Variable frequency screw air compressor is mainly composed of rectification (AC to DC), filtering, inverter (DC to AC), braking unit, driving unit, detection unit and micro-processing unit.

1. Why can the rotation speed of the motor be changed freely?

Motor rotation speed unit: r/min Number of rotations per minute, also can 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 the motor is proportional to the frequency

The rotation speed of an induction ac motor (hereafter referred to as a motor) is approximately determined by the number of poles and frequency of the motor. The number of poles of the motor is fixed by the working principle of the motor. Since this value is not a continuous value (a multiple of 2, such as 2, 4, 6), it is generally not suitable to adjust the motor speed by changing this 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 for the purpose of controlling frequency is the preferred equipment for motor speed regulation equipment.

n = 60f/p

N: Synchronization speed

F: Power frequency

P: polar logarithm of motor

If you change the frequency without changing the voltage, the motor will be overvoltage (overexcited) when the frequency decreases, which may cause the motor to burn out. Therefore, the frequency converter must change the voltage at the same time as changing the frequency. When the output frequency is above the rated frequency, the voltage cannot continue to increase, but can only be equal to the rated voltage of the motor.

For example, in order to halve the rotation speed of the motor, change the output frequency of the frequency converter from 50Hz to 25Hz, then the output voltage of the frequency converter needs to be changed from 400V to about 200V.

2. When the rotation speed (frequency) of the motor changes, how will its output torque be?

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

When the motor is powered by power frequency power supply, the impact of starting and accelerating is very large, and when using frequency converter power supply, these impacts are weaker. Direct power frequency starting will produce a large starting current. When using the frequency converter, the output voltage and frequency of the frequency converter is gradually added to the motor, so the motor starting current and impact is smaller.

In general, the torque produced by the motor decreases as the frequency decreases (speed decreases). The actual reduction is explained in some frequency converter manuals.

By using flux vector controlled frequency converters, the torque deficiency of the motor at low speed will be improved, even in the low speed region, the motor can output sufficient torque.

3. When the frequency of inverter speed regulation is greater than 50Hz, the output torque of the motor will be reduced

Normal motors are designed and manufactured at a voltage of 50Hz, and their rated torque is given within this voltage range. Therefore, speed regulation under rated frequency is called constant torque speed regulation. (T=Te, P<=Pe)

When the output frequency of the frequency converter is greater than 50Hz, the torque generated by the motor will decrease in an inverse linear relationship with the frequency.

When the motor is running at a frequency speed greater than 50Hz, the size of the motor load must be taken into account to prevent insufficient output torque of the motor.

For example, the torque produced by the motor at 100Hz is about half that produced at 50Hz.

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

4. Application of frequency converter above 50Hz

As we all know, for a particular motor, its rated voltage and rated current are constant. If the frequency converter and motor rating are: 15kW/380V/30A, the motor can work above 50Hz.

When the speed is 50Hz, the output voltage of the frequency converter is 380V and the current is 30A. At this time, if the output frequency is increased to 60Hz, the output voltage and current of the inverter can only be 380V/30A, obviously the output power remains unchanged. So we call it constant power speed regulation.

What is the torque situation? Because P=wT (w: angular velocity, T: torque). Since P stays the same and W increases, the torque will decrease accordingly.

We can also change another Angle: the stator voltage of the motor U = E + I*R (I is the current, R is the electronic resistance, E is the inductive potential), it can be seen that when U and I are unchanged, E is also unchanged. And E = K * F *X, (k: constant, F: frequency, X: magnetic flux), so when F from 50- >60Hz, X will decrease accordingly

For a motor, T=K*I*X(K: constant, I: current, X: flux), so torque T decreases as flux X decreases. At the same time, when I*R is smaller than 50Hz, the flux (X) is constant and the torque T is proportional to the current because U/f=E/f is constant because I*R is small. This is why the overload (torque) capacity of a converter is usually described in terms of its overcurrent capacity. And called constant torque speed regulation (rated current unchanged -> torque unchanged)

Conclusion: When the output frequency of inverter increases from 50Hz, the output torque of motor decreases.

5. Other factors related to output torque

The heat and heat dissipation capacity determines the output current capacity of the converter, thus affecting the output torque capacity of the converter.

Carrier frequency: generally, the rated current of the frequency converter is carrier frequency, which can ensure continuous output at ambient temperature. By lowering the carrier frequency, the current of the motor will not be affected, but the heating of the components will be reduced.

Ambient temperature: it will not increase the frequency converter protection current value because the ambient temperature is low.

Altitude: The increased altitude affects heat dissipation and insulation performance. Generally, less than 1000M can not be considered. It is ok to reduce the capacity by 5% per 1000 meters above.

Source: Inverter world - air compressor network

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