Types of DC motors, DC compound motors, AC servomotors and DC servomotors, Asynchronous motors - applications

Types of DC Motors

DC Motors can be categorized into four types depending on the connection of the field and the armature windings:

DC Shunt Motors
In these motors, the field and the winding are connected in parallel. They are used in applications where there is minimal change in speed as the motor is loaded. The provide medium torque while starting.

DC Series Motors:
These motors are used for applications requiring high starting torque. Here, the field and the winding are connected in series. These motors can be used in applications requiring high starting torque such as in traction related applications. The load on these motors must never be reduced to zero as this may result in excessive speed.

Permanent Magnet Motors:
These are used in applications which require greater reliability. Here, the field is made up of permanent magnets. The efficiency of the motor is higher. Speed can be controlled by varying the voltage of the armature.

Compound wound motors:
These motors combine the features of shunt and series motors. They have on field winding connected in series to the armature and another field winding connected in parallel. They provide a heavy starting torque. This kind of motor can be used for loads which are not sensitive to speed variations.

DC Compound Motors

A compound motor is a combination of shunt and series motor i.e., a series field winding, wound with heavy copper conductor on top of the shunt field winding. The series field winding is connected in series with the armature. So that its mmf will be proportional to the armature current and in the same direction as the shunt field mmf

Typical compound motors designed for industrial application obtain approximately 50% of their mmf from the series field wen operating at rated load.

There are two types of compound motors connection,

If the connection to the series and shunt winding is in such a way that their respective mmfs are additive is called cumulative compound motor.

If the series field is reversed with respect to the shunt field, its mmf will subtract from the shunt field mmf, causing the net flux to decrease with increasing load, resulting in excessive speed, which is differential compound motor.

Stabilized - shunt motor
Compound motors, whose series field are designed to provide just enough mmf to nullify the equivalent demagnetizing mmf of armature reaction and provide a very slight speed droop, are called stabilized – shunt motors. The series field winding of such machines generally have one – half to one and half turns / pole and depending on the application, provide approximately 3 to 10 percent of the total field mmf at rated load. The speed of stabilized – shunt motors is fairly constant, with only a slight droop in speed with increasing load. Stabilized – shunt motors are used in applications that require a fairly constant speed and a moderate starting torque.
Reversing the direction of rotation of compound or stabilized – shunt motors is accomplished by reversing the armature branch or reversing both the series field and the shunt field.

AC Servomotors and DC servomotors-A comparison

AC Servomotors DC Servomotors
Suitable for Low Power applications
They have low efficiency
The operation is stable and smooth
It has low Maintenance as there is no commutator

Used for High Power Applications
They have high efficiency
Noise is produced during operation
Relatively more maintenance is required due to the presence of the commutator.

Asynchronous motors

Asynchronous motors are motors which run below the synchronous speed.  The term "asynchronous motors" is usually used to refer to induction motors.

Motors which run at super synchronous speed are also asynchronous motors.  The doubly fed induction motor is an example of a super synchronous motor.

By "asynchronous" we mean that the speed of the rotor is not equal to the synchronous speed of the rotating magnetic field of the stator.  The difference between the speed of the rotor and the speed of the stator is called slip.  In asynchronous motors, the slip is not equal to zero.  In synchronous motors, the slip is zero.

Synchronous motors are motors that run a a constant speed, known as the synchronous speed.  The speed of the synchronous motor is constant regardless of the load.

Applications of Synchronous Motors

Synchronous Motors are used in the following situations

Applications which require constant Speed
Applications which require load as well as power factor improvement

Synchronous Motors are used in applications which require constant speed.  Examples can be escalators which need to rotate at a constant speed as people get in and get out.  In industries, synchronous motors are used in systems which have to operate in synchronism such as in bottling plants, in robotics and in conveyors.

Fractional Synchronous motors are used for small applications such as in microwaves, electric clocks and in data storage devices.  Fractional Synchronous motors do not have wound excitation system.  The rotor contains permanent magnets which run in synchronism with the stator.
Synchronous Motors for Power Factor Correction.

Synchronous Motor operated in an overexcited condition can be used for power factor correction.  The overexcited synchronous motor draws active power from the grid and supplies reactive power.  This helps improve the power factor of the system.  A motor run in this manner is called a Synchronous Condenser