# Need to have a set RPM



## sbgff (Nov 14, 2008)

I have 1 916100 cat drive motor, 36/48vdc is about 18hp
I have 1 916855 cat hyd pump motor, 36/48vdc don't know it's hp?
I have 1 918590 cat power steering motor,36/48vdc don't know it's hp?

I would like to know how I can control the RPM of these three motors using 48 vdc. I want to use this on a unique setup. It has to turn 3600 rpm at a low torque situation and torque will be slowly increased depending on hydraulic demands. How can I simply accomplish this? I don't know much about electric motors and controling them.

Thanks Blair


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## Georgia Tech (Dec 5, 2008)

sbgff said:


> I have 1 916100 cat drive motor, 36/48vdc is about 18hp
> I have 1 916855 cat hyd pump motor, 36/48vdc don't know it's hp?
> I have 1 918590 cat power steering motor,36/48vdc don't know it's hp?
> 
> ...


Well it sounds like what you’re wanting to do is create some sort of servo situation. Where you have a small tachometer or generator as feedback on your motor shaft. Then what your wanting to do is feed this signal in some sort of micro controller setup that is constantly correcting the speed. Its not that hard to do. With your small generator or Tach condition your signal and run it to a A/D then have your micro read it and constanly make correction to the motor's speed.


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## Georgia Tech (Dec 5, 2008)

The answer above will give you a constant speed to control torque you have to do more of the same servo action but now you have to sense the current through the motor. Some how sense the current and have a signal fed to another A/D then have your uP perform correction on it as well. Its not that elaberate to do.


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## sbgff (Nov 14, 2008)

Thanks for your help, but I'm pretty numb and new to this stuff. The tach you speek of, is it mechenical, where would I find one? For my application I need to maintain a some what steady RPM, but mainly only so I don't over speed my hyd pump. As for torque or current draw, when I am in a idle or no work mode there would be a very low current draw, Right! and when I am under load, which is not a constaint load, I would draw what ever current was needed to get the work done, wether that is 50 amp or it is 300 amp, Right! The maximium hp rating on the hyd system is 25hp @ 12 gpm @ 3000 psi. So!

Lets just suppose, for simplicity sake, I just wire the motor direct with a heavy battery disconnect switch which could carry say 500 amp, it should work, except under no or low load it would over rev. To prevent that I need the tach and microcontroler! What and how does this controller work and where can I find one, how would it be wired into the system?

Like I said I am pretty dumb about these things. The building or puting it all together is not a problem for me, it's the knowledge of the electrical side of it I don't understand.

thanks Blair


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## Georgia Tech (Dec 5, 2008)

sbgff said:


> Thanks for your help, but I'm pretty numb and new to this stuff. The tach you speek of, is it mechenical, where would I find one? For my application I need to maintain a some what steady RPM, but mainly only so I don't over speed my hyd pump. As for torque or current draw, when I am in a idle or no work mode there would be a very low current draw, Right! and when I am under load, which is not a constaint load, I would draw what ever current was needed to get the work done, wether that is 50 amp or it is 300 amp, Right! The maximium hp rating on the hyd system is 25hp @ 12 gpm @ 3000 psi. So!
> 
> Lets just suppose, for simplicity sake, I just wire the motor direct with a heavy battery disconnect switch which could carry say 500 amp, it should work, except under no or low load it would over rev. To prevent that I need the tach and microcontroler! What and how does this controller work and where can I find one, how would it be wired into the system?
> 
> ...


 
Well the Tach would be nothing more than a little DC motor. The kind you would find in toys or at Radio Shack. You take the output of this and you would need to go to some electronics to condition this signal then take it and run that to an A/D. From there the speed of your BIG motor is in the Digital Domain and its either FPGA or Microcontroller software from there to process this RPM Data. As the Motor reach you desired speed the uP would cut current to the motor in a PWM fashion it would feed just enough to keep the motor at speed. As your load got bigger the error signal would feed from you generator and your uP will then open up More PWM to your motor.
The trick is o mount that little motor to the Big shaft of you Big motor! This is an overall summery of how this thing would work. Just going to take a little doing to implement it.


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## Georgia Tech (Dec 5, 2008)

sbgff said:


> Lets just suppose, for simplicity sake, I just wire the motor direct with a heavy battery disconnect switch which could carry say 500 amp, it should work, except under no or low load it would over rev. To prevent that I need the tach and microcontroler! What and how does this controller work and where can I find one, how would it be wired into the system?
> 
> Like I said I am pretty dumb about these things. The building or puting it all together is not a problem for me, it's the knowledge of the electrical side of it I don't understand.
> 
> thanks Blair


I guess I'm taking alot for granted in my last post. Do you understand how basic DC motor controllers work? If not I know where to begin I can come back latter today and make a GOOD detailed Lengthy post. To help you, we can go through this part by part.....in the mean time do a Google in terms like "PWM DC Motor Control"...this will be a good starting point..


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## sbgff (Nov 14, 2008)

Ya I'm pretty green. What is an A/D, uP, and a PWM. I will do a search and see what I can find. I would greatly appreciate your explanation post.

Do you understand how basic DC motor controllers work?

No! not at all

Thanks Blair


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## Georgia Tech (Dec 5, 2008)

OK let’s talk about how motor controllers work. Typically one would think the best way to control a motor is through some sort of Variable resistor or some big rheostat device. However there are MANY drawbacks to this approach. One that would be a Very Large device and second it would consume lot of power in the form of heat. Modern Motor Controllers Whether its Curtis or the Zilla use a technique call Pulse Width Modulation or PWM. This technique involves the switching of a hefty silicon device, whether its an IGBT or a MOSFET, for a certain "on" time over a the "off" time. This On off type is a ratio that determines effect "net" or "average" current to the motor, we call this ratio a Duty Cycle. Let say that in one cycle we have the on time for 2 units of time and the off time is off for 8 units of time. We then have a 20% duty cycle. Let’s say we have 6 units of time on and 4 units of time off, then we have a 60% duty cycle! A 60% duty cycle will run the motor faster than the 2% duty cycle. So if you looked at it on a scope as you pressed the gas pedal you would see this square wave with the top half getting bigger while conversely the bottom half getting smaller. The overall cycle time is the frequency at which the thing runs at typically you’re looking at around 2Khz.This is a rough explanation of how a motor controller works.
Now your wanting to add another element to this and its speed feed. Your wanting to sense the speed of your motor and send that speed back to your controller so your controller can adjust the real speed of the motor. A very small generator would generate a voltage directly proportional to the speed at which the Motor is running. Then we would take that voltage and condition it and filter to give us a voltage range of let’s say 0 to 6 volts. Now we have a clean signal that goes form 0 volt to 6 volts that represents the RPM of our motor. Now we have to do something with this signal. If we digitize it we can have a computer with software read it and react to it. So thats what we do we run our 0 to 6 volt signal to an Analog to Digital converter or A/D. There are all sorts of A to D's(A/D) out there. But basically that A/D is going to take our 0 to 6 volt signal and give us a Digital Number that represents that signal. Now with it in this digital domain we can have our computer read it and change the Duty cycle when it sees the speed drop. If the speed is down lets say by a slight bit do to an increase load or in your case hydraulic load, it can keep upping the Duty cycle until the speed is back to where you had it set. Think of it as sort of a cruise control but only the loop and response is Much Much faster.


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## major (Apr 4, 2008)

sbgff said:


> Do you understand how basic DC motor controllers work?
> 
> No! not at all
> 
> Thanks Blair


Hey Blair,

What you need for a motor controller is something like this http://curtisinst.com/index.cfm?fuseaction=cProducts.dspProductCategory&catID=11

The input to the motor controller is normally a potbox, or spring loaded pot (varialble resistor). So a few milliamps in the pot controls 100's of amps in the motor. This controls the speed and current (torque) of the motor.

On the typical EV, the driver is the feedback control. He knows how fast the car is going and adjust the pot with his foot to get the desired speed. You want to eliminate him and use an automatic speed control for the motor. This is not easily done with existing motor controllers for series wound DC motors.

Your Cat lift truck had some type of motor controller. Is it still functional? You might be able to get it wired up with a hand control pot instead of the foot pedal. Then set it to the speed you want with an unloaded motor. Leave it there. Trouble is that when you load the series motor, the speed will drop, probably quite a bit. But the pump will compensate with more flow. It might work well enough for starters. If you crank up the pot when the motor loads, then when it unloads, it will overspeed. It is conceivable you could devise a linkage between the pot and pump slosh plate to turn the pot up when the plate is moved and the pot back down when it returns. Might be tricky to get adjusted. So try it at a lower safe speed first.

Obviously the electronic automatic feedback system is preferable. Or later on, maybe a SepEx motor and control. More of a constant speed system to start with.

Good luck,

major


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## sbgff (Nov 14, 2008)

Thanks Georgia Tech and Major 

It's been snowing here and my sat. Internet was down due to snow load, now it's raining. One thing about N.S. is if you don't like the weather, just wait a few minuites and it will change 

This info will give me something to chew on and try to figure it out. Can you explain the different types of motors avalible. you say a series wound motor is hard to control rpm. Is there other motors that are easy to do what I want. I am trying to use the motors I got because of expence and the fact I have them Free, sort of, but if there is a better way with a different type of motor I may be better of to change my approch. My main intrest is to do it right, not cheep, it has to work and work well. It is a comercial feeding machine and has to be reliable.

Thanks again for all your help
Blair


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## Georgia Tech (Dec 5, 2008)

sbgff said:


> Thanks Georgia Tech and Major
> 
> It's been snowing here and my sat. Internet was down due to snow load, now it's raining. One thing about N.S. is if you don't like the weather, just wait a few minuites and it will change
> 
> ...


Well there are 3 major types of DC motors and they all three have there pluses and minuses.

1) 
The series motor is one we all use here most of the time for car conversions. It is made up of very think stator (field coil) windings, and they are as thick as your armature windings. All the current that flows through the armature flows through the Stator as well. The performance characteristics is that if it is ever powered up unloaded it will turn faster and faster and faster until it just burst! It also makes a very poor generator which makes it difficult to have regenned braking in EVs. Couple of big pluses is that it has FANTASTIC starting torque! Some people even say it might be infinite of it survives the current! But that’s of course farfetched, it also seems to have a high efficiency across a wide rpm range. Also it seems to have very little commutator noise when it runs.

The Shunt motor has a stator that has very thin coils but has lot of turns of this thin coil. This coil or stator is usually powered separately from the Armature. The Goal of the Stator is to use a little power as possible to energies the magnetic field. Most of the power is to be consumed in the Armature doing work; this is where all the heavy lifting of the motor is to be done. This motor is the easiest motor to control out of the three. You can vary both the current in the Stator and in the Armature. The less current you have on the Stator the higher the RPM and the less torque the motor will make....Not to mistake you don't get more power by making the field coil less all you do is shift the Horse Power up the RPM band. Conversely if you strengthen the Stator you get less RPM and more torque. Therefore you push the Horse power down the RPM band. You can also vary the Armature current like in traditional controllers as well. The Shunt motor has a very stable RPM under no load and makes a Good generator for regen braking. The starting torque is not bad either you can get near the starting torque of a series by putting full current to the field coil at start up.

The permanent Magnet motor is just like the Shunt motor but it does not require an extra supply for the stator, but it has magnets for the stator. It has good regen and has a very stable RPM under no load. It has a very narrow efficiency range when running. It also does not have near the durability as the field wound motors do. Big torques and lot of current surges and heat seem to damage magnets over time. But they are good for small EVs or go-karts...


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## major (Apr 4, 2008)

sbgff said:


> Can you explain the different types of motors avalible.


Hi Blair,

O.K. So you don't need to use the Cat motors. Then I'd recommend using a SepEx set-up. The ones currently available will look underpowered at first, but may do the job for you.

The series wound DC motor uses the armature current to excite the field. Armature current is what produces torque, in conjunction with flux from the field. The motor RPM is inversely proportional to the field flux. The armature current is proportional to the load, or torque opposing shaft rotation. So, in the series motor, at light loads, the armature current is low and the field flux is low so the RPM is high. Conversely, at high loads, the armature current is high and the field flux is high, so the RPM decrease. The motor slows down as it is loaded. 

In a shunt (or Separately Excited, SepEx for short) DC motor, the armature current is not used to excite the field coils. A separate source is used for field excitation, so the field flux is no longer dependent on the armature current. So one can keep the field flux reasonably high at no load and limit the RPM. Also SepEx systems give you the capability to "map" the relationship of field flux to armature current so that the motor will emulate the high load torque capacity of a series motor.

Here is a SepEX controller which I have used. Quite pleased with it.

http://www.sevcon.com/PDFs/Millipak_MP0106EN.pdf 

I use it with a 6.6 inch diameter SepEx motor from D & D. Like I said above, from first glance, the rating will appear wimpy. But it in fact can do 500 amps at 48 volts for short times. And with a continuous rating of 180/200 amps, would seem able to suck down a reasonable battery without too much trouble.

The Sevcon controller can be programmed for your particular application in regards to field map and top speed and a few other things. You'd need a programmer pendant or PC pack to do this.

You should be able to find some educational web sites or books which can explain the DC motor types.

Regards,

major


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