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

Rotary Phase Converters

Rotary phase converters produce three-phase power from a single-phase source and are relatively versatile.  They are capable of powering resistive, capacitive and inductive loads and can power multiple loads.  They have been in use since the 1960s and are especially popular with home machinists that operate a variety of small three-phase machine tools.  Their simple design makes them reliable and relatively inexpensive.  They are manufactured by hundreds of companies large and small, and in fact can be built by almost anyone with components that are cheap and easy to find.  Their main shortcomings are in the quality of the three-phase power they produce and in efficiency.  Voltage balance can be very important to the safe and efficient operation of three-phase motors and other three-phase equipment.

A rotary phase converter consists of a three-phase motor (usually without external shafts) and a bank of capacitors wired together to act as a single large capacitor. Two of the leads to the motor are connected to the single-phase power source and the third lead to the motor is connected in series with the capacitor bank to either one of the single-phase inputs. The output leads from the phase converter are connected across the three motor terminals. Typically the motor used in the phase converter is larger than the loads it is supplying. For example, a rotary converter designed for a 7.5 Hp load might use a 10 Hp motor frame. The electrical interaction between the capacitor bank and the free-running phase converter motor generates a voltage on the third motor terminal which approximates the voltage needed for a balanced three-phase system.

The voltage on the third leg generated by the converter is affected by the incoming voltage on the single-phase line, by the amount of capacitance wired in series with the motor winding, by the motor frame size, and by the amperage draw on the load side. By knowing something of the load demand and relying on past experience, the right combination of motor frame and capacitors approximately produces a voltage equal to the other two legs. However, it usually isn't a very good approximation. For example, measurements on a 7.5 Hp rotary converter in an actual machine shop installation resulted in line-to-line voltages of 252 V, 244.2 V and 280.5 V, which is about a 12% imbalance in the voltages. With this much voltage imbalance, a motor should not be loaded anywhere near its rated capacity or it will suffer damage. In fact, the lead with the lowest voltage could be completely disconnected and it would not significantly change the performance of the motor.

Theoretically you could install a rotary converter and operate the load while measuring the output voltages phase to phase. The voltages could be balanced by adding and subtracting capacitance to the phase converter winding This is not very practical for the average end-user and the system remains balanced only if nothing changes. Unfortunately in the real world, utility voltages fluctuate and amperages demanded by the load change, all of which will upset the voltage balance of the converter.

For loads that require good voltage balance it is necessary to increase the size of the converter motor frame. It is also advisable to restrict the converter to operation of one machine so that the range of amperage the converter has to supply is not as wide.

A phase converter must also be able to supply the current needed for across-the-line-starting of motors. These currents are 5-6 times the full load running currents of the motor, and even greater for high efficiency motors. The voltage on the generated phase of a rotary drops as the current demanded by the load increases, so it is easy to see how starting currents can render that phase useless in accelerating the motor up to speed unless the motor frame of the converter is large enough. Motors that start under load require a larger rotary converter than those that are lightly loaded.

While increasing the size of the rotary converter can improve its voltage balance and ability to start motors, that strategy has some drawbacks. The overall efficiency of the system will decrease as the size of the converter increases, with some systems having efficiency as poor as 70%. Care must also be used when operating small motors alone on a large rotary converter. Because the amperage demand of the small load does not reduce the voltage of the generated leg like a full load on the converter would, that voltage might remain high enough to damage the small motor.

Rotary phase converters offer an affordable, versatile solution for many users, especially those with lightly loaded, simple motor loads. For larger loads and demanding applications like CNC machines, converters with higher efficiency and better power quality may be a better choice.