STARTING OF DC MOTOR:
NECESSITY OF A STARTER
What is starter?
Ans: Starter is a protective device used to start an electrical motor. For DC motor there are various type of starter- 3 point starter ,4 point starter, 2 point starter.
For dc shunt Motor,
3 point started is used.
For DC compound motor 4 point starter is used
For DC series motor 2 point starter is used.
Why starter is used for dc motor?
Ans:
To start the DC motor and to limit the inrush current at start starter is used for DC motor.
What is the necessity of starter for DC motor?
Ans: Starter is necessary to start the DC motor and as it is a protective device it protect the DC motor from the following.
Heavy inrush of current at the starting instant may cause
1.heavy sparking at the commutator and even flash-overs,
2.damage to the armature winding, either by the heat developed in the windings, or by the mechanical forces set up by electro-magnetic action.
3.damage to the rotating parts of the motor and load due to development of large starting torque and quick acceleration and
4. large dip in the supply voltage.
What is the significance of starter in DC motor.
When the motor is at rest the speed of the motor is zero, therefore, back emf E, is zero and if a de motor is connected directly to the supply mains, a heavy current will flow through the armature conductors. When running, of course, the applied voltage V is opposed by the induced back emf and, therefore, a much smaller current flows.
Heavy inrush of current at the starting instant may cause
1.heavy sparking at the commutator and even flash-overs,
2.damage to the armature winding, either by the heat developed in the windings, or by the mechanical forces set up by electro-magnetic action.
3.damage to the rotating parts of the motor and load due to development of large starting torque and quick acceleration and
4. large dip in the supply voltage.
Hence for the protection of the motor against the flow of excessive current during starting period (say 5 to 10 seconds), it is necessary that a high resistance be connected in series with the armature of the motor at the instant of starting and gradually cut in steps as the motor gains speed and develops back emf and ultimately when the motor
attains it’s normal speed , the additional resistance from the armature circuit is totally disconnected .If this additional resistance inserted is left in the armature circuit, it would causes (1) additional loss of of energy resulting in reduced operating efficiency and ii) reduction in operating speed of the motor.
Why started is not necessary for very small motors that is for fractional kilwate motor?
Ans: Starter is a protective device and it is used to start and limit the inrush current at the time of starting.
However, a very small motor (fractional kW motor) may be started simply by closing
the switch which connects it to the supply mains.
The reasons for it are given below.
1. The resistance and inductance of the armature winding in case of small motors are generally sufficiently large to limit the initial inrush of current to values that are not particularly serious.
2. The inertia of a small armature is generally so low that it comes upto speed very quickly, thereby minimizing the detrimental effects that might otherwise result from the excessive sustained current.
Describe 3 point starter.
Ans:
Description of 3 point starter and connection procedure:
Three-Point Starter. The three point starter with its electrical connections and protective devices (no-volt release and over-load release) is illustrated in fig. It consists of a series starting resistance divided into several sections and connected to brass studs, brass arc by which the connection to shunt field is made, no-volt release and over- load release. Since only three terminals (L,F and A) are available from the starter, it is called a three-point starter.
The last stud of the starting resistance is connected to terminal A to which STARTER one terminal of the armature is connected.
The + ve supply line is connected to the line terminal L through main switch.
From line terminal L supply is connected to the starter arm through over-load release.
A spiral spring S is placed over the lever to bring the starter arm to the OFF position in case of failure of supply.
A soft iron keeper is attached to the starter arm which is pulled by the holding coil or no-volt release under normal running condition.
The far end of the brass arc is connected to the terminal F through the no volt release and to the terminal F one terminal of the field is connected.
The- ve supply line is connected directly to the remaining ends of armature and field winding of the dc shunt Motor.
Operation:
When the Moto is at rest the starter arm is in the OFF position due to action of strong spiral spring S.
For starting the motor the dc supply is switched on by closing the main switch keeping starter arm in OFF position.
The starter arm (or handle) is then turned clockwise to the first stud and brass are (or strip).
As soon as it comes in contact with first stud, whole of the starting resistance R is inserted in series with the armature, the field winding is directly connected across the supply through the brass are and the holding coil is also energized.
As the starter arm is turned further the starting resistance is cut out of the armature circuit in steps and finally entire starting resistance R is cut out of armature circuit. When the starter arm reaches the ON position, it is held against the action of spiral spring S by the force of attraction between holding coil magnet and soft iron keeper attached to the starter arm.
** The starter arm should not be held for an unduly long time in an intermediate position as it is likely to burn out the starting resistor.
NO- VOLT RELEASE:
No-Volt Release. When the starter arm reaches "ON" position, the resistance is completely cut off and motor starts running at normal speed.
If the supply gets interrupted or disconnected, the starting arm will remain in the same position i.e. "ON" position and when the supply is switched on or gets restored, no back emf will be acting in circuit, the armature being directly across the supply mains, and resistance of armature being low, the motor will draw excessive current and will get damaged. Hence for the protection of the motor, some device must be provided, so that starter arm may reach the "OFF" position automatically as soon as the supply is cut off or disconnected or fails and for this purpose "no-volt" release coil is provided.
Function of No-Volt release:
No-volt release coil consists of an electro-magnet connected in series with shunt field which holds the arm in the "ON" position. Now when the supply fails or gets disconnected the electromagnet demagnetizes and so releases the starting arm A, which goes back to off position due to the spring attached to it and gets disconnected from the supply mains.
The other important advantage of connecting the no-volt release theerupply thains. sunt field winding is that it prevents the motor from running away owing to with the Sunt file starter arm A tad shunt field will demagnetise the electior tegne open ase the starter arm A and, thus the starter arm will go back to its OFF position and the supply will be disconnected.
Over-Load Release Coil.
Over-Load Release Coil. This coil is provided for the protection of the motor against he flow of excessive current due to over-load.
This coil is connected in series with motor so carries full load current.
When the motor is overloaded, it draws heavy current , which also flows with this coil and magnesites it to such an extent , that it pulls it’s arms upward in and so short-circuits the no-volt release coil, as shown in fig.
The no-volt lease coil, being short-circuited, demagnetises and releases the starting arm, which goes ack to "OFF" position with the action of spring attached to it and the motor is automatically connected from the supply mains.
Thus the motor is disconnected from the supply and protected against over-loading.
**The starters for motors upto 15 kW are provided with over-load release to disconnect the motor from the supply mains in the event of an over-load. Larger motors are provided with separate automatic circuit breakers.**
Q. Describe the 4 point starter.
Ans: Four-Point Necessity of 4 point starter:
In three point starter no-volt release coil connected in series with the shunt field and the field rheostat and, therefore, the current flowing through the field is the same current that flows through the holding coil. Thus if sufficient resistance is cut in by the field rheostat so that holding coil current is no longer able to create sufficient electro-magnetic pull to overcome the spring tension, the starter arm will fall back to the "OFF" position. It is this undesirable feature of the three point starter that makes it unsuitable for use with speed-controlled motors and that has resulted in the wide spread application of four point starters.
A four point starter with its internal wiring connected to a long shunt compound wound motor is shown in fig From fig it is obvious that when the arm touches stud no. 1, line current divides into three parts:
1. One part passes through starting resistance, armature and series field.
2. Second part passes through the shunt field winding.
3. The third part passes through no-volt release coil and protective resistance. in this arrangement, "no-volt release coil" circuit is independent of shunt field
Circuit, it will not be affected by the change of the current in the shunt field circuit.
It means that the electromagnetic pull exerted by the holding coil will always be sufficient and will prevent the spiral spring from restoring the arm to the "OFF" position, no matter how the field rheostat is adjusted.
The possibility of accidently opening the field circuit is quite remote; hence there is mater acceptance of the four-point starter over the three-point starter.
Starting of Motor:
A motor is started with a four-terminal starter in the same way as with a three- terminal starter. Any desired speed, above normal, of the motor can be obtained by adjustment of the field rheostat in series with the shunt field.
It is necessary to ensure, before starting a dc motor that the field circuit is closed, the rheostat in series with the shunt field winding is at zero resistance position and starting resistance in series with the armature circuit is at maximum value.
Stopping procedure:
For stopping the motor, the line switch should always be opened rather than throwing back the starting arm. In shunt motors, the line switch can be opened without any appreciable arc, since the motor develops a back emf nearly equal to applied line voltage and the net voltage across the switch contact is small. The electro-magnet energy stored in the field does not appear at the switch but is discharged gradually through the armature. On the other hand if the starting arm is thrown back the field circuit is broken at the last contact button. Owing to the inductive nature of the field, this will cause a hot arc and burn the contact.
Precaution.
While stopping the motor, all the resistance in the field rheostat is cut- out, so that motor speed falls to its normal value; then the line switch is opened. This procedure ensures that, the next time the Moto is started, it will be with a strong field and resultant strong starting torque.
Series motor starters: In series motors also, a starting resistance is inserted in series with the armature, for limiting the starting current to a safe value.
As usual, this start-ing resistance is cut out gradually as the motor accelerates. Fig. (a) illustrates à series motor starter used in applications, where the removal of load is remote. Holding coil in series with protective resistance R, acts as no-volt release, like the no-volt release of three-point or four-point starter. In view of this, starter of Fig. (a) is referred to as the no-volt release type of starter.
In case there is a possibility of removal of or reduction in load, starter illustrated in Fig. (b) may be used. In this, the load current passes through the series field, armature and holding coil. The holding coil consists of a few turns which are capable of carrying the load current. In case of removal of load or reduction in load below a safe value, the line current flowing through the holding coil is reduced. This reduced value of current decreases the strength of the holding magnet and the spring pull brings the starter handle to OFF position. This type of starter is, therefore, referred to as the no-load release type of starter.
Note.
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