How does the inverter control motor speed?

1 Why does the rotational speed of the motor change freely?

Motor rotation speed unit: r/min rotation per minute, can also be expressed as rpm.

For example: 2 pole motor 50Hz3000[r/min]

4 pole motor 50Hz1500[r/min]

Conclusion: the rotating speed of the motor is proportional to the frequency

The rotational speed of the induction motor (hereafter referred to as the motor) is approximately determined by the number and frequency of the motor. According to the working principle of the motor, the number of poles of the motor is fixed. Since the pole value is not a continuous value (a multiple of 2, for example, the number of poles is 2, and 4, 6), the general discomfort and the speed of the motor can be adjusted by changing the value.

In addition, the frequency can be adjusted outside the motor and then supply the motor, so that the rotation speed of the motor can be freely controlled.

Therefore, to control the frequency for the purpose of frequency converter, is as a motor speed control equipment preferred equipment.

N=60f/p n: synchronous speed f: power frequency p: motor pole

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

If you change the frequency without changing the voltage, the frequency will decrease when the motor out of the voltage (Guo Lici), leading to the motor may be burned out. So the frequency converter must change the voltage at the same time. When the output frequency is above the rated frequency, the voltage can not continue to increase, the maximum can only be equal to the rated voltage of the motor.

For example: in order to make the motor rotation speed halved, the inverter output frequency from 50Hz to 25Hz, then the inverter output voltage needs to change from 400V to about 200V

2 when the motor rotation speed (frequency) changes, the output torque?

The starting torque and the maximum torque of the inverter drive are less than that of the direct power supply.

Motor power supply in the power supply when the start and acceleration impact is very large, and when the use of inverter power supply, these shocks will be weak. The direct starting of power frequency will produce a large starting current. When using the inverter, the inverter output voltage and frequency is gradually added to the motor, so the motor starting current and impact to be smaller.

In general, the torque produced by the motor decreases with the decrease of frequency (speed). The reduced actual data will be given in some of the inverter manuals.

Through the use of flux vector control of the inverter, the motor will improve the torque at low speed, even in the low-speed motor can also output enough torque.

3 when the frequency converter speed is greater than 50Hz frequency, the motor output torque will be reduced

The usual motor is designed and manufactured according to the 50Hz voltage, and the rated torque is given in this voltage range. Therefore, the speed regulation under the rated frequency is called constant torque speed regulation (T=Te, P<=Pe)

When the output frequency of the inverter is greater than 50Hz, the torque generated by the motor should be decreased linearly with the frequency.

When the motor is operating at a frequency greater than 50Hz, the magnitude of the motor load must be taken into account in order to prevent the output torque of the motor being insufficient.

For example, the torque generated by the motor at 100Hz is reduced to about 50Hz when the torque is generated by 1/2.

Therefore, the speed above the rated frequency is called constant power speed regulation. (P=Ue*Ie)

4 the application of inverter 50Hz above

As we all know, the rated voltage and rated current of a particular motor are the same.

Such as inverter and motor ratings are: 15kW/380V/30A, motor can work more than 50Hz.

When the speed is 50Hz, the output voltage of the inverter is 380V, the current is 30A. when the output frequency increased to 60Hz, the maximum output voltage and current of inverter is only for 380V/30A. obviously the same output power. So we called the constant power speed.

What about the torque?

Because P=wT (w: angular velocity, T: torque)

Because P does not change, the w increases, so the torque decreases accordingly. We can look at it another way:

Motor stator voltage U=E+I*R (I for the current, R for electronic resistance, E for the induced potential)

It can be seen that U, when I is constant, E also remains unchanged, while E=k*f*X, (k: constant, f: frequency, X: flux), so when f by 50-->60Hz, X will be reduced accordingly.

For the motor, T=K*I*X, (K: constant, I: current, X: flux), so the torque T will decrease as the flux X decreases

At the same time, less than 50Hz, because I*R is very small, so the U/f=E/f at constant flux (X) is a constant torque and current is proportional to T. This is why usually used to describe the flow capacity of the inverter (torque) and overload ability. Called constant torque speed (rated current maximum torque constant -- > unchanged)

Conclusion: the output torque of the motor can be reduced when the frequency of the inverter is increased from 50Hz

5 other factors related to output torque

The heating and cooling capacity determines the output current capability of the inverter, which affects the output torque of the inverter.

Carrier frequency: the general inverter rated current is the highest carrier frequency, the maximum ambient temperature can ensure the continuous output value. However, the heating component will be reduced.

Ambient temperature: it is not because of the detection of the surrounding temperature is relatively low when the frequency converter to protect the current value

Altitude: altitude increases, the heat dissipation and insulation performance are affected. Generally 1000m the following can not be considered. More than 1000 meters per volume reduction of up to 5%