reduce a motors rpm

[nate4548]

Hi all,

just want to know,

if I have a motor that turns say... 10,000 rounds per minute at 9 Volts , and I wanted to reduce that to about 60 rounds per minute, would I just be able to do that by inserting a couple of resistors? or is that going to burn out the motor or be a fire hazard?

Thanks,
Nate,

 

[Marks256]

couldn't you use gears?

 

[dknguyen]

Motors get most of their power from speed than from torque. You use either use something that needs their speed (and low torque) like a fan/propeller or use gears to decrease their speed (and increase torque) like wheels. If you just slow them down directly it will take away most of the motor's power (which comes in the form of speed) and since it's torque is probably too low already it won't be able to do anything useful.

You can use resistors if they are large enough to handle the power. It also wastes a lot of energy since it's just being turned to heat by the resistor. Assuming it does do whatever you want it to do at 60RPM (which it probably doesn't)- to slow it down your way from 10kRPM, about 0.25% of the power drawn from the battery would get to the motor, the other 99.75% would be getting turned to heat. THat's a really terrible way to do anything.

Reduce the voltage or use gears. Using gears which will decrease speed, increase torque on for the same voltage which means the current draw will be reduced and that means the energy not required by the motor stays in the battery, where it belongs. Reduce the voltage by using a lower voltage battery or a PWM-based motor controller. These do not burn off the excess energy as heat, but control the voltage that the motor sees coming from the battery meaning excess power from the battery not needed by the motor stays in the battery.

 

[nate4548]

well it would definately be simpler just doing it with resistors that is why I want to know if it's safe.

ps. reducing from 10,000 rounds to 60 with gears is going to be a major if not impossible feat.

Nate,

 

[dknguyen]

well it would definately be simpler just doing it with resistors that is why I want to know if it's safe.

ps. reducing from 10,000 rounds to 60 with gears is going to be a major if not impossible feat.

Nate,

It's a high gear ratio, but not impossible. It certainly beats 0.25% efficiency or near zero torque. You could just find another motor with a gearbox. Is the required motor anything like one of these?

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What is this motor for? Because if it's for moving anything, I can tell you right now that it probably won't without gears unless the wheel is less than 1mm-2mm in diameter (which really means the motor shaft is acting directly as the wheel). And with that radius at 60RPM it's going to be travelling at a top speed of 1-2mm per second. You can always use gears to get the torque up to what you need and use a PWM controller to get it down the rest of the way.

 

[user_88]

Motors produce mechanical power...They convert electrical power into mechanical power .... If you use a resistor to decrease the current to the motor, you are decreasing the available power... both electrical and mechanical.

Power is the quantity that is transferred from the motor to the wheels ... power is the mathematical product of torque and rotational velocity.

Use a gear train, or a pulley and drive belt set, to transform a high rpm motor operating speed to a lower shaft speed at the wheels.

If you have access to a lathe, you should be able to fashion some pulleys out of plastic, nylon, or some other convenient material. Find some elastic bands to use as the drive belts.

 

[audioguru]

The motor won't start if you limit its current with a series resistor.

 

[MrAl]

Hi,

It all depends on how much power you really need from the motor.
Almost anything will work with a system that doesnt require a large amount
of mechanical power, but as the load gets bigger the motor has to supply
enough torque to get the load going.
DC motors seem to have the most torque at zero speed, so it might work
for your app if it doesnt require too much power.

 

[audioguru]

A DC motor has a lot of torque at zero speed because then it draws more current. But with a resistor in series limiting the current the motor cannot draw more current so it will not start.

 

[Boncuk]

Calculating the voltage to rpm ratio linear the motor would theoretically rotate at 60rpm applying 54mV supply voltage.

No motor starts at that low voltage, not even a special solar motor.

Using a reduction gear with a 6-12V motor you can achieve the desired rpm with high torque at the drive shaft.

Boncuk

 

[duffy]

Use a short PWM with a low frequency, like 60hz. You will be amazed at the results.

It will start, and run, v-e-r-r-y s-l-o-w-l-y.

 

[ericgibbs]

As per Duffy, use a 555 timer ic.

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Google for: 555 PWM circuits

 

[dknguyen]

Hehe, that's my favorite 555 timer PWM example too.

 

[proffski]

Use a short PWM with a low frequency, like 60hz. You will be amazed at the results.It will start, and run, v-e-r-r-y s-l-o-w-l-y.

Yup, twin stage PWM is the way I'd go or even a basic ubiquitous 555 driving a large power transistor or MOSFET would probably do the trick...

 

[muntadar]

thak you and very happy

 

[Ubergeek63]

or switch to a stepper that is meant to work down in the mud...

 

[MrAl]

As per Duffy, use a 555 timer ic.

**broken link removed**

Google for: 555 PWM circuits

Hi,


Looks like the output protection diode is missing from that circuit
diagram. There should be a diode or zener or diode/resistor from
the transistor drain to V+ to make sure peak voltage stays
at or near the V+ line to protect the transistor, or at least
so that the drain voltage stays under the rating of the transistor.

For a diode, the anode connects to transistor drain, cathode to
V+.
For a diode and resistor, the anode connects to transistor drain,
resistor to diode cathode, other end of resistor to V+.

Without this diode the transistor drain can reach voltages as
high as 1000v or more when the transistor turns off due to
the inductive effects of the motor winding.