Measuring Reflected Wave Voltage at Motor Terminals
Measuring Reflected Wave Voltage at Motor Terminals
(OP)
Greetings,
I am trying to satisfy myself that a VFD/motor combo isn't going to suffer damages induced by CIV. Some notes on the application:
*2HP 460 VFD with 4kHz carrier
*2HP 460V motor
*VFD rated cable @ 250~500' length
*Output reactor installed at the drive
I'm using a Fluke 196C scope-meter in conjunction with a 7000V differential probe. My question is this:
Should I be measuring Vp-p between phases, or Vp to ground? Currently when measuring Vp-p between phases, I'm getting readings in the neighborhood of 2100 volts. I was always of the understanding that the main area of concern with CIV was between phases, due to the possible break-down of phase paper installed between phases. NEMA MG-1 requires that a motor withstand 1200V I think, and our particular motor is rated with a CIV of 1600V. 1600V what? Between phases or from phase to ground?
I am trying to satisfy myself that a VFD/motor combo isn't going to suffer damages induced by CIV. Some notes on the application:
*2HP 460 VFD with 4kHz carrier
*2HP 460V motor
*VFD rated cable @ 250~500' length
*Output reactor installed at the drive
I'm using a Fluke 196C scope-meter in conjunction with a 7000V differential probe. My question is this:
Should I be measuring Vp-p between phases, or Vp to ground? Currently when measuring Vp-p between phases, I'm getting readings in the neighborhood of 2100 volts. I was always of the understanding that the main area of concern with CIV was between phases, due to the possible break-down of phase paper installed between phases. NEMA MG-1 requires that a motor withstand 1200V I think, and our particular motor is rated with a CIV of 1600V. 1600V what? Between phases or from phase to ground?





RE: Measuring Reflected Wave Voltage at Motor Terminals
There will be some actual motor experts along shortly, I suspect.
h
RE: Measuring Reflected Wave Voltage at Motor Terminals
At 250-500feet at this low hp, you will definitely need at least a dv/dt filter and possibly a sine filter to properly protect an MG1-P31 rated motor.
RE: Measuring Reflected Wave Voltage at Motor Terminals
I know the biggest concern as was mentioned above it the turn insulation which is sensitive to rate-of-rise of voltage (not just the peak).
My guess is that votage to ground would be of interest although I'm not positive. I'm pretty sure that is the way that a surge test is conducted. The high frequency components are capacitively coupled to ground.
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RE: Measuring Reflected Wave Voltage at Motor Terminals
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Measuring Reflected Wave Voltage at Motor Terminals
If you're seeing the voltage ringing up to 2100V then you've got a problem. I would be really surprized if the motor survives much longer. Install a good dv/dt filter with the goal to get rid of most if not all of the voltage ringing.
RE: Measuring Reflected Wave Voltage at Motor Terminals
RE: Measuring Reflected Wave Voltage at Motor Terminals
Given a 2hp motor, a PWM output, a 4khz carrier, an IGBT drive output section, and 250-500 foot motor leads, you are in trouble, period.
In my judgement, given these conditions, motor leads up to 60 feet are ok without any extra. Beyond that, figure on either reactors, dv/dt filters, or sine filters in increasing order of severity.
RE: Measuring Reflected Wave Voltage at Motor Terminals
When I measure the combo that's been working from phase to phase, I'm seeing surge voltages in the neighborhood of 2kV, which is 200 over the published limit. This makes me think I'm measuring it wrong, considering that the unit's been in service for 3+ years and still operating normally. So I guess the root of my question is: How do I measure RWV properly? Phase to Phase? Or Phase to Ground?
Does a CIV rating of 1600 mean between phases? or from phase to ground? or both? If between phases is the case, then I'm stuck in some parallel dimension where exceeding the limit has been fine for 3+ years.
RE: Measuring Reflected Wave Voltage at Motor Terminals
Unfortunately, it's not always that simple and phase-to-ground voltages can be destructive too. I generally think of phase-to-ground voltages being the source of common mode noise which, while not a motor insulation threat, is still a messy problem in buildings with poor or high impedance grounding systems.
As for not having any problems with the long motor lead lengths, you must be buying very good grade motors. Otherwise, the basic ingredients are all there for premature motor insulation failures.
RE: Measuring Reflected Wave Voltage at Motor Terminals
So, I can think of two explanations:
1. Your mains voltage is on, or above, upper limit. That could give you the readings mentioned.
2. Either your differential amplifier or probes needs to be HF compensated. A probe that peaks when measuring a square wave will definitely show too high a value when measuring high speed signals (less than microseconds, I presume).
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Measuring Reflected Wave Voltage at Motor Terminals
RE: Measuring Reflected Wave Voltage at Motor Terminals
Drivesrock.