Phase Converters and Variable Frequency Drives

Variable frequency drives (VFDs) are designed primarily to control the speed of AC motors, but can be adapted to function as phase converters.

While a phase converter will supply a three-phase output at the same frequency as the input voltage from the power line, a VFD has the ability to create voltages that vary in frequency. A VFD has an input rectifier (either 4 or 6 semiconductor diodes) which charge up a DC link capacitor. Three pairs of semiconductor switches are also connected to the DC link capacitor. These switches generate a pulse-width-modulated (PWM) voltage for each of the three-phases on the output.

A VFD cannot produce a sinusoidal output voltage. The inductance of a motor powered by a VFD responds to the area beneath the curve of a plot of the voltage as a function of time.  So, even though the voltage isn't sinusoidal, if the on/off times of the switches are chosen correctly then the current in the leads to the motor can be sinusoidal as long as the average value of the voltage is sinusoidal. Since the torque generated by the motor is proportional to the currents and not the voltages, then to a first approximation the motor behaves as if it had sinusoidal voltages applied to it.

Problems can arise with VFDs if they are used to power loads other than motors, if there are multiple loads on the VFD, if the motor needs to provide braking action, if the distance between the motor and the VFD is appreciable, or if the current drawn by the VFD is large compared to the rating of the utility step-down transformer.

VFDs were not originally designed to function as phase converters, in fact most VFDs are powered from a three phase source. When used as phase converters, single-phase input powers 4 of the 6 diodes on the rectifier, so the drive must be de-rated.  Usually you have to double the size of the drive to the load.

A diode or SCR input rectifier typically produces large harmonic distortion in the input current. Table 2 below gives typical values of the harmonic distortion expressed as a percentage of the fundamental component of the input current at 60 Hz.

Table 2                                 VFD Input Harmonic Content

The harmonic component of the current will be a problem when the current flowing into the VFD is a significant portion of the total current load that the step-down transformer is capable of delivering. If a very large VFD is used or if multiple smaller VFDs are all attached to the same line then there may be problems. The relatively large current drawn by the input circuit of the VFD at the peak of the voltage sine wave can distort the voltage waveform and cause problems for other users on the power system. Input line reactors are often used between the VFD and the power system to help alleviate this problem.

VFDs are designed to drive a single motor load.  The manufacturer's recommendations usually are that the wires to the motor be solidly connected to the VFD and that the connections not be broken under normal operating conditions.  That is, one would not normally install a contactor between a VFD and a motor because the high voltage and arcing that are a normal part of the contactor opening and closing can have unpredictable effects on the semiconductor switches in the VFD and increase the risk of failure. If multiple loads are connected to a VFD with individual contactors for each separate load, the VFD may not be able to handle the current surges which occur when individual loads are switched on and off. 

If a VFD were connected to a piece of equipment which contained three-phase motors as well as other controls, it is very likely that both the VFD and the equipment would be damaged.  For example, if there were any capacitors in the equipment connected directly across the VFD outputs, the VFD would have to shut down immediately or be destroyed by the extremely high currents that would flow when the output voltage pulses were applied to the capacitors.