DC motor, bulbs

Question

What happens when you are getting low voltages ? Why fan becomes slow and bulb become dim ? Dye to low voltage or current? If power is V*I then due to low voltage current becomes low or not Many people say current becomes high in order to maintain constant power but if this is so then fans n bulb must not become dim or slow. And why current increases due to low voltage because resistence is const so voltage is directly proportional to current.??

2 what is meant by it draws more current . I think it stupid how a device drwas current to maintain its constant power . It has fix resistence not piwer so if voltage decreases it allow less current to pass through so what is drawing factor.

What

Answer 1

Pls don't use the word "stupid" when asking question.

Any load which obeys ohm's law will definitely behave in the way you expecting. When voltage goes low so does the current. But there are few types of loads which not obeying ohm's law.

One of such thing is the switching mode power supplies. You may have noticed all modern chargers are rated 100V to 250V. They works perfectly well throughout the range. If the charger can do at 100V what it does at 250V, it certainly draws more current at 100V compared to 250V.

You may loosely call this as negative resistance. There are other gadget operates as the negative resistance load. Such as the arc lamp, discharge lamp and simple DC motor with carbon brush.

Feel free to Google around to find out how and why they exhibit the negative resistance characteristics.

Answer 2

If you have something like a light bulb then when the voltage drops so does the current and so the bulb becomes dimmer and if the voltage becomes too low the bulb will not any visible light.
A fan motor which have mentioned has a specific job to do and is designed to work at a certain voltage and current and hence with a certain power input. If the voltage drops so does the current and as there is less electrical power entering the motor the motor can do less work and so rotates at a slower rate.

However there are other electrical motors which are designed to do a variable amount of work eg household electric drills, electric trains etc. In those cases the amount of electrical power entering the motors is sometimes controlled by the motor itself. So when an electric drill is used as more demand is made on the drill the coil of the motor rotates slower and more current (and hence more electrical power) is delivered to the drill. It is not acting like a resistor. If the power supply voltage drops then that would mean that the drill will not be able to get as much electrical power and will therefore not work as well. If you have used a rechargeable drill you will have noted this effect.

Some devices eg televisions, computers, chargers etc are fairly immune to a drop in supply voltage because they have electronic circuitry which keeps the output voltage constant. If you look at a typical charger it will allow and input voltage of anything between 110 V to 240 V to give a roughly constant output voltage which is usually much less than the input voltage.
When you switch a television from standby to on the current into the television increases because a lot more of the electronic circuitry including the screen need electrical power to function.

If things are working well at home then the voltage of the supply will stay constant but as more devices are used then the supply will have to deliver more electrical current - more current is drawn.