Resistive and inductive loads?

[SceneMaster]

Hi

 

I've heard the terms use about lights such as resistive and inductive (refering to howthey consum power?) I guess that resistive lights resist the power and produce light and inductive suck power out to produce light. Is this right or am I complety loony and don't have a clue what I'm one about please help.

 

Thanks

 

:huh:

[Brian]

It's all to do with power in a.c. systems.

 

When we have a d.c. power supply (a battery) we are only concerned with good old ohms law which we know is V=IR and the derived power formula P=VI. So a 9 volt battery with a 180 Ohm resistor will be delivering 50mA (0.05A) and the resistor will be dissipating 450mW (0.45W).

 

It gets tricky when we have an a.c. supply though. With a.c. it is possible for the current waveform to be out of phase with the voltage waveform. It is indeed possible for the voltage to be at the top of the sine wave (ie maximum voltage) whilst the current is at zero.

 

This is a feature of reactive loads, of which there are 2 basic types - inductive and capacitive. Inductive loads are things with loads of coiled wire in like transformers and capacitive loads are things like, well, capacitors so something like switch-mode power supplies.

 

So a normal filament lamp is pretty much a perfect resisitve load and the voltage will be in-phase with the current.

 

With a low-voltage lamp fed from a transformer, like an pin spot, we have a transformer inside it and it will appear as an inductive load.

 

Why does this matter I hear you ask. 2 main reasons...

 

1. Some dimmers do not like the current to be out of phase with the voltage, it stops them working. A triac in a dimmer (the bit which controls the power) needs a current flow to stay on, if the control circuit is looking at voltages to decide how much power to let through it can get confused.

 

2. The electricity board like things to be not too reactive as well. Generators don't like reactive loads, it causes them to run hotter than you might expect. The supplier has to fit bigger generators and bigger wire in their network.

 

Look at a typical florescent fitting, it has a choke in which makes it look like a inductive load so to counter this they fit them with a capacitor to attempt to make it look like a resitive load (inductive loads and capacitive loads cancel each other out).