Difference Between Servo Motor and Induction Motor

Servo Motor vs Induction Motor

Motors are the class of electromechanical devices which convert the electrical energy into mechanical energy. In some applications, pure torque is needed to drive a mechanism, and in some applications, the position and the rotational speed of the mechanism have to be controlled. Induction motor delivers pure uncontrolled torque, while servo motors deliver controlled torque, where speed and the position of the shaft (rotor) can be adjusted.

More about Induction Motors

Based on the principles of electromagnetic induction, the first induction motors were invented by Nikola Tesla (in 1883) and Galileo Ferraris (in 1885), independently.

The induction Motor consists of two main parts, the stator, and the rotor. Stator in the induction motor is a series of concentric magnetic poles (usually electromagnets), and the rotor is a series of closed windings or aluminum rods arranged in a way similar to a squirrel cage; hence the name squirrel cage rotor. The shaft to deliver the torque produced is through the axis of the rotor. The rotor is placed within the cylindrical cavity of the stator, but not electrically connected to any external circuit. No commutator, brushes, or other connecting mechanism is used to supply current to the rotor.

As any motor, it uses magnetic forces to rotate the rotor. The connections in the stator coils are arranged in a way that opposite poles are generated at the exact opposite side of the stator coils. At the start-up phase, magnetic poles are created in a periodically shifting manner along the perimeter. This creates a change in the flux across the windings in the rotor and induces a current. This current creates a magnetic field in the rotor and the interaction between the stator field and the induced field drives the motor.

Induction motors are made to operate with both single and poly-phase currents; latter for heavy duty machines that require a large torque. The speed of the induction motors can be controlled using either number of magnetic poles in the stator pole or regulating the frequency of the input power source. The slip, which is a measure to determine the motor’s torque, gives an indication of the motor efficiency. Since the short-circuited rotor windings have small resistance, a small slip induces a large current in the rotor and produces large torque. Yet the rotational speed of the rotor is slower than the input power source frequency (or the rate of rotation of the stator field). Induction motors do not have any feedback loops for control of the motor.

More about Servo Motors

Technically, a servo motor is any motor that has feedback and closed loop control, and it is only a part of a servo mechanism in which the negative feedback is used to control the performance of the motor.

But, commonly used industrial servo motors are normal AC induction motors with added features such as Low inertia rotor, high torque brake, and inbuilt encoder for speed and position feedback. All these components combine to work with servo drive. Servomechanisms with DC motors are commonly used in radio controlled devices, usual instruments that require low power and high precision.

The DC servo motor stator is usually formed with permanent magnets placed at 900 around the rotor. Servomotors are designed to deliver considerably constant levels of torque and have low inertia. Input to a servomotor is in the form of pulses, and at every pulse, the motor will turn by a finite, exact amount.

Servo motors can deliver high torque and the position and the speed of the motor can be controlled. Therefore, servomotors are extensively used in robotics and control systems related applications.

What is the difference between Induction Motor and Servo Motor?

• Servo motor has a closed loop negative feedback system whereas the general induction motor has feedback mechanisms (in inbuilt encoder).

• Servomotor speed and position can be adjusted and controlled with greater precision while, in induction motors, only speed can be adjusted.

• Servo motors have low inertia, while induction motor rotor has a higher inertia.

• The Servo motor is a class of controlled motors, and it can be, an induction motor or other type.