VFD get clear picture of output on a scope ??

Am teaching a VFD class at North Seattle Comm College for VFD and motor used in electric vehicles. This class wherever possible uses actual equipment in operation.

VFDs are 5 HP or less and made by either Hitachi or AB.

To show the actual operation of VFD we do two things.
1. Measure the current using a shunt resistor connected in series with a phase. This gives us an almost clean sine wave with some sawtooth in it. We can easily show a difference between varying frequencies.
2. Measure line to neutral on motor. We are unable to get a clear picture of PWM. At best using a digital scope we can get a trace much like that of six step but cannot get down to individual pulses to show width and more pulses with frequency change. Scopes are good units ie Hewlitt Packard or Techtronix. What I want to do is in real operation show the output as it is shown in manuals and ideally show what happens when you double frequency.

Am I just trying to do too much?

Suggestions?

Dan Bentler

I do the same thing in the seminars that I teach, Leitmotif, and use a Fluke Scopemeter to show the pulses. But, we always look at them from phase to phase, not phase to neutral. I'm not sure what you would see to neutral.

In fact, where would you get a neutral? The drive outptu doesn't give you one.

Try phase to phase and see what you get.

DickDV said:

I do the same thing in the seminars that I teach, Leitmotif, and use a Fluke Scopemeter to show the pulses. But, we always look at them from phase to phase, not phase to neutral. I'm not sure what you would see to neutral.

In fact, where would you get a neutral? The drive outptu doesn't give you one.

Try phase to phase and see what you get.

Click to expand...

Dick
I should have been more accurate and said the star point of the motor.

Hmmm this may be the source of my problems. Harmonics add in the neutral. Even though the star point is not connected and hanging in free air am I getting all the harmnics off three phases in the motor?
Dan Bentler

I would measure Phase to Ground. I suspect that the drive dc bus is referenced + and - with ground being the midpoint.

Nick