DC link voltage and VFD question 2

[NIDinc]

We manufacture a three phase induction motor, 230VAC, 250 Hz. It operates on a VFD for variable speeds. Normal operation is about 200 Hz and 200 volts. The proper ratio for the drive is about 1:1, Hz to Volts.

I have a overseas customer asking the following question:
Are the windings strong enough for the use with a 400 V frequency converter. The DC link voltage of this converter will be about 540 V.

I confess to not understanding what he means by the DC link voltage, in relation to the operation of our motor and the drive.

A rough draft of my reply is as follows:
The source electricity for your VFD is not an issue as long as the drive is supplying balanced, full sine wave AC to all three phases of the motor at said ratio of approximately one hertz to one volt. For example, at 200 Hz and no load, the motor should be drawing about 2.6 amps, with about 200 VAC.

With some of the older drives that do not provide clean, balanced three phase electricity, the amps may be noticeably higher, even though the voltage and hertz are correct. All the modern VFDs we have used and are aware of being used have worked fine. We have used VFDs with 400VAC 3/phase and 240VAC 1/phase. What matters is the output, not the input.

Is this answering his question? Any input is much appreciated (before I hit the send button on my email)

 

[jraef]

What they are referring to is that the output voltage of a VFD is not actually AC, it is a series of high speed pulses of DC that make the motor REACT to it as if it is AC by virtue of the motor coils being an inductor. So what a VFD does is to rectify your incoming AC to DC, then fire high speed transistors in what’s called a PWM (Pulse Width Modulation) pattern that emulates AC of varying frequency and voltage. The frequency is determined by how often the strings of pulses change from positive to negative, and the RMS (average) voltage is controlled by altering the width of the pulses and the width of the gaps between them. But regardless of the outcome of the RMS voltage, ALL of the pulses are of the same magnitude; essentially the peak voltage of the incoming sine wave, which is the RMS x 1.41. So with a 240VAC input the DC bus ends up at around 338VDC. The insulation used in your motor must be capable of handling that but in most cases, it is.

If they use a 400VAC input to the VFD, you can program the VFD to give a 200VAC RMS output to run your motor, that’s not a problem. BUT, the pulses that make up that PWM output will all be at approx. 546VDC, not 338VDC, and they are asking you if the insulation that you are using can handle that additional voltage stress. In addition, VFDs can cause an additional amount of voltage stress in motor windings due to what are called “standing waves” or “reflected wave” phenomena in which spikes occur on the circuit that can sometimes exceed 2-3x the line voltage under the worst case scenarios. So when making motors for use with inverters, it’s now common practice to use what’s genetically referred to a “inverter spike resistant magnet wire”. Some of those are rated for up to 2000V. If that is what you are using, even though that would be over-kill for a 230V application, you should be fine.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[NIDinc]

Very helpful. Thank you. Yes, we use a inverter duty, spike resistant magnet wire. So that makes sense. But we used what I think is one of these older pulse generator VFDs in the past and found that. although the frequency and voltage were correct, the motor drew more amps than on regular VFDs. Does that sound right?

 

[itsmoked]

You say the "motors drew more amps". You generally cannot use standard meters to measure the output of a VFD because the output is a complex waveform made up of many kilohertz pulses. All meter readings on the motor side are suspect.

A VFD is not 100% efficient so running a motor across the line will typically be more efficient than adding a VFD into the mix. Measuring the current into the VFD will result in a slightly higher draw than if the motor was directly driven across-the-(power)line due to the small loss of efficiency.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Not sure what you meant by "older pulse generator VFDs". The only viable "older" VFD technology were what are called "six step" inverters and "current source" inverters, neither of which have been used in Low Voltage (600V and under) for 30+ years now. Older CSI drives had issues with Power Factor, so they did appear to draw more current and some older six-step voltage source inverters were notoriously inefficient. But PWM drives are all around 95-97% efficient, have been for decades.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden