Factors Affecting the Design of an
Ac Electric Induction Motor1
Author: BaljeetFactors affecting the Design of an ac electric induction motor
Design of an ac electric motor is directly affected by the length of the air gap. Ampere Conductors value also affects the design of an ac electric motor.
The value of average flux density over the air gap of an ac electric motor also affects the design of an ac electric motor. The size or dimensions of an ac electric motor depend upon the speed of an ac electric motor. It can also be said that the volume of active parts of an ac electric motor varies inversely as the speed of an ac electric motor. The value of output co-efficient is directly responsible for the dimensions of an ac electric motor. In other words the volume of active parts of an ac electric motor is inversely proportional to the value of output co-efficient of the ac electric motor.
The total flux around the armature (or stator of an ac electric motor) periphery at the air gap is called the total magnetic loading. While total electric loading is the total number of ampere conductors around the armature (or stator of an ac electric motor) periphery. Since the output coefficient of an ac electric motor is proportional to the product of specific magnetic and specific electric loading of an ac electric motor, we conclude that the size and hence the cost of ac electric motor decreases if increased values of specific magnetic and electric loading are used. The flux density in iron parts of an ac electric motor is directly proportional to the average flux density in the air gap of the ac electric motor. In a well designed ac electric motor the maximum density occurs in the teeth of the ac electric motor and therefore let us relate the flux density in the teeth with flux density in the air gap of ac electric motor.
The magnetizing current of an ac electric motor is directly proportional to the mmf required to force the flux through the air gap and the parts of the ac electric motor. The mmf required for the air gap of an ac electric motor is directly proportional to the gap flux density i.e. the specific magnetic loading of an ac electric motor. The consideration of magnetizing current is very important in ac electric induction motor(s) as an increased value of magnetizing current means of a low operating power factor of ac electric motor. Therefore specific magnetic loading in the case of ac electric induction motor(s) is lower than that in dc electric motor(s).
The core loss in any part of the magnetic circuit of an ac electric motor is directly proportional to the flux density for which the ac electric motor is going to be designed. Thus a large value of specific magnetic loading in an ac electric motor indicates an increased core loss in ac electric motor and consequently a decreased efficiency of ac electric motor and an increased temperature rise of ac electric motor. In case of high frequency ac electric motor, specific magnetic loading must be reduced in order to get lower iron losses in ac electric motor so that reasonable values of efficiency may be maintained in an ac electric motor. The maxmium temperature rise of an ac electric motor is determined by the type of insulation material used in the ac electric motor. If the cooling co-efficient of the ac electric motor is small, a high value of specific loading may be used in the ac electric motor.
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Factors affecting the speed-torque characteristics of an Induction motor :
The speed-torque characteristics are affected by various factors like applied voltage, R2’ and frequency.
(a) Applied voltage : We know that T µ V2. Thus not only the stationary torque but also the torque under running conditions changes with change in supply voltage.
(b) Supply frequency : The major effect of change in supply frequency is on motor speed. The starting torque is reduced with increase in frequency.
(c) Rotor resistance : The maximum torque produced does not depend on R2’. However, with increase in R2’, the starting torque increases. The slip at which Tmax is reached increases too which means that Tmax is obtained at lower motor speeds
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