The induction of a DC motor while rotation lead to a non continuid current. This AC part of the DC csupply current is called ripple current. You can measure this and see if the motor is moving and also you can count the ripple per second and have the motor speed "RMS".
V(ripple)= V(rms) / V (DC)
when a dc motor armature current is fluctuated in a range then it is call hunting in a d.c motor.
In an ideal DC power supply, there is no ripple.
Bushes, with reference to DC motors is a brushed DC motor which is a motor that designed to run from a direct current (DC) power source. This type of motor is an internally committed electric motor.
In a switching DC-DC voltage converter, the oscillatory nature of the switching circuit generates a small "ripple" effect in the output voltage which is supposed to be minimized via careful design of the overall circuit. The output current of this type of converter typically flows through a diode into the rest of the system. The voltage measured at the cathode of this diode will exhibit the aforementioned ripple.
V(ripple)= V(rms) / V (DC)
Its a very difficult subject to which most of my fellow designers estimate the ripple current in the caps and then select capacitors that can handle that current. The idea of voltage droop om most DC links can be forgotten because of other compensators such as a lower BEMF motor. The problem is to get as much life out of the caps as you can and that's to keep them cool as you can. Assume that every amp delivered from the DC like is an amp of ripple current and you should be OK.
RMS current is converted to DC current by using a rectifier. Capacitors and other components are used to make a cleaner DC output with less ripple.
The field current of a dc motor provides the magnetic field that allows the motor to develop a torque when current flows through the armature..
What supplies a dc motor is not current, but voltage. Current is what flows in the circuit formed by the dc motor and for the charge. That current varies according to the resistance of the circuit.
rectifierac motor - dc generator set
dc motor
when a dc motor armature current is fluctuated in a range then it is call hunting in a d.c motor.
In an ideal DC power supply, there is no ripple.
A DC motor is an electric motor that runs on direct current (DC) electricity. A DC motor is designed to run on DC electric power. Two examples of pure DC designs are Michael Faraday's homopolar motor (which is uncommon), and the ball bearing motor, which is (so far) a novelty. By far the most common DC motor types are the brushed and brushless types, which use internal and external commutation respectively to reverse the current in the windings in synchronism with rotation.
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AC stands for alternating current, and DC is direct current. A motor is a device that uses electrical energy to spin. An AC motor uses AC voltage, and a DC motor uses DC voltage. http://ezinearticles.com/?The-Difference-Between-AC-and-DC-Electric-Motors&id=193767