VFD Output Voltage, Current, Frequency 7

[RogerMM]

I would like to know what the actual output curves look like from a VFD. I have read several comments and from what I have read there is a difference of opinion. If the PWM voltage is constant and only the frequency is varied what is going on with the volts/hertz ratio? It may be that we must include the motor inductance to be able to see what the curves the motor is seeing. I have read much about VFd's but, it is still confusing to what is actually taking place at the motor. The motor seeing constant voltage with a varying current frequency?

 

[RogerMM]

The clamp on meter is a Fluke that one of the mechanic's own. It is about a year old and has been used to check other motors on VFD's. The VFD in question was working fine as long as we had the FLA set twice as high as the motor nameplate. Internally there had to be a problem with the unit and therefore the software required higher settings.

 

[aolalde]

RogerMM

Can you provide the current and Voltage readings before and after replacing VSD's?

If possible Current and Voltage Input to the driver, current and voltage output to the motor by the driver meter and by Clamp on Fluke.

 

[buzzp]

Clamp-ons can be bad or good. You need to look at the max crest factor the meter can measure when your talking about making PWM measurements as well as determine the crest factor at the time the measurement is desired based on the drive parameters. However, the current waveform the motor sees is not quite what the voltage signal will look like. The signal is 'smoothed' out some by the inductance of the motor itself. So the current measurement is not near as difficult to measure as the voltage from one of these types of drives. Hence, you don't see the significant errors you would with a standard voltmeter. But you do see errors, no doubt about it.

PWM crest factors can be all over the map (usually high) depending on the frequency/voltage of the drive at that particular time as well as the switching frequency. Most meters on the market can only handle crest factors of 3 with some handling crest factors up to 5. Here is a link that lists some examples of crest factor-

http://www.tinaja.com/glib/muse125.pdf

Drive mfgs running amps can be inaccurate as well. I did some testing on a 5HP single phase drive using a clamp-on, scope (with series resistor), and the drives running amps. Neither the drive nor the clamp-on were that accurate compared with the scope. So I still don't depend on the drive too much but this was a small drive (probably cheaply made so they can sell the thing).

Dont forget about PAM drives (these are the ones I am used to working on - used in the oil patch alot).

 

[RogerMM]

The new drive is working and the correlation between the clamp on meter and the drive display are within tolerance. We suspect the drive was at fault.