I have an experimental generator and an off-the-shelf motor that I intend to arrange like in Fig. 1. The problem is that if I connect the required load to the output of the generator the power drawn from the DC source is too big and the electricity bill too high. I cannot afford it.
I intend to connect the motor as a load for the generator to run this generator much longer and pay just the losses in the motor and generator not the power dissipated in the load.
I tried to close Kmotor, keep it like this for a time and then open it and close Kloop and keep it closed for another interval of time then open it and close Kmotor and repeat the process. In PSIM (software used for simulating electrical circuits) I get zero current in the loop no matter that Kloop is closed or opened (some large ideal spikes appears but they disappear soon). Despite this zero current in the loop, both motor and generator quickly reach the nominal speed and voltage and stay like this.
I also added a flywheel with a big moment of inertia on the shaft of the generator starting from the idea that when the motor is not powered at all the only source of energy for the generator is the mechanical energy stored in the spinning rotors of the motor and generator and the flywheel. I got no positive results. The current in the loop continues to be zero.
I do not expect to run a loop with 100% efficiency but even a 75% efficiency will save me a lot of money.
Fig. 1. Generator powering the motor that rotates the generator.
After asking the question and getting some answers I decided to use as motor and generator a predefined DC machine available in PSIM that has the following characteristics (unfortunately I get the same zero current in the loop):
and this symbol:
Ra (armature): Armature winding resistance, in Ohm
La (armature): Armature winding inductance, in H
Rf (field): Field winding resistance, in Ohm
Lf (field): Field winding inductance, in H
Moment of Inertia: Moment of inertia J, in kg x m2
Vt (rated): Rated terminal voltage, in V
Ia (rated): Rated armature current, in A
N (rated): Rated mechanical speed, in RPM
If (rated): Rated field current, in A
This DC machine is described by the following equations:
Vt : terminal voltage
Ea: back emf
Ia: armature current
If: field current
Wm: mechanical speed, in rad./sec.
Laf: mutual inductance between the field and the armature windings
The term Laf*If is often defined as kφ in many text books. Note that the relationship between the flux phi and the field current If is assumed to be linear. Magnetic saturation is not considered. The mutual inductance Laf is calculated as follows based on the specified rated operating condition: