VFD Variable Frequency Drive | Different input output readings

By Eddie Mayfield and Abdou Barrow, EMA Inc.

To begin, the design of modern variable frequency drives (VFD) takes output ampere readings very seriously. The drive protection circuits in modern drives are excellent, which is why output device failures are very low given the fact that they routinely switch large currents.

Although its not advised, most modern VFDs can take a phase to phase or phase to ground short, and trip off in time to save the output devices. This is only possible because the current measurements are sampled rapidly and accurately.

At EMA we’ve been applying and servicing VFDs for over two decades now, and we’ve learned to trust the drive output readings. The measurement methods are almost always considerably more precise and reliable than external measurements.

This isn’t to say that there could not be a failure, but rather to say that all things being equal, we would trust the internal drive readings over external measurements.

There are several reasons why input currents, measured externally, can differ considerably from the drive output currents.

1. Voltage Differences

The 3 phase input voltages to a VFD are more or less static. They may vary by a small amount, but in general, a 2300 volt input is going to be relatively stable.

However, VFDs provide not only a variable frequency output to the motor, they provide a variable voltage output as well. In most modern drives this is accomplished by using Pulse Width Modulation (PWM) .

Since the power being consumed by a VFD will always equal the power output of a VFD plus the internal drive losses, this means that if the output voltage is less than the input, then the amperes will be higher than the input. (Power = Volts times Amps)

2. Drives are non-linear loads.

VFDs do not draw sinusoidal current from a power line, they draw pulsating currents, which can appear to many measuring instruments as DC currents. This non-linear amp draw can be mitigated somewhat by using phase shifted input bridges, which have become common in high horsepower VFD equipment, but nonetheless, it remains non-sinusoidal. This often causes errors in external current measuring equipment.

3. Harmonic currents.

Due to the non-sinusoidal currents, the input voltage can be distorted by currents flowing at frequencies other than the power line 60 hz. The equal harmonics tend to cancel each other, but the odd harmonics do not. Consequently, one can find significant currents flowing at the 3^rd and 5^th harmonics especially (180 hz and 300 hz) and these currents can, if unmitigated, not only cause power line and equipment problems, but can result in erroneous power and current measurements.

EMA can supply your VFDs, we can repair your existing VFDs, and troubleshoot VFD problems.. we work on both low voltage and medium voltage VFD s. Call your nearest EMA office as listed below, or use our contact page to email us.