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INDUCTION MOTOR LECTURE NOTES
Explain the principles and characteristics of induction motors. Explain how torque is produced. Calculate slip. Solve a problem involving slip, given number of poles, frequency and motor speed. Sketch and label the torque/speed characteristic for a single cage induction motor, indicating starting torque, pull-out torque and synchronous speed. Explain how the torque/speed characteristic is affected if the rotor resistance is increased. Draw a power flow chart for a three phase induction motor. Calculate useful shaft output power and efficiency at full load. Explain how speed control is achieved in an induction motor Explain Calculate efficiency for induction motor.
Construction and Operation
Explain the working principle of a 3phase, induction motor. What are the various types of rotors? Differentiate between squirrel cage and wound rotor motor of the three phase a.c. induction. Torque List the factors that determine, the starting torque of the three- phase induction motor. In respect of the following Rotor construction, torque characteristic, speed variation. Indicate on a sketch of typical characteristic curves of current and torque against speed, disadvantages of a DOL start squirrel cage induction motor. Sketch a graph of starting current, and torque against the speed of rotation for a single cage ' motor. How does this torque generally compare with the value of the rated torque? List the factors that determine the maximum developed torque of the induction motor Condition for maximum torque under running condition Describe the effect of change in speed, rotor current and torque as load is applied to an induction motor How does the motor adjust to its stator current with changes in mechanical load? What is slip? What are the factors on which the torque of an induction motor depends?
Breaking and speed control
With reference to deck machinery driven by A.C induction motor describe each of the following types of braking. Regenerative. Plugging. Dynamic. Describe how speed change and braking are achieved Explain the principle of AC motor starting and speed control POWER FLOW Show how the power that is transferred across the air gap of a 3phase induction motor is represented explain the trms what portion of this is useful power. .
Problems
An 18.65 KW, 6pole, 50 Hz, 3-phase slip-ring induction motor runs at 960 rpm on full load with t rotor current per phase of 35 A. Allowing 1 KW for mechanical losses, find the resistance/phase; of 3-phase rotor winding. A three phase induction motor is wound for four poles and is supplied from a 5.0 Hz system. Calculate. (1) The synchronous speed; (2) The speed of the rotor when the slip is 4 per cent. (3)The motor frequency when the speed of the rotor is 600 r.p.min
Performance
Describe the effects of frequency and voltage change on an induction motor.
Maintenance
A cage rotor motor has been flooded with sea water, the insulation resistance can drop down to zero MΩ. describe the procedure for putting the motor back into serviceΩ. describe the procedure for putting the motor back into service What are routine maintenance carried out on an a.c. motor and
Protection
With reference to the condition monitoring, of electrical machinery: (a) State TWO important parameters that may be recorded; (b) Explain how the parameters are measured and what defects may be revealed. Describe the devices used to protect a 3 phase a.c. induction motor against the following faults. Short circuit Overload Single phase fault. a) With reference to single phasing applied to a.c motors i) Explain the meaning of single phasing ii) Describe it's effect
Direct on line starting for large induction motors such as those for bow thrust units, may not be viable, (a) State the reasons for this. (b) Sketch a starling system that may be useful for such motors. (c) Describe the starting method sketched in (a, b ). Why is squirrel-cage windings used on main propulsion motors? What are the three basic types of ac motors? State their advantages, Disadvantages The direct on line start of squirrel cage motor is used for most electrical drives on a.c. powered ships. Sketch the lay out of a solid state soft starter motor for a bow thruster unit powered from a dedicated diesel alternator
