000A for a few cycles, then it would be a race and I have no problem reporting that. If that is what happened, I just want to be able to say why that was a possibility.

Everything after the event tested SAT. Some want me to say "spurious trip". But wth does that really mean? We have documented procedures for setting breaker trip units. They are tried and true and based on industry standards. I have only a small amount of flexibility with the trip settings.

I am not familiar with using "plugging" current. Is that additive? I am still looking for allowable locked rotor time for this motor.


Man is troubled by what might be called the Dog Wish, a strange and involved compulsion to be as happy and carefree as a dog --- James Thurber

 

[waross]

Plugging was fairly common with the "U" frame motors, the predecessors to the "T" frame.
It is putting full reverse current to a forward running motor. Basically a reversing contactor with no time delay between forward and reverse.
There were two main applications; Rapid reversing of equipment and rapid stopping.
For stopping, the motor was energized in reverse and a switch called a "Plugging switch" or a "Zero speed switch" determined the moment that the motor stopped and disconnected the power.
A plugging switch was a zero speed switch with a latch attachment. A zero speed switch was not safe for plugging to a stop. In the event that a stopped motor was rotated backwards, even at a fairly slow speed, the switch may close and energize the motor. For example, if a mechanic were to put a wrench on the motor coupling and turn it fast enough to close the switch, the motor could kick back towards him with full force. A plugging switch was a zero speed switch with a latch that prevented the switch from re-closing once the motor was at rest. I haven't seen a plugging switch for years, but variations of the zero speed switch may still be used for detection of broken belts.
Bottom line, the current when the motor is spinning backwards is about the same as the DOL starting current.
BUT
Do you have capacitors connected to the motor terminals? That can cause issues with a back spinning motor.
Capacitors will allow the back EMF to flow and may allow the buildup of a fairly high voltage. The actual voltage and current comes under the heading of 'It depends". However, the initial current may be in the order of twice the normal starting current.


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