What are types of dc and ac induction motor
Different Types of dc motors:
Depending on the type of combinations of windings used on the stator and the rotor, electrical machines are classified in different types as follows:
1. DC machines:
The DC machines have an edge over AC machines when it comes to the speed control of a motor. It is easier and cheaper.
(a) Shunt motor and its characteristics of dc motors
Diagram:
 |
| dc shunt motor and torque characteristics |
This machine has field winding mounted in yoke and the armature winding is mounted on rotor. The shunt motor is used where speed regulation is important.
Self excited Field:Dc motor winding is connected in parallel (shunt) with the armature winding on the same supply. Changing the field current can vary the speed. Torque is proportional to armature current.
This machine can also act as a generator. To limit the high starting current of the motor drive release the voltage in the ramp. For this motor, a variable resistor is connected in series with the field circuit to change the flux value and the speed by a small amount.
This machine can also act as a generator. To limit the high starting current of the motor drive release the voltage in the ramp. For this motor, a variable resistor is connected in series with the field circuit to change the flux value and the speed by a small amount.
Separately excited:Dc motor field winding is connected in parallel (shunt) with the armature winding with separate excitation.
Torque is proportional to armature current. In a separately excited shunt motor, speed can be varied up to a certain limit by changing armature voltage. After that using field weakening (reducing field current), it is possible to increase the speed of motor above
base speed. Other features remain same as that of the self-excited one.
base speed. Other features remain same as that of the self-excited one.
(b) dc Series motor:
Diagram:
 |
| dc series motor and torque characteristics |
As the name suggests in this type of motors, field winding is connected in series with the armature winding. Naturally, heavy current will pass through it; hence field winding of a
thicker gage is used. A series motor is used where speed regulation is not important.
thicker gage is used. A series motor is used where speed regulation is not important.
The main advantage of this motor is that a high torque can be obtained, which makes it useful for applications such as diesel locomotives, cranes, etc.
The relationship between Torque and current is as follows:
Ta ∝ ɸ.Ia
Ta ∝ ɸ.Ia
It is important to start this motor in a loaded condition else it could lead to damage of the motor and its surroundings.
(c) Compound dc motor:
 |
| dc compound motor |
motor and the speed regulation of a shunt motor.
2. Types of ac induction motor :
(a) Squirrel-cage induction motor:
AC machines are simple and sturdy. The most common machine of this type is the
Squirrel cage induction motor (the name was derived from its construction type). The basic working of this was dealt with in the previous sections. The following relation gives
the speed of this motor:
Where
f= frequency
p= number of poles.
Squirrel cage induction motor (the name was derived from its construction type). The basic working of this was dealt with in the previous sections. The following relation gives
the speed of this motor:
Where
f= frequency
p= number of poles.
For example, if the motor has two poles, then at 50 Hz frequency the motor rpm will be
3000 (rpm).
However, you will not find 3000 or 1500 rpm on the motor nameplate because the
motor rpm will not be 3000 rpm at full load. This is because of a slip associated with an
induction motor.
The RPM of the motor is controlled by controlling the frequency ( f ) – as frequency
increases, motor speed will also increase. High starting current is limited using a
star/delta starter or reduced-voltage starters.
3000 (rpm).
However, you will not find 3000 or 1500 rpm on the motor nameplate because the
motor rpm will not be 3000 rpm at full load. This is because of a slip associated with an
induction motor.
The RPM of the motor is controlled by controlling the frequency ( f ) – as frequency
increases, motor speed will also increase. High starting current is limited using a
star/delta starter or reduced-voltage starters.
(b) Wound rotor induction motor:
This is similar in construction to the squirrel cage and works similarly too, except that slip rings are provided. The main feature of the slip ring motor is that resistors, which are connected in series with the rotor circuit, limit the starting current
The motor starts with a full resistance bank, but as speed of the motor increases, the resistances are shorted, one by one. As the motor reaches full speed, the whole bank of
resistance is shorted out and the motor now runs as a normal induction motor.
resistance is shorted out and the motor now runs as a normal induction motor.
(c) Synchronous motors:
Synchronous motor is a constant-speed motor, which can be used to correct the power factor of the three-phase system. Like the induction motor in terms of the stator, the synchronous machine has either a permanent magnet arrangement or an electromagnet (with current supplied via slip rings) rotor. In simple terms, the rotor will keep locking
with the rotating magnetic field in the stator. So, a two-pole machine will run at exactly
3000 rpm. In many synchronous machines, a squirrel cage is incorporated into the rotor for starting. Therefore, the machine acts as an induction motor when starting and as it approaches synchronous speed, it will suddenly ‘lock in’ to the synchronous speed.
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with the rotating magnetic field in the stator. So, a two-pole machine will run at exactly
3000 rpm. In many synchronous machines, a squirrel cage is incorporated into the rotor for starting. Therefore, the machine acts as an induction motor when starting and as it approaches synchronous speed, it will suddenly ‘lock in’ to the synchronous speed.
Read also
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