# effect, if any, of input voltage on RPM of DC motor

I bought a home coffee roaster which is designed to sit on a gas cooktop. It has a small geared motor which turns the drum, which sits above the flame. The target rotational speed of the shaft turning the drum should be about 60RPM.

The motor has these markings:

output: 6W
voltage: 24V
speed: 2950RPM
current: 0.5A


The (overseas) seller shipped a 220V AC 12V DC 850mA adapter but I have 120V AC. So I'm looking to replace the adapter with one that can take 120V input and supply the needed amps.

But does input voltage have an effect on RPM?

I'm wondering why the seller provided a 12V adapter when the motor says 24V. Is the seller maybe relying upon the 12V DC adapter to reduce the RPM? Should I be searching for a 12V DC (as supplied by the seller) or a 24V DC adapter?

• I might assume the motor marking is for the motor only, and not the gearbox. Does that seem reasonable to you? Can you estimate the gearbox ratio? Maybe by rotating the motor shaft by hand? I'd expect the motor to run at less than 2950RPM powered with 24V when under load, and less than 1/2 that at 12V. I'd ask the seller what to do. They should replace the adapter, or at least tell you the right spec. Commented Mar 31, 2016 at 19:51

But does input voltage have an effect on RPM?

Yes. A DC motor speed will be proportional to voltage (for a given load resistance).

I'm wondering why the seller provided a 12V adapter when the motor says 24V. Is the seller maybe relying upon the 12V DC adapter to reduce the RPM?

Possibly to reduce the RPM but more likely to limit the current should the grinder stall. The motor is rated at 6 W. Power, voltage and current are related by $P = VI$ (where I is current). At 24 V and 6 W we can calculate maximum continuous current, $I = \frac {P}{V} = \frac {6}{24} = 0.25~A$.

Current will increase with increasing load reaching a maximum at stall. You could estimate what the stall current will be by measuring the motor coil resistance and calculating the stall current from Ohm's Law, $V = IR$, as follows: $I_{STALL} = \frac {V}{R} = \frac {24}{R}$. I suspect that you'll find that it's more than 0.25 A.

By reducing the voltage to 12 V the stall current will be reduced by half but since both V and I have been halved the power dissipated during a stall will be one quarter of the 24 V value. $I_{STALL} = \frac {V}{R} = \frac {12}{R}$ in this case.

Should I be searching for a 12V DC (as supplied by the seller) or a 24V DC adapter?

Does it turn the coffee? Try it out with a 12 V, 0.5 or 1 A, wall-wart PSU and see if you can make some measurements. Report back!

• Thanks for this info. It roasts over gas-heat, not a grinder. I cannot try it out unless I plug the 220V DC adapter into my 120V mains.
– Tim
Commented Mar 31, 2016 at 20:14
• Sorry I missed that. Any 12 V DC, 0.5 to 1 A, wall-wart style adaptor should be fine to test it out. If you're happy with the speed then order the correct voltage unit. Commented Mar 31, 2016 at 20:33

A lower voltage will make a standard two-wire, run of the mill DC motor turn slower.

But even more simply: If you buy something with an adapter supplied which outputs 12V, then any replacement should be outputting 12V for it to function properly.

Aside from that, a supply that only takes 220VAC is very old-fashioned and you probably want to replace it with a nice, light-weight, hopefully more energy efficient switching adapter any-way. But that, of course is up to the user and the use-case.

You or a friend is likely to have a shoe-box full of adapters that will provide 12 volts DC, from broken or worn out gadgets. I get a lot of them from WiFi routers. The shape of the connectors is somewhat standardized too. You need an adapter that outputs at least 850ma, and make sure the plug fits and the polarity is the same, usually, the tip is + and the shell is -.

A simple motor is unlikely to fry itself if polarity is reversed momentarily, but it'll shorten the motor life.

• Q1: Why would he need 850 mA at 12 V when the motor is rated at 0.5 A (500 mA) on full 24 V? Q2: Reversing the polarity of a DC motor won't "fry" it or shorten it's life. It will run in reverse. Why do you think it will shorten its life? Commented Mar 31, 2016 at 23:25
• Because (based on his post on diy.se) his existing adapter is 850ma and I'm not one to second-guess the factory. I don't know his motor but if it's optimized for one direction, it means brush timing is advanced for efficiency, and in reverse it's now retarded, so less efficient and making more heat. Meanwhile the cooling fan is spinning the wrong way. It's also possible the motor will spin the same way anyway, and then the question is whether there's anything else that cares about polarity. Commented Apr 1, 2016 at 0:48
• That makes sense for a bigger motor anyway. Have you ever seen directional sensitive motors at 6 W? Commented Apr 1, 2016 at 8:35