Three phase problem

[Harry_Worth]

A friend asked me to have a look at a piece of equipment he had recently had installed. It's three phase with an internal transformer (415v to 24v) that provides 24v for the hand switch to operate a relay to switch on the motor. It doesn't work. I measure 8v on the relay rather than 24v. I checked between earth and phase and read circa 150v, I also measure 150V across phases. The motor doesn't run if I manually operate the relay. The three phase is produced via an Allen Bradley device from a single phase supply. The Allen Bradley device is displaying 330 which I take to be a voltage value.The three phase supply is also used for a milling machine that's been working fine for several years and continues to do so. Any thoughts on this. The plate on the milling machine suggests it needs 415v 3 phase. The wiring is dreadful with an isolator being used as a connector block with two outputs one on the switched side and the other confusingly, on the unswitched side so even in off it's on but feeding another isolator which in turn feeds the none functioning machine. The cabling from the Allen Bradley to this isolator is some old twin and earth with little of the outer sleeve. Why do I read 150V both across phases and to earth??

 

[TJ Anderson]

Wiring fault. It is floating. Prove supply upto device. Isolate and dead test wiring and terminations to locate the break.

 

[Darkwood]

What kind of meter are you using?
Like TJ has asked, is the 3ph supply floating?
If you drop off the TX outputs what voltage do you get with nothing connected?
Is the SELV strapped down to earth on one side?

 

[Harry_Worth]

Hi Darkwood, many thanks, good question. Meter's a cheap old DVM ( I was visiting for free tea). Haven't been back to see if 3ph is floating although wouldn't the phase to phase voltage still read correctly? Voltage didn't change on no load. What would be SELV in this other than the transformer for the relay?

 

[Darkwood]

Can you draw out a wiring detail so we are all singing on the same hymn_sheet, the way you describe it, it seems --- about face way of doing things, a 1-3PH VSD using the output control voltage and terminal inputs would have been a simpler cheaper way of doing things but it sounds like you have a upfront 1-3phase power converter that then get reduced again for control purposes or am I reading this wrong... remember the voltage output of a motor inverter is subject to the speed setting of the VSD so at half speed the voltage will be approx half. Also to note is the 3ph is generated via pulse width modulation and most digital meters will struggle to read the output voltage correctly.

 

[Harry_Worth]

Hi Darkwood, really appreciate your help. The PWM output throwing my meter reading makes a lot of sense as the other piece of kit hanging off this 1-3ph device works OK. I was beginning to doubt my meter. I've possibly confused things mentioning the transformer. Will try and draw it all out.

 

[Rigpig]

If you are going to be doing more of this type of work it would be worthwhile investing in a meter with a lo-pass filter setting. Saves this type of problem.