Induction Machines - Basics and Classifications

For better understandings, machines or more accurately motors are divided in two groups AC and DC. These groups are further divided into four basic groups as given below:

• Synchronous Machines
• D. C. Machines
• Induction Machines
• Special Purpose Machines

Here is a brief introduction and classifications to induction motors.

Induction Motor : -

An induction motor is a motor that uses the principle of induction of e.m.f. in an inductor due to the effect of flow of current in other coil that is magnetically attached but physically insulated with the coil. For detail on induction please visit Basics of Electromagnetic Induction. As I said above induction motor is a motor or device that operates on the principle of induction and converts electrical energy into mechanical energy, which is essentially function of a motor. Basically I have divided motors in four groups for better understanding of the field machine design. These are :

Principle of Operation of Induction Machines

The principle of operation of induction machine is something similar to that of operation of Transformer. First of all a supply is given to the machine and due to this supply a rotating magnetic field i.e. A field whose magnetic axis of rotation changes with respect to time is produced. In three phase induction motors this rotating magnetic field is produced due to the simultaneous maximum of phase voltages. It means that voltage in phase R will reach its maximum first say at wt=90 degrees. Where w is angular frequency. Now the next phase Y is 120 electrical degree lags the R phase so it will reach its maximum at wt=210 degrees and similarly B phase will reach its maximum at wt=330 degrees. Now as voltage reaches its maximum at different time levels so the current will also maximum at corresponding time values and thus the flux produced will have a rotating magnetic field with axis changing with time. Due to this flux similar flux will induced in rotor as in secondary of a transformer due to inducion. This rotor flux will induced e.m.f. in rotor and due to this e.m.f. current will produced as the rotor bars are short circuited. This current will produce the torque and the rotor tends to follow the stator magnetic field, which is rotating. Thus rotor will also start rotating.

Classification of Induction Machines

There are three main criterion for classifying induction machines. There are :

• Based on Supply
  • Single Phase Induction Motors
  • Three Phase Induction Motors
• Based on Type of Rotor
  • Squerrel Cage Induction Motors
  • Slip Ring Induction Motors
• Form of Energy Produced
  • Induction Generator
  • Induction Motor

Based on Supply

This classification consider the number of phases in the machine.

Single Phase Induction Machine

If in an Induction machine there is only one phase of supply, the machine is called single phase induction machine.

Three Phase Induction Machines

If the input of electrical motor or output of induction generator has three phases then this is called a three phase induction machine.

Based on Type of Rotor

There are many rotor type in induction machines like squirrel cage, slip rings, double cage and deep bar rotors but among above said following two are very important.

Squirrel Cage Induction Motors

If the rotor have simply bars on it and there bars are short circuited with the help of end rings, the rotor and hence the machine is called squirrel cage type. Due to low cost there are widely used.

Slip Ring Induction Motors

If in an induction motor's rotor external resistances are connected, it is called slip ring type induction motor. It is more costly and only 10% of total induction motors are slip ring type.

Form of Energy Produced

This classification consider the input energy form and output energy form.

Induction Generator

The induction generator is the machine which produce electrical energy from mechanical energy. It is not practically used as a source of electrical energy or power generation but commonly used in co-generation. It is used where speed is varying and its efficiency of energy conversion is small as compare to synchronous generators.

Induction Motor

Induction motor is the induction machine which convert electrical energy into mechanical energy. It takes the electrical power as input and produce mechanical power along its rotor shaft. There are widely used in industrial and domestic applications.