Terry Burger

iTrader: (4)

I'd like to setup something that allows me to slow down my electric water pump on the highway, to see if I can keep the engine from overcooling (I have a problem with that now).

Basically I need a circuit that will reduce the voltage from 12v at the pump, down to around 6v at the pump. I want to run a wire off the hotwire, to a high watt resistor (or light bulb of approp resistance), to a 30a relay, to ground. So when I click the relay on, the water pump slows down a lot. The problem is I can't remember much from second semester physics, can anyone help me figure out what the right resistance is? The water pump draws around 6amps.

Camaroholic

iTrader: (1)

Volts = Current * Resistance

Resistance of pump = 14 / 6 = 2.3 ohms

Switch a 2.3 ohm resistor in series with your pump to cut the current flow thorough the pump in half. Although you may find that you need more than half the current to get only half the flow... It's probably not a linear relationship.

Power = (Current)^2 * Resistance

= 9 * 2.3 = 25 watt (rough) minimum, I'd do at least 50 watt for safety.

The way you'll need to do it is to run the normally closed side of a relay directly to the pump, and then when the relay 'opens', there should be a second contact that closes up. Make the common wire of the relay the 12V source, the normally closed wire of the relay your 'normal' source to the pump, and the normally open (activated closed) wire of the relay have your resistor in it that leads to the pump. 'zat make sense? <img src="gr_grin.gif" border="0">

Of course, a real EE would build a variable frequency generator for it and use a switching SCR to drive it. <img src="gr_images/icons/wink.gif" border="0"> Resistors are so wasteful. <img src="gr_grin.gif" border="0">

-Andrew

Terry Burger

iTrader: (4)

Is there such thing as a 50 watt 1-10ohm POT?

Terry Burger

iTrader: (4)

So you are suggesting I put the resistor in line rather than in parallel to ground? I guess that makes more sense. <img src="gr_images/icons/wink.gif" border="0">

zzz

Terry,

If you cut your voltage in half your pumping power reduces to 25%. You can try putting two Radio Shack 1ohm, 10 watt ressitors in series to get 2 ohms. This will probably put you on the ragged edge of the resistor's power rating. You can look at the switches/resistors for the blower motors instead of getting your own circuit together. You could possibly have several speed positions and adjust it from the inside.

just me

I would run a duty cycle type controller instead of just dropping the voltage.
This way you still have full torque from the motor but can control the speed.
I think their are some cheap DC motor controllers out there in kit form.
Not sure on the amperage they can handle though.

Terry Burger

iTrader: (4)

I'd love to run a speed control, but I'm trying to come up with something simple here. <img src="gr_images/icons/wink.gif" border="0">

just me

here you go, 20amp model for 18$.
http://www.oatleyelectronics.com/kits/k098.html

ChrisB

What kind of motor does your electric pump use? (Brushless, etc?)

The big problem with cutting the voltage to the motor is not only do you slow the RPM you also reduce the torque output of the motor - this could be a real problem with a heavy load like coolant.

You are also going to be radiating a BUNCH of heat through your resistor if you just use it to drop the voltage - I would get a large HD one and heatsink it at a minimum.

The problem with using a potentiometer/rheostat is because of the way the are constructed they are hard to heatsink properly, and you will probably end up burning them out.

How much current does your pump pull? Using a voltage regulator would be a much better idea, or you could even use a step down transformer (though that's a PITA).

All these methods are still linear regulation and will cause problems(mainly related to torque output = stalling waterpump). You can't just reduce flow rate like this (with linear regulation).


I think a PWM setup would be a MUCH better alternative - this would basically apply full voltage to the pump - and give you full torque, but would only apply it for a certain amount of time at a certain frequency - allowing you to control rpm independant of torque (duty cycle).

I built something along the same guidlines to give me control over some electric fans - basically the circuit below uses two 555 timer chips to give you control of both frequency and pulsewidth of the voltage cycling - feeding the signal into an op-amp to get us a square wave (as opposed to sine, sawtooth, etc.) - we then run the signal (2-9v) to a NPN transistor to convert it to 0-12v, and then use that to drive a MOSFET - you will just have to spec a mosfet that can handle the power requirements of your pump (wich should be easy - even radioshack has ones that can safely handle 4a - and you can order ones for 75+ amps).





Chris

[ December 11, 2001: Message edited by: ChrisB ]</p>

just me

you could easily wire the speed control in and have full manual control at cruise and have it bypassed at WOT for straight battery voltage.
Seems simple to me and would be a really good system.
I am following your project. Good luck with it!

Steve

ChrisB

Whoa, I wasn't ignoring justme - his post wasn't there when I started typing <img src="gr_stretch.gif" border="0"> .

Chris

Terry Burger

iTrader: (4)

Quick idea, I know the stock fans have a "high" and "low" speed. Is there a low voltage power source somewhere on the fuse box I could use?

I could then buy a thermo fan switch, at temps < 180 use the lower voltage, otherwise the full voltage.

1 Bad Z

iTrader: (8)

[quote]Originally posted by Terry Burger:
<strong>I'd like to setup something that allows me to slow down my electric water pump on the highway, to see if I can keep the engine from overcooling (I have a problem with that now).

Basically I need a circuit that will reduce the voltage from 12v at the pump, down to around 6v at the pump. I want to run a wire off the hotwire, to a high watt resistor (or light bulb of approp resistance), to a 30a relay, to ground. So when I click the relay on, the water pump slows down a lot. The problem is I can't remember much from second semester physics, can anyone help me figure out what the right resistance is? The water pump draws around 6amps.</strong><hr></blockquote>

Please, tell me what kind of problems you are having when your engine is "overcooling" and what exactly is "engine overcooling"

<img src="images/icons/confused.gif" border="0">

[ December 12, 2001: Message edited by: 1 Bad Z ]</p>

Terry Burger

iTrader: (4)

The engine temp is too low on the highway for my taste, so I am trying to heat it up a bit.

ChrisB

I don't think slowing down your pump is the best way to achieve this - and infact, depending on your coolant flow rate presently, may actually increase cooling. Flowing water to quickly through the radiator can also hurt cooling effeciency - if you modelded it you would probably see a small bell shaped peak around some flow rate, with a slightly lower flat line on either side.

A different thermostat would be a better solution.


But regardless, I would again suggest a PWM as opposed to linear regulation approach.

Chris

zzz

A chopper circuit would be best for power dissipation and efficiency and be infinitely variable. ChrisB is right too slow can be bad as well as too fast.

ChrisB,

You will need a diode with good reverse recovery characteristics across the motor load to provide a path for the inductive current or you will see a pretty big inductive voltage spike when the MOSFET shuts off. The kick may be big enough to breakdown the MOSFET or the pump motor insulation.

ChrisB

Good point - I wasn't really terribly worried about that with my computer fans pulling 2A total, but in this application you are right!

Chris