Polyphase Induction Motor

• polyphase induction motor - asynchronous motor, it has multiple phases in its stator winding. principle of electromagnetic induction to convert electrical energy into mechanical energy.
• Ferraris - established the foundation for the idea when he presented his study on a "rotating magnetic field" engine to the Royal Academy of Sciences in Turin.
• Tesla - built and tested a functioning polyphase induction motor, earning significant patents in the process.
• A major step toward broader use was made when Westinghouse created a range of 60 Hz polyphase induction motors.
• induction motor - is a commonly used AC electric motor, the electric current in the rotor needed to produce torque and it is obtained via electromagnetic induction from the rotating magnetic field of the stator winding.
• DC motor is an electrical machine that transforms electrical energy into mechanical energy by creating a magnetic field that is powered by direct current.
• commutator and brushes - anong parts ng dc motor ang wala sa dc motor
• Wound motors - can carry a high load without causing over-heating. Cranes, accelerators, elevators, hoists, and conveyors are the common 3 phase motor uses in the lifting industry.
• The class-A squirrel cage induction motors - have normal starting torque, high starting current and low operating slip (from 0.005 to 0.015). This motor design has a low-resistance single-cage rotor. The full-load efficiency of class-A motors is high. Therefore, these motors are suitable for loads such as −

  Blowers,
• Class-B motors have normal starting torque, low starting current and low operating slip. By increasing the leakage reactance, the starting current can be reduced and the starting torque is maintained by using double-cage or deep bar rotor. These motors are most popular and used for full-voltage starting. Therefore, these are also used for loads such as fans, blowers, machine tools and centrifugal pumps etc.
• The class-C motors have high starting torques and low-starting current. These motors have a high-resistance double-cage or deep-bar rotor construction. The applications of class-C motors are for practically constant-speed loads that require fairly high torque with low starting current. Hence, these motors are used for driving the loads such as compressors, crushers, conveyors and reciprocating pumps.
• A single phase induction motor is rated 5hp, 75% power

  factor and 220 volts. What is its full phase current.

  
• An induction has rotor impedance standstill of 0.1 + j2 ohms.

  When the motor is running at a slip of 4%, the rotor current

  is observed to be 25 A. Determine the voltage induced to

  rotor circuit at a standstill.

  
• Why do we conduct equivalent circuit tests?

  • Equivalent circuit of an induction motor is used to determine how the rest of the ‘circuit’ will respond to the motor.
  • This includes overcurrent protection and voltage load to the rest of the system
  • In a way it is a form of preventive maintenance to upkeep its efficiency.
• No Load Test - Rated voltage is applied to the machine and is allowed to run on no-load.

  
• Locked Rotor Test - Reduced voltage is applied to the machine so that the rated current flows through the stator winding

  
• Torque - twisting force that tends to cause rotation.
• Starting Torque - torque that is transferred by the shaft coupling during run-up.
• 2 types of starting Torque of motor

  
  

  • Squirrel cage slip ring motor
• Direct On Line - It is a starter by which the motor is switched ON direct to the supply mains by switching conductor. With normal industrial motors this operation results in a heavy rush of current of the order of five to seven times of the normal full load current.
• Primary resistance (or inductance) starter [Stator Resistance (or Reactance) Starter] -

  • A variable resistor (or inductor) is connected in series with each phase of the stator winding of a 3-phase squirrel cage induction motor.
  • Very low voltage is supplied to each phase of the winding at the start by inserting more resistance (or inductance) in series with the winding. As the motor picks-up the speed, the resistance (or inductance) is taken out of circuit.
• Star/Delta Starter - connections of the motor are made in star fashion so that reduced voltage is applied across each winding. After the motor attains speed the same windings through a change-over switch, and are connected in delta across the same supply.
• Auto-transformer Starter - The current can only be reduced to 1/3 times the short circuit current. Whereas, in this method, the voltage applied across the motor and hence current can be reduced to a very low value at the time of start.

  At the time of start, the motor is connected to supply through the auto-transformer by a 6 pole double throw switch. When the motor is accelerated to about full speed, the operating handle is moved to run position.