Motor Circuit Breaker Tripping

[coulter]

...in our lab we actually witnessed that a lot of large frame motors, i.e. over 10HP 460V, actually exhibited Negative Coefficient of Resistance properties...

Even more interesting. I checked the two references. Neither mentions (or aludes to) anything about motor windings having a Negative Temperature Cooeficient. I'm not sure why you cited them. All of the data/equations are consistant with the Copper PTC that has been known for perhaps the last 250? years.

Perhaps I don't understand your definition of NTC.
Are you saying the winding resistance goes down with increasing temp?

Are you saying the LRC goes up with increasing temp?

Is there some other measurement you are using to infer this NTC?

I am not surprised that you have seen motors exhibit an NTC. For many analysis a motor is a transformer that has a moving secondary. Transformer damage curves are often published with two curves; one for cold energization and one for hot.

I am. There is no known physics that will make copper winding resistance go down with incresaing temp? Jim - you already knew that.

carl

 

[jim dungar]

I am. There is no known physics that will make copper winding resistance go down with incresaing temp? Jim - you already knew that.

I know that the resistance of the windings is not the only thing that affects the inrush of a magnetic device. I do not remember if there is a difference in the properties of warm vs cold core/laminated steel.

It is a common practice to have different protection levels for warm and cold starting in part because a warm core can not absorb as much heat as a cold one which is one reason that the are often limits on starts/hr and time between starts.

 

[dtiller]

... All of the data/equations are consistant with the Copper PTC that has been known for perhaps the last 250? years.

Perhaps I don't understand your definition of NTC.
Are you saying the winding resistance goes down with increasing temp?

If the winding resistance decreased with increasing temperature, you'd have a condition called thermal runaway that would guarantee the destruction of the motor. This is what destroys bipolar transistors in such a spectacular fashion - trust me, I know!

 

[Jraef]

Re: my statement of NTC on motors.
Don't know what happened to me there, brain fart I guess. Just went back and read my old notes, my memory was 180 degrees flip flopped. My curiosity at that time was that we observed a NTC on smaller motors, which was unexpected and unexplained, but repeated in testing. One of the team members added notes a month later assuming it was probable instrument error and the issue was dropped because it was not germane to what we were working on at that time. I just remembered the initial issue and discussions we all had about it, then drew on my faulty memory when composing that post.

So in the words of Gilda Radner as Rosanne Rosannadana,,, "Never mind".

Sorry to raise everyone's hackles, I promise to stop posting when tired and stressed, as has been the case of late.

 

[whillis]

When the motor is running the armature poles are synchronized to the magnetic field rotating around the stator. Once the motor is de-energized, the magnetic fields collapse and synchronism is lost. If the power is reapplied before the motor stops, the armature poles try to "snap" back into sync but cannot because of the inertia of the armature and connected load. The sudden spike of torque requires a spike of current which will blow the protection.

I have a graph somewhere that shows the torque produced by a 100HP squirrel cage motor that was re-energized while it was spinning down. At the moment the power was applied the motor produced a torque in the opposite direction of rotation (like a brake) that was more than 1000% of the rated operating torque as the poles tried to synchronize. Once the poles were back in step the torque quickly went to zero then started climbing in the positive direction.

The reason the fuses don't blow when the motor starts from a stop is that in a squirrel cage motor, the manufacturer will usually optimize the design of the armature so that it looks like a high impedance when it's stopped and a low impedance when it's running at speed. (This is accomplished by engineering the shape and depth of the conductor bars in the armature core.)

Anyhow, that's a very basic explanation of what's happening. I can hunt up a detailed document that describes the principles of these motors but it's pretty dry and boring.

 

[GeorgeB]

When the motor is running the armature poles are synchronized to the magnetic field rotating around the stator. Once the motor is de-energized, the magnetic fields collapse and synchronism is lost. If the power is reapplied before the motor stops, the armature poles try to "snap" back into sync but cannot because of the inertia of the armature and connected load. The sudden spike of torque requires a spike of current which will blow the protection.

I'm not sure that an induction motor has any of that characteristic; slip is what produces the torque; if at sync speed, there is no torque.

I have a graph somewhere that shows the torque produced by a 100HP squirrel cage motor that was re-energized while it was spinning down. At the moment the power was applied the motor produced a torque in the opposite direction of rotation (like a brake) that was more than 1000% of the rated operating torque as the poles tried to synchronize. Once the poles were back in step the torque quickly went to zero then started climbing in the positive direction.

I _SUSPECT_ this was a wound rotor, not squirrel cage is it braked, but your follow on agrees with my memory of electric machines many years back.

The reason the fuses don't blow when the motor starts from a stop is that in a squirrel cage motor, the manufacturer will usually optimize the design of the armature so that it looks like a high impedance when it's stopped and a low impedance when it's running at speed. (This is accomplished by engineering the shape and depth of the conductor bars in the armature core.)

Anyhow, that's a very basic explanation of what's happening. I can hunt up a detailed document that describes the principles of these motors but it's pretty dry and boring.

I believe you are 100% on here; at standstill, slip is 60Hz, the smaller section bars are near the surface and you have fairly high impedance to limit inrush. As the motor speeds up, slip drops, the lower, usually thicker, rotor conductors have lower impedance (or similar at the lower frequency). When closing on a spinning motor, slip is lower so inrush is higher.

There is a pretty good discussion on http://www.lmphotonics.com/m_control.htm which Google helped me find.

George (Electrical engineer by education, not a PE)

 

[whillis]

Thanks George, you're right on with your comments. I should've gone to bed and wrote that in the morning with a clearer head. I don't know what I was thinking as those first two paragraphs really apply to a synchronous motor.

Sorry for any confusion...

 

[kingpb]

Phase angles of heavily loaded motors drop very quickly out of phase with the supply voltage when disconnected and therefore an attempt to restart the motor prior to allowing the residual voltage to decay before power is applied, can cause excessive shaft torques and can cause abnormally high motor inrush currents. The abnormally high inrush currents can be high enough to trip overcurrent breakers, fuses, and relays, and can damage the motor.

The solution would be to put in a timer (I believe already mentioned) that would allow a delay of anywhere from 3-10s before allowing the source to be reapplied should be sufficient. Keep in mind that frequently cycled motors require special consideration, since heat build-up that can cause damage to the insualtion is obviously an important consideration.

 

[mull982]

Phase angles of heavily loaded motors drop very quickly out of phase with the supply voltage when disconnected and therefore an attempt to restart the motor prior to allowing the residual voltage to decay before power is applied, can cause excessive shaft torques and can cause abnormally high motor inrush currents. The abnormally high inrush currents can be high enough to trip overcurrent breakers, fuses, and relays, and can damage the motor.

This seems to be the case from what I've heard and read from others. But what about after the voltage supply has dropped off and therefore completely dissipating the magnetic field. I've come to understand that the magnetic field dissipates after a couple of seconds, so what if we are talking about restarting the spinning motor after 5 or 6 seconds when the voltage has totally decayed and the magnetic field has dropped off?

Is there some sort of residual magnetic field that can still exist after the voltage has completely decayed after a couple of seconds??

 

[hdpeng]

Instananeous Trip

Instananeous Trip

I have been having a problem with a 480V circuit breaker tripping for a 125hp motor. The circuit breaker is a 3phase, 480V, 250A instantanous breaker...

NEC Table 430-152 allows up to 800% FLA for instantaneous trip (unless it's type E which is 1100% FLA), so it sounds to me like the breaker is simply undersized. Assuming 140A, you're using an inst. trip that's only 178.6% over FLA.