Phase Converters and Electric Motor Starting Currents

When an electric motor is started at full voltage and frequency it requires an inrush current that is several times the maximum running current of the motor.

This type of start is commonly referred to as across-the-line-starting and has important consequences for any phase converter operating the motor.

The current drawn by a motor during across the line starting is often specified on the motor nameplate specifications as locked rotor amps (LRA). This current is typically five to six times the full load amps (FLA) of the motor, and as high as ten times FLA for high efficiency motors. This inrush current lasts only until the motor shaft has reached its rated speed, so it is typically of very short duration, perhaps less than one second. For example, the typical FLA for a 230 volt,10 HP three-phase motor is 29A and the starting current approximately 150A. When there is a large mass attached to or driven by the rotor, the motor will take longer to reach its rated RPM, thus the starting current will be longer in duration and put a larger burden on the electrical system.

If the electrical system supplying these starting currents does not have enough ampacity, the inrush current will cause a voltage drop on the utility line. These line disturbances are what causes lights to flicker and can create more serious problems for other users on the line. When motors start, electric utilities can become concerned when a large phase converter on single-phase service operates large three-phase loads

because of the potential for line disturbances. A phase converter operating a motor must be able to supply most or all of the starting current, especially for motors starting under load. If it cannot, the motor may not start and the converter and motor may suffer damage from over-current.

The sometimes confusing sizing scheme for rotary phase converters is related in large part to motor starting currents. It is common to see rotary converters rated to run more horsepower than they can start. This is because a rotary converter can deliver balanced power only at one level of current. When more current is demanded from the converter, the voltage on the generated phase drops creating phase imbalance. Hard starting loads commonly require a phase converter motor frame 2-3 times the size of the operated motor.  This leads to increased expense and decreased efficiency to supply current for a momentary condition.

A digital phase converter is much better equipped to handle starting currents than a rotary converter. Because it is equipped with software and system monitoring and feedback, it can be programmed to deliver large momentary currents while maintaining phase balance and power quality. Because the generation of the large current produces heat, the current can only be delivered for a short time, usually 4-10 seconds.  This is however long enough to start the vast majority of motors.

The electronic power factor correction of a digital phase

converter also helps reduce chance of starting current producing a line disturbance. During across the line starting most of the power delivered to the motor is not real power, but apparent power commonly referred to as “volt amps reactive” or VARs. During starting thepower factor of the motor is very poor, so a large component of the current is reactive current. Because the digital phase converter has electronic power factor correction, it supplies the reactive component of the current to the motor, reducing the current supplied by the single-phase line to the phase converter.

There is an increase current above FLA because it takes real power to accelerate the rotor from standstill to several thousand RPM instantly. Generally, when a digital phase converter starts a three-phase motor across the line, the current drawn from the single-phase line is equal to that which would be drawn from any one line if the motor were started across the line on three-phase service. As a rule, up to 30 HP motors can be started on single-phase service without significant line disturbances.

Starting currents can be dramatically reduced by a variable frequency drive (VFD) when used as a phase converter.

Because the VFD can control the speed of the motor, it gradually ramps up the speed on start-up, virtually eliminating any problems with line disturbances. In fact, VFDs are commonly used to soft start large three-phase motors on three-phase service.