VFD & Energy Saving-3: Efficiency of a VFD

Thanks for your interest.There is no doubt that your Post is almost a complete answer.

I am going to share some of the other informations

1. We have both type of Loads, CTL and VTL.Most of the Loads operate in the range between 65 -83 % of there rated capacity. Transformers used are Converter Transformers. Efficiency approximately remains in the range between 87 % to 91 %. 91 % is the Maximum one I noticed

2.When we came to the Motors that are below 40 KW and if these motors are operated below the 35 % of there rated capacity, We have even worse cases regarding efficiency even below 80 %

One major reason why the efficiency appears to drop off so much as load goes down, is because there are fixed losses in the VFD output, what's called "Switching Losses". These are the result of the high speed switching of the transistors that make up the PWM output, the core technology behind what a VFD is. The switching losses are somewhat constant regardless of output speed because it is based on the carrier frequency of the switching. As load decreases with speed, the constant level of switching losses begins to represent more and more of the total load percentage, which ends up as a lower throughput efficiency. So it isn't that the drive wastes more internal energy as the motor slows down, it uses the same amount, but the useful energy is dropping, so the waste looms larger by comparison.

@Jef,

Statement 1 : A three phase squirrel cage motor needs both active power as reactive power to run. If supply voltage and frequency remains the same, the reactive power also remains the same, regardless the actual motor load.

Statement 2 : A frequency drive without an active front end, has mostly a simple 6 or 12 pulse passive rectifier at the input.

This means :

• no power can be send back into the grid,
• it can only take active power out of the grid
• power factor of a frequency drive is almost unity

Is it correct to decide out of statement 1 and 2 : A three phase squirrel cage motor takes the reactive power out of the frequency drive and the frequency drive takes this same amount of reactive power as active power out of the supply grid?

This means that when we run a motor with a frequency drive we now have to pay for the reactive power where in the past we took the reactive power (almost free of charge) out of our power factor capacitors.

As a tangent to the OPs thread this may have been better suited to be it's own thread, but too late now.

The flaw in your hypothesis is the "reactive power for free" part. In any circuit, you pay for WATTS, which know not from active or reactive. Watts are watts and none of them are free. In a DOL circuit with PFC capacitors, the reactive CURRENT is stored and released by the capacitors, but it all came from the same input watts. So adding caps does not change the watt consumption, but it does releive the supply system from having to supply EXTRA current to facilitate the reactive current. So that is why PFC caps do not save ENERGY (kWh), but they do allow the utility to use more of their available capacity, so they relive you of kVAR demand penalties.

With a VFD, you can think of that reactive current for the motor as being stored and released by the VFD's internal caps, so it essentially performs the same function. This of course is leaving out the entire discussion of displacement vs distortion power factor of course. I was just addressing your question as stated.

Dear Mr. Aghvel Niazi,

I tried to reply this yesterday but some internet connectivity problem was there and hence posting to-day.

You know well that any equipment at full Load or near to full load gives best efficiency. But your load is 30% only as stated by you. The VFD Manufacturer will have to give a Performance Graph at various Loads say 25%, 50%, 75% and Full Load and compare the actual performance at your site.

Secondly, pl.do conduct HORMONIC ANALYSIS as it causes some problem and energy is wasted as additional heat which can be controlled.

As indicated by other members, Power Input to be measured at the Input of VFD only and Not even in the Secondary Terminal of the transformer.

Thanks,

DHAYANANDHAN.S