Motor tripping breaker when generating.

kalabdel

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I have very little information thus far and would like some general understanding to prepare for site visit in a couple of days.



So we have a machine that lifts a container (load) up and down about feet. The weight doesn't change much.
The machine is very basic, 3HP motor, gear box, reversing motor starter and a braking resistor. There's a circuit breaker supplying the machine with 600VAC located about 20' before the machine. There's no drive and it's a simple one speed up and down operation.

On commissioning the machine the technician reported that the machine lifts up the load with no problem at all but lowering form any position will immediately trip the breaker not the overload. We will have an electrician on site when I go there in case the breaker needs to be replaced.



I would like to understand how the generated current affects the circuit. Let's assume there's no resistor at all would the breaker trip first or the overload? (assume both are sized and working properly).




Thanks
 
does the motor starters have a manual interlock to prevent both starters from being energized at the same time?

it could be that the up motor starter has a phase on the contactor welded together thus creating the problem when you try to lower the lift.

double check the wiring to the motor starters.

if everything checks ok, raise the lift and then disconnect the power to the lift contactor and then try to lower the system. If the breaker doesn't trip, you are energizing both contactors at the same time.


this is what I would check first.

james
 
if everything checks ok, raise the lift and then disconnect the power to the lift contactor and then try to lower the system.
Be careful. One or more of the wires you disconnect in order to perform this test may become live. Dont let these wires dangle freely. Connect them to some terminals so they dont cause any problems.

If the breaker doesn't trip, you are energizing both contactors at the same time.
Or there is a welded contact in the contactor that is disconnected.
 
You said braking resistor? Where do you have a braking resistor while using a contactor?

If the circuit breaker trips immediately then your not generating. As the others have said I would look at the contactor
 
You said braking resistor? Where do you have a braking resistor while using a contactor?

Thanks everyone. The above quote interests me. Where should the resistor be connected when using a contactor?

I should clarify that the motor runs to lower the load a few feet (2 or 3) then the breaker trips.


What I gather from the replies that the overload should trip in case of over current and the fact that tje brwaker is tripping indicates a direct short (very fast over current). Am I correct?
 
Some motor overloads are not instantaneous. They may let the overload happen for a few seconds. Some are, and will trip immediately. Also, the circuit breaker may be sized closer to the motor spec's than the overload.

As for a braking resistor on a 3 phase motor- that would require another contactor that is energized when the motor is off. This would short the 3 motor leads across the resistor assembly. Braking resistors only help slowing a moving motor quicker than it would stop spinning on its own, and extend the life of a physical brake.

If the hoist has a physical brake, see if it is released in both directions. It could be that only the lift contactor releases the brake and dropping is driving against the brake.
 
Some motor overloads are not instantaneous. They may let the overload happen for a few seconds. Some are, and will trip immediately. Also, the circuit breaker may be sized closer to the motor spec's than the overload.

As for a braking resistor on a 3 phase motor- that would require another contactor that is energized when the motor is off. This would short the 3 motor leads across the resistor assembly. Braking resistors only help slowing a moving motor quicker than it would stop spinning on its own, and extend the life of a physical brake.

If the hoist has a physical brake, see if it is released in both directions. It could be that only the lift contactor releases the brake and dropping is driving against the brake.

Thanks a lot. My questions go beyond troubleshooting as I'd like to understand how motors on a crane, winch or similar applications work when powered across the lone without a VFD.

Let's say we have a crane or winch lifting and lowering a load that does not change. As the load is being lowered the motor will generate excess current as it is rotating, doesn't that current have to go through a resistor to "burn it off"? If the resistor is used only after the motor is stopped where does that excess current go?
 
Kalabdel,

The resistors are only used when the contactor to the motor is off, or a VFD has shut off the output.

A load pulling on a motor would only generate when the motor wasn't powered. The load pulling should actually reduce the power consumption of the motor, as it is getting an assist.

The more I think about it I am thinking there is a brake on the hoist that is not releasing during a drop, causing the motor to fight the brake.
 
What is happening is the weight of the object being lowered is causing the motor to regen back to the supply breaker. Usually you would use a three phase resistive load bank to dissipate the regen power.
 
What is happening is the weight of the object being lowered is causing the motor to regen back to the supply breaker. Usually you would use a three phase resistive load bank to dissipate the regen power.

Thanks you. I thought I had completely lost my mind. The load is causing the motor to rotate faster that ist's magnetic field is faster than the line frequency. Did I get the theory right?

If so where would the resistive load bank go, between the motor and the overload?
And, calling that resistive load bank as I did "braking resistor" technically incorrect?

Thanks a lot.
 
Sounds like a direct short. Measure across the motor contacts, all 3. Should get 0 ohms when not moving. If one of them is different then you have a welded contact.
Overloads work by opening due to generated heat from excess current in the wires. If a mechanical brake was really good and motor is fighting it then it should trip overloads. If not then overloads are set too high. Use clamp on current meter to check current at any point on all 3 phases.
The Cont. current draw on the motor should be on the nameplate. If not you'll have to calculate it.

Only time I've seen the disconnect trip is when we had a motor poor wire connection in the peckerhead heat up, burn through the electrical tape and short out to another of the phases.
 
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