Inverter is mainly constituted by Rectifier（AC Variable DC）、 filtering、Re-rectification（DC Variable AC）、Braking Unit.、driving unit，Detection unit as well as microprocessing unit.

    Why is the motor rotation speed can be freely changed?

Motor rotation speed unit：r/min  the number of rotations per minute，it can be expressed as rpm.

    Example  2 poles motor  50Hz 3000 [r/min]

             4 poles motor  50Hz 1500 [r/min]

    Conclusion：The motor rotation speed is proportional to the frequency.

    AC Induction motors’ rotation speed is indeed approximately decision on the number of the motor pole and frequency . The working principle of electric motor is fixed, As the number of the pole is not a continuous numerical ,(it is a multiple of 2. such as the number of poles is 2,4,6) ,so It is generally not appropriate to adjust the speed of the motor by changing the numerical.

    Other, The frequency can be adjusted outside of the motor and supply the motor, that the motor rotation speed can be freely controlled. Therefore, the converter for the purpose of controlling is the preferred equipment for motor speed control .

n = 60f/p

n:Sync speed

f:Power Frequency

p:Number of motor pole pairs

    Conclusion：To change the frequency and voltage is the optimal method of motor control

    If only changing the frequency instead of changing the voltage，over-voltage will occur when The frequency reduce，This leads to motor may be burned. Therefore, changing the frequency converter at the same time must also change voltage. And the Voltage can not continue to increase, it Can utmost only be equal to the rated voltage of the motor.

    such as:In order to halve the motor rotation speed, The inverter output frequency will change from 50Hz to 25Hz, Then the output voltage of inverter is required to change from 400V to about 200V.

    When the motor rotation speed (frequency) changes, how will the output torque?

    when Inverter driven, starting torque and maximum torque is less than the direct labor-frequency power-driven.

    A big impact for starting and speeding up will occur when supplying the power, When using the inverter supply the power  , these shocks must weaker, Direct starting frequency will produce a great start starting current, and When using the inverter, the output voltage and   frequency of Inverter is gradually added to the motor. Therefore, the shock and starting current of motor will be smaller.

    Usually , the torque of motor will be reduced with the frequency decreases，the decreases actual data in some converter manual will give instructions.

    Through useing magnetic flux vector control inverter，it will improve the lack of torque in low speed situation.

    When the speed control of inverter is greater than 50Hz, The motor output torque will be reduced.

    Usually ，the motor is designed and manufactured according to 50Hz Voltage, its rated torque is within the scope of this given voltage. Therefore the rated speed under the frequency .

    when output frequency Inverter is greater than 50Hz, inversely proportional to the linear drop relationship will be between the motor torque and frequency.

    when the motors running above 50Hz frequency, The size of the motor load must be taken into account in order to prevent  deficiencies of the output torque.

    For example, the motor torque on 100hz will be reduced to the 1/2 of torque on 50hz.

    Therefore the rated speed above the frequency is called constant power speed.

    The application of inverter above 50hz frequency.

    As everyone know, for a particular motor, its rated voltage and rated current is constant.

    The inverter and motor ratings are；15kW/380V/30A, Motor can operate above 50Hz.When the speed is 50Hz, the inverter output voltage is 380V, current is 30A，Then if the output frequency is increased to 60Hz, the maximum output voltage and current of Inverteris also only for 380V/30A. Obviously the output power is fixed, So we called the constant power speed.

    So how about the torque？

    Because P=wT（W: angular velocity，T: torque）， as P is fixed， w is increased, So the torque can be correspond reduced.

    We can also find another point of view.

    The voltage motor stator is U = E + I*R(I:current R: Electronic Resistance E: inductive kick)

    As can be seen, U and I are fixed，E is also fixed.

    And E = k*f*X （K: constant, f: frequency, X: magnetic flux）, So when the f from 50 to 60Hz，X will be a corresponding decrease.

    For the motor，T=K*I*X, （K: constant,I: electric current, X: magnetic flux）, Therefore, the decrease of torque  will be reduced along with the magnetic flux.Simultaneous，when frequency is under 50HZ, because I*R is small，so U/f=E/f is fixed，magnetic flux is constant，Torque is proportional to the current. This is why usually the inverter over-current ability to describe their overload (torque) capacity,it is called constant torque speed.Conclusion; When the output frequency of inverter is increased from 50Hz, The output torque will decrease.

    Other relevant factors about the output torque.

    Heating and cooling ability decide frequency converter output current capacity, which affect the output torque converter.

    Carrier frequency: generally the rated current of inverter is the highest carrier frequency, the maximum ambient temperature can ensure continuous output values, Reduceing the carrier frequency, motor current will not be affected, but the fever of components will reduce.

    Ambient temperature: it would not because of the low ambient temperature that increased in the value of inverter current protection.

    Altitude: Altitude increased,these will be an impact on thermal and insulating properties, it will not take into account below 1000m, above 1000m, 5% can let down every 1000m.