Reference to the chart reveals useful performance information valid for all MCP DC brushed servomotors.It shows speed n, current I, output power P and efficiency η 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:
Is = U/R
Ms = Is·k
With increasing speed, an increasing back EMF E is induced in the armature which tends to reduce the current:
The value of E is the product of angular speed ω (expressed in rad/s) and the torque constant (expressed in V/rad/s=Vs=Nm/A):
Thus the supply voltage splits into two parts: RI, necessary to establish the current I in the armature, which generates the torque M, and kω to overcome the induced voltage, in order to generate the speed ω:
U = RI + kω
No-load speed n0 is a function of the supply voltage and is reached when E becomes almost equal to U; no-load current I0 is a function of friction torque:
Power output P is the product of angular speed ω and torque M (P = M · ω); 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 η is the mechanical to electrical power ratio (η = Pm / Pel). Maximum efficiency ηmax 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:
The maximum continuous torque depends upon dissipated power (I2R), its maximum value is determined by:
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.