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Old 06-17-2015, 05:14 AM
bistander bistander is offline
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Originally Posted by bistander View Post
BEMF is half of the equation for the power converted from mechanical to electrical (generator) or from electrical to mechanical (motor). That conversion power is the generated voltage (BEMF) times the armature current. Without BEMF, a motor would not rotate due to an electric input.

Originally Posted by mbrownn View Post
In the last sentence do you mean EMF? It does not make sense to me that BEMF, the force that opposes applied current, is the force that causes a motor to turn.
Actually BEMF is confusing. That is why I like to use "generated voltage" or Eg (E_sub_g). Eg is present in any conductor moving in the presence of a magnetic field (commonly viewed as cutting lines of flux). This generated voltage (Eg) in the conductor (or coil) is present regardless of the current in the conductor. Current can be either direction or zero and Eg will still exist in the conductor cutting flux.

So this generated voltage (Eg) occurs in the armature of a motor or generator. It is calculated (or measured) exactly the same way whether it is a motor or generator. It actually is the same physical attribute whether in a motor or in a generator. I guess you call that generated voltage (Eg) the BEMF only in the motor (because it opposes the current flow) and not in the generator where it sources the current flow. Confusing to me because it is the same thing. But whatever.

My point was that whether a motor or generator, this generated voltage (Eg) times the armature current (Ia) is equal to the power converted (Pconv). So in the motor, you have the electrical losses (IČR) subtracted from the electric input power to give you the power converted at the air gap (Pconv) and then the rotational losses (Iron and friction) subtracted to yield the mechanical output power on the shaft. From the generator viewpoint, the rotational losses subtract from the input shaft power and the IČR from the Pconv. Either way, Pconv = Eg * Ia.

With the motor you have an applied voltage, Vm. I guess what you call EMF. In the motor you have the voltage drop due to resistance (and brush). Then you have the generated voltage, Eg (you call BEMF). Both the voltage drop across the resistance and the Eg oppose the applied voltage, Vm. The electrical power input to the motor is not the power converted to mechanical power. Some of it is lost as heat in the resistance. In fact, at stall, all of the electric input power is lost as heat and none is converted to mechanical power. So what is converted to mechanical power? The generated voltage (Eg or BEMF) times the armature current.

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