7.2volt Dc motor too quick for water pump

Theres plenty of fancy electronic solutions.

But a simple solution might be to power the motor with a long length of very thin, low current cable, so that when it draws more power at startup, the resistance in the cable keeps the current flow down. This might take a bit of fiddling to get it right.

Manufacturers often use thin cable so that light globes don't blow - its a similar analogy.

Cheers
Phil

Hi Phil,

Thanks for your reply. But the problem with that, is that i need the full 7.2v at 3Amps after it gets going.

So i agree it will start off slow, but it won't (If i have read you reply correctly), get up to the power i need. Your method keeps it slow [low].

Your method also, would be building up heat in the wire due to resistance (i think), so that's not good for my application. I need the pump to get the full 7.2volt under load and getting a good 3 Amps, after starting low - about a second would be great.

Good reply though.

Alan.

G'day Alan,
I guess you're thinking to put a capacitance in parallel which takes time to charge when the power is switched on. Yes? I suppose that could work, though I would think that an inline choke would be what you are looking for.

A choke provides inductance and this resists a change in voltage/current; in your case from 0 to 7.2V (hopefully). They are normally used to filter AC and are the opposite of a capacitor which is used to filter DC. The problem with a resistor is that it would reduce the voltage when the pump is up to speed and waste energy as heat in the resistor. An inductance will slow the change in voltage but have no (very little) effect when the voltage gets to a constant state.

This seems like the wrong solution to your problem to me.

I use quite a few types of DC and AC water pumps on my boat and I don't know of any that would be bothered by what you describe. Maybe the problem is mechanical.

You say it's to "kick start a small water pump". If you're using this pump to prime another pump and the second pump then puts any pressure back to the first, it can hold the outlet valve closed and the stripped gears would be pretty likely. If you use a centripetal pump with a magnetically driven impellor you won't have any gears to strip.

Cheers Frank.

try a headlight globe in series. This will limit current and torque until motor starts and hen short out the headlight giving full power to motor.
Just a suggestion
Techie

Hi Frank and techie,

Frank. Yes i would prefer to use capacitors and as in techies very clever and correct reply, alltough i would prefer to use Electronics (Solid State), as against a globe. But very well thought out techie.. very good. So maybe a 12vlt globe (don't know what wattage though, would absorb the intial 7.2vlts, but what about it providing the full 7.2vlt under load and would it let through the 3Amps (Ni-MH Battery), till the battery drained?

Frank. You refer to inductance, but i thought inductance was not what i was looking for here. Isn't inductance (spelling?), in all forms of electric conductivity - even in a single wire, you can have inductance?

Can you throw any more light on the capacitor side Frank? It's just that i have the space (an enclosed - lid fitting space - with a bit of ventalation i guess, so again i can't have a globe burning away inside), to fit and play with them. And also they are cheap and i can play with certain values farily easy.

Cheers,

Alan.

Alan.
You would be better of using the heater switch from a car, if you take the switch and the loom to the resistors in the blower you will have a 3 speed pump. Eric.

if your stripping the gears sounds like a mismatch....need to know what is driving what and by how...ie direct drive or belt...is gear train separate from motor or is gear train part of motor?

if gears are part of motor ie before output shaft then sounds like load (pump) is too much....
if gear train is after motor output shaft (not part of motor) then motor is too small for application/load.

Ok People.

I think Klaus has hit the nail on the head with me :(

No need to continue the post as i can't provide more detailed information.

But as a last mention - Please no flames!

I was getting together, a windmill with 12volt motor in, to generate electricity (I have the windmill).
Since i have a supply of small tiny light 7.2volt recharable batteries, i want to use that 7.2volt battery with a 7-8volt motor which connects to the pump to drive water into the tips of the blades to increase inertia or outside mass or whatever (Apparently it works).
The pump is a tiny pump and obviously it was not designed to be drived or started up that quick..
So all i wanted to do was slow down the motor to have it gradually starting up.
THERE. That's it....

Apologies for vaige (Spelling?), but i'll have to look at the Water/blade issue some other way.
Thanks for all your replies.
Alan.

For what it is worth IMO a 2 ohm resister in series will slow startup a little and when upto running speed still give you about 7.2 volts at 3 amps . Try it and you may have to tailor the resistance to get the results you need . Keep in mind that the resistor will need handle in excess of 20 watts . There will be fancier methods but for what you are trying to achieve this is the simplest.

Cheers Dave L.

One last parting shot/ information.
I healight globe would be about the right wattage.
8V 3A has internal resistance of 2.6ohms
12v 50W has resistance of 2.6 ohms.
Add in series will give about 4v to start (less torque) and when started turning short the bulb out to give full 8V.
There is another way: by building a mark space ratio regulator.
This would pulse the full 8V with different on/ off periods to enable the motor to run up to speed.
To start, the pulses will be at 10% then work their way up to 100%.
This idea will need
1/ time to find/ design circuit (not complicated)
2/ time to build (are you any good at constructing kits?)
Regards
Techie.
ps: there are no problems, just opportunities.

have designed a quick analogue regulator that may dothe job.
A transistor, a cap and a resistor.
How do I upload it???
Techie

Hi Techie,
If you want to send it to me, that'll be great and i'll let you know when i have made it.
Can your 'device' be modified for say a maximum of 3.5amps?

If it's just a matter of increasing this and that, then send me the original, letting me know what 'bits' i have to change.

Can you send it to aheywood@bigpond.net.au
I do appreciate the time you have taken.

I'll certainly let you know how it fairs.

Your's,
Alan.