A frequency converter for motor control that changes both voltage and frequency. However, the inverter used for fluorescent lamps is mainly used to adjust the frequency of power supply. Equipment used in automobiles to generate AC power from batteries (DC) is also sold under the name “inverter”. The working principle of the frequency converter is widely used in various fields. For example, the power supply of a computer power supply, in this application, the frequency converter is used to suppress reverse voltage, frequency fluctuations and instantaneous power failure of the power supply.
1. Why is the rotational speed of the motor freely changeable?
*1: r/min Motor rotation speed unit: The number of rotations per minute, also expressed as rpm.
For 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 inductive AC motor, and most of the motors used in the industry are motors of this type. The rotational speed of an inductive AC motor (hereinafter referred to simply 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 the pole value is not a continuous value (a multiple of 2, for example, the number of poles is 2, 4, 6), it is generally uncomfortable and the speed of the motor is adjusted by changing the value.
In addition, the frequency can be supplied to the motor after being adjusted outside the motor, so that the rotational speed of the motor can be freely controlled.
Therefore, the inverter for the purpose of controlling the frequency is the preferred device for the motor speed control device.
n = 60f/pn: Synchronous speed f: Power supply frequency p: Motor pole pairs
Conclusion: Changing frequency and voltage is the optimal motor control method
If only the frequency is changed without changing the voltage, the frequency will decrease and the motor will be over-voltage (overexcitation), causing the motor to be burned out. Therefore, the inverter must change the voltage at the same time while changing the frequency. When the output frequency is above the rated frequency, the voltage cannot continue to increase, and the maximum can only be 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, then the output voltage of the inverter needs to be changed from 400V to about 200V.
2. What is the output torque when the motor’s rotational speed (frequency) changes?
*1: Power frequency power supply Power supply from the power grid (commercial power supply)
*2: Starting current The output current of the inverter when the motor starts running
The starting torque and maximum torque when the inverter is driven are smaller than those driven by the commercial power supply.
When the motor is powered by the commercial frequency power supply, the starting and acceleration shocks are large, and when the inverter is used for power supply, these impacts are weaker. A direct start of the power frequency produces a large starting and starting current. When the inverter is used, the output voltage and frequency of the inverter are gradually added to the motor, so the starting current and impact of the motor are smaller. Generally, the torque produced by the motor is reduced as the frequency decreases (the speed decreases). The reduced actual data is given in some of the drive manuals. By using a flux vector controlled inverter, the torque of the motor at low speeds is improved, and even in the low speed range, the motor can output sufficient torque.
3. When the inverter is adjusted to a frequency greater than 50Hz, the output torque of the motor will decrease.
The usual motor is designed and manufactured at a voltage of 50 Hz, and its rated torque is also given within 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 should decrease in a linear relationship inversely proportional to the frequency. When the motor is running at a frequency greater than 50 Hz, the size of the motor load must be considered to prevent the motor from outputting insufficient torque. For example, the torque generated by the 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. Application of inverter above 50Hz
As you know, the rated voltage and current rating of a particular motor are constant. If the inverter and motor are rated: 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. If the output frequency is increased to 60Hz, the maximum output voltage of the inverter can only be 380V/30A. Obviously, the output power is unchanged. 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 does not change, w increases, so the torque will decrease accordingly. We can also look at another angle:
The stator voltage of the motor is U = E + I*R (I is the current, R is the electronic resistance, and E is the induced potential). It can be seen that when U, I are constant, E is also unchanged. E = k*f* X, (k: constant, f: frequency, X: flux), so when f is from 50–>60Hz, X will decrease 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. Meanwhile, when it is less than 50 Hz, since I*R is small, U/f=E/f When constant, the flux (X) is constant. The torque T is proportional to the current. This is why the overcurrent capability of the inverter is usually used to describe its overload (torque) capability. (The rated current does not change -> the maximum torque does not change). Conclusion: When the inverter output frequency increases from above 50Hz, the output torque of the motor will decrease.
5. Other factors related to output torque
The heat and heat dissipation capacity determine 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, and the value of continuous output can be guaranteed at the highest ambient temperature. When the carrier frequency is reduced, the current of the motor will not be affected. However, the heat of the components will decrease. Ambient temperature: It does not increase the protection current value of the inverter because it detects that the ambient temperature is low. Altitude: The altitude increases, which has an effect on heat dissipation and insulation performance. Generally, it can be ignored below 1000m. 5% of the rice can be reduced.
6. How does vector control improve the output torque capability of the motor?
*1: Torque boost
This function increases the output voltage of the inverter (mainly at low frequencies) to compensate for the output torque loss caused by the voltage drop across the stator resistance, thereby improving the output torque of the motor. The technique of improving the low output torque of the motor at low speed, using “vector control”, can make the output torque of the motor at low speed, such as (without speed sensor) 1Hz (for a 4-pole motor, its speed is about 30r/min) The torque of the motor at 50 Hz output is reached (maximum approximately 150% of rated torque). For conventional V/F control, the voltage drop of the motor increases relatively as the motor speed decreases, which results in the motor not being able 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 motor speed reduction. 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 torque component produced by the motor and other components (such as the excitation component). The “vector control” distributes the current value of the motor to determine the value of the motor current component and other current components (such as the excitation component) that produce the torque. The “vector control” can be optimally compensated by responding to the voltage drop at the motor end, 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 speeds.
7. Inverter braking situation
*1: Braking concept: means that the electric energy flows from the motor side to the inverter side (or the power supply side). At this time, the motor speed is higher than the synchronous speed. The energy of the load is divided into kinetic energy and potential energy. Kinetic energy (by speed and weight) Determine its size) as the object moves. When the kinetic energy is reduced to zero, the thing is in a stopped state. The method of mechanically holding the brake device is to use the brake device to convert the kinetic energy of the object into friction and energy consumption. For the frequency converter, if the output frequency is reduced, the motor speed will also decrease with the following frequency. This will generate a braking process. The power generated by the brake will return to the inverter side. These powers can be dissipated with resistance heating. When used to lift the load, when it is falling, the energy (potential energy) is also returned to the inverter (or power supply) side for braking. This method of operation is called “regenerative braking” and the method can be applied. The inverter brakes. During deceleration, the power generated is not consumed by the method of heat consumption, but the method of returning energy to the power supply side of the inverter is called “power return regeneration method”. In practice, this application requires an “energy feedback unit” option. How to improve braking ability? In order to dissipate the regenerative power with heat dissipation, it is necessary to install a braking resistor on the inverter side. In order to improve the braking capacity, it is not expected to solve the problem by increasing the capacity of the frequency converter. Please use options such as “brake resistance”, “brake unit” or “power regeneration converter” to improve the braking capacity of the inverter.