CN0182 PULSE INCREMENTAL SERVO DRIVE
POWER SUPPLY = 10 VDC
TERMINAL RESISTANCE = 1 OHM
TORQUE CONSTANT = 13.52 OZ-IN/AMP
SPEED CONSTANT = 100 RPM/VOLT
SPEED
1000 RPM
500 RPM
SPEED
TORQUE
67.6 OZ-IN
135.2 OZ-IN
MECHANICAL
PEAK POWER
25 WATTS
CURRENT
10 AMPS
5 AMPS
CURRENT
Figure 16
Mechanical power is the product of speed times torque. A motor running at its no-load speed
is producing zero power since torque applied is zero. At the other extreme, a stalled motor is
producing zero power as well, since its speed is zero. Therefore, it can be inferred that output
power must increase from zero, reach a peak, and then fall back to zero again as applied
torque goes from zero to stall. In fact the power versus torque curve is parabolic and peaks at
½ of stall torque (Figure 16).
Motor efficiency is the percentage of electrical power delivered to the motor that is converted
to mechanical power; the rest is dissipated as heat. As seen above, the motor delivers
maximum output power at one half of its stall current. At this point the series resistor has half
of the power supply voltage across it, leaving the other half for the ideal motor. Since the
same current flows through both, half the power is converted to heat. This means the real
motor efficiency cannot be more than 50% at peak power output.
If there were no friction in the motor, efficiency would be 100% for an infinitesimal torque
load, decreasing linearly with load to 0% at stall torque.
The continuous rated torque is typically one tenth of the motor’s stall torque. Because motor
heating goes up with the square of the current (W = I
2
R ), peak heat dissipation in a typical
motor is 100 times higher than what the motor can dissipate continuously. Heat dissipation at
peak motor power output is 25 times the motor’s continuous dissipation. Be careful what
percentage of time the motor must operate above its continuous rated torque!
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Summary of Contents for CN0182
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