Methods of starting Squirrel Cage Induction Motors

METHOD OF STARTING ELECTRIC MOTOR

There are several method of starting Induction Motor namely:

• ·     Direct on line starter commonly called DOL
• ·         Star Delta starter
• ·         Rotor resistance starter
• ·         Stator resistance starter
• ·         Forward and Reverse starter
• ·         Auto transformer starter
• ·         Frequency inverter starter
• .      Servo starter ( position/timing based)

Before exploring the methods mentioned above, it is of higher importance to understand very well the makeup of an electric motor with emphasis on Three Phase Motor. There is virtually no process or packagining equipment that could operate in the industries without the need for motion transmission hence Electric motors are used for all forms of motions being rotational, translational or otherwise. Electric motor transforms electrical energy into mechanical energy. Electric motors are of many types and designs based on application. It has proven advantages ranging from simplicity of design, better load characteristics, ruggedness, constant efficiency at maximum speed, constant torque, durability when operated within the design conditions and also very easy to maintain. The makeup of the squirrel-cage induction motor are a stator, rotor and end shields. The stator houses the field windings (which is connected to power supply) and serves as the enclosure for the motor component parts, a squirrel cage rotor known as the armature, and end shields that holds the bearings and keeps the rotor in free position within the stator. Squirrel-cage induction motor uses Alternating current A.C. and is available in both single and three phase.

Other types of motors aside from Squirrel cage induction motor are D.C motor, Synchronous motor, Universal motor, centrifugal motor, servo motor, stepper motor and e.t.c

Because of the high current required to bring the squirrel cage motor from its stand still position to attaining maximum torque, it is important to adapt best starting method that will place less or no risk on the motor windings during the starting period since the starting current goes as high as 2 to 4 times the normal operating current.

For a three phase motor, at the terminal box, six leads from the coils are available. Two leads for each set of windings of the three pair that are 120degrees arranged apart from each other within the stator shell. These terminals required connection manipulation in order to achieve the desired starting method. The two possible arrangements are known as STAR and DELTA connections as shown below.

DIRECT-ONLINE STARTER
DOL is a method of starting three phase induction motor. It is a method by which the supply voltage is apply to the motor directly without any reduction or manipulation. The motor may be connected in star or delta and its power rating should not exceed 7.5KW or  10HP.

Components required to wire a DOL Starter

• a contactor capable of handling the motor rated current C1
• an overload relay that its current rating range can carry full load current of the motor F2
• start button S2
• stop button S1
• three pole circuit breaker or fuse for the power circuit protection F1
• 2 pieces of indicator lamps (green for motor running status and amber for motor tripped status)
• Three phase electric motor M1

OPERATION OF DIRECT ON LINE STARTER

After Power and Control circuits have been correctly wired, the following steps will confirm if the intended operation is achieved. In the control circuit, press Start button S2 and current will flow from L1 (power fupply) through F2 (Overload relay protective contact) to S1 (Stop button) to S2 (Start button) to C1 (Contactor coil) and to neutral N in order to energise the contactor C1 in the Power Circuit. ImmediatelyContactor C1 is energised, current in the power circuit will flow from L1, L2,L3 (3-Phase Power supply) through Contactor C1, through Overload relay F2 and to the Electric Motor M1 then the Motor will start running if all conditions are satisfied. on the control circuit, immediately current gets to C1 (contactor coil), 1L1 (Run lamp) comes ON and retaining contact of contactor C1 connected across the start button S2 allows current to keep flowing even after you have remove your finger from start button S2. If you want to bring the motor to a stop condition, you will need to activate stop button S1 by pressing it. this will situation will play out. current will flow in the control circuit from L1 to F2 to S1 but S1 would have become open and stop the flow on current to retaining contact C1 of contactor C1 thereby stops flow of current to coil C1 and that in respective order opens the C1 contactor in the Power circuit and the electric Motor goes on stop condition.

In the event of failure due to exceed mechanical load on the motor or loas of any otf the three phase (L1, L2,L3) feeding the motor, the overlaod relay F2 which is working on the principle of bimetallic strips opens the associated contact NO F2 on the Control circuit and same process for stop the Motor is activated, lamp 1L2 comes ON because contact NC F2 gets closed.

STAR-DELTA STARTER

STAR-DELTA is a method of starting three phase induction motor. It is a method by which the supply voltage is apply to the motor first on Star connection as revealed earlier taking advangage of the low voltage ( Phase voltage, between 220V to 240V) that is apply to each set of motor winding thereby reducing the starting current. A timer is used to delay the period with which the motor will run on Star connection before automatically switched to run on Delta connection with high voltage(Line voltage, bettween 380V to 415V). The motor will now continue to run on Delta connection with constant speed, maximum torque and designed operation current.  The motor terminal is niether connected in star or delta connection but instead all the six leads from motor terminal are connected via cable unto contactor which are connected in such a way that Star-Delta connection may be done automatically with the contactors following the control wiring logics.

Components required to wire a STAR-DELTA Starter

• three contactora capable of handling the motor rated current C1, C2, and C3
• an overload relay that its current rating range can carry full load current of the motor F2
• start button S2
• stop button S1
• Timer T1
• three pole circuit breaker or fuse for the power circuit protection F1
• 2 pieces of indicator lamps (green for motor running status and red for motor tripped status)
• Three phase electric motor M1

OPERATION OF STAR-DELTA STARTER
It is important to mention that the power circuit shoul be carefully studied before connection, taking notice of the motor connections according to the terminal label. From the control circuit, once the start button S2 is pressed, current flows from supply L1 to NC of overload relay F2 and then through S2 to NC of Timer T1 to NC of contactor C2 to coil of contactor C3 and start  contactor C3 gets energised creating the star point. NO on contactor C3 becomes closed and  allows current to flow to coil of contactor C1 and Timr T1 which the in turn energised contactor C1. immediately this happens, motor start running in Star and NO of C1 closes and retain current in the circuit. the timer at the same time start counting to attain preset time and automatically opens NC of T1 and closes NO of T1 which will enable current flow to coil of contactor C2 thereby allow the motor to run on Delta

FORWARD-REVERSE STARTER

Forward-Reverse is a method of starting three phase induction motor. It is a method by which the supply voltage is apply to the motor directly without any reduction or manipulation like in DOL starter but the only differene is that two contactors are used. One for forward and the other for the reverse operation. To reverse the direction of a three phase induction motor, two out of the three supply cable is interchanged so that if the initial connection following the phase sequence makes the motor run in clockwise direction, after interchanging the two leads, the motor direction of rotation will goes anti-clockwise. This type of connections is applicable where motor is connected to a load which is required to be tranported to and fro, or of a lift that could lift a load up and down and many more.

Components required to wire a FORWARD-REVERSE Starter

• two contactora capable of handling the motor rated current C1, C2, and C3
• an overload relay that its current rating range can carry full load current of the motor F2
• stop button S1
• forward start button S2
• reverse start button S3
• three pole circuit breaker or fuse for the power circuit protection F1
• 3 pieces of indicator lamps (2 green for motor running status and red for motor tripped status)
• Three phase electric motor M1

OPERATION OF STATOR RESISTANCE STARTER
Do not get lost with the size of the two diagram above. The first is simply the power circuit which reveal the connection along the three phase path while the one beneath shows the control circiut that reveals how each component on the power circuit are connected to achieNevertheless care should be taken in order to avoid explosion because if both contactors are activated mistakenly at the same time due wrong connections, two phase will jam at the contactor output. From the control circuit, whenever S2 is pressed, it allows flow of current to contacto C1 coil and the motor runs in clockwise direction. whenever S1 is pressed either the motor is running in clockwise or anticlockwise direction, pressing S1 will make it goes off. if S3 is pressed, current flows to contactor C2 coil and motor runs in anticlockwise direction. to bring the motor to stop position, press S1. under fault condition F2 tripped off and allow the motor to have a safe stop.

STATOR RESISTANCE STARTER

Stator resistenace  starter is a method of starting high power three phase induction motor. It is a method by which the supply voltage is not directly apply to the motor winding. Resistance are wired with contactor swiching at different level to the motor windinging in order to start the motor on low current. The one shownn below is three steps Stator resistance stater. At first, the resistor shared the motor current in two steps and then goes on star connection befor e finally swicthes to delta connection when the motor would have gained momentum and then continue to run on maximum speed and torque.

Components required to wire a 3-STEPS STATOR RESISTANCE  Starter

• Six contactora capable of handling the motor rated current C1, C2, C3, C4, C5  and C6
• an overload relay that its current rating range can carry full load current of the motor F2
• stop button S1
• start button S2
• Resistor R1, R2, R3, R4, R5 and R6 Resistor
• three pole circuit breaker or fuse for the power circuit protection F1
• 2 pieces of indicator lamps (21green for motor running status and red for motor tripped status)
• Three phase electric motor M1