Electrical discharge from Inveter

[TailSpin]

Hi All

Is there any regulations regarding motor isolation feed from inverters. As I have had quite a bite when checking a three phase motor which was locked off being feed from an inverter. The isolator was before the inverter and was left for a good five minutes to allow for discharge from inverter unit. It seems its would be better to put the isolator after the inverter ?
Motor was checked for dead before work commenced ,
Any thoughts ?

Regards

John

 

[silva.foxx]

Definitely isolator in motor tails from inverter, fitted with an auxiliary contact of the late-make-early-break type tied in with the inverter run signal.

Discharge time can range from 5-15 minutes and beyond. As usual; consult manufacturers manuals.

 

[SirKit Breaker]

All the ones i have met, have had a contactor between the Inverter and the motor which pulls in with the run signal. so if the inverter drops off the motor cant restart unless the inverter is reset, and they also had a local isolator next to the motor.

 

[TailSpin]

Thanks for the replies
The inverters on the site I work at just have one Isolator before the inverter but no Isolator after. This can cause problems as the inverters needs time to discharge, as a college of mine found to his cost. I was still wondering what regulation would cover this , at lease some sign on the motor door ? or is it just down to experience to discharge the load to earth before work commences.

Regards

John :confused5:

 

[netblindpaul]

Well, just to throw in the proverbial spanner!
I used to work for an Inverter & Servo Drive manufacturer on service & applications.
Any job where there was an isolator in the supply lines from the drive to the motor was immediately "out of warranty"!
It was not acceptable under any means to open circuit the inverter power outputs.
You must control the whole system and make the whole system safe.

 

[Knobhead]

So how did you isolate the supply to a motor then?

All systems I’ve worked on have had the isolator close to and in line with the motor with control going back to a contactor fitted before the inverter. To say it would invalidate the warranty indicates poor design.

 

[netblindpaul]

Tony,
We required that the whole system was shut down, that was the nature of the industry or of the beast as it were.
However, there was no need for an isolator in the motor "tails" as the drives themselves were suitable isolation and were accepted as such by HSE (I was personally involved in this), and other "authorities".

 

[Knobhead]

Tony,
We required that the whole system was shut down, that was the nature of the industry or of the beast as it were.
However, there was no need for an isolator in the motor "tails" as the drives themselves were suitable isolation and were accepted as such by HSE (I was personally involved in this), and other "authorities".

I can see this being the case for an individual machine. Most plants I’ve worked on have had multiple product flow, so it would be common to isolate a section while the rest of the system carried on working. The entire plant would be fed from one MCC panel so it would not be possible isolate a particular section with a common isolator. To have a MCC with 150 drive cubicles would be quite common.

The site rules for all personnel other than electricians was quite specific “the drive and associated drives shall be isolated at the local isolator(s)”.

 

[silva.foxx]

Tony,
We required that the whole system was shut down, that was the nature of the industry or of the beast as it were.
However, there was no need for an isolator in the motor "tails" as the drives themselves were suitable isolation and were accepted as such by HSE (I was personally involved in this), and other "authorities".

Hi NBP

So... if you wanted to work on the motor, whether it be test or replace, you'd have to lock-off the power to the inverter then wait for around 15 minutes whilst the drive discharges!

Takes around 15 mins to get and down a nice coffee anyway!