Why current decreases when increase in viscous friction of dc motor?

Answer 1

I think your question could be rephrased as:

"When viscous friction increases, why does currentdecrease?". Though the latter is not directly dependent on the former, or vice versa, in most motors this is the case. To understand exactly why this happens we need to look into the constructional features of a motor.

All motors have a rotor (the spinning part) and a stator (the stationary part). In linear motors the part the slides along can be thought of as the 'rotor', and the non-moving base, as the 'stator'. The motor spins by causing like poles on the stator and rotor to oppose each other (different motors - AC, DC, brushless - do this in different ways). Read up about this on Wikipedia, HowStuffWorks, or just Google around. For maximum efficiency it is desirable to build the rotor and stator in such a way that there is very little space between the two. This allows a high degree of magnetic flux linkage/concentration between the rotor and stator. Between the rotor and stator is a small air gap. Air, like all fluids, has viscosity (a sort of 'stickyness', if you will). Viscous force appears between two sliding objects which have a liquid in between them. The closer the sliding surfaces, the greater is the viscous force. Viscous force also depends on the area of the surfaces, their relative velocity, and the liquid's properties. For a detailed derivation please refer to any text on fluid dynamics.

Another important motor characteristic is back EMF (BEMF), which is induced in motor coils due to magnetic interaction. The faster a rotor spins, the greater the magnitude od back EMF. Back EMF, as its name suggests, has a polarity opposite to the applied (battery/power source) EMF. It is the back EMF which causes a reduction in motor current, since it partially cancels out the applied EMF : (Applied EMF-Back EMF)=(Effective EMF). Since the coils in the motor have unchanging resistance, a lower apparent EMF imples lower current.

Now let us link these two phenomena:

low speed-->low BEMF-->high current

also;

low speed-->low viscous friction

but:

higher speed-->higher BEMF-->lower current

and:

higher speed-->higher viscous friction

I hope this clears things up.

Rohit de Sa,

Engineer,

Mercedes-Benz,

India