The brushless motors are excellent torque transducers which can develop peak torques by far exceeding the nominal torque. Consequently, the achievable peak torque as a rule is purely determined by the electric drive. The correct dimension of the motor is therefore both thermal and electric; the most suitable motor is the one which stabilizes at the preset temperature, typically 40-50°C above the ambient temperature.
The complete testing of the dimension of the motor is made up of three elements:
- check of the peak torque or demagnetisation;
- thermal dimensions
- electric dimensions
- Check of the demagnetization current
It is obtained by comparing the maximum value of the peak current, through the following formula:
and the demagnetisation current of the motor, considering that the motor demagnetisation current will increase to a temperature lower than the highest rates represented on the catalogues.
2 – Check of the thermal dimension
Roughly, it will be sufficient to verify that the Ceff, eff point is included in the running area in continuous duty of the motor.
More in detail, the rise in temperature of the motor can be quite correctly estimated with the use of the following formula:
where Ln stays for the nominal losses of the motor referred to an overheating of 65 °C.
If the preset temperature exceeds the highest temperature of the motor or in any case the required data, it will be necessary to opt for a larger motor.
Attention: the temperature in excess is the only true reason leading to opt for a motor of larger size.
3- Check of the electric dimension
It is necessary to verify that at the highest speed the voltage required by the motor to deliver the highest useful torque is inferior or lower to the rate supplied by the electronics, for the lowest foreseeable mains supply (usually for mains equal to 90% of the voltage of the nominal mains). Emin considered as the voltage value which can be supplied by the electronic feeder at the minimum feeding voltage, we must verify that:
In case this condition has not occured, it will be necessary to opt for a motor with a winding suitable for a higher speed, considering that this will involve a higher voltage rate, too.