The frequency converter is mainly composed of rectification (AC to DC), filtering, re-rectification (DC to AC), brake unit, drive unit, and detection unit micro-processing unit.
1. Why can the rotation speed of the motor be freely changed?
*1: r/min
Motor rotation speed unit: number of rotations per minute, can also be expressed as rpm.
Example: 2-pole motor 50Hz 3000 [r/min]
4-pole motor 50Hz 1500 [r/min]
Conclusion: The rotational speed of the motor is proportional to the frequency
The motor referred to in this article is an induction AC motor, and most of the motors used in the industry are of this type. The rotational speed of an induction type AC motor (hereinafter simply referred to as a motor) is approximately determined by the number of poles and frequencies of the motor. The principle of the motor determines that the number of poles of the motor is fixed. Since the pole value is not a continuous value (a multiple of 2, for example 2, 4, 6 poles), it is generally uncomfortable and the speed of the motor is adjusted 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 freely controlled.
Therefore, the inverter with the purpose of controlling the frequency is the preferred device for the motor speed control device.
n = 60f/p
n: Synchronous speed f: Power frequency p: Motor pole pair number Conclusion: Changing the frequency and voltage is the best motor control method
If only the frequency is changed without changing the voltage, the motor will be out of voltage (over-excitation) when the frequency is reduced, causing the motor to be burned out. Therefore, the inverter must change the voltage while changing the voltage. When the output frequency is above the rated frequency, the voltage cannot continue to increase. The highest value is equal to the rated voltage of the motor.
For example: In order to reduce the rotational speed of the motor by half, change the output frequency of the inverter from 50Hz to 25Hz. At this time, the output voltage of the inverter needs to be changed from 400V to about 200V.
2. What happens to the output torque when the speed (frequency) of the motor changes?
*1: Power supply for commercial frequency power supply from the grid (commercial power supply)
*2: Starting current When the motor starts running, the output torque of the inverter is less than the starting torque and maximum torque when the inverter is driven, and the starting and accelerating shock is large when the motor is directly driven by the power frequency power supply. When using frequency converters, these shocks are weaker. Power frequency direct start will produce a large starting starting current. When using the inverter, the output voltage and frequency of the inverter are gradually added to the motor, so the motor starting current and impact are smaller.
In general, the torque generated by a motor decreases with decreasing frequency (decrease in speed). Reduced actual data is given in the manual of the frequency converter.
By using a magnetic flux vector controlled inverter, the shortage of torque at low speeds of the motor will be improved, and the motor may output sufficient torque even in the low speed range.
3. When the inverter speeds to a frequency greater than 50Hz, the output torque of the motor will be reduced. Normally, the motor is designed and manufactured at a voltage of 50Hz. The rated torque is also given in this voltage range. Therefore, the speed regulation below 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 decreases in a linear relationship inversely proportional to the frequency.
When the motor is running at a frequency speed of more than 50Hz, the size of the motor load must be taken into consideration to prevent the output torque of the motor from being insufficient.
For example, the torque generated by a motor at 100 Hz is reduced to approximately 1/2 of the torque at 50 Hz.
Therefore, the speed regulation above the rated frequency is called constant power speed regulation. (P=Ue*Ie)
4. The application of the inverter above 50Hz It is known that for a specific motor, its rated voltage and rated current are constant.
If the rated values ​​of the inverter and motor are: 15kW/380V/30A, the motor can work above 50Hz.
When the speed is 50Hz, the output voltage of the inverter is 380V, and the current is 30A. At this time, if the output frequency is increased to 60Hz, the maximum output voltage and current of the inverter can only be 380V/30A. The output power is obviously unchanged. So we call it constant power speed regulation.
What about the torque situation at this time?
Because P = wT (w: angular velocity, T: torque). Because P is constant, w increases, so the torque will be reduced accordingly.
We can also look at another perspective:
The stator voltage of the motor U = EI*R (I is the current, R is the electronic resistance, E is the induced potential)
It can be seen that E is unchanged when U and I are unchanged.
And E = k*f*X, (k: constant, f: frequency, X: magnetic flux), so when f is 50-->60Hz, X will decrease accordingly. For the motor, T=K*I *X, (K: constant, I: current, X: magnetic flux), so the torque T will decrease as the magnetic flux X decreases.
At the same time, when the frequency is less than 50Hz, since I*R is small, when U/f=E/f is constant, the flux (X) is constant. The torque T is proportional to the current. This is why the inverter is usually used. Overcurrent capability to describe its overload (torque) capability. Also known as constant torque speed regulation (nominal current -> maximum torque constant)
Conclusion: When the frequency converter output frequency increases from above 50Hz, the output torque of the motor will decrease.
5. Other factors related to the output torque The heat and heat dissipation capacity determines the output current capability of the inverter, which affects the output torque capability of the inverter.
Carrier frequency: Generally, the rated current of the inverter is the highest carrier frequency, which can ensure the continuous output at the highest ambient temperature. If the carrier frequency is reduced, the motor current will not be affected. However, the heating of the components will be reduced.
Ambient temperature: As if the ambient temperature is detected to be low, increase the inverter protection current value.
Altitude: The increase in altitude will affect the heat dissipation and insulation performance. Normally, the following 1000m can not be considered. The above derating of 5% per 1000 meters is sufficient.
6. How does vector control improve the output torque capability of a motor?
*1: Torque boost This function increases the output voltage of the inverter (mainly at low frequencies) to compensate for the loss of output torque caused by the voltage drop across the stator resistance, thereby improving the output torque of the motor.
The technique of improving the low speed output torque of the motor using the "vector control" can make the motor at low speed, such as (without speed sensor) 1Hz (for a 4-pole motor, its speed is about 30r/min) can output torque The motor's torque output at 50Hz is reached (maximum is approximately 150% of the rated torque).
For the conventional V/F control, the voltage drop of the motor is relatively increased as the motor speed decreases, which results in the motor being unable to obtain sufficient rotational force due to insufficient excitation. In order to compensate for this deficiency, the inverter needs to increase the voltage to compensate for the voltage drop caused by the reduced motor speed. This function of the inverter is called "torque boost" (*1).
The torque boost function is to increase the output voltage of the inverter. However, even if a lot of output voltage is increased, the motor torque cannot be increased corresponding to its current. Because the motor current contains the motor generated torque component and other components (such as excitation components).
"Vector control" distributes the motor's current value to determine the value of the motor current component and other current components (such as the excitation component) that produce the torque.
"Vector control" can be optimized and compensated by the response to the voltage drop at the motor terminal, allowing the motor to produce large torque without increasing the current. This function is also effective for improving the temperature rise of the motor at low speed.