Star-delta motor starter explained in details

Introduction to Star-Delta motor starter

Most induction motors are started directly on line, but when very large motors are started that way, they cause a disturbance of voltage on the supply lines due to large starting current surges.

To limit the starting current surge, large induction motors are started at reduced voltage and then have full supply voltage reconnected when they run up to near rotated speed.

Two methods used for reduction of starting voltage are: Star delta starting and Auto transformer starting.

Working Principle of Star-Delta Starter

This is the reduced voltage starting method. Voltage reduction during star-delta starting is achieved by physically reconfiguring the motor windings as illustrated in the figure below. During starting the motor windings are connected in star configuration and this reduces the voltage across each winding 3. This also reduces the torque by a factor of three.

After a period of time the winding are reconfigured as delta and the motor runs normally. Star/Delta starters are probably the most common reduced voltage starters. 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.

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 contractor 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 (33%) of the motor rating.

Star-delta Starter Consists following units

1. Contactors (Main, star and delta contactors) 3 No’s (For Open State Starter) or 4 No’s (Close Transient Starter).
2. Time relay (pull-in delayed) 1 No.
3. Three-pole thermal overcurrent release 1 No.
4. Fuse elements or automatic cut-outs for the main circuit 3 Nos.
5. Fuse element or automatic cut-out for the control circuit 1No.

Power Circuit of Star Delta Starter

The main circuit breaker serves as the main power supply switch that supplies electricity to the power circuit.

The main contactor connects the reference source voltage R, Y, B to the primary terminal of the motor U1, V1, W1.

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:

The star contactor serves to initially short the secondary terminal of the motor U2, V2, W2 for the start sequence during the initial run of the motor from standstill. This provides one third of DOL current to the motor, thus reducing the high inrush current inherent with large capacity motors at startup.

Controlling the interchanging star connection and delta connection of an AC induction motor is achieved by means of a star delta or wye delta control circuit. The control circuit consists of push button switches, auxiliary contacts and a timer.

Control Circuit of Star-Delta Starter (Open Transition)

The ON push button starts the circuit by initially energizing Star Contactor Coil (KM1) of star circuit and Timer Coil (KT) circuit. When Star Contactor Coil (KM1) energized, Star Main and Auxiliary contactor change its position from NO to NC.

When Star Auxiliary Contactor (1) (which is placed on Main Contactor coil circuit ) become NO to NC it’s complete The Circuit of Main contactor Coil (KM3) so Main Contactor Coil energized and Main Contactor’s  Main and Auxiliary Contactor Change its Position from NO to NC. This sequence happens in a friction of time.

After pushing the ON push button switch, the auxiliary contact of the main contactor coil (2) which is connected in parallel across the ON push button will become NO to NC, thereby providing a latch to hold the main contactor coil activated which eventually maintains the control circuit active even after releasing the ON push button switch.

When Star Main Contactor (KM1) close its connect Motor connects on STAR and it’s connected in STAR until Time Delay Auxiliary contact KT (3) become NC to NO.

Once the time delay is reached its specified Time, the timer’s auxiliary contacts (KT)(3) in Star Coil circuit will change its position from NC to NO and at the Same Time  Auxiliary contactor (KT) in Delta Coil Circuit(4) change its Position from NO To NC so Delta coil energized and  Delta Main Contactor becomes NO To NC. Now Motor terminal connection change from star to delta connection.

A normally close auxiliary contact from both star and delta contactors (5&6)are also placed opposite of both star and delta contactor coils, these interlock contacts serves as safety switches to prevent simultaneous activation of both star and delta contactor coils, so that one cannot be activated without the other deactivated first. Thus, the delta contactor coil cannot be active when the star contactor coil is active, and similarly, the star contactor coil cannot also be active while the delta contactor coil is active.

The control circuit above also provides two interrupting contacts to shutdown the motor. The OFF push button switch break the control circuit and the motor when necessary. The thermal overload contact is a protective device which automatically opens the STOP Control circuit in case when motor overload current is detected by the thermal overload relay, this is to prevent burning of the motor in case of excessive load beyond the rated capacity of the motor is detected by the thermal overload relay.

At some point during starting it is necessary to change from a star connected winding to a delta connected winding. Power and control circuits can be arranged to this in one of two ways – open transition or closed transition.

What is Open or Closed Transition Starting

1. Open Transition Starters

Discuss mention above is called open transition switching because there is an open state between the star state and the delta state.

In open transition the power is disconnected from the motor while the winding are reconfigured via external switching.

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 current 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.

Open transition starting is the easiest to implement in terms or cost and circuitry and if the timing of the changeover is good, this method can work well. In practice though it is difficult to set the necessary timing to operate correctly and disconnection/reconnection of the supply can cause significant voltage/current transients.

In open transition there are four states:

1. OFF State: All Contactors are open.
2. 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.
3. Open State: This type of operation is called open transition switching because there is an open state between the star state and the delta state. The Main contractor 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.
4. 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

2.   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.

This is achieved by introducing resistors to take up the current flow during the winding changeover. A fourth contractor is required to place the resistor in circuit before opening the star contactor and then removing the resistors once the delta contactor is closed.

These resistors need to be sized to carry the motor current. In addition to requiring more switching devices, the control circuit is more complicated due to the need to carry out resistor switching

In close transition there are four states:

1. OFF State. All Contactors are open
2. 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.
3. Star Transition State. The motor is connected in star and the resistors are connected across the delta contactor via the aux [KM4] contactor.
4. 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.
5. 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.

Effect of Transient in Starter (Open Transient starter)

It is Important the pause between star contactor switch off and Delta contactor switch is on correct. This is because Star contactor must be reliably disconnected before Delta contactor is activated. It is also important that the switch over pause is not too long.

For 415v Star Connection voltage is effectively reduced to 58% or 240v. The equivalent of 33% that is obtained with Direct Online (DOL) starting.

If Star connection has sufficient torque to run up to 75% or %80 of full load speed, then the motor can be connected in Delta mode.

When connected to Delta configuration the phase voltage increases by a ratio of V3 or 173%. The phase currents increase by the same ratio. The line current increases three times its value in star connection.

During transition period of switchover the motor must be free running with little deceleration. While this is happening “Coasting” it may generate a voltage of its own, and on connection to the supply this voltage can randomly add to or subtract from the applied line voltage. This is known as transient current. Only lasting a few milliseconds it causes voltage surges and spikes. Known as a changeover transient.

Size of each part of Star-Delta starter

1.   Size of Over Load Relay

For a star-delta starter there is a possibility to place the overload protection in two positions, in the line or in the windings.

Overload Relay in Line:

In the line is the same as just putting the overload before the motor as with a DOL starter.

The rating of Overload (In Line) = FLC of Motor.

Disadvantage: If the overload is set to FLC, then it is not protecting the motor while it is in delta (setting is x1.732 too high).

Overload Relay in Winding:

In the windings means that the overload is placed after the point where the wiring to the contactors are split into main and delta. The overload then always measures the current inside the windings.

The setting of Overload Relay (In Winding) =0.58 X FLC (line current).

Disadvantage: We must use separate short circuit and overload protections.

2.   Size of Main and Delta Contractor

There are two contactors that are close during run, often referred to as the main contractor and the delta contactor. These are AC3 rated at 58% of the current rating of the motor.

Size of Main Contactor= IFL x 0.58

3.   Size of Star Contractor

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 1/ √3= (58%) of the current in delta, so this contactor can be AC3 rated at one third (33%) of the motor rating.

Size of Star Contactor= IFL x 0.33

Motor Starting Characteristics of Star-Delta Starter

• Available starting current: 33% Full Load Current.
• Peak starting current: 1.3 to 2.6 Full Load Current.
• Peak starting torque: 33% Full Load Torque.

Advantages of Star-Delta starter

• The operation of the star-delta method is simple and rugged
• It is relatively cheap compared to other reduced voltage methods.
• Good Torque/Current Performance.
• It draws 2 times starting current of the full load ampere of the motor connected

Disadvantages of Star-Delta starter

1. Low Starting Torque (Torque = (Square of Voltage) is also reduce).
2. Break In Supply – Possible Transients
3. Six Terminal Motor Required (Delta Connected).
4. It requires 2 set of cables from starter to motor.
   .
5. It provides only 33% starting torque and if the load connected to the subject motor requires higher starting torque at the time of starting than very heavy transients and stresses are produced while changing from star to delta connections, and because of these transients and stresses many electrical and mechanical break-down occurs.
   .
6. In this method of starting initially motor is connected in star and then after change over the motor is connected in delta. The delta of motor is formed in starter and not on motor terminals.
   .
7. High transmission and current peaks: When starting up pumps and fans for example, the load torque is low at the beginning of the start and increases with the square of the speed. When reaching approx. 80-85 % of the motor rated speed the load torque is equal to the motor torque and the acceleration ceases. To reach the rated speed, a switch over to delta position is necessary, and this will very often result in high transmission and current peaks. In some cases the current peak can reach a value that is even bigger than for a D.O.L start.
   .
8. Applications with a load torque higher than 50 % of the motor rated torque will not be able to start using the start-delta starter.
   .
9. Low Starting Torque: The star-delta (wye-delta) starting method controls whether the lead connections from the motor are configured in a star or delta electrical connection. The initial connection should be in the star pattern that results in a reduction of the line voltage by a factor of 1/√3 (57.7%) to the motor and the current is reduced to 1/3 of the current at full voltage, but the starting torque is also reduced 1/3 to 1/5 of the DOL starting torque.
   .
10. The transition from star to delta transition usually occurs once nominal speed is reached, but is sometimes performed as low as 50% of nominal speed which make transient Sparks.

Features of star-delta starting

1. For low- to high-power three-phase motors.
2. Reduced starting current
3. Six connection cables
4. Reduced starting torque
5. Current peak on changeover from star to delta
6. Mechanical load on changeover from star to delta

Application of Star-Delta Starter

The star-delta method is usually only applied to low to medium voltage and light starting Torque motors.

The received starting current is about 30 % of the starting current during direct on line start and the starting torque is reduced to about 25 % of the torque available at a D.O.L start. This starting method only works when the application is light loaded during the start.

If the motor is too heavily loaded, there will not be enough torque to accelerate the motor up to speed before switching over to the delta position.