1. Why can the rotational speed of an electric motor be changed freely?
Motor rotation speed is measured in r/min, which is the number of revolutions per minute, or rpm.
For example: a 2-pole motor, 50Hz, 3000 [r/min]
4-pole motor, 50Hz, 1500 r/min
Conclusion: The rotational speed of an electric motor is proportional to its frequency.
The rotational speed of an induction AC motor (hereinafter referred to as a motor) is approximately determined by the number of poles and the frequency of the motor. The number of poles is fixed due to the motor's operating principle. Since this number of poles is not a continuous value (it is a multiple of 2, for example, 2, 4, 6 poles), it is generally not suitable to adjust the motor speed by changing this value.
In addition, the frequency can be adjusted outside the motor before being supplied to the motor, so the rotational speed of the motor can be freely controlled.
Therefore, frequency converters, designed for frequency control, are the preferred choice for motor speed regulation devices.
n=60f/p
n: Synchronization speed
f: Power supply frequency
p: Number of pole pairs of the motor
Conclusion: Changing the frequency and voltage is the optimal motor control method.
If only the frequency is changed without changing the voltage, the motor will experience overvoltage (overexcitation) when the frequency decreases, potentially burning it out. Therefore, the inverter must change the voltage simultaneously with the frequency. However, when the output frequency is above the rated frequency, the voltage cannot be increased further; it can only be equal to the motor's rated voltage.
For example, to halve the motor's rotational speed, the inverter 's output frequency is changed from 50Hz to 25Hz. In this case, the inverter's output voltage needs to be changed from 400V to approximately 200V.
2. How will the output torque of a motor change when its rotational speed (frequency) changes?
The starting torque and maximum torque of the inverter drive are less than those of the direct power frequency drive.
When a motor is powered by mains frequency, the starting and acceleration shocks are significant, while these shocks are reduced when powered by a frequency converter. Direct starting at mains frequency generates a large starting current. However, when using a frequency converter, the output voltage and frequency are gradually applied to the motor, resulting in a smaller starting current and less shock.
Typically, the torque produced by a motor decreases as the frequency decreases (speed decreases). The actual amount of this decrease is usually provided in the inverter manual.
By using a frequency converter with flux vector control, the insufficient torque of the motor at low speeds can be improved, and the motor can even output sufficient torque in the low-speed range.
3. When the frequency converter is adjusted to a frequency greater than 50Hz, the motor's output torque will decrease.
Typical electric motors are designed and manufactured for a 50Hz voltage, and their rated torque is also given within this voltage range. Therefore, speed regulation below the rated frequency is called constant torque speed regulation (T=Te, P<=Pe).
When the inverter output frequency is greater than 50Hz, the torque generated by the motor will decrease in a linear relationship that is inversely proportional to the frequency.
When the motor is running at a frequency greater than 50Hz, the size of the motor load must be taken into account to prevent insufficient motor output torque.
For example, the torque produced by a motor at 100Hz is reduced to about half of the torque produced at 50Hz.
Therefore, speed regulation above the rated frequency is called constant power speed regulation. (P=Ue*Ie)
4. Applications of frequency converters above 50Hz
As we all know, for a specific motor, its rated voltage and rated current are constant.
If both the inverter and the motor are rated at 15kW/380V/30A, the motor can operate at frequencies above 50Hz.
When the speed is 50Hz, the inverter's output voltage is 380V and the current is 30A. If the output frequency is increased to 60Hz, the inverter's maximum output voltage and current will still be 380V/30A. Obviously, the output power remains unchanged. Therefore, we call this constant power speed regulation.
What is the torque situation at this point?
Because P = wT (w: angular velocity, T: torque), and since P remains constant while w increases, the torque will decrease accordingly.
We can also look at it from another angle:
The stator voltage of the motor is U = E + I * R (where I is the current, R is the electronic resistance, and E is the induced electromotive force).
It can be seen that when U and I remain unchanged, E also remains unchanged.
Since E = k * f * X (k: constant, f: frequency, X: magnetic flux), X will decrease accordingly when f changes from 50 to 60 Hz.
For a motor, T = K * I * X (K: constant, I: current, X: magnetic flux), therefore the torque T will decrease as the magnetic flux X decreases.
Meanwhile, below 50Hz, since I*R is very small, the magnetic flux (X) is constant when U/f=E/f remains unchanged. The torque T is proportional to the current. This is why the overload (torque) capability of a frequency converter is usually described by its overcurrent capability, and it is called constant torque speed regulation (rated current remains constant --> maximum torque remains constant).
Conclusion: When the inverter output frequency increases from above 50Hz, the motor output torque will decrease.
5. Other factors related to output torque
The ability to generate and dissipate heat determines the inverter's output current capability, which in turn affects the inverter's output torque capability.
Carrier frequency: The rated current marked on a frequency converter is the value that can guarantee continuous output at the highest carrier frequency and highest ambient temperature. Reducing the carrier frequency will not affect the motor current, but the heat generated by the components will be reduced.
Ambient temperature: Just like the inverter's protection current value will not increase when the ambient temperature is detected to be low.
Altitude: Increased altitude affects both heat dissipation and insulation performance. Generally, it can be disregarded below 1000m. Above that, a 5% derating is sufficient for every 1000m.
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