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The principle of frequency converter to control motor speed

Time: 2017/5/24 10:19:33

The frequency converter is mainly composed of rectifier (AC DC converter), filter, re rectification (DC AC converter), brake unit, drive unit, detection unit, micro processing unit and so on.

1. why is the rotational speed of the motor freely changed?

*1: r/min

Rotational speed of a motor: the number of turns per minute can also be expressed as rpm.

For example: 2 pole motor 50Hz 3000 [r/min]

4 pole motor 50Hz 1500 [r/min]

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

The motors in this paper are induction type AC motors, and most of the motors used in the industry are of this type. The rotational speed of an induction type AC motor (hereafter referred to as a motor) approximately determines the pole number and frequency of the motor. The working principle of the motor determines that the number of poles of the motor is fixed. Since the pole value is not a continuous value (multiples of 2, such as the number of poles 2, 4, 6), it is generally uncomfortable and adjusts the speed of the motor by changing the value.

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

Therefore, the frequency converter with the purpose of controlling the frequency is the preferred equipment for the motor speed regulation equipment.

N = 60f/p

N: sync speed

F: power frequency

P: motor pole count

Conclusion: changing frequency and voltage is the best method of motor control

If only the frequency is changed without changing the voltage, the frequency is lowered and the motor may be burned out due to over voltage (Guo Lici). Therefore, the inverter must change the voltage while changing the frequency. When the output frequency is above the rated frequency, the voltage can not continue to increase, and the maximum is only equal to the rated voltage of the motor.

For example, in order to reduce the speed of rotation of the motor by half, the output frequency of the converter is changed from 50Hz to 25Hz, then the output voltage of the converter needs to be changed from 400V to about 200V

2. what is the output torque when the rotational speed (frequency) of the motor is changed?

*1: power frequency power supply

A power supply supplied by a power grid (commercial power supply)

*2: starting current

When the motor starts running, the output current of the inverter

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

When the motor is powered by the power frequency, the starting and acceleration impact is great, and the shock will be weaker when the inverter is used to supply power. Power frequency direct starting will produce a big starting starting current. When using a converter, the inverter output voltage and frequency is gradually added to the motor, so the motor starting current and impact is smaller.

In general, the torque generated by the motor decreases as the frequency decreases (the speed decreases). The reduced actual data will be illustrated in some frequency converter manuals.

By using flux vector control, the torque of the motor at low speed will be improved, even in the low speed area, the motor can also output enough torque.

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

The normal motor is designed and manufactured according to the 50Hz voltage, and its nominal torque is also 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 inverter output frequency is greater than 50Hz frequency, the torque generated by the motor will decline linearly with the frequency.

When the motor runs 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 from being insufficient.

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

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

4. application of inverter above 50Hz

As you know, the rated voltage and rated current are constant for a specific motor.

Such as frequency converter and motor rating 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.

How about torque at this moment?

Because the P=wT (w: angular speed, T: torque), because the P constant, w increased, so the torque will be reduced accordingly.

We can also look at it from a different angle:

The stator voltage of the motor is U = E + I*R (I is current, R is electronic resistance, and E is inductive potential)

It can be seen that U and I remain the same when E is invariant

And E = k*f*X, (k: constant, f: frequency, X: flux), so when f is 50-->60Hz, the X will decrease accordingly

For the motor, the T=K*I*X, (K: constant, I: current, X: flux), therefore, the torque T decreases 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)

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