presentation on starter
TRANSCRIPT
presentation on Starter in three phaseInduction motor
Prepared By:
M. AFTAB ALAM 2097-FET/BSEE/F-13
Course Instructor: DR.SUHEEL ABDULLAH
Faculty of Engineering and TechnologyDepartment of Electronic EngineeringInternational Islamic University, Islamabad
Need of starter A starter is needed for a three phase induction motor because at the time of starting, if induction motor is started directly, it will draw large amount of current which causes damage to adjoining equipments.Thus, a starter is needed in order to limit the starting current.If a rated stator voltage is applied to the motor at the time of starting, then the motor will draw heavy starting current.
This will increase i2R losses in the winding which will overheat the motor.
The heavy starting current may damage the motor windings
Starter means at the time of starting our motor takes morecurrent due to high EMF in starting so its function is tolimit the starting current. we know in starting motor takes5 to7 times current of rated current.
In order to avoid these problems, we can use some kind of a starter to start the induction motor safely.
Stator resistance starter Auto transformer starter Star-delta starter Rotor resistance starter Direct on line (DOL) starter
TYPES OF STARTER
STATOR RESISTANCE STARTER
A starter resistance is connected in each line in series with each phase winding of the stator.Initially all the starter resistance are kept in “Start” position so that they offer their maximum resistance .The switch is turned ON to connect the three phase AC supply to the stator winding.Due to starter resistance in series, each phase winding will receive a reduced voltage. Due to reduction in the value of V1
, the starting current is limited to a safe value.As the motor accelerates, the starter resistance is reduced by moving the variable contact of the resistance towards the “Run” position.In the “Run” position, the starter resistance is shorted out and full stator voltage is applied across the stator winding.
Advantages and disadvantages
The starter is simple in construction and cheap. It can be used for both star and delta connected
stator. There are large power losses due to resistances. Also the starting torque of the motor reduces due
to reduced voltage applied to the stator.
. AUTO-TRANSFORMER STARTER
An autotransformer is used to apply a low voltage to the stator winding at the time of starting. When the motor speed reaches the desired level, autotransformer is disconnected and motor is connected directly across the supply. The stator of the motor is connected through a 6-way double throw
switch. While starting, the switch is thrown to ‘Start’ side so that a reduced
voltage is applied to stator. This keeps the starting current safe limits. Once motor take up the speed, the switch is throw to ‘Run’ side so
that full supply voltage is applied to stator. A specific advantage of this starter is that reduction in voltage during
starting, can be done to any desired level by selecting proper tapping of the autotransformer.
Diagram
ROTOR RESISTANCE STARTER
This starter is used with a wound rotor induction motor. It uses an external resistance/phase in the rotor circuit so that rotor will develop a high value of torque.
High torque is produced at low speeds, when the external resistance is at its higher value.
At start, supply power is connected to stator through a three pole contactor and, at a same time, an external rotor resistance is added.
The high resistance limits staring current and allows the motor to start safely against high load.
Resistors are normally of the wire-wound type, connected through brushes and slip rings to each rotor phase. They are tapped with points brought out to fixed contactors.
As the motor starts, the external rotor resistance is gradually cut out of circuit ; the handle or starter is turned and moves the three contacts simultaneously from one fixed contact to the next.
The three moving contacts are interconnected to form a start point for the resistors.
Star Delta 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.
D.O.L.(direct on line) starter
A Starter Which Connects A Motor Directly Across The Line Is Called D.O.L. Starter.In This Method, The Motor Is Connected By Means Of A Starter Across The Full Supply Voltage.It Is Very Simple, Inexpensive, Easy To Install And Maintain.It Consist - START Button - OFF Button - Overload RelaySwitching By This Starter Is Directly From Line Without Any Provision To Control The Starting Current i.e. There Is No Device To Reduce The Starting Current In This Starter.
SOFT STARTERS
Soft-Starter is an electronic starter designed to accelerate, de-accelerate three-phase induction motors and to control the voltage applied on the motor during start and stop.
Block Diagram Starting Principle of Soft Starter
By using six SCR’s in a back to back configuration , the soft starter is able to regulate the voltage applied to the motor during starting from 0 volts up to line voltage.Feedback from the motor to the logic circuit used to control the SCR firing for stabilization of motor acceleration
Frequency do not change as in VFD Only the voltage and current changes.
Difference between DOL/Star delta /Autotransformer
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