With the development of frequency converters, the use of frequency converters is becoming more and more popular. However, most of the time, we are only using frequency converters, but never understand it. Today, Xilinx Electric is going to explain to you many of the frequency converters A – how to control the motor speed inverter
1. Why can the rotation speed of motor change freely?
Motor rotation speed unit: r / min that the number of revolutions per minute, can also be expressed as rpm.
For example: 2 pole motor 50Hz3000 [r / min] 4 pole motor 50Hz1500 [r / min]
Conclusion: The speed at which the motor rotates is proportional to the frequency.
The rotational speed of an induction AC motor (hereinafter referred to as a motor) is approximately determined by the number of poles of the motor and the frequency of the motor. The working principle of the motor determines the number of poles of the motor is fixed. Since the pole value is not a continuous value (a multiple of 2, for example, poles 2, 4, 6), it is generally uncomfortable and the motor speed is changed by changing this straightness.
In addition, the frequency can be adjusted outside the motor and then supplied to the motor, so that the motor speed can be freely controlled.
Therefore, to control the frequency of the purpose of the inverter, as a motor speed control equipment, the preferred equipment.
n = 60f / p
n: synchronous speed
f: power frequency
p: Motor pole pairs
Conclusion: Changing the frequency and voltage is the optimal motor control method.
If only the frequency is changed without changing the voltage, a decrease in frequency will cause the motor to over-voltage (over-excitation), resulting in possible motor burn-out. Therefore, the frequency converter must also change the voltage while changing the frequency. The output frequency above the rated frequency, the voltage can not continue to increase, the maximum can only be equal to the rated voltage of the motor.
2. When the motor rotation speed (frequency) changes, the output torque will be what kind of change?
When starting the inverter, the starting torque and the maximum torque should be less than the ones directly driven by power frequency.
The acceleration of the motor during start-up with commercial power supply is very strong, and these impacts are weaker when using the inverter. Direct start frequency will have a much higher than the rated current of the large starting current. When using the inverter, the inverter output voltage and frequency is gradually added to the motor, so the motor starting current impact to be reduced.
By using a flux-vector-controlled inverter, the shortcoming of insufficient torque at low speed of the motor is improved, and the motor can output sufficient torque even in a low speed region.
3. When the inverter speed to more than 50Hz frequency, the motor output torque will be reduced
The usual motor is designed and manufactured at 50Hz, so its rated 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) <span = “”>
When the inverter output frequency is greater than 50Hz, the torque generated by the motor will decrease in a linear relationship inversely proportional to the frequency.
When the motor is operated at a frequency of more than 50Hz, the size of the motor load must be taken into consideration to prevent the motor from being insufficient in output torque.
For example, the torque generated by the motor at 100Hz is approximately reduced to 1/2 of the torque generated at 50Hz.
Therefore, speed regulation above rated frequency is called constant power speed regulation (P = Ue * Ie)
4. Inverter 50Hz above the application
As we all know, for a particular motor, its rated voltage and current rating is unchanged.
Such as inverter and motor rating are: 15kW / 380V / 30A, the motor can work in more than 50Hz.
When the speed is 50Hz, the output voltage of the inverter is 380V and the current is 30 A. At this time, if the output frequency is increased to 60Hz, the maximum output voltage and current of the inverter can only be 380V / 30 A. It is obvious that the output power Will not change, so we call it constant power speed regulation.
What is the torque situation at this time?
Because P = wT (w: angular velocity, T: torque).
Since P is constant, w is increased, so the torque is reduced accordingly. We can also look another way:
Motor stator voltage U = E + I * R (I is the current, R is the electronic resistance, E is the induced potential)
It can be seen that when U and I are constant, E does not change, and E = k * f * X (k: constant, f: frequency and X: flux) , X will be reduced accordingly.
For a motor, T = K * I * X, (K: constant, I: current, X: flux) and therefore the torque T decreases as the flux X decreases.
At the same time, less than 50Hz, due to I * R is very small, so U / f = E / f unchanged, the flux (X) is constant torque T and current is proportional to That is why the inverter Overcurrent capability to describe its overload (torque) capacity and called constant torque speed (rated current constant -> maximum torque unchanged)
Conclusion: When the inverter output frequency increases from above 50Hz, the motor output torque will decrease.
5 and other factors related to the output torque
The ability to heat and dissipate heat determines the inverter’s output current capability, which affects the drive’s output torque capability.
Carrier frequency: The rated current of the general inverter is the highest carrier frequency, the maximum ambient temperature to ensure continuous output value. Reduce the carrier frequency, the motor current will not be affected. However, the components of the heat will be reduced.
Ambient temperature: Just as it does not increase the inverter protection current value when it is detected that the ambient temperature is low.
Altitude: The increase in altitude will have an impact on heat dissipation and insulation properties, and the following can generally not be considered below 1000 meters. If it is over 1000 meters, the above 5% will be derated for every 1000 meters.