I'm looking for some basic guidance to point me in the right direction on a project I'm involved in, as I'm overwhelmed with all the different types of electric motors and associated equipment available. I'm an electronics novice, so I'm very sorry if I've left out any important information.

Here's roughly what I would like to do: I need to flip a switch and have a shaft go up to a constant speed of about 200 RPM...actually, it could go up to anywhere between about 100 RPM and 300 RPM. What matters here is that it goes up to a constant speed...the exact speed isn't too important.

The load on the shaft will vary with time, but the load will always be between 5 in-lbf and 25 in-lbf of torque.

I don't need extreme amounts of accuracy here...I'd just like to keep the motor in the vicinity of a constant RPM. If it fluctuates up and down in speed by 10% as the load changes, that is OK, assuming it can correct itself somewhat quickly keep its average speed close to a constant value.

Is this sort of thing possible to achieve without spending tons of money and without being an electronics expert? Just for reference, I was looking at something like this for the motor.

Is something like that in the ballpark of what I should be looking at? Can something like that be made to do what I want, if paired with some sort of controller or something? Or, am I not even close?

  • \$\begingroup\$ Are you open to off-the-shelf electronics that track the RPM and adjust accordingly, such as PID controllers with shaft encoders on the motor? As far as I know the motor you linked isn't designed for precision speed, but just has precision-machined components in its gearbox and body. \$\endgroup\$ – Reinstate Monica - ζ-- Sep 20 '15 at 18:24
  • \$\begingroup\$ What about voltage, max motor speed, regulted or unregulated speed? From your data the minimal power is something arround 100W. \$\endgroup\$ – Marko Buršič Sep 20 '15 at 18:41
  • \$\begingroup\$ @hexafraction, I'm definitely open to off-the-shelf components...in fact, I probably prefer them! I'm pretty much at the very basic level of being able to hook up wires to a switch to turn the motor off and on. I could probably do stuff slightly more complicated than that with proper instruction, but I don't see myself succeeding with wiring up my own complex circuits or anything like that. \$\endgroup\$ – user3208212 Sep 20 '15 at 18:48
  • \$\begingroup\$ If it has to spin only in one direction, than a cheap barbecue motor is the right chice \$\endgroup\$ – Marko Buršič Sep 20 '15 at 18:56
  • \$\begingroup\$ @MarkoBuršič, I love the simplicity of that idea. Wouldn't a barbecue motor be a bit slow for my application? I need somewhere around 100 to 300 RPM, with a load of between 5 and 25 in-lbf. \$\endgroup\$ – user3208212 Sep 20 '15 at 19:02

To achieve a constant speed on your motor over varying loads you will need to build a closed loop control system. If you are into ready made subsystem components you could look at the Roboclaw motor speed control modules at the same ServoCity web site.

Use of a closed loop system does require the application of a quadrature encoder mounted on either the motor shaft or on the gear box output shaft. Speed control off the motor shaft can be better for the life of the gearbox. It a encoder is placed on the output of the gearbox and if there is backlash in the gearbox the servoing done by the controller can cause the gear box to be exposed to more wear than otherwise would be the case.

The Roboclaw controller is advertised to support quadrature encoders but it is not certain that you can get them through ServoCity or if their specific motors allow the attachment of an encoder onto the motor shaft.

  • \$\begingroup\$ Thanks Michael for the suggestion. Is that the sort of thing that a novice could figure out how to use? Are there any good tutorials anywhere that you might recommend? Also, could you point me towards a suitable motor that I might be able to use with that controller? Any help is appreciated! \$\endgroup\$ – user3208212 Sep 20 '15 at 19:57

Frem your explanation I think, you don't need any regulation closed loop, rather an induction motor /w gearhead. With load varying the speed will also vary, but with very small difference, some 5%.


You just need to calculate the required torque. Let's say 3Nm @ 200rpm, PMomega;
omega = 2*pi*200rpm/60s = 21 rad/s
P = 63W
The synchronous speed (zero torque)of 4 pole induction motor is 1500rpm @ 50Hz, or 1800rpm @ 60Hz. At full torque usually the speed is 1300rpm @ 50Hz, 1500@60Hz. Therefore you will need a reduction ratio of 1500/200 = 7.5

The gear reducer itself has a loss, then multiply calculted power with some margin.

This is very close hit, 60W, gear reducer ratio = 9 , 2.7Nm of output torque http://www.ebay.com/itm/Oriental-Motor-5IK60GU-SWT-Induction-Motor-with-5GU9RH-Gear-Head-/221479577093?hash=item3391362605


In industry we would use a Synchronous Motor. This is typically a 3-phase AC motor where the motor will accelerate to match the AC line frequency, up to some maximum specified load.

If you don't have a 3-phase supply handy but you want to use that kind of motor you can use a Variable Frequency Drive (a.k.a. VFD) which can output a 3-phase signal at any frequency, and depending on how much current you need out of it, you can run it off a single phase (because it just turns it into DC and then inverts it back into 3-phase AC). A VFD also has the ability to run that motor at any frequency from zero up to twice the normal frequency (so 120 Hz in North America) so you have much more speed regulation. Some models also allow you to compensate for a bit of slip based on load, so you can open-loop correct the speed based on how much load you have. If you really need good regulation, many also support an external encoder or tachometer input for closed-loop speed control. Note that to use a VFD then the motor you select has to be "inverter rated." This means the core is constructed in a laminated way to reduce eddy currents from the high frequency elements of the AC signal being reconstructed.

For less expensive alternatives, you could use a synchronous gearmotor or a stepper motor. The latter are particularly cheap and give you lots of torque at slower speeds. They are heavily used in 3D printers, disk drives, normal printers, and low end CNC routers. You can check out stepper motors at Automation Direct. Note that you need to buy a motor, a power supply, and a drive. I believe their SureStep drives can be programmed to do certain things, like run at a speed you set with a potentiometer. For even less expense, you can run a stepper from an Arduino. One way is using the EasyDriver board. Here's a tutorial.


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