Electrical Laws

Ohm's Law Coulomb's Law Kirchoff's Law Faraday's Law Ampere's Law Joule's Law Lenz's Law Biot Savart Law

Electrical Theorems

Thevenin Theorem Nortons Theorem Super Position Theorem Reciprocity Theorem Compensation Theorem Maximum Power Transfer Millmans Theorem Tellegans Theorem

Electrical Rules

Flemings Left Hand Rule Flemings Right Hand Rule Cork Screw Rule

Electrical Network

Network Terminologies

Electrical Terms

Electrical Terms Materials Capacitors Resistors Inductor Self Inductance Mutual Inductance Magnetic Flux Magnetic Characteristics EMF MMF Permeability Sources Reluctance Torque

Electrical Transformer

Transformers How Transformer Works Transformer Classifications Types Transformers Core Type Transformers Ideal Transformers Parallel Operation Transformer Cooling Transformer Forces Transformer Losses Transformer Testing Transformer Bushing Transformer Windings

Types of Transformer

Auto Transformer Current Transformer Potential Transformer Rectifier Transformer Converter Transformer

AC Motor

Stator and Rotor Three Phase Induction Motor Induction Motor Transformer

AC Generator

AC Generators Alternator Stator Construction Alternator Rotor Construction Alternator - Parallel Operation Synchronizing AC Alternator Losses in Alternator

DC Motors

DC Motors Commutator Braking of Electric Motors Dynamic Rheostatic Braking Regenerative Braking Plugging Braking Speed Control DC Motor Losses DC Motors

Types Of DC Motor

DC Motors Types DC Series Motors DC Shunt Motors DC Compound Motor Brushless DC Motors Permanent Magnet DC Motor

Starter For DC Motors

Starters DC Motors

DC Generator

DC Generator Types DC Generators Sparking DC Generators Why Generator Overloading Losses DC Generators

Parallel Operation

PO - DC Generator Series DC Generator Shunt DC Generator Compound DC Generator
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Stator and Rotor

An electric motor is a device which converts an electrical energy into a mechanical energy. This mechanical energy then can be supplied to various types of loads. The motors which are all operating under DC supply is called DC motor. The motors which are all operating under 1Φ AC supply or 3Φ AC supply is called AC motor. Out of all these motor, 3Φ AC supply motors are widely used as conventional current itself AC supply.

Major parts of electric motor

Basically, the electrical motor consist of two main parts, they are

  • Stator
  • Rotor


Stator is a stationary part of any electrical motor, to start any electrical motor, the supply must be given to the stator.


The part which rotates and is connected to the mechanical load through shaft called rotor. There are two types of rotor construction, they are

  • Squirrel Cage Rotor
  • Slip Ring Rotor

Stator Construction

The stator is made up of laminated stamping which are 0.4 mm to 0.5 mm of thickness. The stampings are slotted on its periphery to carry the stator winding. The stampings of stator are insulated from each other to minimize the iron loss. The material used for stampings are generally silicon steel, which is highly capable of minimizes the hysteresis loss. For 3Φ motor, the slots on the outer surface of the stator core carries 3Φ winding, connected eitheir in star or delta. This 3Φ winding on the stator is called stator winding.

Squirrel Cage Rotor Construction

The outer core of the rotor is slotted and cylinderical. The rotor consist of uninsulated copper or aluminium bars called as rotor conductors. These aluminium bars are placed in the slots of the rotor. These aluminium bars are permanently shorted at each end with the help of copper ring called end ring. The aluminium bars are usually brazed to the end rings to provide good mechanical strength. As the name itself implies that the rotor looks like a cage, forming a closed electrical circuit. Hence the rotor is called squirrel cage rotor.

Applications of squirrel cage rotor

  • Squirrel cage rotors are used in fans, blowers, water pumps.
  • Printing machine uses squirrel cage rotor type construction.
  • Lathes, drilling machines are also uses squirrel cage rotor type construction.

Slip Ring Rotor Construction

Slip ring rotor construction is very much likely to the stator. Slip ring rotor carries a 3Φ star or delta connected, distributed winding, wound for same number of poles as that of stator. Slip ring rotor construction is laminated and slotted. The slots of the rotor contains the rotor conductor or rotor winding. The three ends of 3Φ winding, available after connecting the winding in star or delta, are permanently connected to the slip rings, which is mounted on the same shaft. As we know that slip ring is an electromechanical device that allow us to connect external stationary circuit to the internal rotating circuit. So by using slip ring rotor, it is possible to add external resistance to the circuit by means of rheostat. Slip ring rotor is also called as Wound rotor.

Applications of Slip Ring Rotor

  • Slip ring rotors are used in lifts, carnes, elevators, hoists.
  • Compressors are also uses Slip ring rotor.

Squirrel Cage rotor vs Slip ring rotor

Squirrel Cage Rotor Slip Ring Rotor
Slip ring and brushes are absent. Slip rings and brushes are present to add external resistance.
Rotor resistance starter cannot be used. Rotor resistance starter can be used.
Speed control by rotor resistance is not possible. Speed control by rotor resistance is possible.
Squirrel cage rotor is very simple in construction. Slip ring rotor is very complex is construction.
Due to simple construction, the rotors are cheap. Due to complexity in construction, the rotors are very costly.
Construction is robust and maintenance free. Construction is delicate and due to brushes and external resistance, maintenance is necessary in frequent.
As permanently shorted, external resistance cannot be added. Resistance can be added externally due to the presence of slip ring and brushes.
In industry, 95% of Squirrel cage induction motors are get used. In industry, only 5% of slip ring induction motors are get used.
Rotor copper losses are less hence have higher efficiency. Rotor copper losses are high hence poor in efficiency.

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