Star / Delta Starters
Star/Delta starters are probably the most common reduced
voltage starters in the 50Hz world. (Known as Wye/Delta starters in the
60Hz world). They are used in an attempt to reduce the start current applied
to the motor during start as a means of reducing the disturbances and
interference on the electrical supply.
In many, if not most, cases the star/delta starter does little to reduce
problems, infact it commonly exacerbates them.
Tradditionally in many supply regions, there has been a
requirement to fit a reduced voltage starter on all motors greater than
5HP (4KW). This regulation was introduced in order to reduce the start
current, but unfortunately, a solution was specified rather than a result.
The Star/Delta (or Wye/Delta) starter is one of the lowest cost electromechanical
reduced voltage starters that can be applied and this is why it has been
so popular. The Star/Delta starter complied with the regulations, but
did not achieve the desired results.
The Star/Delta starter is manufactured from three contactors,
a timer and a thermal overload. The contactors are smaller than the single
contactor used in a Direct On Line starter as they are controlling winding
currents only. The currents through the winding are 1/root 3 (58%) of
the current in the line.
There are two contactors that are close during run, often referred to
as the main contactor and the delta contactor. These are AC3 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 AC3 rated at one third of the motor rating.
In operation, the Main Contactor (KM3) and the Star Contactor
(KM1) are closed initially, and then after a period of time, the star
contactor is opened, and then the delta contactor (KM2) is closed. The
control of the contactors is by the timer (K1T) built into the starter.
The Star and Delta are electrically interlocked 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
contacor 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.
Open Transition Starters.
When a motor is driven by the supply, either at full speed
or at part speed, there is a rotating magnetic field in the stator. This
field is rotating at line frequency. The flux from the stator field induces
a curent in the rotor and this in turn results in a rotor magnetic field.
When the motor is disconnected from the supply (open transition) there
is a spinning rotor within the stator and the rotor has a magnetic field.
Due to the low impedance of the rotor circuit, the time constant is quite
long and the action of the spinning rotor field within the stator is that
of a generator which generates voltage at a frequency determined by the
speed of the rotor. When the motor is reconnected to the supply, it is
reclosing onto an unsynchronised generator and this results in a very
high current and torque transient. The magnitude of the transient is dependent
on the phase relationship between the generated voltage and the line voltage
at the point of closure, but typically can be much higher then DOL current
and torque and can result in electrical and mechanical damage.
Closed Transition Star/Delta Starter.
There is a technique to reduce the magnitude of the switching
transients. This requires the use of a fourth contactor and a set of three
resistors. The resistors must be sized such that considerable current
is able to flow in the motor windings while they are in circuit.
The auxiliary contactor and resistors are connected across the delta contactor.
In operation, just before the star contactor opens, the auxiliary contactor
closes resulting in current flow via the resistors into the star connection.
Once the star contactor opens, current is able to flow round through the
motor windings to the supply via the resistors. These resistors are then
shorted by the delta contactor. If the resistance of the resistors is
too high, they will not swamp the voltage generated by the motor and will
serve no purpose.
In effect, there are five states:
- OFF State. All Contactors are open
- Star State. The Main [KM3] and the Star [KM1] contactors are closed
and the delta [KM2] contactor is open. The motor is connected in star
and will produce one third of DOL torque at one third of DOL current.
- Star Transition State. The motor is connected in star and the resistors
are connected across the delta contactor via the aux [KM4] contactor.
- Closed Transition State. The Main [KM3] contactor is closed and the
Delta [KM2] and Star [KM1] contactors are open. Current flows through
the motor windings and the transition resistors via KM4.
- Delta State. The Main and the Delta contactors are closed. The transition
resistors are shorted out. The Star contactor is open. The motor is
connected to full line voltage and full power and torque are available.