WORKING
PRINCIPLE OF STAR-DELTA STARTER
There are three states
of Star-Delta starters, Connected
State. During starting time Main and Star Contactors remain closed and complete
Circuit.
In Star Connected State, voltage applied is reduced to 1/√3 of the
Line Voltage across each winding. As and when motor attains good rotational speed,
say about 90% of full R.P.M after few seconds, timer connected in starter
disconnects Star Contactor first and then connects Delta Contactor. Between
these two, Star connected and Delta connected states, circuit becomes open and
motor neither remains in Star nor in Delta State. This is called open
transition switching.
In Delta connected state voltage applied to windings is
equal to Line Voltage. These items Required to Make Star Delta Starter Three
Contactors (One Main Contactor, One Star Contactor and One Delta Contactor),
Over Load Relay (or OLR), Star - Delta Timer, Miniature Circuit Breaker (MCB),
Start Push Button (NO) Stop Push Button (NC) and some NO, NC Auxiliary
contacts.
WORKING OF
There are two contactors that are
close during run, often referred to as the main Contactor and the delta
contactor. These are AC 3 rated at 58% of the current rating of the motor. The
third contactor is the star contactor and that only carries star current while
the motor is connected in star. The current in star is one third of the current
in delta, so this contactor can be AC 3 rated at one third of the motor rating.
In operation, the Main Contactor (K3) and the Star Contactor (K1) are closed
initially, and then after a period of time (setting time of KT), the star
contactor is opened, and then the delta contactor (K2) is closed. The control
of the contactors is by the timer (KT) built into the starter. The Star and Delta are electrically
interlocked (K2, K3 NC Auxiliary contacts) and preferably mechanically
interlocked as well. In effect, there are four states: OFF State. All
Contactors are open Star State.
The Main and the Star contactors are closed and
the delta contactor is open. The motor is connected in star and will produce
one third of DOL torque at one third of DOL current, Open State. The Main Contactor
is closed and the Delta and Star contactors are open. There is voltage on one
end of the motor windings, but the other end is open so no current can flow.
The motor has a spinning rotor and behaves like a generator. Delta State. The
Main and the Delta contactors are closed. The Star contactor is open. The motor
is connected to full line voltage and full power and torque are available.
This
type of operation is called open transition switching because there is an open
state between the star state and the delta state.
