Breaker Dilema

[BobLfoot]

Last night I had to upgrade a 480 three phase breaker from 7A/58A {continuous/instant} to 15A/150A in order to get a 6.4 FLA motor to start without tripping the breaker. Two motors produced the same result. The 6.4A heaters in the Nema starter where not tripping so it must have been the instant current that tripped it. Wiring showed good when ohmed/megged and so did both motors. What could this be? I will check the starter contact points tonight. Did not think of it till the drive home. Others with ideas. Wire is 12 AWg and distance it 250 feet or so from starter to motor. Just my luck motor is on top of 100 ft tower and MCC on 25 ft level in neighboring building.

[PdL]

Don't know what clamp you have but some can measure the inrush current, have you measure what is was drawing on all three phases with the new breaker ? *edit aha you tried two motors. So probably not motor related. In that case, do you have a similar 7A breaker to test with ?

[P Daniil]

Maybe the breaker is acting faster than it should be (twice as fast?). Is its "trip class" spec right for the application?

[GerryM]

I would guess the same as P Daniil. Check with the CB manufacturer, they should have suggested CB sizes for standard Motor sizes. I know the Square-D Digest has a table of recommendations.

[BobLfoot]

This breaker and motor has been in service for past 12 months that I know of and proably for past 5 years. Trying to figure what changed and why it became a problem during this startup.

[PdL]

In that case it sounds more towards a defect breaker instead of wrong CB characteristic.

[TimWilborne]

Were there any repairs or modifications made during the shutdown?

[ECSI]

My guess is something changed with the motor load. What is the motor driving? Also, a 7A breaker on a 6.4A motor seems to me to be too low to begin with. I rarely put anything less than a 15A breaker on a small HP motor. It's only for short circuit protection anyway since the overload heaters take care of the overload conditions.

[BobLfoot]

That is why we went with the 15A/150A breaker as a next step. I am just trying to figure what might have changed. The load appears not to have changed and we saw the same behavior with no load on the motor. I would suspect wire but it checks ok. The only portion I did not check was the starter contact points.

[TimWilborne]

Are there any other motors on the supply? Trying to figure out if it is upstream or downstream

[BobLfoot]

Power Company Supplies 13.8KV power to substation. Substation converts 13.8Kv to 480V. Substation has a 1600 A 480 breaker and feeds the Motor Control Center. MCC has a 400A breaker feeding the bus to the starter buckets. The 7A or now 15A is fed from the MCC bus. After the breaker is the starter then heaters then wire to motor. Hope this explains it all.

[TimWilborne]

That takes care of the upstream. My next step would be to change the contactor and overload if you have a spare. Not very scientific but...

[BobLfoot]

I actually just called the plant and asked that they do just that, also take instant readings before and after.

[TimWilborne]

On thing just came to thought that help me once. We had a circuit that kept tripping. Sometimes it would be hours in between and sometimes seconds. We tried all the normal voltage and current test, but in the end we took an infrared thermometer and looked for the point of the most temperature change when the breaker was reset. Ended up being the hardware that attached the lug to the breaker had burnt off and it was pretty much just laying there. Just brainstorming

[P Daniil]

Chances are that the load during starting has changed. We once had a similar problem with one of our soft starters driving a 25 HP centrifugal fan which generated the suction vacuum in a paper cuttings/shredings collection duct system. It turned out that during maintainance the air filters were changed to a finer mesh and as a result during starting the vacuum took a few seconds longer to build up in the ducts causing an overload trip. After readjusting the ramp-up timing the trippings stopped and our sanity was finally recovered.

[BobLfoot]

The breaker problem existed with no load on motor. For both the old and new motors.

[P Daniil]

Can it be an installation earth or neutral return problem? That extra current has to flow from/to somewhere.

[newpageboba]

Per NEC, the Instantaneous trip of the Motor Circuit Protector (MCP) (not breaker) can not be set to more than 13 X the full load rating of the motor (13 X 6.4 = 83 Amps). New MCPs are usually set at minimum. You are supposed to turn the setting up one step at a time (with out going over 13X) and try it until the motor runs. This inrush current will be slightly higher when the load is connected. I have had problems coordinating this, so I usually set it at the highest setting with out going over 13X. If this doesn't help, then Megger to check for grounded leads or motor. Use a digital meter and compare the 3 motor phase resistances (low ohms) - should be very close to each other. Do this test with the motor leads. And last but not least...make sure the motor is wired Y (480 Volt) and not delta (277 Volt). One other thing, make sure all three phase are continuous from the MCP to the Motor. An open phase will cause high single phase current. Other checks that can be done if the motor runs for a short time is to check the current on each phase. This will tell you if there is an open phase, voltage imbalance or looped motor. You could also take voltage readings at the motor with the motor disconnected, but be very careful. Good Luck, keep us posted! Edited January 8, 2007 by newpageboba

[andybr]

The load on the motor should have little effect on the inrush current as this is primarily used to build the magnetic field in the motor. I would check whether the site voltage has changed recently. This could be a result of changes on the utilities network (ie transformer tap changes) or even changes to the site load. Andybr