1. Why can the rotation speed of the motor be changed freely?
Motor rotation speed unit: r/min 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 divided into two parts of the frequency
The rotational speed of an induction AC motor (henceforth 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. Because the pole value is not a continuous value (a multiple of 2, e.g. the pole number is 2, 4, 6), it is generally not suitable to adjust 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 freely controlled.
Therefore, the inverter with the intention of controlling the frequency is the preferred equipment for motor speed regulation equipment.
n = 60f/p
n: Synchronization speed
f: Power frequency
p: Number of pole pairs of the motor
Conclusion: Changing the frequency and voltage is the best way to control the motor
If only the frequency is changed but not the voltage, the motor may burn out due to overvoltage (overexcitation) when the frequency drops. Therefore, the frequency converter must change the voltage at the same time as changing the frequency. When the output frequency is above the additional frequency, the voltage cannot be further added, and the maximum voltage can only be equal to the additional voltage of the motor.
For example, if the output frequency of the inverter is changed from 50 Hz to 25 Hz in order to halve the rotation speed of the motor, the output voltage of the inverter needs to be changed from 400 V to about 200 V
2. What happens to the output torque of a motor when its rotation speed (frequency) is changed?
The starting torque and maximum torque when driven by the inverter are less than those driven by direct industrial frequency power supply.
The motor starts and accelerates when the power frequency power supply is very large, and when the inverter is used for power supply, these impacts are weaker. A large starting current will occur when the power frequency is directly started. 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.
In general, the torque generated by the motor decreases as the frequency decreases (speed decreases). Practical data for the reduction is illustrated in some inverter manuals.
By using a magnetic flux vector-operated inverter, the lack of torque in the motor at low speeds will be improved, and the motor can output sufficient torque even at low speeds.
3. When the speed of the inverter is adjusted to a frequency greater than 50Hz, the output torque of the motor will decrease
The general motor is planned and made according to the voltage of 50Hz, and its additional torque is also given in this voltage range. Therefore, the speed regulation under the extra frequency is called constant torque speed regulation. （T=Te， P）
When the output frequency of the inverter is greater than the frequency of 50Hz, the torque of the motor decreases in a linear relationship inversely proportional to the frequency.
When the motor is running at a frequency speed greater than 50Hz, the size of the motor load must be considered to prevent the lack of motor output torque.
For example, the torque of a motor at 100 Hz drops to about 1/2 of that at 50 Hz.
Therefore, the speed regulation above the additional frequency is called constant power speed regulation. （P=Ue*Ie）
4. The use of the inverter above 50Hz
As you know, for a particular motor, the extra voltage and extra current are constant. For example, the additional value of the inverter and the motor is: 15kW/380V/30A, and 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, and the output power is obviously unchanged. That's why we call it constant power speed regulation.
What about the torque? Because P=wT (w: angular velocity, T: torque). Since P does not change, w is added, so the torque decreases accordingly.
Let's also look at it from another angle: the stator voltage of the motor U = E + I*R (I is the current, R is the electronic resistance, E is the induced potential) It can be seen that when you and I are unchanged, E is also constant. E = k*f*X, (k: constant, f: frequency, X: flux), so when f is from 50–>60Hz, X decreases accordingly
For motors, T = K*I*X (K: CONSTANT, I: CURRENT, X: MAGNETIC FLUX), SO THE TORQUE T WILL DECREASE WITH THE DECREASE OF MAGNETIC FLUX X. Together, when it is less than 50Hz, because I*R is very small, when U/f=E/f is constant, the magnetic flux (X) is constant, and the torque T is proportional to the current. This is why the overcurrent ability of the inverter is generally used to describe its overload (torque) ability. It is also called constant torque regulation (the additional current does not change – > the maximum torque does not change)
Conclusion: When the output frequency of the inverter is increased from 50Hz or more, the output torque of the motor will decrease.
5. Other elements related to output torque
Heat generation and heat dissipation can determine the output current of the inverter, which affects the output torque of the inverter.
Carrier frequency: Generally, the additional current marked by the inverter is the value that can ensure the continuous output at the highest carrier frequency and the highest ambient temperature. If the carrier frequency is lowered, the current of the motor will not be affected, but the heat generation of the components will be reduced.
Ambient temperature: Just like the inverter maintenance current value will not be increased because the ambient temperature is low.
Altitude: Altitude is added to have an impact on heat dissipation and insulation. Generally, less than 1000m can be considered. The above 5% reduction per 1000 meters is enough.
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