Factors affecting the Design of an ac electric induction mot
by BALJEET VASEER
Factors affecting the Design of an ac electric induction motor The value of cooling co-efficient while designing an ac electric motor depends upon the ventilation conditions in the ac electric motor. An ac electric motor with better ventilation has a lower value of cooling co-efficient. If the current density in conductors of an ac electric motor and the slot space factor of ac electric motor are assumed constant, then specific electric loading of an ac electric motor is proportional to the diameter of the core of ac electric motor as depth usually depends upon the diameter of core of ac electric motor. Typical values of current density are in the range of 2 - 5 A/mm sq. in case of an ac electric motor. While designing an ac electric motor, temperature rise is usually 40 deg C for normal applications of ac electric motor and co-efficient is between 0.02 to 0.035 CW-m sq. The larger ac electric motor is intrinsically more efficient. As the size of an ac electric motor increases, better ventilation and cooling conditions have to be provided in the ac electric motor. Iron loss in an ac electric motor increases with increased value of air gap flux density. The value of air gap flux density of an ac electric motor determines the overload capacity of the ac electric motor. A large value of ac in an ac electric motor means that a greater amount of copper is employed in the ac electric motor.
This results in higher copper losses in the ac electric motor and large temperature of embedded conductors in ac electric motor. A small value of ac should be taken for high voltage ac electric motor. And also higher value of ac in an ac electric motor, the lower would be the overload capacity of ac electric motor. For a normal design of an ac electric motor the diameter of core of ac electric motor be so chosen that the peripheral speed of rotor of ac electric motor does not exceeds about 30 m/s. The three phases of an ac electric motor can be connected in either star or delta. The power factor of ac electric motor with a greater gap length is smaller. The harmonic fields in an ac electric motor are due to: i) windings of ac electric motor, ii) Slotting of ac electric motor, iii) Saturation of air gap of an ac electric motor and iv) Irregularities in the air gap of ac electric motor. The harmonic fields in an ac electric motor are responsible for increase in stray load losses in ac electric motor and increased ac electric motor heating. The choice of rotor slots of an ac electric motor is particularly important in the case of ac squirrel cage electric motors. The Cogging effect can be over come in an ac electric motor by the way that the number of stator slots of ac electric motor should not be equal to the number of rotor slots of ac electric motor. Better to have 15 to 30 percent more or less rotor slots of ac electric motor than the number of stator slots of ac electric motor. The difference of stator slots of an ac electric motor and rotor slots of an ac electric motor should not be equal to 2p or 5p to avoid synchronous cups in ac electric motor.
The difference between the number of stator slots of ac electric motor and rotor slots of ac electric motor should not be equal to 3p for 3 phase ac electric motor in order to avoid magnetic locking in ac electric motor. The difference between the number of stator slots of ac electric motor and rotor slots of ac electric motor should not be equal to 1, 2, (p /-1) or (p /-2) to avoid noise and vibrations in ac electric motor. The ac electric motor noise and vibrations, cogging effects in ac electric motor and synchronous cups in ac electric motor can be reduced or even entirely eliminated be skewing either the stator slots of ac electric motor or the rotor of ac electric motor. Also, in order to eliminate the effects of any harmonic in the ac electric motor, the rotor bars of ac electric motor should be skewed through an angle so that the bars lie under alternate harmonic poles of the polarity or in other words, bar must be skewed through two pitches in ac electric motor. A rotor of an ac electric motor with a high resistance has the disadvantage the I sq. R loss is greater in the ac electric motor and therefore its efficiency is lower under running conditions of ac electric motor.
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