DC motor/changing RPM

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DC motor/changing RPM

Post by Russell » Mon Jan 27, 2003 2:47 pm

I have a DC motor operating a water pump that I would like to change the RPM of the pump/motor(the motor is direct drive to the pump). Is there an easy way to slow the RPM of the DC motor & not lose the H.P. rating?

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Post by Forrest_Addy » Mon Jan 27, 2003 3:42 pm

First look at the physics of your question: "Is there an easy way to slow the RPM of the DC motor & not lose the H.P. rating?"

Electric motors are constant torque devices. It's easy to slow a permanent magnet DC motor via some kind of control and still have full motor torque and good speed regulation but HP won't stay constant.

HP is a function of torque times RPM. Lower the RPM and the HP will diminish in proportion. That's physics.

As for a motor control you can get a nice DC motor control suited for a permanent magnet motor from W. W. Grainger for $85 to $500 depending on bells and whistles.

Maybe this link will work:

http://www.grainger.com/Grainger/search ... .jsp?xi=xi

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Re: DC motor/changing RPM

Post by Harold » Mon Jan 27, 2003 8:39 pm

As Forest stated - DC controllers are readily available. You must know the HP rating and maximum amps when selecting one. I have used ones from Minarik Corp. for many applications with no failures. They have economical "roll your own" units whaich are basically the circuit board/chassis with terminal strips. You add a pot for speed control and start/stop buttons. Also has fwd/rev control. Most have a ramp-up/ramp-down feature which eases starting load.
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Re: DC motor/changing RPM

Post by philinmt » Mon Jan 27, 2003 9:01 pm

Well you didnt not say how much you want to slow it down or why, also you didnt not say if it was a pm field or if it was shunt wound. But if you want to drop the rpm with full HP the only thing you can do is make the field stronger, this can be done only on a shunt wound motor. You would have to feed the field more voltage, but give it too much or leave it on to long and it will over heat and let the smoke out...Or you can reduce the air gap between the arm. and the field shoes by shimming...a 1800 rpm motor will slow to 1400 easly with a field change..This is based on the same arm voltage and amps.. if your goal is less water just machine a little off the pump impeller and leave the motor alone....Phil in Mt

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Re: DC motor/changing RPM

Post by willjordan » Mon Jan 27, 2003 9:02 pm

Have you thought about the effect of changing the rpm on the pump. While positive displacement pumps (piston, gear, roller) are not very sensitive to rpm changes (the output is directly proportional to the rpm) other types of pumps, most notably centrifugal pumps, operate only in a narrow rpm band. While generally more efficient as the rpm goes higher, there comes a point where cavitation sets in and output falls off dramatically.

And of course, there is a difference in whether you are moving water, air, or hydraulic fluid.
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Re: DC motor/changing RPM

Post by J_Tiers » Tue Jan 28, 2003 12:55 am

Is the HP a real need? Because HP can be expressed in flow also, and by reducing the pump rpm, you reduce flow, "flow HP" and hence motor HP needed.

In fact, that will be automatic, since when you reduce some motor input, like voltage, the rpm will slow until the "flow HP" is just what the new electrical input allows. Pumps can be like fans that way.

If you have a hydraulic system , and want to slow a piston, etc, same thing may apply, as the rate of power input is related to pressure and distance per unit time.

If you wan t the same max pressure but lower flow, you STILL might be Ok with a positive displacement pump if the current isn't limited, because the motor TORQUE is related to the current and mag field. And the HP is lower as mentioned. So if the motor can still draw full load current at the lower voltage, you can still get the same torque input and pressure output.

So what do you need?

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Re: DC motor/changing RPM

Post by Russell » Tue Jan 28, 2003 10:56 am

Heres the story: we are using a positive displacement pump (gear) direct mounted to a 1/2 H.P. DC motor (1750rpm) in the cooling system of a small block chevy in place of the belt driven stock pump. I don't know what type of DC motor it is but can find out if it does make a difference. I may have been overly concerned trying to keep the HP the same, or even the the torque the same as this is a closed system and the only back pressure will be what is caused by the radiator, I guess the main concern is reliability. We have the system operating well now but would like to try to experiment a bit to get the most out of our cooling system as we are currently running the engine on alcohol (which runs quite a bit cooler than gasoline) but would like to have the option to run gas carb as there are a couple of race tracks that have a "gas only" rule. We are thinking that it might be more advantages to keep the water in the radiator longer for cooling purposes and wanted to try something without losing the reliabiity of what we already have. I really didn't think it would be as easy as wiring a DC controller inline if that is in fact what we need.

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Re: DC motor/changing RPM

Post by fredwhite » Tue Jan 28, 2003 1:36 pm

As I read your last post, it appears that you want to increase the capacity of the cooling system in your vehicle because you want to run gasoline instead of alcohol and gasoline throws off more heat during the combustion process than alcohol.

Slowing the pump down would probably be opposite to what you want to do. Your objective is to get rid of the extra heat - that is usually accomplished thru higher volume and/or a better heat removal media ( larger radiator, better radiator, more air across the radiator, higher flow rate of the system, different heat transfer media - somthing other than pure water-, etc. ) BTW - don't the Hot Rodders have a water replacement product that carries heat away better than water?

But if you really want to reduce the flow rate in your system consider this.

You are probably running the motor from a battery - pure DC.
I think the others have assumed you were starting with AC from a power line and thus recommended the electronics.

For pure DC source, you could use a voltage dropping resistor to drop the motor voltage and thus reduce the motor RPM and the pump capacity. Or, to reduce the capacity (flow) you could add a restrictor in the discharge of the pump ( carefully if its a positive displacement pump! ) or add a by pass line with a valve to recycle some of the pump output directly back to the intake of the pump.

Hope some of this is helpful.

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Re: DC motor/changing RPM

Post by Steve_in_Mich » Tue Jan 28, 2003 5:16 pm

I thought that was the main reason for Non positive displacment pumps and selecting the correct thermostat - i.e., run the engine at optimum temperature conditions.
Just because you don’t believe it - doesn’t mean it’s not so.

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Re: DC motor/changing RPM

Post by Butch » Wed Jan 29, 2003 12:44 am

Don't know whether this helps or not, but. on the big Chrysler engines that were used for HD purposes, like motor homes, the water pumps had much smaller fins on the impeller. I know of a couple instances where guys trying to get off cheap put the car water pumps on motor homes and had overheating problems. Seems bassakwards but that the way they are.
Just thought I would throw that in.

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Wow, Deja vu all over again !

Post by Dave_V » Wed Jan 29, 2003 5:21 am

You don't indicate if you run on a circle track or a drag strip. This story and information may not exactly parallel your needs.

This goes back to the '70's. I have a friend and former co-worker that had a Chevy that was turning in the low 10 seconds at the time. He wanted to change the weight distribution of the car.
I built a regulator that would operate from a 24 volt bus battery that was going to go in the trunk and also built a jet action pump with a through bore of 1 1/2 inches. The jet was formed on the circumference of the bore by a thin sheet of water from a gear pump capable of developing 100 psi into the jet pump.
This allowed him to remove 45 pounds from the front of the car and put the weight directly over the back of the car ! The regulator held the voltage to the BEI ignition at 14.7 volts. As a result, he was able to increase the jets two sizes larger in the Holly carburetor and actually cleaned up spark plugs that fouled with the smaller jets in the Holly !

Now for the good part. The jet water pump moved way too much water and he wound up taking the two piece aluminum housing apart ( that held the cone that developed the jet action ) and sealing it off with a piece of SS shim stock so the circulation was directly from the small gear pump ( it had 1/4" NPT ports ). I don't recall that I ever knew the actual hp. of this little motor, but it was probably more on the order of 1/8 hp. It was designed for and initially connected to 24 volts and it alone moved too much water at that voltage - without the large jet. When wired to run on the 14.7 regulated supply, it was perfect. That puts the hp. that was actually necessary to move the proper amount of water - way down. As was stated below, and you already know, faster coolant flow can result in poorer engine cooling.

I suggest that you find a smaller motor / pump combination and move the weight to where it will do more good.

Another option to consider would be that of bypassing the pump so excess flow can recirculate through the pump that runs at it's present constant speed.

For the lack of more information, it is guess work as to what the best electrical solution would be. A simple reostat would have to be quite large to directly control this motor.

If you want an electronic speed control, the simplest form is an adjustable voltage regulator fitted with ample pass transistors. Assuming 12 or 14 volts I would guess that it draws about 35 amps at full load so you will need a healthy design to hold up. The application notes in the product guides of the companies that make the basic regulator IC's show such circuits. These designs usually need to be ' tuned ' for the environment where they are expected to function.

Good luck,


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Re: DC motor/changing RPM

Post by JOHN_HUMPHREY » Wed Jan 29, 2003 7:38 am

Most SBC race engine parts suppliers have a cooling water restricter kit, consisting of several alum. disks with different sized holes to be used in place of the thermostat. I can give you the speedway #, or look in Jegs or Summit.

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