One of the morriss Crane in a factory was retrofitted last year by a contractor. But since then it is been used very rarely.

Last month this crane failed four times in the span of 15 Days.

All the four times problem was in its Incoming section of rectifier and precharge circuit.

Inverter section never went faulty. All the failures were only in Long Travel drive. L.T drive is used to drive four motors parallel.

Replacing the defective cards Drive was repaired every time. But reason for the failure was untraceable. The input contactor found short few times. And was replaced.

The fourth failure was with Damaged Precharge resistor as well as the welded input Contactor.

Interaction with customer has enabled us to think more, following points were enlightened.

• Last time input contactor of L.T drive founds chattering during the operation and was been replaced .
• All the drives were used to trip on under voltage fault.
• The current collector leads were found damaged last time ,two out of eight were wearied off, and never been aligned properly since beginning. They had caused direct short circuit many times.
• During one of the operation the swing drive had misbehaved and people escaped narrowly.
• The crane is operated on radio remote control, which is locally made. Without any CE or UL ratings. Operation frequency for it is 440MhZ.

We strongly suspect on the operation and installation of this radio receiver. This is mounted just near to L.T panel. And have used many interlock for operation. Mains contactor was also enabled through Radio remote.



Hence we suggest to by pass this radio remote temporarily. And simultaneously replaced the Breaking Module.

Since then the drive is working very fine. Trials were taken continuously for 1 week And drive never tripped with Under voltage fault.

As it was hard to believe for people that radio remote can produce this type of damage to drive, as they had multiple reasons in favor of it. Everyone was suspecting the replaced Breaking Module.

The breaking Module (50Kw and 40Amp max) is tested separately and found perfectly ok.

I have read somewhere that range of RFI generated by drive is around 7 MHz. And Radio remote is working on 443Mhz.Again it is difficult for me to explain how can these create damage to inverter.

Do any one have some inputs for me.

Hello VST

It is difficult to give a definite answer to your problem, however there are a number of issues that are probably relevant and may help to gain a resolution.

1. It is not normally a good idea to have a contactor on the input of a drive where the contactor can be operated to start and stop the drive. When power is removed from the drive, the DC bus discharges and the drive shuts down. When the conductor is closed, the DC bus capacitors are charged via a soft charge circuit which is the switched out connecting the DC bus direct to the output of the rectifier. If there is frequent operation of the input conductor, there is a lot of power dissipated in the soft charge resistor and it is prone to failure. The soft charge resistor is rated for very intermittent operation only, and frequent operation will exceed the rating of the resistor and it will often fail.
2. Sudden changes in voltage after the soft charge circuit is taken out of circuit, can put a high stress on the rectifiers and the capacitors effectively try to limit any sudden change in voltage. If the incoming supply is very erratic due to very bad sliding contacts on the supply rails, there will be high current transients which can damage the rectifier and associated circuits.
3. If the conductor is not controlled correctly, and is able to chatter or switch frequently high charging currents will damage the rectifier and the contactor.
4. If the connections between the drive and the motors are not done correctly, the drive will create a very high level of noise. This noise can interfere with other equipment nearby and cause all sorts of problems. In this situation, it is very possible that the output of the radio control is being interfered with by the noise from the drive. This could cause the outputs to be erratic.
5. If you are controlling the input contactor by the radio control receiver, it could very easily operate and do frequent switching and damage the drive.

Recommendations :

1. Remove the input contactor and control the drive rather than the contactor.
2. Fit line reactors at the input of the drive. This will reduce the effects of bad contacts on the input supply.
3. Ensure that the connections from the drive to the motors are run in a EMC screened cable, with the screens correctly terminated to the drive output and to the motors. NB the screen MUST be continuous and terminated either by a screened gland, or clamped to the panel with a clamp that goes right around the cable. Do not use pigtails to terminate the cable screens. Do not run an earth conductor from the motors to the supply, any earth conductor from the motor MUST go to the output of the drive.

Best regards,
Mark.

Thank you Very Much Mr. Marke,
And Thanks for your prompt support.
Can you help me by explaning something more about the installation and use of the radio remote for such appication.
This customer have already started rectifying the installation mistakes. But at the end i want to be assured that this Radio Remote will work successfully with drives. The next thing i have suggested him to relocate the receiver.And to use screen cable for signals going to VFD.
Thank you.
Regards,

Hello VST

Most VFD manufacturers include detailed installation instructions covering the installation for CE marking and EMC compliance. You need to follow these instructions accurately.
There are some basic rules that apply to all VFDs, plus additional steps for some VFDs depending on the brand.
The most important issue, is the connection between the VFD and the motor(s).
Some types need additional EMC line filters to comply, and others need output filters as well. It depends on the design of the internal circuitry of the drive.

Moving the receiver may have an effect, but it depends on what noise is getting in where. It may be simply a case of moving the antenna for the receiver only.

Good luck,
Best regards,
Mark.