Motor selection The drive requirements must be defined before proceeding to motor selection. − How fast and at which torques does the load move? − How long do the individual load phases last? Often the drive is indirect, this means that there is a mechanical transfor- mation of the motor output power using belts, gears, screws and the like. The drive parameters, therefore, are to be calculated to the motor shaft. Additional steps for gear selection are listed below. − What accelerations take place? − How great are the mass inertias? Furthermore, the power supply requirements need to be checked. − Which maximum voltage is available at the motor terminals? − Which limitations apply with regard to current? The current and voltage of motors supplied with batteries or solar cells are very limited. In the case of control of the unit via a servo amplifier, the amplifier’s maximum current is often an important limit. Selection of motor types The possible motor types are selected using the required torque. On the one hand, the peak torque, M max , is to be taken into consideration and on the other, the effective torque M RMS . Continuous operation is characterized by a single operating or load point ( M L , n L ). The motor types in question must have a nominal torque (= max. continuous torque) M N that is greater than load torque M L . M N > M L In operating cycles, such as start/stop operation, the motor‘s nominal torque must be greater than the effective load torque (RMS). This pre- vents the motor from overheating. M N > M RMS The stall torque of the selected motor should usually exceed the emerging load peak torque. M H > M max Selection of the winding: electric requirement In selecting the winding, it must be ensured that the voltage applied directly to the motor is sufficient for attaining the required speed in all operating points. Uncontrolled operation In applications with only one operating point, this is often achieved with a fixed voltage U . A winding is sought with a speed-torque line that passes through the operating point at the specified voltage. The calcula- tion uses the fact that all motors of a type feature practically the same speed-torque gradient. A target no load speed n 0,theor is calculated from operating point ( n L , M L ). Δ n Δ M n 0 , theor = n L + M L This target no load speed must be achieved with the existing voltage U , which defines the target speed constant. n 0 , theor U mot k n, theor = Those windings whose k n is as close to k n, theor as possible, will approxi- mate the operating point the best at the specified voltage. A somewhat larger speed constant results in a somewhat higher speed, a smaller speed constant results in a lower one. The variation of the voltage adjusts the speed to the required value, a principle that servo amplifiers also use. The motor current I mot is calculated using the torque constant k M of the selected winding and the load torque M L . M L k M I mot =
Advices for evaluating the requirements: Often the load points (especially the torque) are not known or are difficult to determine. In such cases you can operate your device with a measuring motor roughly estimated according to size and power. Vary the voltage until the desired operating points and motion sequences have been achieved. Measure the voltage and current flow. Using these specifications and the part number of the measuring motor, our engineers can often specify the suitable motor for your application. Additional optimization criteria are, for example: − Mass to be accelerated (type, mass inertia) − Type of operation (continuous, intermittent, reversing) − Ambient conditions (temperature, humidity, medium) − Power supply, battery When selecting the motor type, other constraints also play a major role: − What maximum length should the drive unit have, including gear and encoder diameter? − What service life is expected from the motor and which commutation system should be used? − Precious metal commutation for continuous operation at low currents (rule of thumb for longest service life: up to approx. 50% of I N ). − Graphite commutation for high continuous currents (rule of thumb: 50% to approx. 75% of I N ) and frequent current peaks (start/stop operation, reversing operation). − Electronic commutation for highest speeds and longest service life. − How great are the forces on the shaft, do ball bearings have to be used or are less expensive sintered bearings sufficient?
U = constant
Speed-torque line high enough for the required load speed
Speed-torque line too low for the required load speed
September 2020 edition / subject to change
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