Motor efficiency

Your motor/ VFD vendor should provide you with turndown curves for the system.. Have you looked at the documentation?

Hi,

As informed keep V/f (E/f) constant. Synchronous speed is known. Measure the actual speed. Find out the slip. Based on that differnce in speed, Torque will dider and therby the efficiey.

In the curve at Rated speed when slip is one Motor efficiency will be maximum. This can checked practically by using an voltmeter in the incoming circuit, measuring current output, and the speed.

You find out the reslus and come back.

N.Suresh

Hi suresh ,

My reply to steve must explain the situation .Now my query is after knowing the torque how do u find the Eff.Could u explain the procedure in detail with some example.

Thanks &regards

Hi,

Efficiency = Output/input

wheras Output = Input - Losses (Fixed & variable Losses)

Hence = Input - Losses (Fixed & variable Losses)/Input

Efficeincy = 1 - Losses.

Losses are No Load Losses & Load Losses (I sq. R)

Collect the Loss details fromthe name plate rating and obtain the efiicency.

N.Suresh

Hi suresh,

Thanks.At reduced speed with V/f Constant will the eff be the same as normal operation? I still don't get it.

regds

Thanks Steve.Unfortunately the installation is 5-6 yrs old and no body has any clue to all documents which must have been supplied at project time.I am quite new to this .Let me try and locate.

Thanks &regards

Nesubra,

Perhaps the question to ask is: "What is the method they used for achieving reduced flow before using a VFD?"

Example #1--building ventilation system. Old method of reducing flow was to use variable air volume dampers to throttle the flow to the desired rate while the fan ran at full speed and its rated static pressure. New method is to vary the fan speed (and thus the static pressure) according to ventilation requirements (perhaps time of day, or area usage, or temperature differential). Measure the air flow rate when the VFD is running at the desired lower speed and its input power; compare these to the input power for the fan at full speed. The savings will be substantial because the energy usage decreases much more rapidly than the flow volume.

Example #2--circulating chilled water system. Old method had no flow reduction. New method is to vary flow according to the temperature differential between inside and outside, or according to the temperature change between the water entering the loop and the water exiting the loop (to hold this change constant and thus match flow rate to load). Again the savings will be substantial for the same reason as above.

Fan and pump affinity laws can give you some fairly good approximations for energy savings from flow reductions of up to 50%. Granted, the fans or pumps will be operating somewhere else in their curves, but this new operating point may be more efficient than the original, or may be less, or may be about the same. At the same time, the reduced flow in the system will translate into a lower pressure loss which becomes a substantially lower energy cost.

What I am saying is that most of your savings will be from significant reductions of friction loss or excess flow rate throughout the system, with only minor contributions of savings because of the fan or pump's design itself. Don't worry about the motor efficiency at reduced speed. If you want to look at it, however, measure the volume flow rate and the pressure differential across the fan or pump and use this to calculate the actual work it is doing. The VFD will give you its power consumption at this level of output, so you can directly calculate the efficiency of the combination of motor and fan/pump.

Regards--John M.

Thanks John.My question is how do u determine the motor efficiency at site when it is running at lower speed.Like for example a 45 kw 1475 rpm motor is running at 60% speed maintaining V/f constant.The power is very low.The normal efficiency of the motor at 45kw 1475 rpm say is 93%.How do we determine the eff. when it is operating at low speeds with v/f constant?

regards