Re: I need Help on internal resistance!

Also Google for motor internal resistance.

Regards,

Boris Mohar

Got Knock? - see: Viatrack Printed Circuit Designs (among other things)

Imagine a "perfect" battery, i.e.-one that will supply its output voltage at *any* current. Of course this type of battery doesn't exist becuase there is always going to be some internal resistance due to the resistance of internal metal connections, the electrolyte interface, etc.

The practical model for a battery (or any other source, AC or DC) is an ideal source with a series resistor.

The way you would be able to measure the internal resistance of a source is to measure the no-load voltage, then place a load resistance that will cause the output voltage to be *half* of the open voltage. The load resistor will be the same as the internal resistance.

If I recall correctly, the rule of thumb for the starting current for a motor is about 5 times the running current. If this assumption is correct, you need to make certain that the source you are using to drive the motor will supply enough starting current to get the motor to run. The internal resistance of your source will be the limiting factor.

Steve

Doing your homework for you... Internal resistance is exactly what the term implies.. the resistance of what's inside the motor. In the case of most motors (not counting the brushless DC types), it will be the resistance of the wire in the motor windings. Its effect will be as you would expect a resistance to exhibit.. as current passes through the wire, it will dissipate power, manifest in the form of heat. I'm sure your instructor has broached the term "heat rise" in the context of motors?? The heat generated by the IIR product in the wire contributes (but not completely) to the heat rise of a motor under load. Your next assignment from this newsgroup is to explain the other major factor causing the heat rise of a motor.