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Crane Resistor Braking Gets Very Hot When Connected To SK Commander skc3400400

01/12/2018 6:54 AM

Hello friends

I have an overhead crane having problem in braking resistor for long travel. I tried changing the braking resistor of 250 ohm 520 Watts rating to drive sk commander skc3400400, but as soon as I connect the resistor to drive its getting heated up to very high temperature. I tried changing the pram ates but all in vain. If any body could help me solve this problem I would be very grateful ful

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Pathfinder Tags: Braking resistor heating up
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#1

Re: Crane resistor braking gets very hot when connected to sk commander skc3400400

01/12/2018 7:10 AM

Why was the change perceived as being needed?

What was the outcome of the telephone calls made to the crane manufacturer about this problem?

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#4
In reply to #1

Re: Crane resistor braking gets very hot when connected to sk commander skc3400400

01/12/2018 1:29 PM

Crane manufacturer recommended to change the braking resistor that I did but still its the same.

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#13
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Re: Crane resistor braking gets very hot when connected to sk commander skc3400400

01/15/2018 7:28 AM

<...telephone calls...>

OK. Keep going. Make more calls.

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#14
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Re: Crane resistor braking gets very hot when connected to sk commander skc3400400

01/15/2018 12:47 PM

In the original post, you say"...as soon as I connect the resistor to drive its getting heated up to very high temperature."

The braking resistor should NOT be connected (by the braking switch/relay/device) to anything until the motor stops after running. If the resistor heats at all when first connected, then (A) The resistor is NOT destroyed, broken, or disconnected, although it could have changed in value, and (B) That braking switch/relay/device and/or its control system is clearly defective.

Since you indicate that the resistor connects directly to the drive, then that braking switch/relay/device and its control system are clearly part of the drive assembly (assuming the resistor is being connected correctly). If you are unable to repair that part of the drive, then you must either send the drive back for repair or replace the drive.

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#2

Re: Crane Resistor Braking Gets Very Hot When Connected To SK Commander skc3400400

01/12/2018 7:49 AM

The braking resistor is switched-in (with ingoing power from mains to motor off) to brake motor.

It seems this switch is permanently on, so the resistor is connected as soon as you power the motor.

Check first that whatever contactors or solid-state switches, which connect the resistor, are not "welded on" or shorted. A resistance test with motor and drive isolated from supply might indicate 250 ohm - you will have to disconnect the motor to take its winding resistance out of circuit.

If that is OK, Second the command signal to the resistor switching might be on all the time due to a hardware fault - check that any coils/Triac gate drives or SSR (solid state relay) commands to the resistor drive switching are not on continuously.

Third - the "brain" of the system might have failed or it thinks the motor is still turning, even though it is de-energised.

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#5
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Re: Crane Resistor Braking Gets Very Hot When Connected To SK Commander skc3400400

01/12/2018 1:31 PM

Thank you..

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#8
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Re: Crane Resistor Braking Gets Very Hot When Connected To SK Commander skc3400400

01/12/2018 10:49 PM

Right!. I once repaired a large VTL (vertical turret lathe), where the silver contact on the braking relay had been completely worn off, leaving bare copper, which then welded itself in the closed position. The next time power was applied, the (several kilowatt) braking resistor was connected directly to the power source, and was glowing yellow-orange by the time they got it shut off.

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#10
In reply to #2

Re: Crane Resistor Braking Gets Very Hot When Connected To SK Commander skc3400400

01/13/2018 3:47 PM

I did isolate drive and motor but still the same.. I think drive IGBT is faulty.. Need to replace the drive?

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#11
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Re: Crane Resistor Braking Gets Very Hot When Connected To SK Commander skc3400400

01/13/2018 9:21 PM

Follow the wires from the braking resistor. One side if it is likely common or ground; the other side must go to some device that is intended to connect the braking resistor to the motor ONLY when NO mains power is reaching the motor. That device could be a relay/contactor or any of several solid state devices (Transistor, SCR, IGBT, Solid State Relay, etc). In any case, that device appears to be shorted, or continually in the ON condition.

The problem could be in the contacting device itself (like the welded relay contacts I mentioned in a previous post), or in the control circuitry that turns that device on.

If it is a three-phase motor, there should be three pairs of contacts and three control circuits. Any one of the three pairs being shorted would cause a braking resistor to overheat quickly.

A faulty drive circuit would NOT by itself cause the braking resistor to heat, because that braking resistor should NOT be connected to the motor until AFTER line power has been removed from the motor.

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#15
In reply to #10

Re: Crane Resistor Braking Gets Very Hot When Connected To SK Commander skc3400400

01/15/2018 5:24 PM

Manual indicates drive gives various error codes. Are any present?

Apart from the braking resistor problem, does the drive work in any way?

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#3

Re: Crane Resistor Braking Gets Very Hot When Connected To SK Commander skc3400400

01/12/2018 11:10 AM

You obviously don't know what the problem is.

Call someone who can find the problem and fix it, like the crane manufacturer.

Yes, it will cost money. You can spend iy now or later after you get someone hurt or killed.

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#6

Re: Crane Resistor Braking Gets Very Hot When Connected To SK Commander skc3400400

01/12/2018 2:13 PM

Seems like troubleshooting as usual. Start by replacing parts that aren't broken, continue until something acts differently, pat yourself on the back for using the process of elimination. Some of us call this "TV troubleshooting".

So, this braking resistor is intended to burn off energy from the inertia in the motor. That is, to slow the motor down while it is dumping inertia and the motor is acting like a generator. The rest of the time, the energy to that resistor should be zero!

Relays and contactors use motion of parts to make them work. That makes them mechanical. Mechanical stuff fails more often than electrical stuff, under normal use.

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#7

Re: Crane Resistor Braking Gets Very Hot When Connected To SK Commander skc3400400

01/12/2018 3:00 PM

Brake resistors are controlled by a "braking transistor" which is in turn triggered by the microprocessor in the VFD based on a DC bus voltage threshold. If your DC bus voltage is excessively high, the braking transistor could be permanently trying to dissipate this excess DC bus energy through the resistor. Your DC bus voltage can become excessively high due to a higher than normal AC line voltage, or also due to resonance with other capacitors on the AC line. Has anything at your installation recently changed?

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#9

Re: Crane Resistor Braking Gets Very Hot When Connected To SK Commander skc3400400

01/13/2018 12:05 AM

Recently same nature problem we faced with Bosch Rexroth Indradrive. Finally we found that motor winding was showing some resistance value with its body. We replaced motor & problem of resistor heating solved.

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#12

Re: Crane Resistor Braking Gets Very Hot When Connected To SK Commander skc3400400

01/14/2018 8:54 AM

It would be a good idea to measure how hot this resistor is getting and comparing the measurement temperature to the resistor spec sheet. 520 Watts is not a trivial amount of electrical energy to dissipate. You might find the resistor has now failed due to overheating from being constantly ON. You might even find out your idea of a very high temperature is actually well within the acceptable temperature range.

You might even find in that spec sheet that the resistor requires a missing heat sink or forced air cooling from a failing fan to dissipate this power.

I'm just trying to "touch all of the bases" here. I like the other suggestions in this thread much better than these ideas.

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#16

Re: Crane Resistor Braking Gets Very Hot When Connected To SK Commander skc3400400

01/31/2018 12:06 PM

The Dynamic braking resistors for a VFD are supposed to get hot. They are usually low Ohms and High watts. This is so that the regenerative energy is dissipated through the resistors and not allowed to get back to the IGBT's.(this will cook them)

If your resistors are getting hot as soon as power is turned on they are more than likely connected incorrectly.

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#17
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Re: Crane Resistor Braking Gets Very Hot When Connected To SK Commander skc3400400

01/31/2018 12:28 PM

As I understand it, the problem developed in an existing/previously operating crane, so it is not likely to be an improper connection. It is conceivable that some cable could have worn through the insulation, but I believe the problem is much more likely to be in the switching device that should disconnect the resistor once the unit stops moving, but has shorted, leaving the resistor connected all the time..

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