Phase Converters and CNC Machines

Computer numeric control (CNC) machines are probably one of the most demanding applications for phase converters. Most often used in metal fabrication, CNC machines use electronics to control the speed and movement of multiple motors within the machine. When operating at maximum productivity, the constantly changing load of CNC machines results in a widely variable current demand from the three-phase power supply.

Of the four phase converter types -- static, rotary, digital and VFD – only rotary and digital phase converters are suitable for the operation of CNC machines. Operating a CNC machine with a static phase converter or phase converting VFD will damage the machine.

Rotary Converters and CNC Machines

The widely variable load of CNC machines exposes the primary weakness of rotary phase converters, namely, the inability to main balanced voltage. A rotary phase converter can provide balanced voltage at only one load point and the more variable the load, the worse the voltage balance. If a CNC machine is operated on a rotary phase converter, the converter must be oversized to minimize the voltage imbalance. Oversizing the phase converter to improve voltage balance will result in poor efficiency, often as low as 70%. It is also not advisable to operate any other loads with the phase converter, as this will make the load demand on the converter more variable.

In all likelihood, a CNC machine operated on a rotary converter will have to be detuned to reduce the large spikes of current that are needed when the spindle motor speed ramps up and down very quickly. This detuning will make the CNC machine less productive.

The large rotary converter needed to operate CNC machines will have a very large inrush current when it is started and is likely to cause line disturbances on the single-phase line. These line disturbances are a severe voltage drop that affects other users on the line. Most electric utilities will not allow across-the-line starting of large motors on single-phase service and may require a soft start device for these loads, which adds significant expense to the installation. A large rotary converter will also operate with a very poor power factor, consuming large reactive currents on the line side. This current requires the wires and breakers feeding the converter to be oversized and leads to losses in efficiency. Even though your meter may not charge for the reactive current, the utility must generate this reactive power and may charge a power factor penalty.

Digital Phase Converters and CNC Machines

Digital phase converters are far superior to rotary converters for operating CNC machines.

A true digital phase converter produces sine wave voltage, making it safe to power the electronics in the machine. It will maintain perfectly balanced voltage from no load to full load and can supply very large momentary currents, allowing the CNC machine to operate at maximum capacity. The converter does not have to be oversized to operate CNC machines, and as a rule of thumb, can be sized by matching the spindle HP of the CNC machine to the HP rating of the converter.

A digital converter has no moving parts, so there are no starting currents to cause line disturbances. With electronic power factor correction, it operates at unity power factor and does not consume reactive power. In fact, it will supply the reactive power needed by any loads it operates, making it a very utility friendly converter. The converter operates at 95-98% efficiency and when energized with no load consumes very little power.

A digital phase converter can operate multiple CNC machines or any combination of loads within its capacity. It supplies clean, balanced three-phase power that is of higher quality than utility supplied three-phase. The only true digital phase converter is manufactured by Phase Technologies. Some companies are marketing phase converting VFDs as digital phase converters, but these cannot safely operate anything other than single inductive motor loads.