Autotransformer Starter Working
Such starters consist of an auto-transformer, with necessary arrangements. At
the time of starting, a reduced voltage is applied across the motor through the
tappings of the autotransformer. The motor gets a lower starting voltage
according to the selection of the tapping. Also, the motor draws less starting
current and develops less starting torque accordingly.
When the motor attains 80% of its normal speed, auto-transformers are cut
out and the full supply voltage is applied to the motor. The switch making
these changes from ‘start’ to ‘run’ may be air-break (for small motors) or
maybe oil-immersed (for large motors) to reduce sparking.
Most of the auto-starters are provided with 3 sets of taps, to reduce the
voltage to 80, 65, or 50 percent of the line voltage, to suit the local conditions
of supply.
Most squirrel cage induction motors can be successfully started at 65 percent
of line voltage. Where this fraction of line voltage does not provide sufficient
starting torque, the 80 percent tap can be used. Where 50 percent tap creates
excessive voltage dip, the 65 percent taps can be used.
Autotransformer tapping 50% 65% 80%
Motor Starting Current 50% 65% 80%
Line Current 25% 42% 64%
Starting Torque 25% 42% 64%
Auto Transformer Starter Power Circuit Diagram
Auto Transformer Starter Power Circuit Diagram
Autotransformer Starter Control Circuit Diagram
Autotransformer
Starter Control Circuit Diagram
The working of the starter is as under:
When the start button is pressed a circuit is completed to the coil
of main contactor M, timer TR and shorting contactor S. And all
these get energized, the motor gets supply through the tappings of
the autotransformer.
The normally closed S contact connected in series with R coil opens
to prevent both S and R contactors from being energized at the
same time.
When the time sequence for TR timer is completed, it operates. Its
normally closed TR contact opens. This disconnects
contactor S from the line and opens the shorting of the
autotransformer. In this state, winding of autotransformer is
connected is series of motor like a reactor and keeps on running
smoothly.
After a very short time delay, normally open contact of
timer TR closes that closes the run contactor R and connects the
mains supply directly to the motor. After that motor reaches full
speed and is then in normal operating mode.
The normally open contact of the timer operates after its
normally closed contact with a very short time delay due to its
special time delay design. This time delay is very essential for the
smooth operation of the motor.
When the stop button is pressed, the main contactor M is de-
energizes. This disconnects all other control components from the
power line and the circuit returns to its normal position.
Advantages of Auto-transformer Starter
Due to the availability of different tappings, the autotransformer
starter is a more flexible option than the other starters. Therefore,
the user can select the best tapping according to the need of
starting torque and local conditions like voltage and voltage dip
during the commissioning of the starter.
These starters provide a continuous power supply to the
machine during the change over from low voltage to full voltage.
This eliminates the high transients switching current, false tripping
of other machines, and unneeded mechanical jerks on the machine.
They produce the highest starting torque per ampere of line
current drawn. So, are the best suited for the applications where
high torque is needed to overcome the inertia of the load, and at
the same time starting current is to be limited to the minimum.
