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Transcript
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.