Starters are used to protect DC motors from damage that can be caused by very high current and torque during startup. They do this by providing externalresistance to the motor, which is connected in series to the motor’s armature winding and restricts the current to an acceptable level.
To see why DC motors experience high startup current, we can examine the voltage equation of a DC motor:
Back EMF is directly proportional to the motor’s speed, so at startup, when motor speed is zero, back EMF is also zero. Therefore, removing the Eb term and rearranging the voltage equation, we can see that, at startup, armature current is inversely proportional to armature resistance.
For the best motor performance, armature resistance in DC motors is kept very low (typically less than 1 ohm).To see how significantly this affects the starting current, we can assume that armature resistance (Ra) is 0.4 ohm and supply voltage (E) is 220 V. This gives a starting current (Ia) of 550 amps, whichcan be more than tentimes the rated current, and high enough to damage the internal motor circuit.
Not only canthe high startup current result inpotential motor circuit damage, it can also produce dangerously high torque, which can cause the rotor to literally break apart. From the DC motortorque equation, we can see that torque is directly proportional to current:
To combat these problems, a motor starter adds external resistance (Rs) to the armature winding, which reduces the armature current:
But this resistance doesn’t need to be present through the motor’s full operating speed range. As motor speed increases, back EMF develops, which counters the supply voltage and also has the effect of reducing armature current:
As the back EMF reaches its maximum, the starter progressively decreases the external resistance, Rs, to zero.
Types of DC motor starters
Shunt wound and compound wound DC motors typically use either a 3 point or 4 point starter. The name comes from the number of terminals that connect to the motor, with 3 point starters having three terminals: the armature terminal, which is connected to the motor armature winding; the line terminal, which is connected to the positive supply pole; and the field terminal, which is connected to the field winding.
Four point starters are virtually identical to 3 point starters, but they have an additional terminal, labeled the “N” terminal, which links the supply to the no voltage coil of the starter. This protects against unnecessary tripping when the motor is run above its normal speed. Series wound motors typically use 4 point starters.
Three point and 4 point starters are manually controlledby an operator, whoadjusts theresistance that’s delivered to the motor through a simple lever or handle. But electronic starters are also available, using a micro controller and thyristors to regulatethe resistance delivered to the motor. A more recent development are starters that use “voltage chopping,” rather than resistance, to limit the startup current. “Chopping” the frequency of the input voltage varies itsratio of on-time versus off-time. Thisallows the average voltage to be controlled, so that armature current is kept to an acceptable level while speed (and, thus, back EMF) is developed.
Starters are used to protect DC motors from damage that can be caused by very high current and torque during startup. They do this by providing external resistance to the motor, which is connected in series to the motor's armature winding and restricts the current to an acceptable level.
Concept: Starter : A starter is necessary to start a DC motor because it restricts the initial high armature current that exists, as the value of starting back-EMF is zero.
6. What will happen if DC motor is used without starter? Explanation: It would cause intolerably heavy sparking at the brushes which may destroy the commutator and brush-gear. Sudden development of large torque will cause mechanical shock to the shaft, reducing its life.
DC motor starter is used to control the initial current.
A starter is necessary to start a DC motor because it restricts the initial high armature current that exists, as the value of starting back-EMF is zero.
The purpose of starter of a DC motor is to limit starting current taken by armature . This is done by having resistance in the armature circuit and as machine picks speed this resistance is progressively cut to zero. The starter is not intended to control speed.
As the name suggests, a starter is an electrical device which controls the electrical power for starting a motor. These electrical devices are also used for the purpose of stopping, reversing and protecting electric motors.
Motor Starters serve the functions of safely starting/stopping a motor, reversing the motor's direction, or protecting the motor from low voltage and overcurrent. A motor starter comprises two main components that complement each other to control and protect the motor.
Unfortunately, if the starter motor has completely gone there's no way to get your car going again. However, when a vehicle doesn't start, it's typically to do with the battery's insufficient voltage to start the motor.
DC motors are always self-starting in nature. In AC motors, the armature is stationary while the magnetic field rotates. In DC motors, the armature rotates while the magnetic field remains stationary.
Very small dc motors, however, may be started directly by connecting them to the supply with the help of a contactor or a switch. It does not result in any harm because they gather speed quickly due to small rotor inertia.
There are mainly 3 types of DC motor starters, such as 2 point starter, 3 point starter and 4 point starter. The basic concept behind every DC motor starter is adding external resistance to the armature winding during starting.
What is a Three-Point Starter? An electrical device known as a three-point starter is used to start and maintain the speed of a DC shunt motor. The connection of resistance in this circuit is in series which decreases the initial high current and protects the equipment against any electrical failures.
Purpose and Control: The purpose of the four-point starter is to control the starting current and speed of the DC motor. By gradually reducing the resistance in the armature circuit, it prevents excessive current flow during the motor startup, which could damage the motor windings.
A DC series motor should always be started with some load because at no load it will rotate with dangerously high speed. Important: When the motor is connected across the supply mains without load, it draws small current from the supply mains.
When you turn the ignition switch, the electromagnet inside the body engages & pushes out a rod to which the pinion is attached. The pinion meets the flywheel and the starter motor turns. This spins the engine over, sucking in air (as well as fuel).
The starter motor in your automobile is a DC motor. If you were to accidentally reverse the battery polarity, the DC motor would still rotate in the same direction. Reversing polarity of the battery will not cause the motor to rotate in the opposite direction.
Initially back emf is zero, therefore starting armature current is high. To limit or minimize starting armature current, we use 3- point starter. A three-point starter helps in starting and running the shunt wound motor or compound wound DC motor.
Introduction: My name is Gregorio Kreiger, I am a tender, brainy, enthusiastic, combative, agreeable, gentle, gentle person who loves writing and wants to share my knowledge and understanding with you.
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