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3 phase - Open Delta Circuit from electrical box

I’ve seen several instances of overvoltage alarms occur with spindle drives running around 240V. It has always been during deceleration on drives that regenerate to the line rather than a brake resistor.
 
Confused.
On any 3 phase machine why would you go one leg to neutral or ground?
Seems like just asking for trouble. Depending on configuration the measured voltage may have next to no current available so no trust here.
VFD or servo decel overvoltage problems are different.
Here the motor is a generator and without a brake resistor the bus pumps up too much as it can't access the lines since there is a brick wall in between.
VFDs take AC, make DC then the new AC.
 
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Sorry, I probably added confusion to this unnecessarily.

Confused.
On any 3 phase machine why would you go one leg to neutral or ground?
Seems like just asking for trouble. Depending on configuration the measured voltage may have next to no current available so no trust here.

Regards to one incoming phase to neutral (or ground), no, that doesn't happen. After the control transformer there are two (or more.. Haas) floating connections that read 120v between each other and neither read anything to ground (by nature of an isolation transformer). That's a no-go because there is no way to detect a fault. The solution when building a control circuit is to connect one floating end to ground so that if the other one gets shorted somewhere the fault detection device triggers. This is the common setup I've seen in CNC machines... Haas, Mitsu, Fanuc, Siemens.... hell, air compressors... HVAC controls, home brew motor starter controls... pretty standard practice to make 120v out of 240v




VFD or servo decel overvoltage problems are different.
Here the motor is a generator and without a brake resistor the bus pumps up too much as it can't access the lines since there is a brick wall in between.
VFDs take AC, make DC then the new AC.

The regen topic is something I didn't consider and I've never had an issue with it but that's great info to have going foreword.

Some VFDs don't like unbalanced line-ground setups (delta). They don't load the ground at all, maybe just use it to sense imbalance for something?? Something with MOFs and surge protection.
 
There are rare cases where some VFDs don’t like high leg delta or corner ground delta.
Corner ground i can understand shit not working especially on a 480vac system. It basically requires twice the dc voltage to ground isolation that would otherwise occur. It may work for a week or a year until dust creeps in and then the circuit board blows up.

But the 120/240-208 system with 208 on one phase and 120 on the other two, yes in theory MOV's can fail. Sometimes they dont.

Slowly at first, because usually they clamp at twice the nominal line voltage. 208 is not quite twice 138

I have typically seen mov's that clamp at 700 volts on 240 systems and 330v on 120v systems.
 








 
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