Method said:
I posted a message about a waterpump i was using as in inter cooler spray
for a car which was getting very hot with continuous use. I have found if i
put ~30 ohm resistor in parallel with the wires, it takes a lot longer to
heat up but the resistor starts to go brown and smoke. I tried using a 100
ohm resistor and it got very hot but didn't smoke for a while.
The Resistors are
RES 0W5 MET 100R 1% 0W25
that is what is written on the packet. Are there any other types i can use
which will not heat up?
What am i missing here??
Thanks
You need to look at the rating on your pump. If it is overheating
when driven by the nominal 12V from an automotive batery, then it may
be that you have a 6V pump.
There are two solutions to this problem. 1, and this is probably the
better of the two, is to replace the pump with a 12 volt version.
However, there may be other factors at play here, for example, how
exactly you are using the pump. If your pump is designed to probide a
steady flow of water without much back pressure, and you are using it
with a spray fitting, then the motor on the pump, having to work far
harder than its designers intended, will overheat. So replacing your
present pump with a 12 volt high pressure version will solve the
problem of overheating.
If, on the other hand, you want to just make it work as it is, then,
as is mentioned elsewhere in this thread, you really should have the
resistor in series with the pump. To choose the correct resistor you
need to know how much current your pump is drawing at 12 volts. To do
this, you will need a multimeter set to Amps (not milliamps) and
connect in series with the motor. When the motor is running you will
be able to measure the current drawn by the motor.
If, for example, your motor draws 2 amps at 12 volts, then its
internal "resistance" (it's not strictly speaking resistance, but for
practical purposes you can think of it as resistance) will be 12/2 = 6
ohms. Its power disipation will be (12^2)/6 = 24 watts when fed with
12 volts. I can see how this would overheat.
So, to drop the voltage for the motor from 12 to 6 volts, in this
example, you need a series resistor of 6 ohms and 6 watts (explained
later) dissipation capability. Assuming that your motor draws half
the current at half the voltage, which may not be exactly true, but
will be close enough for practical purposes, this will reduce the
voltage over the motor to 6 volts.
The current through the overall setup, 6 ohm resistor in series with a
6 ohm motor, will now be half what it was previously, or 1 amp. The
power, which shows mainly as heat, developed in the motor will be
current squared times resistance (I^2*R)or 12 watts overall; 6 watts
in the motor and 6 watts in the resistor. Hence the call for a
resistor capable of dissipating 6 watts in the previous paragraph. To
be on the safe side, a 10 watt resistor is probably best, since this
will have a greater surface area and will dissipate the heat at a
lower temperature.
At the same time you have reduced the power (heat) produced by the
pump to 6 watts, which is probably enough to prevent overheating.
I hope this helps and that your car goes like a striped ape!
regards
John