# How to model a load as combination of RLC (resistance, inductance capacitance?)

I have a constant voltage DC power source and a load connected accross it.

The load consists of various sensors, modems, some small actuators, and something else which I don't know (mostly resistive.)

I want to model the behavouir of this load and see the current consumption in start, stop, and other operating conditions.

How do I model the load?

• series RLC
• parallel RLC
• mix of the two
• Start by defining each load. Commented Jan 29, 2020 at 12:43
• Maybe you can measure its step function. Commented Jan 29, 2020 at 13:32
• For an aggregate load, your best bet is to use a RL serie load for the static load and to add in parallel, a 'fictionnal' DC motor which aggregate all the loads that will vary on a Torque-speed characteristics. Don't expect great results! In my past experience, aggregate model used by utility can easily have 10-15% error during transient because you are not modelling precisely the transient behaviour Commented Jan 31, 2020 at 21:20
• The main usage of aggregate loads is to see the behaviour of governors during transient and the regional dispatcher and not really the EMT behaviour Commented Jan 31, 2020 at 21:21
• If your load has nonlinear behaviour, I don't see how you could model that with linear components. I'd like to think you'd get what @MathieuL says. Commented Feb 1, 2020 at 20:55

RLC in whatever arrangement is a linear load. The load you describe has nonlinearities.

A small series resistance lets you observe and measure the load current under all conditions. From that you can infer the load characteristics. If you think the load might have significant inductance, I would start with a suppression diode across the load. The sense resistor will create a small amount of isolation between the load impedance and power supply output impedance, and you can see if there are any inductive spikes at turn-off.

You should calculate the impedance of your circuit knowing that : Zr=R ZL=jLw Zc=1/jCw

RLC series : Z= R + jLw + 1/jCw RLC parallel : Z=1/(1/R + 1/jLw + jCw )

If it's a mix, you follow the same principle if it's either series or parallel

• Impedance is not good enough, there is an electromechanical loop which you neglect. Commented Jan 31, 2020 at 21:18

If you do have the load in your bench setup, you don't need to model the load, but just measure the current flowing out of the DC power supply.

1. Use a multi-meter to measure the current.
2. Or, use a current probe with oscilloscope which provides the instant current readings.

BTW, series RLC means no DC current, which cannot be true normally for your load.

• what? you can feed a DC voltage to a RLC load with no problem.... Commented Jan 31, 2020 at 21:17
• I actually mean DC current. Added the word "current" to clarify.
– X J
Commented Jan 31, 2020 at 21:24