Tolerances Tolerances must be considered in critical ranges. The possible deviations of the mechanical dimensions can be found in the overview drawings. The motor data are average values: the adjacent diagram shows the effect of tolerances on the curve characteristics. They are mainly caused by differences in the magnetic field strength and in wire resistance, and not so much by mechanical influences. The changes are heavily exaggerated in the diagram and are simplified to improve understanding. It is clear, however, that in the motor’s actual operating range, the tolerance range is more limited than at start or at no load. Our computer sheets contain all detailed specifications.
Tolerance field presentation for maxon motors
Tolerance for starting current
Thermal behavior The Joule power losses P J in the winding determine heating of the motor. This heat energy must be dissipated via the surfaces of the winding and motor. The increase D T W of the winding temperature T W with regard to the ambient temperature arises from heat losses P J and thermal resistances R th1 and R th2 . T W − T U = D T W = ( R th 1 + R th 2 ) · P J Here, thermal resistance R th1 relates to the heat transfer between the wind- ing and the stator (magnetic return and magnet), whereas R th2 describes the heat transfer from the housing to the environment. Mounting the motor on a heat dissipating chassis noticeably lowers thermal resistance R th2 . The values specified in the data sheets for thermal resistances and the maximum continuous current were determined in a series of tests, in which the motor was end-mounted onto a vertical plastic plate. The modi- fied thermal resistance R th2 that occurs in a particular application must be determined using original installation and ambient conditions. Thermal resistance R th2 on motors with metal flanges decreases by up to 80% if the motor is coupled to a good heat-conducting (e.g. metallic) retainer. The heating runs at different rates for the winding and stator due to the different masses. After switching on the current, the winding heats up first (with time constants from several seconds to half a minute). The sta- tor reacts much slower, with time constants ranging from 1 to 30 minutes depending on motor size. A thermal balance is gradually established. The temperature difference of the winding compared to the ambient temperature can be determined with the value of the current I (or in inter- mittent operation with the effective value of the current I = I RMS ) . ( R th 1 + R th 2 ) · R · I mot 2 1 – α Cu · ( R th 1 + R th 2 ) · R · I mot 2 Δ T W = Here, electrical resistance R must be applied at the actual ambient temperature.
Influence of temperature An increased motor temperature affects winding resistance and magnetic characteristic values. Winding resistance increases linearly according to the thermal resistance coefficient for copper ( α Cu = 0.0039): R T = R 25 · (1 + α Cu (T −25°C)) Example: a winding temperature of 75°C causes the winding resis- tance to increase by nearly 20%. The magnet becomes weaker at higher temperatures. The reduction is 0.5 to 5% at 75°C depending on the magnet material. The most important consequence of increased motor temperature is that the speed curve becomes steeper which reduces the stall torque. The changed stall torque can be calculated in first approxi- mation from the voltage and increased winding resistance: U mot R T M H = k M · I A = k M ·
September 2020 edition / subject to change
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