Which of the following motors is preferred when quick speed reversal is the main consideration?
Right Answer is:
DC motor is preferred when quick speed reversal is the main consideration.
There are three different types of DC open-loop motors. All DC motors have an armature, field windings, and housing. How the field windings are set up changes the operating characteristics of the motor. The three ways to set up field windings are
- To use permanent magnets
- To use series wound electromagnets
- To use parallel or “shunt” wound electromagnets.
In a permanent magnet DC motor, permanent magnets line the inside of the motor housing. The armature is made from a hunk of iron wrapped in multiple turns of copper wire. In series and shunt-wound motors, the field windings are electromagnets, wired to the same power source as the armature. Each of the three methods of DC motor winding has its advantages and disadvantages. In general, permanent magnet and shunt motors are more easily adapted to use as the main drive motors for a robot that are series-wound motors. The problem with series-wound motors is that they are harder to reverse. While permanent magnet and shunt wound motors can be easily reversed by simply switching the direction of the armature current, a series wound motor must be completely rewired in order to enable the motor to reverse.
Permanent magnet DC motors
The small DC motors (which has 12V or below 12V) made up of a permanent magnet i.e. it contains the permanent magnetic field. If you want to change the direction of the shaft you change the polarity only. Because it contains only armature winding only.
Temporary magnet DC motors
The high voltage DC motors (which have 220V or above 220V) made up of temporary magnet i.e. field and armature have separate winding. So if you change the polarity of the supply the total circuit will change. Due to that, the motor will rotate in the normal direction.
If you want to change the direction of the shaft you need to change either field or armature supply. Take care, that you change either the field or the armature wires. If both are changed at the same time, the direction remains same.
For a brushless DC motor, reversing the supply polarity won’t work and will possibly damage the motor. The electronic commutation needs to be reversible, which has to be part of the motor design.
In case of single phase AC motor the direction can be changed by changing the wiring in the starting circuit
In case of three phase induction motor, the direction of rotation can be changed by changing any two of the phases.
The direction of rotation of the synchronous motor is determined by its starting direction, as initiated by induction motor action. Thus, to reverse the direction of a 3 phase synchronous motor, it is necessary to first stop the motor and then reverse the phase sequence of the 3 phase connections at the stator.
REVERSING THE CURRENT TO FIELD WINDINGS WILL NOT AFFECT THE DIRECTION OF ROTATION.
Hence from the above discussion, it is clear that for quick speed reversal the dc motor is more suitable than any other type of motor.