For a number of reasons, it may be desirable to start a pump motor slowly, using one of the following soft-start techniques.
simplest method of starting a pump motor is by closing a contactor and allowing
the motor to start at full voltage, or “across the line,” as it is called.
However, when a pump motor is started at its full-rated voltage, the current
drawn by the motor will be as high as six times to eight times its normal,
full-load running amps for a short period of time while the motor accelerates
up to speed. Referred to as “locked-rotor amps,” this can cause a momentary
voltage drop in the motor circuit, which can dim lights, affect other
electrical equipment and possibly overload distribution transformers. The
larger the motor, the greater the effect. In fact, electrical utilities often
will limit the size of motor that can be started across the line to protect
their distribution systems.
Additionally, starting a large pump motor at full voltage may cause water
hammer in the piping system, or damage the pump due to high torque. For these
reasons, it may be desirable to start a pump motor slowly, using one of the
following soft-start techniques. Although soft starters are available for both
single- and three-phase motors, this article will focus on three-phase
Autotransformer motor starters use a transformer with several voltage taps
(usually 50, 65 and 80% of full voltage), multiple contactors and a timer to
switch from one of the reduced voltage taps to full voltage after a few
seconds. Autotransformer starting delivers the highest starting torque per amp
of line current, thus providing reduced in-rush current with minimum sacrifice
of starting torque. These starters are inherently closed transition, thus they
provide a relatively smooth transition from reduced voltage to full-voltage
Franklin Electric makes the following recommendation for using autotransformer
starters with its motors: If the pump cable length is less than 50 percent of
the maximum allowable, either the 65-percent or 80-percent taps can be used.
When the pump cable length is more than 50 percent of the allowable, only the
80-percent tap should be used. This is because there is an inherent voltage
drop in the cable, which must be accounted for. Franklin’s maximum cable length
charts are based on a 5-percent voltage drop at the motor. This 5-percent drop,
by itself, will reduce the starting current by 20 percent and the starting
torque by 36 percent, compared to having the rated voltage at the motor, which
may be enough of a reduction in starting current on some applications to
preclude the need for a reduced-voltage starter.
Wye-delta motor starters are used in conjunction with a specially wound motor,
having leads from each individual winding brought to the outside of the motor.
In other words, wye-delta motors have only one set of windings, like a standard
three-phase motor, but each end of each winding has a connection wire on the
outside of the motor. These six wires then can be hooked in one of two ways. In
the wye configuration, one leg of each winding is brought to a common point,
and the three legs of the three-phase power are hooked to the other end of each
winding. This configuration increases the impedance of the motor, reducing the
current and torque to 33 percent of normal.
In the delta configuration, the windings are wired in the normal way, producing
full torque and current draw. The transition from wye to delta is made using
three contactors and a timer. During this transition, the motor is taken
off-line for an instant to avoid short-circuiting the contactors, so wye-delta
starters typically are open-transition types. There are some closed-transition
wye-delta starters available on special order, but the circuitry required to
make them closed-transition makes them prohibitively
Part-winding starters also require the use of specially wound motors, but,
unlike the wye-delta motors, have only one set of windings with six leads.
Part-winding motors, on the other hand, have two sets of windings and six or 12
leads. One set of windings is the start windings, and the other set is the run
windings. The starter, inherently a closed-transition starter, starts the motor
on the start windings, and after a preset time interval – typically 2 seconds
to 3 seconds – connects the other set of windings in parallel with the start
windings. A part-winding starter will reduce the starting current draw to
approximately 65 percent of normal locked rotor amps, and the torque to 45
percent of normal motor torque. A part-winding starter uses two contactors, two
overload relays and a timer.
Solid-state starters utilize solid-state devices called silicon controlled
rectifiers (SCRs) to decrease the motor voltage according to user-defined
parameters. These starters can be used with standard induction motors. In the
case of water-cooled submersible motors, the soft starter has to be programmed
to ramp up the motor to speed within the time period specified by the motor
manufacturer (usually within 3 seconds). The in-rush current can be reduced to
less than 50 percent of full voltage start amps (locked rotor amps), and the
starting torque can be controlled to closely replicate the starting torque
requirements of the pump, reducing mechanical stress on the system. Soft starts
have become very reliable, and the cost is coming down to the point where they
are an attractive alternative to electro-mechanical reduced voltage starters. Also, VFDs (variable frequency drives) have
soft-start capabilities, and have become more affordable.
Table 1 shows the relationship between line current, motor current and motor
torque for the different types of starting methods.
To summarize, if the utility can provide enough power to your motor for a
full-voltage (across the line) start, most people go that way because it is
cheaper. Sometimes, however, the utility does not have enough capacity to
accommodate the in-rush starting current of a large motor, and may ask you to
provide a reduced current starter. If you have a conventionally wound motor,
your choices are autotransformer or solid-state soft-start. If you need a new
motor, you can buy one that is wound for wye-delta or part-winding starts, and
go that way. The choice is yours.
Next month, we will take a closer look at one of the reasons mentioned above
for using a soft-start – water hammer. ’Til then ....
Tech Topics: Soft-start Motor Controls
July 1, 2009