potcore
Electrical
- Feb 26, 2007
- 28
Hi,
I am getting back into drive testing and am hoping to clear up a safety issue first, due to a previous accident...
Myself and a colleague were doing tests on electric drives. The drives had a three phase 415V (rms) input which was fed to a 6 pulse diode rectifier. This then produced the Drives 600V DC Bus which was switched by the inverter to supply the servo motor.
Some of our tests necessitated connection of our scope leads across this DC bus. -Or sometimes just the scope "ground" clip to the "negative" rail of this DC bus.
I was using a Fluke scope which was plugged into the mains (240V RMS). Only two wires came from this plug into the scope (so obviously there was no earth wire coming into the scope).
My colleague was using a Tektronix TDS210 scope which had a 3 pin plug for its mains power connection. My colleague was told to cut off the earth connection in the 3-pin plug, and also to plug this plug into an isolation transformer, which in turn was plugged into the 240V RMS mains electricity socket.
(The isolation transformer was inside a plastic housing some 30cm cubed -it was specifically an "isolation" transformer for 240V mains electricity).
I could not see why it was necessary to use the isolation transformer as well as cutting off the earth connection in the plug. -After all, i was not using an isolation transformer and was having no problem.
Anyway, another worker (accidentlly) swapped my colleagues supply lead (the one with the earth connection cut off) for a lead with the earth pin connected. My colleague then applied the 415V rms mains to his drive with his scope connected across the drive's DC Bus and the drive promptly exploded in my face. (no lasting damage to me but i can still see the white flash in my "mind's eye"). The thing is, he had been using the isolation transformer even though his earth lead was (unbeknown to him) connected in.
I cannot understand why it exploded since he was using the isolation transformer. Also i was not using an isolation transformer and was getting away with it. (maybe there was an isolation transformer in the "wall-wart" plug of my scope -but i doubt it because it was only small (~palm size).
Does anyone know why the drive exploded and why The isolation transformer did not prevent the explosion?
I am getting back into drive testing and am hoping to clear up a safety issue first, due to a previous accident...
Myself and a colleague were doing tests on electric drives. The drives had a three phase 415V (rms) input which was fed to a 6 pulse diode rectifier. This then produced the Drives 600V DC Bus which was switched by the inverter to supply the servo motor.
Some of our tests necessitated connection of our scope leads across this DC bus. -Or sometimes just the scope "ground" clip to the "negative" rail of this DC bus.
I was using a Fluke scope which was plugged into the mains (240V RMS). Only two wires came from this plug into the scope (so obviously there was no earth wire coming into the scope).
My colleague was using a Tektronix TDS210 scope which had a 3 pin plug for its mains power connection. My colleague was told to cut off the earth connection in the 3-pin plug, and also to plug this plug into an isolation transformer, which in turn was plugged into the 240V RMS mains electricity socket.
(The isolation transformer was inside a plastic housing some 30cm cubed -it was specifically an "isolation" transformer for 240V mains electricity).
I could not see why it was necessary to use the isolation transformer as well as cutting off the earth connection in the plug. -After all, i was not using an isolation transformer and was having no problem.
Anyway, another worker (accidentlly) swapped my colleagues supply lead (the one with the earth connection cut off) for a lead with the earth pin connected. My colleague then applied the 415V rms mains to his drive with his scope connected across the drive's DC Bus and the drive promptly exploded in my face. (no lasting damage to me but i can still see the white flash in my "mind's eye"). The thing is, he had been using the isolation transformer even though his earth lead was (unbeknown to him) connected in.
I cannot understand why it exploded since he was using the isolation transformer. Also i was not using an isolation transformer and was getting away with it. (maybe there was an isolation transformer in the "wall-wart" plug of my scope -but i doubt it because it was only small (~palm size).
Does anyone know why the drive exploded and why The isolation transformer did not prevent the explosion?