Ordinary three-phase induction motors are all designed with Constant Voltage Constant Frequency,so it is impossible for them to adapt to the frequency conversion completely.The influences that the frequency converter produces to ordinary three-phase induction motors are showed as follows:

   1.the efficiency and the temperature rising of motors Frequency converters of all kinds generate sinusoidal voltage and current of different levels during the operation,which makes motors running at non-sinusoidal voltage.According to the datum,for example PWN frequency converter is widely used at sinusoidal voltage,and its low-level harmonic is nearly zero,while the frequency of the remaining high-level harmonic is 2u+1(u stands for the modulation ratio),about twice of the frequency of the carrier. High-level harmonics will lead to the risng of the consumption of the stator copper、the rotor/aluminum copper、the iron and the others,of which the consumption of the rotor/aluminum copper is the most notable.Because the induction motor goes round at the synchronous rotating speed close to the fundamental frequency.So,after it is cut by the high-level harmonic with a large disparity between rotating speed,the consumption of rotor is very big.In addition,the rising of the consumption of copper due to the skin effect should also be taken into account.These consumption will all cause the additional heating、low efficiency and low output power of motors.If the ordinary three-phase induction motor is running at non--sinusoidal voltage output by frequency converter,the rising of the temperature will increase 10%~20%。

   2.the strength of the motor’s insulation At present,many small and medium-sized frequency converters use the PWM control mode.It’s carrier frequency is about a few thousand to more than a dozen KHZ,which makes the motor’s stator windings have to bear very high ratio of the voltage’s rising ,and this is equal to add a impactive voltage with great gradient to motors which leads motors’ inter-turn nisulation to bear a austere test.In addition,if the rectanglar impulse voltage generated by the PWN frequency conversions is added to the voltage that the motor is running at,it will be minatory for the motor’s insulation to the ground which will aging faster under the repeated impact of the high voltage.

    3.the harmonic electromagnetic noise and the vibration When the ordinary induction motor’s power is supplied by the frequency conversion,the vibration and the noise generated by the electromagnetic,machines,ventilation and so on will become more complex.The various time harmonics in the inverter power and the inherent space harmonics of the motor’s electromagnetic interfere mutal ,which lead to the formation of a variety of the electromagnetic excitation force.When the frequency of the electromagnetic is close or equal to  the motor’s inherent frequency,the resonance phenomenon will emerge.Because the range of the motor’s working frequency is large and the range of the speed’s changing is large,it is hard for varietious of electromagenetic wave’s frequency to avoid the inherent frequency of the motor’s 

every components.

   4.the motor’s adaptability to the frequent starting and braking

Because of the use of the frequency conversion to supply power,the motor can start at very low frequency and voltage without impact current,and brake rapidly in any way provided by the frequency conversion.This creates the conditions of realizing the starting and braking frequently,so the motor’s mechanical and electromagnetic systems are under a cycle of alternating force,and this will lead to the fatigue and fast aging problem of the mechanical structures and the insulations.

   5.the cooling problem at low speed First of all,the impedanceof the induction motor is not ideal.When the frequency of the power supply is low,the consumption caused by the high frequency harmonics of power supply is large.Secondly,when the ordinary induction motor’s rotating speed is reducing,the volume of the cooling air will reduce in proportion of the rotating speed’s cube.This will lead to the condition of the motor’s cooling at low speed turning bad,the sharp increasing of the temperature rising and hard to achieve the constant torque output.