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DC Motor Performance
Newton’s First Law:
Objects at rest remain at rest unless acted upon by and external force.
Or more generally:
Objects moving at a constant velocity continue at that velocity unless acted upon by an
external force.
Consequences:
1. When the forces on an object are unbalanced, the object accelerates.
2. When the forces on an object are balanced, the object moves at a constant
velocity.
When we accelerate an automobile, it goes faster until the forces of friction and air
resistance balance the forces that are causing acceleration.
Faraday’s Law
When a wire is moved through a magnetic field, an electrical potential difference or
voltage is created between the ends of the wire.
Consequences:
1. When the coil of a motor moves through the magnetic field of the motor magnets,
a voltage is created that is of polarity opposite to that which is applied to the
motor terminals, i.e a counter-electromotive force or counter-emf.
2. The faster the motor goes, the bigger the counter-emf.
3. In the absence of a mechanical load, friction, etc., the steady-state or constant
motor velocity is when the counter-emf equals the applied voltage.
4. In the presence of a mechanical load, the steady-state velocity decreases as the
load increases.
5. In any real system, the steady-state velocity of a DC motor is a function of:
a. The applied voltage
b. The mechanical load including any system friction or other forces.
We found experimentally that  = 0 – kT, where 0 is the angular velocity at no
external load but finite and real friction, etc.
T is the torque produced by the motor to balance the external load, and k is the system
constant that describes the rate at which  decreases as T increases.
The steady-state velocity for an electric motor is reached very quickly; acceleration is not
easily observed in our experiments.
The system performance is described by the  versus T curve. The mechanical power
curve can be calculated by multiplying T = P and plotting P versus T.
It can be shown that the maximum mechanical power is at that torque that is one-half the
torque at which  = 0, i.e. one-half the “stall” torque.