Rotary phase converter

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A simple homemade phase converter
A simple homemade phase converter

Contents

[edit] Definition

A rotary phase converter, abbreviated RPC, is a Rotary converter that produces Three-phase electric power from Single-phase electric power. This allows three phase loads to run using generator or utility-supplied single phase electric power. The RPC can be connected directly to single phase power and will output three phase electrical power. A common measure of the quality of the power produced by an RPC or any phase converter is the voltage balance, which may be obtained through Three-phase testing while the RPC is driving a balanced load such as a three phase motor. Other measures of the quality of an RPC are the harmonic content of the power produced and the power factor of the RPC motor combination as seen by the utility. The best phase converter for any application depends upon the sensitivity of device being run to all of these factors. Three phase induction motors are very sensitive to voltage imbalance.

A rotary phase converter may be built as a Motor-generator set. A motor-generator set has the advantage that it isolates the generated three phase power from the single phase supply and that the three phase voltages can be nearly perfectly balanced. Because of cost and other reasons, most RPC's are not built this way.

Instead, they are built out of an induction motor or generator - called an idler - on which two of the terminals (the idler inputs) are powered from the single phase lines . The rotating flux in the motor produces a voltage on the third terminal, that is the terminal not connected to the single phase power. A voltage is induced in the third terminal that is shifted by +/- 120 degrees from the voltage between the first two terminals . In a three winding motor, two of the windings are acting as a motor, and the third winding is acting as a generator. The quality of three phase power generated by such a phase converter depends upon a number of factors including:

  • Capacity of the phase converter (idler horsepower rating)
  • Power level demands of the equipment being supplied. For instance, "hard starting" loads such as heavily loaded machinery or well pumps may have higher requirements than other loads rated at the same horsepower.
  • Power quality demands of the equipment being supplied (CNC equipment may have more stringent power quality requirements than a welding machine )
  • Use of techniques to balance the voltage between the three legs.

RPC manufacturers use a variety of techniques to deal with these problems. Some of the techniques include,

  • The insertion of capacitors between the terminals to balance the power at a particular load.
  • The use of idlers with higher horsepower ratings than the loads.
  • The construction of special idler motors with more windings on the third terminal to boost the voltage and compensate for the sag caused by the load.
  • The use of electronics to switch in capacitors, during start up or otherwise, based on the load.
  • The use of filters.

[edit] How RPCs work

The main principles of RPC operation in which a motor generator set are not used are as follows:

  • Three phase induction motors and generators have three terminals called "legs", usually numbered (arbitrarily) L1, L2, and L3. Their regular use under three phase power is to apply three phases to legs L1, L2, and L3 respectively, to convert electrical energy into mechanical power, or the generator is spun by an external power source and power is extracted from L1, L2 and L3. There are no fundamental differences between an induction motor and an induction generator, although due to other factors, their manufacture may differ somewhat, but either can be used in either mode, although possibly with some loss in efficiency.
  • A three phase induction motor can be run at two-thirds of its rated horsepower on single phase power applied to legs L1 and L2 (or any pair of legs), once spun up by some means.
  • A three phase induction motor that is spinning under single phase power applied to legs L1 and L2, generates an electric potential (and can deliver power through) leg L3, although without some form of current injection, special windings in the idler, or other means the voltage will sag when a load is applied.
  • Power factor correction is a very important consideration when building or choosing an RPC. Power factor correction is desirable because an RPC that has power factor correction will consume less amperage from the single-phase service supplying power to the phase converter and its loads. It is also better for the equipment that the power is supplied to.

Balanced voltage between the three legs of power is important for operational life of the equipment receiving that power. Unbalanced three phase power can damage the equipment that it is meant to operate.

[edit] Use of RPCs

RPCs may be used anywhere three phase devices need to be used, but only single phase power is available, although they are increasingly being replaced with solid state inverter devices (see below). Three phase motors cannot run on single phase power without a device to generate three phase power. Since prices (and quality) of used three phase motors are usually more favorable than those of their single phase counterparts, demand exists for phase converters. This is also true because single phase electric motors generally are not available in sizes over 15HP due to their complexity, starting requirements and relative high expense and low efficiency compared to three phase motors. In fact, single phase motors larger than 5HP, though available, are rarely seen in use for these reasons.

RPCs are sold by various vendors, but also are popular items to attempt to be made by do it yourselfers. Of practical importance to a do it yourselfer attempting to make a phase converter, is to understand the issues related to sizing and quality of three phase power produced.

Besides RPCs there are other technologies available today that can be used either to convert phase, or to enable three phase machinery to run without conversion on single phase. All techniques have potential problems. Some of the problems are in this list, although every device from every manufacturer is different and so these are generalizations applied specifically to running motors with phase converters.

  • Static phase converters require that the motor being run be significantly derated. Typically 60%. A 10hp motor can be run only at 6hp. The power balance here is extremely poor.
  • Three phase inverters and related Variable Frequency Drives, which are also a form of inverter, at one time produced large amounts of harmonic distortion in their output, and the efficiency of the inverter motor combination was poor. Early inverter drives could damage some motors not rated for use with an inverter and shortened their lifespan. Motor manufacturers may void warranties on non-rated motors if they are run at a variable speed or with an inverter, but most three phase motors over 10HP sold today are rated for use with an inverter drive. Modern phase inverters and Variable Frequency Drives (most of which can also accept single phase input and act as a phase inverter) are very efficient (upwards of 95% for some drives) and produce far less distortion, upstream and down, than early models.
  • Rotary phase converters have difficulty producing highly balanced power. Rotary phase converters must always be sized larger than the power needed to run the equipment in order to handle motor starting currents.

In every case there are manufacturers of the device that have done a better job, and the only way to really know about the quality is through published specifications, reviews, tests and published papers. There are a large number of ways to enable a three phase motor to run on single phase and the best solution always depends on the application.

[edit] See also

[edit] External links