Motor starters are electrical devices used to safely start, stop, and reverse the direction of a motor as well as protect the motor from low voltage and overcurrent. Overcurrent, or electrical overloading as it is often called, can result in the excessive accumulation of heat, posing the risk of fire or damage to assets or equipment. For this reason, if the output current surpasses the safe limit of your starter, the motor starter will deactivate the equipment in question.
A motor starter is composed of two main parts, the electrical contactor and the overload protection circuit. The electrical contactor serves as an ON/OFF switch that either makes or breaks the contact terminals that supply power to the motor. The overload protection circuit on the other hand, protects the motor from potential damage inflicted by the overload condition. As massive amounts of current travel through the rotor, the winding as well as other appliances connected to the power supply may become damaged.
Motor starters are essential for a number of applications, but especially for starting an induction motor with a low rotor impedance. The rotor impedance relies on the slip, or other in other words, the relative speed between the rotor and the stator. When the slip is at a standstill, the impedance is at its minimum and has the ability to draw a huge amount of inrush current. The inrush current magnetizes the air gap between the rotor and stator, inducing an electromagnetic field (EMF) in the winding of the rotor. The resulting magnetic field generates torque in the rotor, allowing the rotor speed to increase, the slip of the motor to decrease, and the current drawn by the motor is reduced. It is important to note that the inrush current is typically 5-8 times stronger than a full load current, which can damage or burn the windings as well as cause an incredible decrease in voltage of the supply line that can damage any other systems connected to the same line. In order to protect the motor from the inrush current, the starter stalls the current at start up. This allows the motor to attain a certain speed before the normal power supply to the motor is resumed.
Starters can be roughly categorized as either a manual starter, an automatic starter, or a magnetic starter. Manual starters are used for smaller rotors where an ON or OFF position is achieved using a hand lever. A drawback of manual starters is their need to manually control each ON and OFF position, resulting in a flow of high currents that can burn the motor. In contrast, automatic starters consist of electromechanical relays and contactors that serve to switch the motor ON and OFF. As current travels through the contactor coils, the resulting EMF pushes the contacts to facilitate a connection between the motor windings and the power supply. As for a magnetic starter, this type of motor starter operates electromagnetically by either making or breaking the contacts using magnetism. Magnetic starters generate a lower and safer voltage as well as provide protection against low voltage and overcurrent. Moreover, during a power failure, the magnetic starter breaks the circuit. Now that you are familiar with the basics of motor starters, we will cover the common types of motor starters and their advantages.
Direct Online (DOL) Starter
A DOL starter consists of a magnetic contactor that connects the motor with a supply line and an overload relay for protection against overcurrent. Since this type of starter cannot reduce voltage for safely starting a motor, the motors used with such starters often have a horsepower rating below 5. Its cost-effective and simple design makes it a popular choice among other starters; however, the high inrush current can damage the windings and cause a voltage dip in the power line.
Stator Resistance Starter
Stator resistance starters are connected in each line series with each phase winding of the stator. The resistor’s role is to reduce the line voltage applied to the stator which reduces the initial current as well. Initially, the resistor is kept at a “start” position in order to offer maximum resistance. As a result, a safe amount of voltage travels across the motor due to the voltage drop across the resistor. The lower voltage prevents the inrush current from damaging the motor windings. As the motor speeds up, the resistance is reduced and the stator phase is directly connected to the power lines. Though this starter can be connected to both a star or delta connected motor and its variable voltage supply enables smooth acceleration, the resistors can be expensive for large motors.
Rotor Resistance Starter (Slip Ring Motor Starter)
This kind of starter only works on a slip ring induction motor, hence its two names. External resistors are affixed to the rotor in a star combination through the slip ring. These resistors limit the rotor current, increase the torque, and successfully reduce the initial stator current. Once the motor achieves the desired speed, the resistors can be removed. Advantages of this starter include its wide range of speed control and low starting current using full voltage.
Autotransformer starters utilize an autotransformer to reduce the voltage applied to the stator winding during the starting phase. During the starting stage, the relay is at a start position so the tap point provides a reduced voltage for the startup. The motor relay switches between the tap points to increase voltage with the speed of the motor. While this kind of starter provides a better starting torque and flexible starting characteristic, its large size makes it unfavorable for compact applications.
Star Delta Starter
For this starter type, the windings of a 3 phase induction motor switch between star and delta connections to start the motor. To start the induction motor, it is arranged in a star by utilizing a triple pole double throw relay. When the motor finally accelerates, a timer relay changes the stator windings from a star connection into a delta connection, allowing the full voltage to travel across the winding. As a result, the motor operates at a rated speed. While the design of the star delta starter is simple and inexpensive, the switch from star to delta connection produces a mechanical jerk that may cause a disruption in the motor.
A soft starter uses semiconductor switches like TRIAC to control the voltages as well as the initial current supplied to the induction motor. Moreover, a phase-controlled TRIAC is utilized to generate variable voltage. This is achieved by modifying the conduction angle of the TRIAC. At maximum conduction angles, the full line voltage is applied to the induction motor, allowing it to run at a rated speed. This kind of starter provides a gradual and smooth increase in starting voltage, current, and torque.
Variable Frequency Drive (VFD)
The last type of motor starter this blog will be covering is the variable frequency drive (VFD). This starter is often used to control the induction motor’s speed as it is largely dependent on the supply frequency. The alternating current (AC) from the supply line is converted into direct current (DC) using rectifiers. Inversely, the pure DC is converted into AC with adjustable frequency and voltage by using the pulse width modulation technique through power transistors like IGBTs. Variable frequency drives offer complete control over the motor speed from 0 to rated speed, and the speed adjust option available for variable voltage provides better starting current and acceleration.
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