Regenerative Drives - Effect of Power Factor

How are you measuring the mechanical & electrical powers to be able to come up with your figure for efficiency?

Hi Adrian

Thanks for your reply

We have measured the output from the generator with an Oscilloscope which provides the power into the VSD. The power exported to the grid is measured on a meter installed in the control cabinet, next to the VSD. The difference between the two readings is about 2kW. Sadly we didn't put the scope on the output terminals from the VSD so we only have the meter values, although there is no reason to suggest they are wrong. 

Separately to this, we have connected the generator up to a resistive load bank to confirm the power measurement; this measurement agrees with the values measured on the scope with the VSD in circuit.

What is strange is that this 2kW can't be felt as heat in the cabinet, or at least it is not obvious.   

That last part about no heat is key - ‘follow the smoke’  - If there isn't any it probably is being more efficient than you think - 2kW is a fan heater or a kettle full - hard to miss in a closed box ?

 Depending on the VSD design, the internal switching waveforms are far from sinusoidal, and it may be that by the time things are measured on the outside, some assumptions about current and voltage waveforms being in step and pure sines is confusing the calculation. Not all meters, even ‘proper’ export electricity meters read the VA of non-sine waveforms very well, and a 10% error is quite credible. (some  light reading on how your leccy bill can be cocked up by electronic loads)

ABB and Siemens both have capable technical help, it may be sensible to get their opinions for this rather unusual case  before embarking on what may be a bit of a wild goose chase.

Mike

To the original question, the ‘unity power factor’ is a stage that is between the mains and the reservoir caps on the DC bus, and is controlled to draw more or less current in time with the rising and falling of the mains voltage, thus spoofing a resistive load.

I'm not sure   how much that helps you when used in reverse as a re-generator.

Mike.

Thanks for your reply Mike,

I suppose the obvious answer to your point would be to stick the scope on the meter terminals and get a true reading of the power. The problem for us is that the income from the electricity export is based on this meter reading.  Just for information the meter is a Schneider PowerLogic PM3500.  

I have contacted ABB and Siemens already; if I get an answer from them I will be sure to share but it is nice to have honest feedback from people not trying to sell me a product. Unfortunately most literature and data relating to drives relates to motors and not generators. 

Does the scope do both current and voltage and do instantaneous multiplication to get true VA ? The meter may not, and the scope may not either.

Mike.

I think Mike is on the right lines - the regen output from the VSD is likely to be far from sinusoidal. Unless the metering equipment is capable of true rms reading then its unlikely to give the correct answer.

So are you saying you are comparing the output of the generator (what kind of machine?) with the output of the VSD? Depending on the kind of motor/generator you are using, where is the excitation coming from? If its an AC machine, the VSD still has excite the motor to make it generate. This may be confusing you measurements of the real generator output when connected to the VSD?

Does the VSD itself tell you the regen output? Some do, others are not so helpful.

I'm having a similar problem with the design of an engine test dyno. The brief wants to use an AC induction motor as the dyno and dump the energy recovered as the dyno brakes the engine back onto the mains. The doing it is easy, its getting the instrumentation right that's the problem (this is for an educational environment so the numbers need to stack up in an energy balance equation…. fuel+air > mechanical + heat + noise > regen electrical etc)

Hmm I'm also not sure what the efficiency or power factor in this regeneration mode should be. When it came out a few years back so called  “Efficient Infeed Technology” was really sold as a bit of a energy saving by not wasting motor braking energy in banks of resistors and especially for things like lift motors when going down to get back some of the work put in when going up. 

The main mode of operation was still expected to be motoring, and the main effort is in making that direction the most efficient.  The generator mode is more of a nice afterthought, though of course in your question the main one…

Mike.