The electrical motors are the machines when connected to electricity, produce mechanical power. The electrical motor is also called the prime mover means the machine that provides the motion to another machine when connected to them. The motor has become a necessity for our life. Electric motors are used in many walks of life. Various equipment is hooked to electric motors for their movement. They may be industrial pumps, domestic water pumps, cars, refrigerators, air conditioners, lorries, power plants, and much more equipment. Without the use of electrical motors, our equipment will be standstill. Our life will be difficult, miserable, and impossible if electrical motors are not moving our equipment.
WORKING PRINCIPLE OF MOTOR:
All electrical motors work on the principle of electromagnetic induction. As per this principle, EMF is the force induced in an electrical circuit that is proportional to the rate of change of magnetic flux linked with the circuit. This principle of Electromagnetic Induction was discovered by British scientist Michael Faraday.
Electrical motors have consisted of two parts:
The stator of the motor is fitted with the three-phase electrical winding on its whole periphery. The rotor of the motor contains the closed electrical circuit coil in it. It is placed in the stator. When a three-phase current is passed through the stator, a magnetic field is produced. Due to the magnetic field current is induced in the rotor circuit and produces a mechanical force, that starts the rotation of the rotor in the direction of the magnetic field generated by the stator, which produces power at the rotor shaft end.
TYPE OF MOTOR
Based on the type of current used in the motors they are classified into two main categories:
DC motors are direct current motors. Unidirectional current is used for the operation of these motors. DC current can be supplied through chemical batteries or Leclanché cell batteries.DC current could also be generated through, generators, photo-voltaic cells, and fuel cells.DC motors are sub-categorized as follows:
1 Shunt winding DC motors
2 Series winding DC motors
3 compound winding DC motors
Alternating current,(current that changes its direction in the cyclic form) is used by AC motors for operation. AC current is generated in the power station through generators. This is obtained through AC mains. AC motors are mainly categorized as follows:
Induction motors are mainly two types:
(1)Single-phase induction motor.
(2)Three-phase induction motor.
Three-phase induction motors are subcategories as follows:
(1)Squirrel cage induction motor.
(2) Slip ring induction motor.
SQUIRREL CAGE INDUCTION MOTOR
Construction of Squirrel cage induction motor is indicated in the above fig. It is consists of a Stator and a Rotor. The stator frame is made from a steel casting. Silicon steel laminations are placed in the stator frame to form a stator core.
These laminations have slots cut on their inside periphery. The laminated core is used to reduce eddy current loss. Three-phase winding is placed in the slots of the stator core. The stator winding is wound for p number of poles. Winding is in delta or star connection. Leads of winding are placed in the terminal box.
Rotor also consists of a laminated core made from silicon steel. These laminations are mounted on the shaft. The construction of the rotor is indicated in the following fig.
Closed slots are cut on the periphery. Copper bars are inserted into these slots. These bars are short-circuited at both ends of the rotor with copper short-circuiting rings. Ventilation holes are made on one end of the core and at the other end, cooling fins are provided. The shape of the rotor is like a cage for the squirrel, therefore this type of rotor is called a squirrel cage rotor. Since the rotor circuit is closed, no more resistance can be connected with the rotor circuit.
MAINTENANCE OF MOTOR
Maintenance of electrical motor is a critical issue. To be addressed carefully. Following steps to be followed for better maintenance of motors:
(1)DO EXTERNAL INSPECTION
This is to be done, to ensure that the motor cooling fan is free from any dirt or dust deposition on the fan cover. If found clean them.
(2)DO BRUSH AND COMMUTATOR INSPECTION
Brush and Commutator of the motor to be checked for wear, tear, or any crack, dent marks. If found replace them.
(3)DO CHECKS OF MOTOR PARTS
Motor rotor, stator, mounting foot, belts, pulleys, or couplings to be checked for any damage. If found replace them.
(4)DO CHECKS FOR MOTOR WINDING
Motor winding to be checked for any burn or crack sign. Any dis-coloring of winding to be viewed seriously. Rewinding and its insulation test to carry out.
(5)DO VIBRATION TESTING
This is to be done on the motor bearing. This can be done with a mobile instrument. The frequency and amplitude of the vibration will indicate its severity. Vibration in the motor may be due to improper grease lubrication, wrong lubricant, excessive lubrication of bearing. This vibration in the motor also can be due to misalignment between motor and equipment. Replace bearing and do realignment of the motor with equipment.
(6)DO INFRARED THERMOGRAPHY
This is to be done to obtain a thermal image of overheated motor components. Replace these components before any failure occurs.