What is the Purpose of Reduced Voltage Motor Control Starter?
Electric Motors | Source: Tekwell Services A squirrel cage motor draws high starting current (inrush) and
produces high starting torque when started at full voltage. High
current draw during a full voltage start can cause the line voltage to sag which may adversely affect other equipment within
the facility. The high torque developed during a full voltage start can cause excessive wear on the driven equipment or damage to material in the process. A reduced voltage start reduces the negative effects of both high current inrush and torque. A typical squirrel cage induction motor started at full voltage will
draw current from four to ten times the current required to run
the motor at full speed, fully loaded. The current drawn during a
full voltage start - or the locked rotor current - can cause negative effects on the distribution system. A motor started across
the line can cause voltage drops which may cause lights to dim,
or other equipment on the feeder circuit to slow down or drop
out. Related Article: Electric Motor Control in Industrial Plants When reduced voltage is applied to a motor, the motor will draw
current in proportion to the voltage reduction:
For example, a typical NEMA design B motor, the inrush or locked rotor
current will be approximately 600% of the motor full load amperage (FLA) rating. If started with 65% of line voltage, the motor
will then draw 0.65 x 600% or 390% FLA. Reduced Starting TorqueWhen started at full voltage, a typical NEMA design B motor will
produce approximately 150% of its rated torque. The torque
produced by the motor in excess of the torque required by the
driven load is used to accelerate the load from rest to full speed. Excess acceleration torque can cause mechanical shocks which can result in twisted shafts and couplings, excessive belt wear, or early failure of moving parts in the driven equipment. If the motor accelerates too quickly, the material in the process can also be damaged during a full voltage start. When a motor is started at reduced voltage, the torque is reduced by the square of the voltage reduction.
For example, if the NEMA B motor described above is started
at 65% of line voltage, the torque developed would then be
(0.65)2 or 42% of the normal starting torque. |
Reduced Voltage Application
Reduced voltage starters are used in a wide variety of applications. It is important to remember that an application that is difficult to start with full voltage will be even more difficult to start
with reduced voltage.
If the motor has high inertia or if the
motor rating is marginal for the applied load, reducing the starting torque may prevent the motor from reaching full speed before the thermal overloads trip. Applications that require high
starting torque should be reviewed to determine if reduced voltage starting is suitable.
Autotransformers
Autotransformer starters are the most widely used electromechanical reduced voltage starter. An Autotransformer starter
may be used on any three-phase AC motor and provides the
maximum starting torque with the minimum starting current.
Autotransformer starters are ideal for applications where available starting current is limited, but a minimal decrease in starting torque is required. Because the Autotransformer provides
the highest torque efficiency, it is also ideal for applications
which may be run from generator power.
Typical applications
for autotransformer starters include:
- Centrifugal pumps
- Fans
- Compressors
- Conveyors
Wye-Delta Starters
Wye Delta starters are recommended for use on applications
that require long start-up time or frequent starts when using a
wye-delta motor. The Wye-Delta starter is available in both
open and closed transition configurations. The open transition
starter is recommended for inertial loads while a closed transition is best suited for frictional loading. Wye Delta starters can
be applied in any application where the low starting current is and
low starting torque are necessary.
Some typical applications
include:
- Elevators
- Chillers
- Large air conditioning units
- Centrifuges
Related Article: Star Delta Motor Starting Explained
Part Winding Starters
Part Winding starters are recommended for use on applications that do not require long start-up time or frequent starts.
Part Winding starters are a cost-effective means of providing a
reduced voltage start where total line current draw is not of primary concern.
No comments: