Derating refers to the operation of equipment at reduced capacity or speed. Derating in motors can be caused due to the following reasons.
When frequency increases, the speed increases and the torque decreases. If the frequency increases by 5%, the speed increases by 5% while the torque decreases by 10%.
Voltage has a direct relation with torque. When the voltage falls the torque also reduces. Equipment has suddenly rotate faster or move faster due to voltage fluctuations.
As the altitude increases, the density of air decreases. This reduces the ability of air to transfer heat and cool the motor. Thus if the motor is to be operated above 1000 metres above sea level, it has to be derated.
The ambient temperature is also a factor in derating. If the ambient temperature is high, the insulation may reach its maximum temperature limit quickly. Hence, the motor may have to be derated.