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DC Brush Motor

Views: 34     Author: Site Editor     Publish Time: 2018-01-10      Origin: Site

DC Brush Motor

DC Brush Motor
Principles of operation
Reference to the chart reveals useful performance information valid for all fulling servomotors.
It shows speed n, current I, output power P and efficiency ¦Ç plotted against torque M for a given supply voltage U. Torque M is a function of the current I and the torque constant k (expressed in Nm/A). The motor develops its maximum torque Ms at stall (n=0), when the current is maximum and determined only by the supply voltage U and the rotor resistance R:
DC1.png

With increasing speed, an increasing back EMF E is induced in the armature which tends to reduce the current:
dc2.png

The value of E is the product of angular speed ¦Ø (expressed in rad/s) and the torque constant (expressed in V/rad/s=Vs=Nm/A):
E = k¦Ø
Thus, the supply voltage splits into two parts: RI, necessary to establish the current I in the armature, which generates the torque
M, and k¦Ø to overcome the induced voltage, in order to generate the speed¦Ø:
U = RI + k¦Ø
No-load speed no is a function of the supply voltage and is reached when E becomes almost equal to U; no-load current Io is a function of friction torque:
DC3.png

Power output P is the product of angular speed¦Ø and torque M (P = M.¦Ø); for a given voltage it reaches its maximum Pmax at half the stall torque Ms, where efficiency is close to 50%. The maximum continuous output power is defined by an hyperbola delimiting the continuous and intermittent operation ranges.
Efficiency¦Çis the mechanical to electrical power ratio (¦Ç= Pm / Pel). Maximum efficiency¦Çmax occurs at relatively high speed. Its value depends upon the ratio of stall torque and friction torque and thus is a function of the supply voltage:
DC4.png

The maximum continuous torque depends upon dissipated power (I2R), its maximum value is determined by:
dc5.png

Where Tmax is the maximum tolerated armature temperature, Tamb is the ambient temperature, Rmax is the rotor resistance at temperature Tmax and Rth is the total thermal resistance (rotor-body-ambient).
At a given torque M, increasing or decreasing the supply voltage will increase or decrease the speed. The speed-torque function varies proportionally to the supply voltage U.
DC6.png


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