STAR DELTA STARTER

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.

Scheme – Working Principle of Star-Delta Starter

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.

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.

Power Circuit of Star Delta Starter

Control Circuit of Star-Delta Starter

WHEN PUSH BUTTON FOR START IS PRESS POWER FLOW START FROM TIMER COIL KT ,STAR COIL KM1,SO STAR CONTECTOR &AUX CONTACTOR IN CHANGE FROM NO TO NC .

NOW  TIMER RUNNING AND CURRENT FLOW IN MAIN COIL KM3 FROM STAR  AUX CONTACT.SO MAIN AUX CONTACT COME INTO NC FORM .SO IF WE RELEASE ON PUSH BUTTON POWER FLOW FROM KM3 LATCH CIRCUIT.

AFTER SOME TIME STAR CONTACTOR DEENERGIZE DUE OPEN OFF TIMER CONTACTOR AND DELTA CONTECTOR ENRGIZE SO POWER FLOW THROUGH KM2 & KM3 ONLY.

 IF WE PUSH STOP THAN POWER FLOW TO MOTOR STOP .

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

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

Size of Main Contactor= IFL x 0.58

Size of Star Contactor= IFL x 0.33

• 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.
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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.
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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.
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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.
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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.
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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.
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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.

DOL: DIRECT ON LINE STARTER

• Different starting methods are employed for starting induction motors because Induction Motor draws more starting current during starting. To prevent damage to the windings due to the high starting current flow, we employ different types of starters.
• The simplest form of motor starter for the induction motor is the Direct On Line starter. The DOL starter consist a MCCB or Circuit Breaker, Contactor and an overload relay for protection. Electromagnetic contactor which can be opened by the thermal overload relay under fault conditions.
• Typically, the contactor will be controlled by separate start and stop buttons, and an auxiliary contact on the contactor is used, across the start button, as a hold in contact. I.e. the contactor is electrically latched closed while the motor is operating.

Principle of DOL:

•  To start, the contactor is closed, applying full line voltage to the motor windings. The motor will draw a very high inrush current for a very short time, the magnetic field in the iron, and then the current will be limited to the Locked Rotor Current of the motor. The motor will develop Locked Rotor Torque and begin to accelerate towards full speed.
• As the motor accelerates, the current will begin to drop, but will not drop significantly until the motor is at a high speed, typically about 85% of synchronous speed. The actual starting current curve is a function of the motor design, and the terminal voltage, and is totally independent of the motor load.
•     Advantages of DOL Starter:
1. Most Economical and Cheapest Starter
2. Simple to establish, operate and maintain
3. Simple Control Circuitry
4. Easy to understand and trouble‐shoot.
5. It provides 100% torque at the time of starting.
6. Only one set of cable is required from starter to motor.
7. Motor is connected in delta at motor terminals.

•      Disadvantages of DOL Starter:
1.  It does not reduce the starting current of the motor.
2. High Starting Current: Very High Starting Current (Typically 6 to 8 times the FLC of the motor).
3. Mechanically Harsh: Thermal Stress on the motor, thereby reducing its life.
4. Voltage Dip: There is a big voltage dip in the electrical installation because of high in-rush current affecting other customers connected to the same lines and therefore not suitable for higher size squirrel cage motors
5. High starting Torque: Unnecessary high starting torque, even when not required by the load, thereby increased mechanical stress on the mechanical systems such as rotor shaft, bearings, gearbox, coupling, chain drive, connected equipments, etc. leading to premature failure and plant downtimes.
• SO WE USE IT FOR BELOW 10KW LOAD MOTOR.

Double-conversion systems

As the name suggests, these devices convert power twice. First, an input rectifier converts AC power into DC and feeds it to an output inverter. The output inverter then processes the power back to AC before sending it on to IT equipment. This double-conversion process isolates critical loads from raw utility power completely, ensuring that IT equipment receives only clean, reliable electricity. In normal operation, a double-conversion UPS continually processes power twice. If the AC input supply falls out of predefined limits, however, the input rectifier shuts off and the output inverter begins drawing power from the battery instead. The UPS continues to utilize battery power until the AC input returns to normal tolerances or the battery runs out of power, whichever occurs sooner. In case of a severe overload of the inverter, or a failure of the rectifier or inverter, the static switch bypass path is turned on quickly, to support

the output loads. 

Multi-mode systems
These combine features of both single- and double-conversion technologies while providing substantial improvements in both efficiency and reliability: • Under normal conditions, the system operates in line-interactive mode, saving energy and money while also keeping voltage within safe tolerances and resolving common anomalies found in utility power. • If AC input power falls outside of preset tolerances for line-interactive mode, the system automatically switches to double-conversion mode, completely isolating IT equipment from the incoming AC source. • If AC input power falls outside the tolerances of the double-conversion rectifier, or goes out altogether, the UPS uses the battery to keep supported loads up and running. When the generator comes online, the UPS switches to double-conversion mode until input power stabilizes. Then it transitions back to high-efficiency line-interactive mode

UPS: Uninterrupted Power System

 What is a UPS? 

 UPS is a device that: 1. Provides backup power when utility power fails, either long enough for critical equipment to shut down gracefully so that no data is lost, or long enough to keep required loads operational until a generator comes online. 2. Conditions incoming power so that all-too-common sags and surges don’t damage sensitive electronic gear. 

What are the main types of UPS?

UPSs come in three major varieties, which are also known as topologies:

 1)Single-conversion systems

 In normal operation, these feed incoming utility AC power to IT equipment. If the AC input supply falls out of predefined limits, the UPS utilizes its inverter to draw current from the battery, and also disconnects the AC input supply to prevent backfeed from the inverter to the utility. The UPS stays on battery power until the AC input returns to normal tolerances or the battery runs out of power, whichever happens first. Two of the most popular single-conversion designs are standby and line-interactive:

MCCB  MCB

MCB:

• MCB stands for “Miniature Circuit Breaker”.
• Rated current under 100 amps.
• Interrupting rating of under 18,000 amps
• trip characteristics may not be adjusted
• Suitable for low current circuits (low energy requirement), i.e. home wiring.
• Generally, used where normal current is less than 100 Amps.

MCCB:

• MCBB stands for “Molded Case Circuit Breaker”.
• Rated current in the range of 10-2500 amps.
• Thermal operated for overload and & Magnetic operation for instant trip in SC (Short circuit conditions)
• Interrupting rating can be around 10k – 200k amps.
• Suitable for high power rating and high energy i.e. commercial and industrial use.
• Generally, used where normal current is more than 100 Amps.

Difference between I_CS & I_CU in term of Circuit Breakers.

I_CS = Service Braking Capacity (means, Circuit breaker can remove the fault, but it may not be usable afterwards.)

I_CU = Ultimate Braking Capacity (means, Circuit breaker can remove the fault and remain usable)