Current Measurement of a Motor Run from VFD

True RMS gives you the heating value of the current, no need to recalculate or adjust for frequency, when working with VFD specifications, for instance. The true heating effect in the rotating machinery due to frequency is taken care of in the design of the equipment. The requirement for cooling is far more affected by the actual speed of the equipment, due to the common practice of applying motors with shaft driven fans.

Over the typical frequency range we normally consider, the thermal effects on the iron and copper of the frequency changes are all lumped in with the other rating factors, so you get a piece of equipment that is pretty easy to apply.

As to the VFD measure, that is what it is using to protect the motor, so if no good, not a very good VFD, and you will end up with lots of customers with burned up motors. Something will eventually give...

GA

That rare(?) mechanical contraption that makes more torque at lower speeds is a DC motor or traction motor. One does not use a VFD to drive such a motor but with so many using VFD to make an AC synchronous motor move at some rate other than power grid frequency rates I feel this true workhorse of simple speed control has been forgotten.

The old work horse is still used in industry,but they are being phased out.

Series wound DC motors have most torque when first starting up,and this torque is limited only by the power supply or current limits within the drive controller.

If current is unlimited,so is speed,and I have seen armatures destroyed by centrifugal force when the controller went bad.

The armature segments actually pulled away and jammed on the motor housing.

They were still making contact with the brushes,and fuses were blown or a fire resulted if someone bypassed the fuses.

As the saying goes "You can't make something foolproof 'cause those fools are so damned ingenious."

The speed is easily controlled by varying the voltage,and synchronization of multiple motors is easily obtained,and once set,seldom needs adjusting,even for varying speeds or loads.

These type drives are used in web systems,such as printing,paper goods,textiles,etc.

These old workhorses are slowly being replaced by the newer AC drives,but they will be around for a long time before they are all relegated to the glue factory.

As for a synchronous AC motor,if it is designed for a slower speed,it will produce more torque at the designed speed than at a higher speed,if power consumed is the same.

Example: A 900 RPM (name plate RPM) motor will have twice as much torque as a 1800 RPM (name plate rpm) motor of the same horse power.

Consider the VF motors in EV's.

Maximum torque from zero speed on up to max speed.

Torque is stuff that is presented mechanically to a motor for it to overcome. A mechanical load that exerts more torque at low speed than at high speed will eventually cause the motor to stall and, if set up correctly, to trip out on overload.

Check out the graphs on this link,espcially the lower one:

https://www.carthrottle.com/post/how-do-electric-vehicles-produce-instant-torque/

There are numerous examples of DC drives controlling a DC motor. These drives are essentially VFDs since the utilize PWM to adjust the speed with the major difference being that they output a DC voltage instead of an AC 3ph. The use of these DC motors is because of the low speed torque as you stated.

current the square of the current

All the VFD operated motors that I've seen recently were on pumps. The pump assembly (including motor) included data sheets that indicated the operating envelope for that assembly.

I seldom saw any that were rated beyond +/-20% of mains frequency and typically these were then also rated to +/-20% nameplate voltage with a separate envelope for total power so that these limiting parameters could be programmed into the VFD device.

When a VFD drives a motor, as the frequency is reduced the voltage applied to the motor is reduced as well. To get an effective measurement of the power being delivered at the various speeds. motor torque T equals the power being developed divided by the motor mechanical angular velocity, which is equal to 4*Pi*f/p (p is the number of poles). I would suggest you use a watt-meter to measure the power because the phase angle varies with load. Using the watt-meter gives you the true power and the torque can be calculated by diving the power my the mechanical angular velocity.

You can carry out a simple experiment to get your mind around the current drawn by using two RMS clamp on meters, one on the supply at 50/60 Hz and one on the motor supply at whatever value of Hz you are using.

At slow speeds motor overheating with shaft driven fans is a big problem and the slow speed running should be limited.

There was a positive displacement pump on a floc dosing system that would always overheat below 20% speed so the solution was to increase the dilution of the floc to keep the motor above this level. On a small motor like this an external fan was not warranted. Remember heat is the killer.

VFD measures the dc current using Hall effect Sensors and gives 4-20 mA outputs for actual drive current hence current measured is not frequency dependent. So there's no need to convert it to 50 hz frequency. If you try to measure the current by using out put CT, it will indicate wrong current because standard CT is rated for standard 50/60hz. Again if you measure the current by using input CT before drive, it will also give wrong value because of harmonics generated by drive. I have checked it practically so better go with current signals given by drive. Hope this helps.

The output of the PWM inverter is not perfectly sinusoidal and average responding ampere meter does not read the current accurately. The true rms meter reads the correct current because the true rms meter measures the current on the basis of the heating value. The current measurement by true rms multimeter gives the accurate reading at all frequencies.

The current measurements from the VFD are readings from CT that are in located in the inverter section of the output circuit. This is measured after the PWM(pulse width modulation) has been performed and can also be measured on the TLeads(U,V,W) with a true RMS clamp meter.

The reason that you may be perceiving erroneous readings is most likely as result of the V/F curve of the programmed application.

You will definitely see higher currents at extremely low speeds due to the need for higher torque to initiate rotation. As motor speed increases and the motor flux is stabilized the current will settle at rated current at rated speed.

Almost always the current from the VFD will be more accurate than any external measurement. Most VFDs offer analog outputs that are either dedicated or selectable as either a 0-10 vdc, -10-+10vdc, or 4-20 mA signal and this will give you the true output of the VFD.