Motor Circuit Breaker Tripping

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mull982

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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, and a solid state overload relay is being used to protect the feeders and motor from overload conditions.

The problem that I am having is that when an operator, or our control system is stopping the motor (through the motor starting circuit) and then trying to restart the motor with the motor still spinning (slowing down) we are tripping the breaker. If we stop the motor and let the motor come to a complete stop before restarting the motor, the motor will start fine. However if the motor is stopped, and then restarted while the motor is still downspinning from the stop command the motor will try to pick backup and trip the breaker.

Should this be happening? Why? I've heard of jogging a motor before which leads me to believe that we should be able to restart the motor without a problem while it is still turning.
 
Can you give more information on your set up . What does this motor actually do ?. When you say solid state overload relay, I take it as a soft starter?Is it a programable one with settings /parameters accelaration, decelaration settings or none of that. Is this a blower, pump, exhaust system ?. I am just trying to get more information on your set up so that I could think of a root cause of this problem.Thanks
 
Controls said:
Can you give more information on your set up . What does this motor actually do ?. When you say solid state overload relay, I take it as a soft starter?Is it a programable one with settings /parameters accelaration, decelaration settings or none of that. Is this a blower, pump, exhaust system ?. I am just trying to get more information on your set up so that I could think of a root cause of this problem.Thanks
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The motor is a 125hp motor used to run a conveyor belt carrying limestone.

The overload relay I am speaking of is an Allen Bradley E3 overload relay. Our new MCC's are the new Allen Bradley smart MCC's so instead of the traditional bi-metal overloads in a starter bucket we are using the solid state E3 relays to provide the overcurrent protection of the motor. Therefore the starter bucket for this motor has the 250A instantaneous breaker coming off of the bus, then going through the E3 overload relay and then out to the motor. Traditional motor setup, just a different type of overload relay.

I dont think this problem has anything to do with the E3 overload, because it is seperate from the breaker and does not provide any means to tripping the breaker. To answer the questions regarding programming of this relay, this is simply an overload relay to control your motor contactor, and does not maninpulate the incoming power in any sort of way. Therefore whatever is causing the breaker to trip is simply on the 480V circuit.

This is a problem we have been having ever since this conveyor was installed back in June. Either an operator will manually try to restart, or our control system will try to restart the motor while it has been stopped and still turning, and it will trip.

Should a motor be able to be stopped and restarted while the motor is still spinning?? (jogging)
 
mull982 said:
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, and a solid state overload relay is being used to protect the feeders and motor from overload conditions.

The problem that I am having is that when an operator, or our control system is stopping the motor (through the motor starting circuit) and then trying to restart the motor with the motor still spinning (slowing down) we are tripping the breaker. If we stop the motor and let the motor come to a complete stop before restarting the motor, the motor will start fine. However if the motor is stopped, and then restarted while the motor is still downspinning from the stop command the motor will try to pick backup and trip the breaker.

Should this be happening? Why? I've heard of jogging a motor before which leads me to believe that we should be able to restart the motor without a problem while it is still turning.
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250A instantaneous seems pretty small for a 125HP motor. NEC FLA (running) is 156A. Starting current could be 6X that or more.

Starting a motor that is already turning has other issues associated with it.
 
Petersona right about the breaker size, starting current of a motor . I was asking questions to find out if there was a connection between the controller type, program settings that would make a difference on the motor when its at complete stop or still turning. Of course overloads have nothing to do with your problem. I THINK , your problem may related to what the motor is driving. While motor is still turning, sudden start on this motor is facing some mechanical resistance increasing the torque and causing breaker the trip. There may be a design issue.When motor is at complete stop, conveyor and everything else is settled. Start up will be smoother.Normally, we do not like to jog or start to motor until it is completely settled
 
When you attempt to restart a motor that is already "spinning" the starting torque can be significantly higher.

What is the instantaneous of the CB set on?

Running current around 135 AMPS?
 
while the motor is decelerating, it is acting as a generator, producing a voltage waveform at its terminals. since the line voltage and the motor terminal voltage is not in phase nor at the same frequency, you have a big issue.

Most VFDs deal with this by having what is usually called a flying start feature where the drive measures the motor frequency and starts the VFD frequency in synch.

You often see this problem on fans where wind might be blowing through the fan causing it to go backwards. this will often trip a breaker on an across the line starter. its why you often see some kind of brake on these type of fans.
 
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When the power is removed from a motor the motor acts like a generator. Lets talk about motor contribution when it comes to calculating the fault current when doing a study. What this means is that not only the available fault from the secondary side if a transformer is calculated you also must consider the motors that were being energized and what they would contribute into a fault if a bolted fault were to occur. When a bolted fault occurs the voltage drops in the system and the motors release energy back into the distribution system. This example was simply made to illustrate that motor do act as generators.
I other words a motor will generate power when voltage is taken away.
They are doing the same thing after a stop bottom is activated and power is removed from the motor. Now consider that individual motor now acting as a generator and all on a sudden you push the start button is the voltage being generated buy the motor in phase or out of phase with the supply source? If out of phase there's going to be opposing voltages that create a high enough current spike that is often is substantially greater than inrush of the motor that can be high enough to trip a breakers.

If the load has a lot of Wr2 which causes is to coast for a long tome before stopping or going slow enough to be restarts you may have to install a time delay for restarting to allow the motor to slow down enough before it can be re-energize.

And a last suggestion. Nothing was mentioned that I saw about where the breakers magnetic adjustment was set. Most have setting from 5 to 10X. They are shipped from the factory set a 5, the lowest setting. I can't count the times where a person has called me because the darned motor circuit protector of even a thermal magnetic breaker with an adjustable magnetic trip setting has been tripping instantaneously. Then I point out that they simply failed to set the adjustable setting.
 
Hmmm... some misconceptions here.

First, the assumption that this is an issue of regeneration. An AC squirrel cage motor will NOT keep generating when coasting. In order to generate, there must be excitation of the magnetic field at a frequency lower than the rotational speed. There is no magnetic field once the power is removed; the field collapses in about 20 - 50 ms (milliseconds), after which the rotor is just a rotating mass.

Caveats to that rule:
  1. Power factor correction capacitors can sometimes continue supplying excitation current for a few seconds after the power is disconnected. If so, simply install a capacitor isolation contactor between the caps and the motor connection.
  2. Some motors, with highly permeable laminations, can retain a small amount of residual magnetism and provide excitation for a second or two longer. IF the load inertia carries the motor speed over unity during that time, it can regenerate. This can sometimes happen on long conveyors because when torque is removed, the belt acts like a rubber band and can drive the motor faster for a brief moment. Installing a coast-down lock out timer will prevent restarting until that magnetic field is gone.
  3. Long motor leads can sometimes have a capacitive coupling effect that works similarly to PFC caps for a short time. The coast-down lockout timer can solve this as well.

Next: the idea that the breaker is "too small". IF this is a standard FVNR starter in an MCC, that breaker is likely going to be a Magnetic Only circuit breaker (a.k.a. MCP) and in that case, there is NO thermal trip element in it. So if the breaker is tripping, it is tripping on the instantaneous magnetic trips, typically set at 10X motor FLA (roughly 1500A in this case). If that is the case, you have some other serious problem. This would NOT be related to starting current, which by definition will only get to 600% FLA, maybe 700% worst case.

Possible caveats to that rule:
  1. If it is a newer high efficiency motor, some of them have very high inrush (instantaneous magnetization) current levels, in the order of 13 to 20 x FLA! If so, only a breaker with much higher magnetic trip settings will solve the problem.
  2. Your breaker instantaneous trip settings are too low. Many people read 430.52 and set the breakers accordingly (700%? I don't have my NEC here), then fail to read the exceptions that allow higher settings IF the breaker fails to hold in. MOST installations will follow that exception rather than the basic rule.

Other more remote possible issues that might be causing the breaker to trip:
  • The contactor coil circuit is allowing the contactor to "bounce" when energized, so it does run in to the regeneration issue because the contactor is closing, re-opening and re-closing in fractions of a second.
  • You have a bad connection in the motor pecker head and when the motor has already been running for a while (hot), the connection is too marginal and fails to ground on re-start.
  • Similar issue with the motor leads; one has a pin hole in the insulation and when warm, finds an easier path to ground on a re-start.
 
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Just a thought. When the motor first starts conditions are such that there is maximum difference in speed between the stator rotating field and rotor windings therefore field strength establishes relatively quickly minimizing duration of peak current. Second start with motor coasting down significantly less difference between speed of field in stator and speed of rotor windings therefore field strength in rotor establishes just a little slower and peak current lasts just a little longer or reaches a slightly higher value. Add settings are borderline breaker trips on second start during coast downs.
 
JRAEF

I went out to the field and found out a little more information on this situation. The 125hp motor itself is a NEMA Design B motor with an efficency of 95.4%. Based on what I have read in the NEC, it appears that this motor may be one of the high efficency motors that you were referring to. The nampeplate FLA for this motor is 140A.

The instantaneous circuit breaker is a Cutler Hammer HMCP 250A breaker. The instantaneous seeting that the breaker is currently set for is "E" which corrosponds to 1875A. This 1875A setting is 13.4 times the FLA of this motor. The NeC table 430.52 says that the max setting for an instantaneous breaker is 800% FLA however like you said there is an exception that allows you to go as high as 1700% for Design B motors. After looking at this I am going to raise the instantaneous setting on the breaker up in increments until I reach the 1700% and see if I can overcome this situation. 1700% of FLA for this motor is 2380A, so with the two highest settings on the breaker being 2500A and 2340A, I am going to raise the setting up in increments until I reach 2340A to see if this will solve the problem.

I follow what you are saying about the motor needing excitation in order to act as a regenerative soure. I am interested in learning more about that if you could point me in the direction of some information.

Also the one thing that has me puzzled is why the motor will start fine from a standstill, but will trip when you start it while it is still spinning? Is it because like others have mentioned the motor and source would now be out of phase causing an additional electrical resistance that takes more current to overcome. Or is this because the spinning motor has an already existant torque that will take more current to overcome? I am interested in learning more about these situations as well.

I will let you know how I make out with changing the breaker settings.
 
mull982,
I'm glad you picked up upon the magnetic pickup adjustment. At lease it was set and not left in the as shipped setting.
When assigned as a breaker appication engineer for 5 years I had made those how asked about this setting awar of the 17X rule. Many may not be awar of the NEC a number of years ago for the 13X maximum. When energy efficient motors started to be in the 80s you couldn'y keep an MCP closed to save your life when set at a max of 13X. Finally the NEC adopted 13X recommended, if the MCP trips you are allowed up to 17X.
But I have always recommended personally that the MCP be set as low as can be without nuisance tripping. Originally the intent of the MCP was to respond to preventing fires due to mmotor failures. If set properly the MCP will trip in response to a winding going to ground when a motor is sarting to fail. If the motor is taken off line soon enough this can reduce motor damage such that it is possible for it to be repaired.

Also, my intent was never to imply that the motor "continues" to generate current as it's coasting, However, when voltage is removed from the motor the current source is removed and the magnetic field in the motor collapses instantaneously. It's that collapse, the movement of the magnetic field, that induces that voltage. After the field collapses then the motor ceases to generate and significant voltage.
I do know there is such a thing as a zero speed switch (sensor), that is wired into the motor circuit which detects if a motor is turning. I used one with three of AO Smith's largest presses when I provided them with (3) 800HP, 2300v, auto transformer, reversing starters. I used it to prevent the operator from reversing a machine until the motor was going slow enough to be reversed because of a very large flywheel.
Being 10 years ago as I can recall that relay was capable of detecting the small amount of current being generated by the motor when it was coasting. That was the first time that I heard of it.
 
Too many issues to address in my limited time, but you are on the right track, as templdl has confirmed.
Also the one thing that has me puzzled is why the motor will start fine from a standstill, but will trip when you start it while it is still spinning? Is it because like others have mentioned the motor and source would now be out of phase causing an additional electrical resistance that takes more current to overcome. Or is this because the spinning motor has an already existant torque that will take more current to overcome? I am interested in learning more about these situations as well.
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Aside from possibly some residual magnetism as mentioned above, another likely reason:
Above 10HP, motor winding resistance has what is called a Negative Coefficient of Temperature meaning that at the temperature goes up, the resistance goes down. So when you start the motor after a rest, winding resistance (the only thing in the circuit at the instant of energization) is higher and prevents the breaker from tripping. On these re-starts immediately after running, the motor is hot so the resistance in the windings is lower. Less resistance, more magnetization current. Your breaker setting was likely right on the ragged edge of being able to hold in, so that difference in winding resistance had enough of an effect to allow tripping.

"Existant torque" as you put it (actually, that would be inertia) would only be a factor in Starting current, not instantaneous current and only if the motor were spinning the opposite direction. It would not be a factor in this case.

I have some resource links somewhere from which you can glean some of this information, but I need to search for them on another PC so check back next week. In the mean time if you want to learn about "regeneration", just use a search engine to look for information on using AC motors as generators in things like wind turbines. That will provide you with the basics. Your specific situation is a little different, but the principals are the same.

Re: "Sensorless" Zero Speed Switches that connect to windings: Most of them impose a low voltage DC signal onto one set of windings after the motor starter has opened and measure the back EMF as the rotor passes through the stationary magnetic field it creates. Some hand-held versions (used to pre-determine rotation direction) can even do that with as little as a 9V battery. Others apply lower voltage current limited AC and monitor the current with a microprocessor, counting blips as the rotor bars pass through the weak magnetic field. It would not be reliable to try to use residual magnetism because that is too variable and likely to not exist in any measurable amount on most AC motors.
 
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Jraef said:
... Above 10HP, motor winding resistance has what is called a Negative Coefficient of Temperature meaning that at the temperature goes up, the resistance goes down. ...
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This is interesting. I had not heard of this before. Perhaps you have some references I could look up.

carl
 
Megger readings on inductive equipment should be temperature corrected to 40C, this is vital to use meggers readings for trend analysis, which is really the whole point, predictive maintenance.


Test Temperature to 40? C
Temperature Multiplier
? C ? F Apparatus Containing Apparatus Containing
Immersed Oil Insulation Solid Insulation
-10 14 0.03 0.10
-5 23 0.04 0.13
0 32 0.06 0.16
5 41 0.09 0.20
10 50 0.13 0.25
15 59 0.18 0.31
20 68 0.25 0.40
25 77 0.35 0.50
30 86 0.50 0.63
35 95 0.71 0.79
40 104 1.00 1.00
45 113 1.41 1.26
50 122 2.00 1.59
55 131 2.83 2.00
60 140 4.00 2.52
65 149 5.66 3.17
70 158 8.00 4.00
75 167 11.31 5.04
80 176 16.00 6.35
85 185 22.63 8.00
90 194 32.00 10.08
95 203 45.25 12.70
100 212 64.00 16.00
105 221 90.51 20.16
110 230 128.00 25.40
 
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coulter said:
This is interesting. I had not heard of this before. Perhaps you have some references I could look up.

carl
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This is something I learned in the lab when working on a design for DC Injection Brakes and Motor Winding Heaters, but later I found it mentioned in the Cowern Papers (available from Baldor). Here is a link to a website that alludes to it, in the appendix at the end.

http://www.motorsanddrives.com/cowern/motorterms4.html

And here;
http://findarticles.com/p/articles/mi_qa3726/is_200412/ai_n9471953/pg_1

Both of these indicate a Positive Coefficient of Resistance, but 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. I can't explain why, that wasn't what we were tasked with, but it was repeatable.
 
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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.
 
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