A test like this is done to get a “baseline” measure of motor performance while the motor is at room temperature. The test was done by quickly loading the motor from no-load to locked rotor (stall) using a fixed terminal voltage from a power supply with low output impedance. For example, the motor curve illustrated in Figure 1 was created using a rapid test on a motor dynamometer. Once the user understands this concept, it becomes clear that motor performance determined using a rapid dynamometer test with a room temperature motor is significantly different than the motor performance when operated at its maximum temperature under load.ĭC motor performance curves can be generated under various conditions. Changes in these two key components of the motor will result in an increase in motor no-load speed and a decrease in motor locked rotor torque altering the overall slope of the motor curve. The flux density of the permanent magnets will also decrease as a function of temperature. As the motor temperature increases, winding resistance will increase based on the temperature coefficient of copper. Winding resistance and permanent magnet flux density will change as temperature changes. The phrase “motor constants,” however, is somewhat of a misnomer. These principles apply to both brush and brushless DC motors. Motor terminal resistance is also determined in the design phase by the number of coils, number of coil turns, and magnet wire diameter. For example, if the motor K T = 0.1 Nm/A, then motor K E= 0.1 V/(rad/s) assuming Nm and V/(rad/s) are the units used. They are always equal when using SI units. The torque constant and voltage constant are determined during the design phase and are a function of the overall magnetic circuit design. Motor terminal resistance (initial “cold”) These three values will determine the output torque, motor speed, and the resulting output power at any point on the motor curve at a given terminal voltage, as well as the overall slope of the motor curve.
Once a motor design is finalized including motor dimensions, magnetic circuit, and motor winding configuration, several characteristics that determine motor performance become “theoretically” fixed the torque constant (K T), voltage constant (K E), and motor terminal resistance (R mt). When reviewing DC motor curves, the user needs to ask the question “Do these curves represent performance of the motor at room temperature, or do these curves illustrate performance at the maximum rated temperature?” Depending on the temperature and the required operating point on the motor curve, the performance difference between “cold” and “hot” conditions can be significant. Performance will change as the motor temperature increases. When applying DC motors to any type of application, temperature effects need to be considered in order to properly apply the motor. Understanding the effects of heat and temperature changes will help in selecting the right motor for an application. Motor performance changes with temperature. By Dan Montone, PITTMAN Motors/AMETEK Precision Motion Control, Harleysville, PA