Singly excited devices are generally employed for motion through a limited distance or rotation through a prescribed angle. Electro-mechanical transducers have the special requirement of producing an electrical signal proportional to forces or velocities or producing force proportional to electrical signal (current or voltage). Such transducers require two excitations one excitation establishes a magnetic field of specified strength while the other excitation produces the desired signal (electrical or mechanical). Also continuous energy conversion devices motors and generators require multiple excitation.

Figure- 6 shows a magnetic field system with two electrical excitations one on stator and the other on rotor. The system can be described in either of the two sets of three independent variables (1, 2, θ) or ( i1, i2, θ). In terms of the first set

**Electromagnetic Force, f**

For the current-carrying conductor, the force (known as Lorentz force) produced on the conductor can be determined from the following equation:

f = Bli

where B, l, and i are mutually perpendicular. The direction of the force can be determined by using the Fleming’s Left Hand Rule.

**Fleming’s Left Hand Rule:**

“Hold out your left hand with forefinger, second finger and thumb at right angles to one another. If the forefinger represents the direction of the field, and the second finger that of the current, then thumb gives the direction of the motion or force.”

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