Starting Method

STARTING METHORDS OF THREE PHASE INDUCTION MOTORS

INSTRUCTED BY: Mr. L.Meegahapola

NAME : H.M.S.L.G.Bandara INDEX NO : 050036J FIELD : EE DATE OF SUBMISSION: 2007.06.12

DISCUSSION: 

Construction and working principle of DOL starters

A direct on line starter or DOL starter is used as a starting method of electric motors. In DOL starting method is done by directly connecting the motor to the power supply. Hence, the motor is subjected to the full voltage of the power supply. Consequently, high starting current flows through the motor. At the starting moment the rotating magnetic field generated in the stator winding induces a voltage drop across the rotor. At the short-circuited end there will be a current governed according to the Faradays law. And also that current will create the torque. The motor starter is used with push button switches, relays, isolators, circuit breakers, transformers and interlock switches. The DOL starter circuit is divided into two circuits called as control circuit and power circuit. The motor terminals connect with the power supply by using the power circuit. It is the higher voltage circuit of the DOL. The control circuit is the lower-voltage circuit and it is electrically isolated from the power circuit. The starter is energized by the control circuit using electro-mechanical relays and push button switches. Modern starters employ solid state relays like electronic relays (over load protection relays). The switch may be a manually operated load break switch or circuit breaker. Although more commonly it would be an electromagnetic contactor which can be opened by the thermal overload relay under fault conditions. And also the contactor will be controlled by using different start and stop buttons.

A picture of a DOL switch Most motors can be run clockwise as well as anti-clockwise. To do that a reversing starter is used which is an electrical or electronic circuit that reverses the speed of a motor automatically. Therefore the circuit is composed of two DOL circuits that are for clockwise operation and the other one for anti-clockwise operation. These types of starters are used to start water pumps, compressors, fans and conveyor belts. The starting current may reach 3-8 times the normal current but the starting torque is tends to be 0.75 to 2 times the full load torque. The motor will draw a very high amount of current for a very short time. That is to establish 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 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. This is the most simple and inexpensive method of starting a squirrel cage induction motor. And also by using this method can avoid excessive supply voltage drops because of large starting currents the method is restricted to small motors only.  Shortcomings of the DOL starter. In DOL method the start time is dependant on the acceleration torque and the load inertia. In this method there is large starting current (That is between 3-8 times the full loads current). And also there is no specific method use to control that current. Because of large starting current used to start the motor there will also be a large voltage drop cross the distribution line. There fore this method is restricted to small motors only. Also due to that will affect on the other electrical consumer’s equipments or some times it may cause a mechanical problems with the driven load. There fore DOL method generally not applied relatively large machines. This method can be use below 7.5 kW. To decrease the starting current cage motors of medium and larger sizes are started at a reduced supply voltage. The reduced supply voltage starting is applied in star-delta and auto transformer methods.

Star-Delta starting The Star Delta starter method is another type of starting method. But it can only be used with a motor rated for connection in delta operation at the required line voltage. And also it should have both ends each of the three windings available individually. At start the winding is connected in a star connection. When the induction motor comes to its normal operation the switch is thrown quickly to the running position and the star connection is opened. After that open circuiting the motor and the ends of the windings are connected the motor in delta for normal operation. The connection changing is done by manually or automatically. In the manual method the switch changing is done by the relays and the switches which are driven by the relays. Although the tap changing should be done before the motor obtain the speed otherwise the fuses will blow up and the system fail occur. In the Automatic mode the connection changing is done by centrifugal switch which is connected the rotating parts in the motor or the timer circuit will make a certain time delay to speed up the motor and it switch on the relays to made connection changes. The timer delay can be adjusted by the user as required. So the best method is the centrifugal one. And also, • The phase voltages & the phase currents of the motor in star connection are reduced to 1/3 of the DOL values in delta. The starting torque is also reduced to 1/3 of its delta value. Because the phase voltage is proportional to the square of the applied voltage and phase voltage is reduce by 1/3 of the value in delta. • The Star Delta starter can only be used with a motor which is rated for connection in delta operation at the required line voltage, and has both ends each of the three windings available individually. The main benefits of the star delta starter are that it puts more money in the pockets of the switchgear supplier, and it is politically correct. Auto-transformer starting This method also reduces the initial voltage applied to the motor and therefore the starting current and torque. The motor, which can be connected permanently in delta or in star, is switched first on reduced voltage from a 3-phase tapped auto -transformer and when it has accelerated sufficiently, it is switched to the running (full voltage) position. The principle is similar to star/delta starting and has similar limitations. The advantage of the method is that the current and torque can be adjusted to the required value, by taking the correct tapping on the autotransformer. This method is more expensive because of the additional autotransformer.

Star-Delta Starting method

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Auto

Transformer

Auto Transformer In Auto Transformer starting, the auto transformer has at least three tapings. If the transformer ratio is 1:k (k