If you read our article Why Do Step Motors Get Hot? you may have wondered "What does this mean to me?” Step motor losses are important because the energy lost in the motor results in heat. Any motor has a thermal constant that can be used to compute how hot the motor will get for a given level of energy dissipation. Once the windings exceed 130°C, the insulation on the motor windings will melt and it’s game over. The subject of this examination is the Applied Motion NEMA 23 step motor HT23-601, which has a thermal constant of 2.24°C/W. If operated continuously at full current and 10 rev/second with a 48 VDC power supply, the motor would be dissipating 54W internally (see chart below), so the motor would eventually reach a case temperature of
If the ambient temperature is 25°C:
Typically the case temperature, which is much easier to measure than the winding temperature, must be kept below 100°C, so in this case, we have an unsustainable situation. But in most applications, step motors are used for incremental motion: to move a load from one place to another. Usually there is some dwell time between moves during which the motor can cool down. If your ambient temperature is 25°C, then we can allow a 75°C rise. By back solving our thermal rise equation, we can find the maximum power that this motor can safely dissipate if operated continuously:
For this scenario, our duty cycle must be limited to
A simpler way to approach this issue is to purchase the motor and drive from a supplier that provides recommended operating conditions and duty cycles, such as the ST series step motor drivers from Applied Motion Products. A sample duty cycle curve from their hardware manual is shown below.
What if my application requires continuous duty? Then, depending on the speed in question, we may have to reduce the internal heating, either by lowering the motor current or reducing the power supply voltage. Which one is more effective? It depends on the motor speed and torque requirement. The chart below shows the effect of reducing power supply voltage on motor losses. Losses are reduced at a high speed, but not at low speeds.
The next chart shows the effect of reducing motor current. Losses are reduced only at low speeds.