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Help with my brainstorm Answered

Hi everyone,

I have an idea for a new gizmo that I'm trying to make a working model of. It will use a 12volt electric motor and want to spend as little as possible on it until I'm pretty sure it will work, so I'm going to scavenge for parts when I can. The most expensive parts that will go into it is the electric motor and the battery and I have a few questions:

1- Does an electric motor draw whatever power it needs or is the power forced into it?
2- If it draws what it needs, can I take a 12volt motor and use an 18v battery with it without problems? So if I scavenge an electric motor from a 12v battery powered drill can I run it with an 18v lithium battery?
3- What about the amps? If I use a 12v motor that normally runs off a 12v car battery, can I run it with an 18v lithium battery that has less amps?
4- Any other specs I need to focus on?
5- When I find the right electric motor, which I think will be a small 12v gearmotor, where should I go to find a low cost supplier?

I'll have more questions later but if you could help me with this I can get started. Thank you!



Hey guys,

I hope you've all been well. This project hasn't been easy but I'm making some good progress and at least have the lifting thing pretty much figured out. Now I'm stuck again with finding the right motor.

I found a source for electric motors but they are all unmarked except for the occasional voltage stamp and I think 12v is what I want. How can the other specs, such as amps, torque, rpm or even voltage be determined on these unmarked electric motors? Is there a machine to connect them to which tells the specs or are there shops that specialize in this? Also, I have a multimeter and will get a benchtop power source if this is typically done with these tools. If so, how? I need to know the specs of a used one that works for me so I can source for new ones later.

Thanks again,

Thanks Josehf. I had considered something like that just to get it to work and maybe workout the size it would eventually be. But in the end, I think new and dedicated parts would be needed if I ever wanted to interest others to buy it. I'll probably give it a try anyway if only to learn and apply the suggestions you guys have helped me with.

Who knows, maybe this will only be an exercise but with what I learn it could lead to an even better idea :-)

You know what might be an idea look for battery powered drills in garage sales second hand stores and the garbage.
Use an ordinary threaded rod and nut from a hard ware store in place of the drill chuck.


Ok steve, I'll remember that about the rating. Cycle? I'd say normal use would be one up-down cycle every few days or so at the most.

Also 12 volts to keep this as simple and low cost as possible but not with a car battery. It needs to be much smaller, like a LiOn power tool battery. My brother has a DeWalt power screwdriver and it seems to have a lot of torque. I checked their site and it's about 70 lbs. From the feel of it it feels like it should be about enough to turn the lead screw and a 400 lb load, but I'm not sure. I thought to get their replacement motor but it's not self encased and the screwdriver housing is its case, plus it's too expensive at over $60.

I'm beginning to think this may not be possible, especially if I want to keep the cost low - like no more than $25. :(

I think that's a bit unlikely. How far are you moving the load again ? If you can do it slower, there are big satellite dish actuators that can shift that kind of load.

Thanks again. I think I now have a foundation of min/max parameters to at least eliminate some dead ends.

Here are a couple resources I found which may help others here:
Parts: http://www.sdp-si.com/
101 level info on lead screws when you register, then you also get more info. from MIT on gears, etc.: http://www.tolomatic.com/library/index.cfm?msg=created&CFID=2038105&CFTOKEN=83796672&jsessionid=8430f4290e5118624bf241131d5336209472

You guys have really helped me on this. It looks like I'm going to have to buy a motor and a 12volt one should be best to keep things as simple as possible now. As for the battery, I think I need to focus on that the amps in the battery are at least what the motor is rated at. Correct? Or, is it the watts? If it's the watts, none of the motors show that spec. Here's a motor (as an example) that would fit: https://sdp-si.com/eStore/Direct.asp?GroupID=803
How do I match the right LiOn battery for it - will 18v safely work with a 12v motor with the resistor previously mentioned or would it be best to stick with 12v if at all possible?

DON'T use a resistor to control the speed.....

The motors you flag are nowhere NEAR powerful enough to meet your spec.

What you need to work off is the RPM you need, and the torque you need. Make sure that torque is the RATED and NOT stall torque

What kind of duty cycle is this thing running ? Is it permanently driving up and down, or once every hour or what ?

A decent battery will supply much more than your motor can handle. A car battery, which is 12V can supply easily in excess of 1000A.....just not for very long....

Thanks boys! The motors around the house are too big. I might call a local electric motor rebuilder, like Josehf, but I have a feeling they will just have industrial size motors. I think my best bet is to search for one with Steve's specs.

I searched for the 5/8" ACME lead screw and I found they are used for scissor jacks and the like, which lift 4000 lbs. or more! Isn't that 10 x more lead screw strength than what I need? The lighter and less expensive my device is the better. What would be a smaller but sufficient lead screw diameter for my measly 400 lbs.- 3/8"?

Depends on how you're loading the screw, what its made of, and how safe its to be. I wouldn't design for anything less than 150% of your rated load, so nothing smaller than 7/16" in steel, and with bronze lift nuts, not plastic. If you use plastic nuts, then my 5/8 is still the correct size.

I use to rebuild electric motors equipment would come from Europe with motors rated differently than North America and we had to test for or rewire the motors to work here.

To do this we had a variable power supply.

We started at the rated voltage and cycles and adjusted the voltage and cycles up until we reached the desired voltage and cycles all the time checking the current with a meter.

If the current went down and then up or if the current just went up we rewired the motor.

Now on AC motors the rpms will change with the cycles and if the rpms changed too much for a pulley wheel size to compensate for we rewired the motor.

As long as the current went down and not up we certified the motor safe for North American standards.

Take the 12 volt motor and connect it to a variable power supply and slowly turn up the voltage from 12 volts to 18 volts all the time checking the current with a meter.

As long as the current goes down and not up it should be safe for 18 volts.


Driving an electric motor can be a fairly complex task depending on how much control you need, so this is just some basic stuff to get started.

The thing to think about is whether you need an AC or DC motor.  It sounds like you want to us a battery so you should be looking for a DC motor.

In general when you are running the motor you will apply a DC voltage to the motor.  In DC motors the speed of the motor is directly proportional to the voltage. One of the simplest ways to control a DC motor is to use a potentiometer as a voltage divider to adjust the input voltage.

When you apply a voltage to the motor it will draw as much power as it needs to spin, provided that the power supply can source that much current. The amount of current it will draw is proportional to the mechanical load on the shaft of the motor (i.e. if you hold the shaft of the motor to slow it down it will draw more current). This means if you have a motor powered by a car battery rated to supply a hundred amps but the motor only draws at most 1 amp, you can use any supply that can supply at least one amp.  If you try to pull more amps out of a battery than it can supply the voltage will drop and the motor will slow down (and the supply may heat up and cause other bad things to happen).

As for running a 12V motor with an 18V battery, running motors at a greater voltage than their rating is not uncommon. That said, if you don't don't have a very good reason to do so it's probably not a good idea. Running at a higher voltage will cause the motor to spin faster than intended and may get hotter that it should, these things could shorten the lifespan of the device or permanently damage it. One way around this would be to use a resistor voltage divider to get the proper voltage or a more complicated motor controller.  If you use a voltage divider you will need to make sure all of the components are rated for the amount of power they will dissipate (ideally two or three times more).

A few of the common specs for a motor you should be looking at are:
- Operating Voltage Range
- No Load Current: The current the motor draws when the motor is spinning free.
- Stall Current: The current the motor draws when the shaft of the motor is stopped.
- Stall Torque: The torque the motor applies when the shaft is stopped.
- No Load RPM: The speed the motor rotates when it is free spinning.

As far as where you look a good place to start would be some of the surplus electronics sites (I've used allelectronics.com but there are may others). Or any robotics sites if you are looking for something more specific (and more expensive).

Hopefully you find this useful.  If you have any more questions don't hesitate to ask.  Also, I'm sure if you posted more specific ideas or what exactly you intended to accomplish the community could provide more insight in to the best ways to go about it.

In DC motors the speed of the motor is directly proportional to the voltage. One of the simplest ways to control a DC motor is to use a potentiometer as a voltage divider to adjust the input voltage.

Mmm,. No, as you say, the speed depends on the load as well.

Running at a higher voltage will cause the motor to spin faster than intended and may get hotter that it should, these things could shorten the lifespan of the device or permanently damage it. One way around this would be to use a resistor voltage divider to get the proper voltage

No, no, no. Using a voltage divider destroys the speed regulation of a PMDC motor.

Using a voltage divider destroys the speed regulation of a PMDC motor.

Sorry about that, I'm obviously missing something in my logic/physics.  Could you explain a a bit more as to why this is a bad Idea? Does adding the resistive network to the mix mess with how the motor works?


The terminal equation for PMDC motor is V terminal = Vbackemf+ IR. Where I is the current in the motor, R is usually considered to be the terminal resistance. For a dynamic motor, we should actually add extra terms in the equation, but this will do.


Now Vb is generated by the speed of the motor, and IR by the load ON the motor. To make the point, make R = 0. Now the motor speed is ENTIRELY controlled by the applied voltage !

If R is KEPT small, which it is, by design, and because it is unavoidable in reality, then there is a inevitably a dependence of speed on load.

Taking the opposing case then INCREASING R, and the motor's "regulation", ie its ability to stay at constant speed as a function of load DECREASES.

Since the effect of adding a voltage divider not ONLY changes the output voltage (open circuit volts = Vin x R1/(R1+R2), but ALSO, and CRITICALLY changes Rout = R1 x R2/(R1+R2), adding a voltage divider ADDS resistance so that our R term is considerably more complex, but the end result it the regulation of the motor is awful.

THE way to do it, of course is to add tacho feedback, but even using a little voltage regulator (Rout~zero, over the operating current range) makes a huge difference.

Yes, that makes perfect sense. I was missing the fact that the drop in speed (and voltage across the motor) due to the load is dependent on the internal resistance.

Thank you for taking the time to explain that.

Thanks so much! That really helps me sort out some major issues. Before I buy DC motors, I'd like to eliminate if any that I have will do, at least to get it to work first. It sounds that since the higher the voltage the faster the motor speed, I might be able to get what I need out of a 6 volt system, unless they are more rare and so cost more.

Basically what I'm making is a DC motor driven lead screw that will have a nut which connects to a rod which will push up something heavy - about 400 lbs. When it's backed down it can't be more than about an inch high. I just need the control to switch it from forwards to stop to backwards. It also doesn't need to move fast - actually it's better if it's slow - say the nut would move an inch every 1 or 2 seconds. This also has to be inexpensive and when I read your post I did some searching and there are some pretty expensive motors, like step/stepper motors. I think a cheaper alternative is a gear motor or a motor with a gear box - but I'm not there yet.

Does that tell you tell you more about what I need? As for looking at the motors around the house first, other than eliminating those too big by size, how do I find out the other specs you mentioned about them? The labels don't say much sometimes.

Well, 400 lbs is going to need a good size leadscrew to hold it safely. Now consider what pitch the leadscrew is going to be as well. Ideally, the best screw to use for a lift is called an Acme thread, a 5/8 Acme has a pitch of around 8TPI You need to turn the screw at its pitch in TPI revs per SECOND to move 1" in a second. An 8TPI screw = 8 RPS= nearly 480 RPM.

Making some horrible assumptions about friction etc, those numbers allow me to say it will take roughly 20 lb-ins of torque to lift, and about 4 to lower your load.

20lb-ins x 480 RPM x some horrible US unit kludge factor yields ~ 120W

A nice speed for a motor like this is probably around 1600 RPM, that means you need to find a motor happy to spin at around 1600 RPM, while delivering about 20 x 480/1600 lb-ins of torque.

There's your spec.

Torque ? Torque depends on the pitch and the diameter.