Different Types Of Special Motors

Hello guys, welcome back to my blog. In this article, I will discuss different types of special motors, what is servomotor, working principle of servo motor, what is a stepper motor, working principle of the stepper motor, what is the universal motor, working principle of the universal motor, what is BLDC motor, working principle of BLDC motor, etc.

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Different Types Of Special Motors

Different Types Of Special Motors

There are mainly 4 types of special motors:

  1. Servomotor. 
  2. Stepper motor
  3. Universal motor.
  4. BLDC motor. ( Brushless DC motor )


A servo system is the one in which the output is a mechanical variable like position, acceleration, velocity. These systems are automatic control systems in which the output is like controlling the position of the shaft etc.

The motors which will be used in such a servo system are called servomotor. Servomotor will have a low power rating and can drive the load directly. That’s why these motors are usually coupled to the load, through a gear train for power matching purpose. 

AC Servomotor

Servomotor used in the low power servo mechanism is AC servomotor. 

Operating Principle Of Servomotor

Operating Principle Of Servomotor

The operating principle of 2 phase AC servomotor will be similar to that of a normal 3 phase induction motor. The voltage control will be applied to the control winding and the voltage applied to the reference winding will be is 90° out of phase. Then flux produces the current with the help of control winding, which is also 90° out of phase with respect to the flux produced by current through the reference winding. 

The resultant flux in the air gap will be rotating in nature hence it is called a rotating magnetic field. Initially, the rotor part will be stationary, then as the rotating flux sweeps over the rotor, e.m.f gets induced in the rotor. This e.m.f will circulates the current through the rotor. The rotor current will produce its own flux that is called as rotor flux. This flux will interact with the rotating magnetic field, which produces a torque on the rotor.

Applications Of Servo Motor

  1. It is used as instrument servos.
  2. Used as process controllers. 
  3. It is used in robotics. 
  4. Used in self-balancing recorders.
  5. It is used in machine tools.
  6. Used as a position control system. 

Advantages Of Servo Motor

  1. Having high output power. 
  2. Accuracy and resolution is determined by encoders. 
  3. Having high efficiency.
  4. Having 5 to 10 times more rated torque. 
  5. Having 2 to 3 times more rated power.

Disadvantages Of Servo Motor

  1. It requires tuning to stabilize the feedback loop.
  2. It becomes unpredictable when something breaks. 
  3. More expensive.
  4. Controlling of the servomotor is complicated.

Stepper Motor


This is the special type of DC motor, that converts input digital Pulses into mechanical shaft rotation. It converts a train of Input pulses (square wave pulses ) into precisely defined increment in the shaft position.

Every pulse of a square wave moves the shaft of the stepper motor through a fixed angle so we can define stepper motor as “ This is as electrochemical which actuates train of step movements of the shaft with with respect to the train of input pulses. “

Working Principle Of Stepper Motor

Working Principle Of Stepper Motor

The stepper motor works on the principle of Electromagnetism. It consists of two main parts. 

• Rotor. 
• Stator. 

For rotor, we can use a permanent magnet or soft iron and rotor part will be surrounded by electromagnetic stators. 

If we give a voltage at terminals, the rotor tries to align with the stator. To maintain a minimum gap with the stator which is due to magnetic effect. Here I would like to take a stepper motor which consists of 4 pole rotor construction. i.e. rotor is having 4 salient poles without any exciting winding.

Stages In Stepper Motor

Stages In Stepper Motor

When we excite AA' phase by keeping SW1 switch as close, then the stator magnetic axis aligns along the poles formed due to AA' ( vertical ). Then the rotor tries to adjust itself in a minimum reluctance position i.e. matching its own axis which passes through the two poles exactly with the stator magnetic axis. 

Then next when we excite phase BB' by keeping SW2switch off and AA' phase de-energized by keeping SW1 switch open. Now the stator magnetic axis shifts towards the pole which is formed by BB'. Then again rotor tries to align in minimum reluctance & it will turn by 30° step angle in an anti-clockwise direction. The axis passing along two diagonally opposite poles of the rotor will match with the stator magnetic axis. The point P will be a new position of minimum reluctance.

Then next when we excite phase CC' by keeping SW3 switch off and AA' and BB' phases are de-energized by keeping SW1 & SW2 switches open. Now the stator magnetic axis shifts towards the poles formed due to CC'. Then rotor tries to align in minimum reluctance & it will turn by 60° step angle in anti-clockwise direction. After exciting the three phases, the motor takes 12 steps to complete one full revolution.

Applications Of Stepper Motor

  1. Used in X-Y plotters.
  2. Used in a floppy disk drive.
  3. Used in machine tools.
  4. Used in the process control system.
  5. Used in robotics.
  6. Used in printers.
  7. Used in tape drives and a variety of others. industrial applications.

Disadvantages Of Stepper Motor

  1. It’s having low efficiency.
  2. Torque drops rapidly with speed.
  3. Having low output power for size.
  4. Missing of feedbacks to indicate missed steps.
  5. Heat experience in the high-performance configuration.
  6. Low accuracy at full load.

Universal Motor


“Universal motor can be operated on both DC & single-phase AC supply. This can be also called a single-phase series motor". These motors will be having high starting torque. This is the main feature of universal motors.

Working Principle Of Universal Motor

As the definition says that, the universal motor works on both DC & single-phase AC supply. If we give DC supply to the universal motor, it works as a DC series motor. i.e. if current flows through the field winding, then it will produce an electromagnetic field and the same current will flows through armature conductors. When we placed a current-carrying conductor in an electromagnetic field, it will experience a mechanical force. Due to this force, the rotor will rotate. The direction of the mechanical force will be explained by using “ Fleming’s left-hand rule".

If we give AC supply, then it produces unidirectional torque. It’s because both the winding of the motor are connected in series, which means they are in the same phase. But polarity of AC supply changes periodically, due to this action the direction of the current in both winding, reverses at the same time. Then the direction of the magnetic field & armature current reverses in such a way that the direction of the force experienced by armature conductors remains the same. Thus matter of AC or DC supply the Universal motor works as similar to DC series motor. 

Applications Of Universal Motor

  1. It can be used for domestic applications such as vacuum cleaners, food, mixers, hair drains, electric shavers, etc.
  2. It can be used in blowers.
  3. Used in portable tools like drilling machines and small drives. Non compensated universal motors can be used whenever there is a requirement of low horsepower.
  4. Compensated universal motors can be used whenever there is a requirement of high horsepower. 

Advantages Of Universal Motor

  1. Less costly as compared to other motors.
  2. The capacity of producing good torque at low speed.
  3. Preferred in many portable tools due to less size.
  4. The capacity of running at an adjustable speed. 

Disadvantages Of Universal Motor

  1. High noise.
  2. It will be difficult to run a universal motor in the opposite direction.
  3. Overheating if the motor takes place, to avoid this, we require fresh air.
  4. It requires frequent service and maintenance.

Brushless DC Motor

BLDC stands for “ Brushless Dc motor". That means these motors do not have brushes in it. Brushless DC electric motor can also be called as electronically commutated motors, which are powered by DC electric source through an integrated inverter or switching power supply, which produces an AC electric signal to drive the motor.

Working Of BLDC Motor

First, let’s see the function of a brushed motor. In brushed motors, there will be permanent magnets on the outside and spinning armature. This spinning armature contains an electromagnet. This electromagnet creates a magnetic field in the armature when supply is given and helps to rotate the armature. 

To keep the rotation of the armature, the brushes will change the polarity of the pole. The working principle of brushed DC motor and brushless DC motor is the same. i.e internal shaft position feedback.

The brushless DC motor has two windings. Rotor and stator. The rotor is rotating part and has rotor magnets in it. Where stator is stationary winding that consists of the stator. 

In the rotor, the permanent magnets are attached and move the electromagnets to the stator. To activate electromagnets, high power transistors are used. The controller performs power distribution by making use of a solid-state circuit.

Applications Of BLDC Motor

  1. It can be used in servo actuation. 
  2. Positioning and variables speed.
  3. It can be used in aerospace. 
  4. Used in air conditioners. 
  5. Used in computer peripheral equipment. 
  6. Used in biomedical instrumentation.

Advantages Of BLDC Motor

  1. Low maintenance cost. 
  2. These are having a long life. 
  3. Low friction and low inertia.
  4. High efficiency (>75%). 
  5. Reliable operation.

Disadvantages Of BLDC Motor

  1. More expensive.
  2. Having a complex network.
  3. It needs complex electronic controllers.
I hope this article may help you all a lot. Thank you for reading. 

Tag: Different Types Of Special Motors