DC machines are the example of rotating machines. Rotating machines are used for electro-mechanical energy conversion i.e., Generator converts Mechanical energy to electrical energy and Motor converts electrical energy to mechanical energy.
When Energy is converted from electrical energy to mechanical energy or vice versa, then it is first converted to magnetic energy and then to mechanical energy and hence magnetic energy acts an intermediate stage between electrical and mechanical energy.
It works on the principle that when an electric current carrying conductor is placed in a magnetic field, mechanical force experienced on the conductor, the direction is given by Fleming left hand rule and hence conductor moves in the direction of force.
TYPES OF DC MOTOR:
1. Separately excited DC Motor : There is no connection between armature & field current.
2. Self excited: There are different connections of self-excitation of DC motor
2.1. Series motor : There is same flow of current in armature & field winding.
2.2. Shunt motor : There is common voltage for field and armature winding.
STARTING OF DC MOTORS:
At starting, the speed of DC motor is zero and hence the induced EMF which is proportional to speed of motor is also zero.
Due to which,
Vt = Iara for shunt & separately excited motor.
Vt = Ia (ra + rse) for series and compounded motor
As a result the current Ia is very high due to small values of ra and (ra + rse).
Such heavy current may cause:
- Sparking at commutator
- Damage to armature winding and isolation.
- High starting torque and acceleration
- Large dips in supply voltage.
So this current must be limited by inserting a resistance in the armature circuit. But this resistance must be cut off as rotor accelerates else-
- Operating speed of motor will be reduced.
- Losses will be higher and efficiency will be less.
To avoid the above dangers while starting a DC motor, it is necessary to limit the starting current. So, a DC motor is started by using a starter. There are various types of dc motor starters, such as 3-point starter, 4point starter, no-load release coil starter, thyristor controller starter etc. The basic concept behind every DC motor starter is adding external resistance to the armature winding during starting.
Hence, 3-point starters and 4-point starters are used for starting shunt wound motors and compound wound motors.
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.
It can be seen that, when the arm is moved from the position 1 to the last position, the starter resistance gets added in series with the field winding. But, as the value of starter resistance is very small as compared to the shunt resistance, the decrease in shunt field current may be negligible. However, to overcome this drawback a brass or copper arc may be employed within a 3 point starter which makes a connection between the moving arm and the field winding.
The main difference between a 3-point starter and a 4-point starter is that the no voltage coil (electromagnet E) is not connected in series with the field coil. The field winding gets directly connected to the supply, as the lever moves touching the brass arc (the arc below the resistance studs). The no voltage coil (or Hold-on coil) relates to a current limiting resistance R.
Delay in stopping a motor may result in heavy damaged to the equipment or to the manufactured products and even the loss of human life.
Requirements of braking:
- Braking should be quick and reliable in action
- Braking torque must be controllable
- Failure of any part of braking system must result in application of brakes
- Provision of some suitable means to dissipate kinetic energy of the moving parts of the motor and its driven machine or machines.
Disadvantages of Electric Braking:
- Shock caused to the motor and the equipment
- Heavy in-rush of current at the time of braking.
- This method is used to get either a quick reversal or get a rapid stop.
This method of braking can be applied to brake with a separate source of dc excitation during braking
- Direct current motors
- Synchronous motors
- Induction motors
TYPES OF BRAKING :
- Regenerative Braking:
Rheostatic or Dynamic Braking:
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