Manually operated face-plate type starters used for starting dc series motors are commonly known as two-point starters as they have only two terminals for their insertion in the motor circuit.
Similar to three-point and four-point starters, it consists of a starting resistance R subdivided between number of contact studs 1 to 5. The starting handle H is pivoted on one side and its other side is free to move against a strong spring S and make contact with each stud during the starting operation.
The starter is provided with a protective device called no-load release. This no-load release consists of an electromagnet M and a hinged soft iron piece P carried by the starting handle. The coil of this electromagnet has a few turns wound on a U-shaped iron core and it is connected in series with the motor as shown in Fig.
For starting the motor, the dc supply is first switched on. The construction of the three-point starter is similar to the four-point starter. The only difference in their construction is that the holding coil is disconnected from the field winding and is directly connected to the line terminal along with the current limiting resistor R in series. The three-point and four-point starter both are similar in construction. But in three-point starter when the speed of the motor varies then the current passes through the field coil and this current affects the no-voltage coil.
The four-point starter is designed for reducing the problem. Your email address will not be published. Save my name, email, and website in this browser for the next time I comment. Skip to content The starter is a device which is used to start and run the motor. Key Differences Between the Three-point and Four-point Starter The starter which consists three terminals is known as the three-point starter. Then, as the lever is moved further, the resistance is gradually is cut out from the armature circuit.
Now, as the lever reaches to position 6, all the resistance is cut out from the armature circuit and armature gets directly connected across the supply. Answer Detailed Solution Below Option 3 : reduce the starting current. Explanation: A starter is necessary to start a DC motor because it restricts initial high armature current that exists, as the value of starting back-EMF is zero. A resistor is used in the starter for a DC shunt motor When the connected dc motor is to be started, the lever is turned gradually to the right When the lever touches point 1, the field winding gets directly connected across the supply, and the armature winding gets connected with resistances R1 to R5 in series During starting, full resistance is added in series with the armature winding Then, as the lever is moved further, the resistance is gradually is cut out from the armature circuit Now, as the lever reaches to position 6, all the resistance is cut out from the armature circuit and armature gets directly connected across the supply So, the purpose of including an external resistance at the time of starting a DC motor is to reduce the starting current.
Get Started for Free Download App. Answer Detailed Solution Below Option 2 : 3. 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. However, very small dc motors may be started directly by connecting them to the supply with the help of a contractor or a switch.
It does not result in any harm because they gather speed quickly due to small rotor inertia. In this case, the large starting current will die down quickly because of the fast rise in the back emf. Answer Detailed Solution Below Option 2 : 3 point. Drawback of 3-Point Starter: The three-point starter suffers from a serious drawback for motors with a large variation of speed by adjustment of the field rheostat.
Additional Information A 4 point starter is used to start and control the speed of a Dc shunt motor with field weakening control. Minimize back emf Minimize armature current Maximize armature current Maximize back emf. Three-Point Starter: The back emf E b is developed as motor armature starts to rotate in presence of the magnetic field.
The four-point starter uses the four terminals for the acceleration of the motor. Terminals Armature terminal, field terminal, and the Line terminal. Two point starter Five point starters Three point starter Four point starters. Control its speed Increase its starting torque Limit the starting current to a safer value Reduce armature reaction effect.
The necessity of D. Motor Starter: At starting, when the motor is stationary, there is no back EMF in the armature because the speed is zero. This heavy starting current may result in: i burning of armature due to excessive heating effect, ii damaging the commutator and brushes due to heavy sparking, iii excessive voltage drop in the line to which the motor is connected. The current drawn from the mains thus decreases and hence the torque.
This continues till the load torque and the motor torque are equal to each other. Machine tends to run continuously at this speed as the acceleration is zero at this point of operation. Various types of manual starters for dc motors, commonly known as two-point, three-point, and four-point starters are available.
A degree of similarity exists among these starters. All have a face plate rotator switch with a connected group of current limiting resistors. The difference lie in the form of protection they contain. A two-point starter is used for starting dc motor which has the problem of over-speeding due to loss of lod from its shaft. Such a starter is shown in figger.
Here for starting the motor, the control arm is moved clockwise from its OFF position to the ON position against the spring tension. The control arm is held in the ON position by an electromagnet.
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