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Phase Converters & Power Factor
Phase Converter Efficiency
Installing a Phase Converter
Rotary Phase Converters
Static Phase Converters
VFDs as Phase Converters
     • Harmonic Distortion
Three-Phase Motors
Phase Converters & Voltage Balance
Phase Converter Applications
     • Submersible Pumps
     • Woodworking Equipment
     • Dual Lift Stations
     • Phase Converters & Welders
     • Phase Converters & CNC Machines
     • Phase Converters & Air Compressors
     • Phase Converters & Elevators
     • Phase Converters & Wire EDM
     Phase Converters & HVAC
Phase Converters & Transformers
     • Step-up Transformers
     • Buck-Boost Transformers
     • Isolation Transformers
Phase Converter Experts
Digital Phase Converters
Regenerative Power
Three-Phase Power
     • Delta vs. Wye Configured Power
Motor Starting Currents

Phase Converters and Variable Frequency Drives

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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

Harmonic

3rd

5th

7th

9th

11th

13th

15th

 Percent

 73.2

 36.6

8.1

 5.7

 4.1

2.9

0.8

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.

The starting sequence of a VFD is carefully controlled to avoid damage. When the start button is pushed, the pulse sequence to the output switches is adjusted so that the average voltage applied across the motor has a low value, with low frequency. As the motor starts to spin, the voltage is allowed to increase and the frequency is increased until the motor reaches full operational speed. A start at full voltage and max frequency would overload the output switches. If a VFD is putting out full voltage at 60 Hz to one motor on its output, and a second motor is suddenly connected by closing a contactor, then the VFD will probably either shut down if it can respond to the overload, or be damaged if it can't.

The circuitry in a VFD does not allow power to flow from the motor back to the power system, as is required when the motor acts as a brake. If the application requires this feature, then one or more braking resistors and additional switches must be added to the VFD so that this power is absorbed without destroying either the output switches or the DC link capacitor. Rotary and static phase converters intrinsically have the ability to absorb braking currents because two of the wires to the motor are connected directly to the supply system. A digital phase converter is able to feed power from the generated phase back into the power system as well.

The output voltage from a VFD is not sinusoidal, but rather a PWM voltage. Because of the high harmonic content of PWM voltages, dangerous voltage rises on the output can occur if the lead length from the drive to the load becomes too great. This distance will vary based on the switching frequency of the drive and the impedance of the electrical system. Inductive output filters can be installed on the output of the drive to reduce the harmonics and mitigate the problem of voltage rises. In general, a drive should not be more than 50 feet from the load without filtering.

VFDs are especially suited as phase converters when it is advantageous to vary the speed of the motor being powered. They are also preferred when the motor load is large enough to cause line disturbances during across-the-line starting. A drive can ramp the motor speed up gradually, greatly reducing the current needed to start the motor.

       
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