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Learning Objectives: After reading this CONTENT, YOU MUST be able to:
1. Define Motor Starters.
2. identify the main functions of a motor starter.
3. Develop safety awareness in the workplace.
This information will cover the importance of motor starter and its main functions. This will also cover on How a Motor Starter Works.
What is a Motor Starter?
What is a Motor Starter?
A motor starter is an electrical device that is used to start & stop a motor safely. Similar to a relay, the motor starter switches the power ON/OFF & unlike a relay, it also provides a low voltage & overcurrent protection.
The main function of a motor starter is;
• To safely start a motor
• To safely stop a motor
• To reverse the direction of a motor
• To protect the motor from low voltage & overcurrent
A motor starter is made of two main components that work together to control & protect the motor;
• Electrical Contactor: The purpose of the contactor is to switch ON/OFF the power supply to the motor by making or breaking the contact terminals.
• Overload protection circuit: The purpose of this circuit is to protect the motor from potential harm due to the overload condition. Huge current through the rotor may damage the winding as well as other appliances connected to the supply. It senses the current & breaks the power supply.
Why We Need a Starter with a Motor?
Why We Need a Starter with a Motor?
A motor starter is essential for starting an induction motor. It is because of its low rotor impedance. The rotor impedance depends on the slip of the induction motor which is the relative speed between the rotor & stator. The impedance varies inversely with the slip.
The slip of the induction motor is at maximum i.e. 1 at standstill (rest position), thus the impedance is at its minimum & it draws a huge amount of current called inrush current. The high inrush current magnetizes the air gap between the rotor & stator that induces an EMF in the rotor winding. This EMF produces an electrical current in rotor winding that creates a magnetic field to generate torque in the rotor. As the rotor speed increases the slip of the motor decreases & the current drawn by the motor is reduced.
The high inrush current is 5-8 times the normal rated full load current. So such amount of current can damage or burn the windings of the motor that will render the machine useless & it can cause a huge dip in voltage of the supply line that can damage other appliances connected to the same line.
In order to protect the motor from such a huge amount of currents, we use a starter that limits the initial current for a short duration at startup & once the motor attains a certain speed, the normal power supply to the motor is resumed. They also provide protection against fault conditions such as low voltage & overcurrent during normal operation.
Although small motors rated below 1 horsepower have high impedance and they can withstand the initial current thus they do not need such motor starter, however, they do need overcurrent protection system which is provided by the DOL (Direct On-Line) starters.
How a Motor Starter Works?
How a Motor Starter Works?
A starter is a control device that is used for switching the motor either manually or automatically. It is used for safe ON/OFF control of electrical motors by making or breaking its contacts.
The manual starter is used for smaller motors where the hand operated lever is manually operated (move the contacts position) to the ON or OFF position. The disadvantage of these kinds of starters is that they need to switch ON after power frailer. In other words, they need manual control for each (ON or OFF) operation. Sometimes, this operation may leads to flow high currents in the motor winding which may burn the motor. This is why it is not recommended in most cases where other alternative motor starters with protection are used such as automatic starters.
On the other hand, the automatic starters which consist of electromechanical relays and contactors are used to switch the motor ON/OFF operation. When current passes through the contactor coils, it energizes and produces the electromagnetic field which pulls or pushes the contacts to make the connection of motor windings to the power supply.
The start and stop push buttons connected to the motor and starter can be used for ON and OFF operation of motors. The contactor coils can be de-energize by pushing the stop button which leads to de-energize the coil. This way, the contactor contacts move back due to spring arrangement to its normal position which leads to switch off the motor. In case of power failure or manual switch-off operation, the motor won’t start automatically until we manually start the motor by pressing the “start push button”. The following diagram shows that how a DOL motor starter operates for ON/OFF operation.
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