How can I cut the RPM of a DC electric motor?

[acesareking]

Is there an easy way to cut the RPM of a DC electric motor by a substantial amount, let's say to about 10% of the present RPM, without adding any parts?

Is there a quick and easy way to do that?

How do I cut the RPM by any amount without adding any parts?

If there is a way to do that, will cutting the RPM way back increase the torque by a proportionate amount?

Failing that, can I choose a motor by RPM, voltage and torque as the deciding factors?

 

[Bill West]

Sorry, I don't think there is a part-less way to cut the speed. About all you can do to the motor is impair and reduce the field but that actually increases the speed!

In crude terms the speed goes with the voltage and the torque goes with the amps. If you want an increase in torque then you need an increase in current. But that will lead to overheating.

Motors are usually specified by voltage, horsepower and speed. The last two inherently define the torque.

Bill

 

[3DDave]

Yes, you can choose a motor by torque, RPM, and voltage, though DC motors are dominated by current. Voltage enters in because, at the no-load RPM in a typical DC motor, the motor is generating a back-emf that is equal to the applied voltage, so that the nominal current is zero. Actual motors have some friction and other losses, but the effect is still there, the motor drawing enough current to overcome those losses. If this was not the case then the motor would increase in speed until equilibrium occurred.

Gear-motors, with integral gear reduction, is the typical solution for low-RPM, high-torque applications.

 

[waross]

You may increase the field strength or reduce the armature voltage.
Either one will reduce the speed.
The torque is proportional to the armature current and the current is limited by the armature heating.
By reducing the applied voltage you may develop full torque at very low speeds, however you may need to add auxiliary cooling fans to avoid overheating when developing the maximum torque at low speeds.
Your best approach is to reduce the applied armature voltage, while keeping the field voltage the same.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

There are several different types of DC motors. Before you try to change the speed you must know what type you have. If you have not yet picked the motor, then make sure when you do that you pick the right type of speed controller to go with it.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[acesareking]

Thanks for all your replies and for increasing my understanding, but that's not to say I'm fully OK on the subject yet and I apologise for that.

I've found this motor that has the 250 RPM I want but it's only 12 volt so will my present 18 volt battery burn it out or will it just make it run 50% faster at 375 RPM?

It says it's a gearmotor and all the motors I'm looking at that run around 500 RPM seem to be gear motors so I'm guessing they have to have internal gearing that slows the original motor speed down to the 250 RPM output I need?

I would prefer a motor that would give me 250 RPM by direct drive (and preferably be 18 volts rather than the 12 volt motor linked below) or do you think that's not possible at all, ie, is the 250 RPM I need only possible through gearing down a high revving motor?

https://www.digikey.ca/product-deta...Pg4eG9zKRb2oBFV3gm2dTsLDWvwkTV1BoCvEsQAvD_BwE

 

[che12345]

Dear Mr acesareking
Q. Is there an easy way to cut the RPM of a DC electric motor by a substantial amount, let's say to about 10% of the present RPM, without adding any parts?

Your pre-conditions are: a) to about 10% of the present RPM, and b) without adding any parts.
A1. a) to run a motor at 10% of its original designed speed would mean lowering the cooling fan speed to about 10% of its original speed. The original (power/torque) ratings would be [heavily reduced] due to the very heavy reduction in cooling. Bottom line, NOT practical, unless a (separate cooling is used).
b) without adding any parts is also a challenge. Bottom line, NOT realistic. Motors are designed with certain speed/speeds with internal gear/gears of different rations or some time with diode or insertion of resistor in series into the armature circuit [internally], which you don't (see them out side) the motor casing.
A2. My proposals are: a) use a proper ratio [gear box]. That is, running the motor at (its original speed).
b) use a set of proper ration [pulley with belt drive]. That is, running the motor at (its original speed).
These proposals [do not] meet your (requirements). These are just proposals as a possible solution to your need, for information purposes.
Che Kuan Yau (Singapore)

 

[waross]

It's a 12 Volt motor. Try using 12 Volts instead of 18 Volts.
The motor is already turning almost 11,000 RPM. It may not be a good idea to spin it up to over 16,000 RPM.
You may have to add a part.
The part may be the proper 12 Volt battery.
Else, look for a suitable PWM driver.
Either way you have to add some parts.
Yes you may be able to find a 250 RPM motor.
Will it be efficient? probably not.
Will it be cheap? Probably not.
For such a small motor at such a relatively slow speed, The motor will have to be quite large to generate the same torque as a smaller motor that is geared down.
A quick comparison:
Your motor is spinning at 11,000 PRM.
To replace the motor-gearbox combination with a direct drive, 250 RPM motor the 250 RPM motor will have to develop almost 150 times as much torque.
It may be larger and more expensive than the motor-gearbox combination.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[acesareking]

Thank you all for your help I appreciate it.

 

[Bill West]

Now that we see your context, let's cut to the chase. Look for an 18v speed controller that will handle the 7A stall current. That will be cheaper in dollars and your time than any other solution. I Googled 18 v speed controller and got this on the first page

https://www.ebay.com/p/890752358

$8 US, looks completely appropriate to me.

You could put a voltmeter on the motor, turn the controller potentiometer up to 12v during full load, unplug the power, disconnect the potentiometer and measure its value from the center pin to the side the knob turns away from during speed increases. Get a resistor of about that size at TheSource and put it in series with the potentiometer on the side that the knob turns towards during speed increases. That way you'll never be able to turn the speed up to more than the motor's rated 12v.

Good luck, Bill

 

[waross]

Good find Bill. I like that.

Bill
--------------------
"Why not the best?"
Jimmy Carter