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
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.
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.