 # When the rotation speed (frequency) of the motor changes, what is the output torque?

Update:29 Apr 2020

1: Power frequency power supply
Power supply provided by the grid (commercial power supply)
2: Starting current
When the motor starts to run, the output current of the frequency converter
The starting torque and the maximum torque of the inverter drive are less than that of the direct power frequency power supply drive
When the motor is powered by power frequency power supply, the impact of starting and acceleration is very large, but when the inverter is used for power supply, these impacts will be weaker. Power frequency direct starting will produce a large starting current. When the frequency converter is used, the output voltage and frequency of the frequency converter are gradually added to the motor, so the starting current and impact of the motor are smaller.
Generally, the torque produced by the motor will decrease with the decrease of frequency (speed). The reduced actual data will be explained in some frequency converter manuals.
Through the use of flux vector control inverter, it will improve the motor torque at low speed, even in the low speed area, the motor can output enough torque.
3. When the frequency converter is adjusted to a frequency greater than 50Hz, the output torque of the motor will be reduced
Generally, the motor is designed and manufactured according to 50Hz voltage, and its rated torque is also given in this voltage range. Therefore, the speed regulation under rated frequency is called constant torque speed regulation (t = te, P < = PE)
When the output frequency of the frequency converter is higher than 50Hz, the torque produced by the motor will decrease linearly in inverse proportion to the frequency.
When the motor is running at a speed greater than 50Hz, the size of the motor load must be considered to prevent the shortage of the motor output torque.
For example, the torque generated by the motor at 100Hz should be reduced to about 1 / 2 of the torque generated at 50Hz.
Therefore, the speed regulation above the rated frequency is called constant power speed regulation (P = UE * IE)
4. Application of frequency converter above 50Hz
As you know, for a specific motor, its rated voltage and current are constant.
For example, the rated value of frequency converter and motor is 15kw / 380V / 30a, and the motor can work above 50Hz.
When the speed is 50Hz, the output voltage and current of the frequency converter are 380V and 30A. At this time, if the output frequency is increased to 60Hz, the maximum output voltage and current of the frequency converter can only be 380V / 30A. Obviously, the output power is unchanged. So we call it constant power speed regulation
How about the torque at this time?
Because P = WT (W: angular speed, t: torque). Because P is constant, w increases, so the torque will decrease accordingly.
We can also look at it from another perspective:
Stator voltage of motor u = e + I * r (I is current, R is electronic resistance, e is induced potential)
It can be seen that when u and I are the same, e is the same
And E = k * f * x, (K: constant, F: frequency, X: flux), so when f is from 50 to > 60Hz, x will decrease accordingly
For the motor, t = k * I * x, (K: constant, I: current, X: flux), so the torque T will decrease with the decrease of flux X
At the same time, when the frequency is less than 50Hz, because I * r is very small, when u / F = E / F is constant, the flux (x) is constant. The torque T is proportional to the current. That is why the over-current capacity of the frequency converter is usually used to describe its overload (torque) capacity. It is also called constant torque speed regulation (rated current is constant -- > maximum torque is constant)
Conclusion: when the output frequency of frequency converter increases from above 50Hz, the output torque of motor will decrease
5. Other factors related to output torque
The output current capability of the converter is determined by the heating and heat dissipation capability, which affects the output torque capability of the converter.
Carrier frequency: generally, the rated current of the frequency converter is the highest carrier frequency, which can ensure the continuous output value under the highest ambient temperature. Reduce the carrier frequency, and the motor current will not be affected. But the heating of components will be reduced.
Ambient temperature: as if the protection current value of the converter will not be increased when the ambient temperature is detected to be relatively low
Altitude: the increase of altitude has an impact on the heat dissipation and insulation performance. Generally, it can be ignored if the altitude is less than 1000m. The above capacity can be reduced by 5% per 1000m

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