Digi-Phase Converters - The Facts
Digi-Phase digital rotary converters offer a better performing and longer lasting alternative to standard rotary and digital inverter type phase converters. It is helpful to understand how a rotary phase converter and a digital Inverter type phase converter works when comparing one to a Digi-Phase. A Digi-Phase is similar to rotary phase converters or a digital phase converter in that two of the phase leads to the load come directly from the power line, through a transformer, and the third lead is generated by the converter.
A conventional standard rotary phase converter is a slightly modified three phase motor used in conjunction with capacitors. After a contactor and a timer plus some start capacitors to kick-start the internal motor, it will run with two of the motor windings powered by the single phase supply. The third winding, sitting in the rotating magnetic field inside the motor, generates the third phase.
Voltage balance is important for safe and efficient operation of three phase motors and other three phase systems. Unbalanced three phase power has the potential to trip fuses in three phase equipment; run hotter than normal; and may require that motors be de-rated.
The challenge for any non-digital rotary phase converter is to generate the new phase so that the phase-to-phase voltages are balanced. With a standard conventional rotary converter, the manufacturer adjusts the third voltage by connecting a motor run capacitor between one of the single phase input lines the third winding of the motor. By adjusting the capacitor value to the selected motor, the voltages can be balanced fairly close to a given fixed load. But voltage imbalance will occur when the load changes. Even though the generated voltage has been adjusted at the factory, load changes will cause this voltage to fluctuate, creating voltage imbalance.
So while some standard rotary
phase converters claim to be balanced; this can be so only at one given
load level. This is usually at around 50% load. Most three phase loads
are variable and many are highly susceptible to damage from voltage imbalance.
A standard rotary phase converter has no voltage monitoring circuits to
know when it is out of balance, and even if it did, it has no way to adjust
the generated voltage to maintain voltage balance. When better phase voltage
balance is required for CNC and similar machines, manufacturers tend to
increase the size of the internal motors. The converter size increases
and the relative load change appears to be smaller. These converters are
often called CNC converters.
Digital inverter type phase converters generate their third phase by using one third of a modified variable speed drive. The drive is modified to produce a fixed frequency at a fixed phase angle positioned between the two existing phases. A large number of electronic components are needed to do this; rectify the incoming voltages to Direct Current, a bank of electrolytic capacitors to store electric energy between sine waves and a group of IGBT transistors to slice and chop DC voltages into groups of pulses in different widths. Feed this through a coil or choke and it appears as a real sine wave. Because sharp pulses tend to produce unwanted radio waves, additional components are needed to suppress radiation and harmonics. Many components generate heat meaning that additional electric fans are needed to remove hot air.
Because variable speed drives can only be overloaded to a certain degree, extra measures are taken to deal with starting motors. These include overload sensors and bypass contactors. IGBTs and their logic are sensitive to over and under-voltage. Voltages are monitored and the unit is disconnected from power when supply voltages are outside a safe narrow voltage bandwidth. This seems to be the reason why digital inverter type phase converters are only available for 230V three phase voltages.
All efforts to rectify, store, chop, cool, check, protect and shield require many electronic and mechanical parts, fans, metal shielding, large coils. Any statistical analysis of system reliability dictates that increased component count translates to lower overall reliability.
The electrolytic capacitors used in a digital phase converter have a limited life span which is directly related to their operating temperature. Contactors and fans used are also prone to failure and will reduce the overall life expectancy.
In contrast, a Digi-Phase
digital rotary converter generates the third voltage with precise and
rigid compact SCR power electronics, similar to devices used in welders
and locomotives. A hermetically sealed power block adjusts load changes
by fast contact-less and stress-free capacitor switching. Switching occurs
at zero crossing moments in the electrical wave when voltages and currents
are zero. No magnetic fields are generated; no unwanted harmonic waveforms
have to be suppressed. The digital controller fitted to our Digi-Phase
Converters monitors the voltage-generating process, ensuring all three
voltages maintain balance at all loads. In the case of our “T”
Range within 5% and our “F” Range within 2.5%.
No “dirty” power, no fast transient, no brown-outs, no over- or under-voltage will cause a Digi-Phase to stop. Variable loads and voltage sensitive equipment can be safely operated on a Digi-Phase digital rotary converter. Applications for a Digi-Phase include CNC machines, variable speed drives, deep submersible pumps, welders, plasma cutters, transmitter stations and all types of highly loaded motors.
are three types of Digi-Phase converter, and we also offer self build
models for your convenience. For further information on how to select
a Digi-Phase digital rotary phase converter, click
Specialist Machine Parts Limited, Baldwin Road, Stourport on Severn, Worcestershire, DY13 9AX