Figure 4: Motor Torque Curve with Operating limits shown

The maximum continuous torque is set mainly by the temperature limits of the internal components of the motor, assuming the motor is operating within specified environmental conditions, such as under 25°C ambient conditions, free air circulation, and no heat transfer through the flange.⁴ It is very likely the environmental conditions of your motor’s installation will be different than these. You will need to either adjust your environmental conditions, such as adding cooling fans, and/or make sure your motors will be operating well within their continuous operating range so as to not overstress and damage or burn out the motor.

The simplest first check to see whether your requirements coincide with a motor’s continuous operating range is to also plot your most demanding torque and speed requirements on the motor’s torque curve. In the ModBot’s case, this would be the point (865 mNm, 189.86 rpm). This point should lie within the continuous operating range. Since this is the point you want your system to operate at, it is commonly referred to as your operating point. In many cases, gearing can be applied to help modify a motor’s effective continuous operating range. This is discussed more in later sections but for now, it is important to remember that you will need to select a motor size whose continuous operation region will contain your operation point whenever possible.

Motors do have the ability to operate outside of the continuous operation zone for short periods of time. The limitation for running in this zone is determined by the thermal limitations and mechanical degradation characteristics of the motor. Thermal limitations occur because the motor is unable to dissipate the heat generated by the inefficiency of the motor. If operated too long in this condition, it will cause demagnetization of the motor magnets and a permanent loss of performance will occur. Mechanical degradation occurs because of the increased wear rate of the brushes and bearings at speeds above the maximum permissible speed. There is no way to calculate the amount of time that you can operate outside of the continuous operation zone from the standard information given on motor data sheets. However, you can monitor your motor’s temperature during operation to estimate the intensity of the stress you are putting on the motor. In general though, it is recommended to only operate within your motor’s continuous operation zone.

Figure 5 shows an example motor having the max permissible speed of 12,000 rpm, with a max continuous torque curve going from 86.6 mNm at low speed to 71.0 mNm at high speed. It also shows short term operation up to about 180 mNm is possible.

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