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Equivalent circuits simplify analysis of existing circuits, with specific parameters such as frequency and temperature. They vary for different circuits and components, such as batteries, transformers, diodes, and motors. The equivalent circuit for a motor is determined by RPM and load torque.
An equivalent circuit is a simplified model of an existing circuit that greatly simplifies the analysis of an original circuit. Any circuit will have an equivalent for specific parameters such as signal frequency, component temperature, and other factors such as transducer inputs. Original circuits may have a voltage source with an internal resistance and several external resistors, while equivalent circuits, in direct current (DC) analysis, will be a voltage source and a single internal resistance, or the net resistance of the resistors internal and external. There are equivalent circuits for all types of circuits with all types of components.
The normal pen battery is rated at 1.5 volts of direct current (VDC). As the battery dies, an equivalent circuit keeps changing until the battery runs out. The ideal voltage source has no internal resistance and, in series with ever-increasing resistance, is the equivalent of a real-world 1.5 volt (V) battery.
Transformers supply power through a secondary winding when power is supplied into the primary winding. The transformer equivalent circuit helps explain the detailed characteristics of the real-world transformer. An ideal transformer will consume no power when there is no load on the secondary winding, but a real-world transformer with an energized primary and disconnected secondary winding will still consume power. The equivalent transformer circuit, due to the nature of core losses, will have parallel core resistance or resistance that does not exist but can be seen from the power source. A transformer equivalent circuit has an ideal transformer at the output with multiple distributed inductance, capacitance, and resistance at the input.
Equivalent circuits for semiconductor circuits vary depending on frequency, voltage polarity, and signal amplitude. The diode equivalent circuit in the forward biased, or conducting state, is a low voltage source in series with a low resistance. For example, a forward biased silicon diode may have an equivalent voltage source of 0.6 VDC in series with a 0.01 ohm resistor.
The equivalent circuit model for motors is also determined by rotor revolutions per minute (RPM) and load torque. For example, a DC motor with a non-rotating rotor looks like two electromagnets in the motor equivalent circuit; at 0 rpm, the DC motor draws more current. If the rotor is allowed to rotate, the net distributed resistance of the motor increases towards normal levels and thus the motor horsepower drops to normal levels. When the load torque is applied, the motor current consumption increases. The induction motor equivalent circuit includes equivalent core resistance and distributed inductance, capacitance, and an ideal transformer driving the armature winding.
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