# Running a brushless DC motor on low rpm using servo tester

## Background

I'm working on a gift for the wife for Christmas - an electric bobbin winder (a winding machine/spooling machine) - which will work in the same way as this machine:

It winds yarn on spools, and you control the rotational speed using a pedal. My idea for the motor and control setup is as follows:

• Brushless motor for durability and low noise
• Brushless motor is connected to an ESC (electronic speed controller)
• Electronic speed controller is connected to power supply AND an analog servo tester
• I utilize the potentiometer on the servo tester, for speed control of the brushless motor.

The motor will have to run smoothly on low RPMs ( < 15 rpm) as well as high RPMs (~5000 rpm).

## Questions:

1. What should be the specs on the motor - will more poles be better? (i.e. 24n22p is better than 14n12p?) Should it be sensored?
2. Will a setup as described, using a servo tester, be able to run the motor on low RPMs ( <15 rpm)? Or do low-RPM running of motor require different type of controller unit? (i.e. arduino, raspberry pi, ...)
• Do winding machines only require unidirectional rotation? Is brushless really required? How much torque is required? – DKNguyen Nov 8 '19 at 19:18
• I would like brushless to reduce noise and get a long-lasting gift. My impression is that brushed motors are way more noisy. This gift is 49% to me also, as the bobbin winder she has already got, is very noisy!!! :D.. Also: the bobbin winder really only need to rotate in one direction yes. – NorwegianDIYer Nov 8 '19 at 19:45

For 15 RPM you want as many poles as possible and sensored. More poles results in smoother operation at low RPM, and sensorless is only reliable for applications with zero startup torque and high running speeds. Even with sensored, I suspect you may have issues with radio controlled equipment since 15 RPM is pretty damn slow. I would run it by some RC truck guys first to see if they think smooth reliable operation is possible at 15RPM. The airplane guys won't be of help since they don't use sensored since it's more expensive and propellers are low starting torque, high RPM applications.

More poles can limit speed at higher RPM since the ESC must commutate much more frequently and faster per RPM, but it shouldn't be an issue at 5000RPM.

You may have resolution issues with a servo tester since the knob will be able to go from 0 to 100% with the maximum RPM probably being in the thousands of RPM. I'm not even sure if a RC ESC has enough pulse width input resolution to reliably read and operate 15 RPM.

I would consider gearing to better accommodate the low RPM side but that makes cost spike considerably. You are probably going to run into torque issues at 15RPM without gearing even if it does rotate smoothly and reliably.

As DKNguyen mentioned if you need to run a BLDC motor smoothly at a slow speed you are going to want it to be sensored.

However I dont think you need to go with a brushless motor to make it quiet. Most of the noise from brushed motors is due to the reduction gearing and a high quality(and cost) brushed motor can be silent. You could definitely find one used for cheap though. This would let you simply control the speed without the need for a ESC. If you look on ebay for Faulhaber you will find a lot of used motors for <30$that retail for 300$+. They have a very wide range of gear reduction, from 3:1 to 500:1 and more that will still be silent when running.

For example here is a DC motor on ebay with 134:1 reduction with a buy it now price for 20\$ https://www.ebay.com/itm/113890961497

That motor will be silent; being brushed or brushless has nothing to do with it.

Also note that even if you do go with a brushless motor you are still probably going to need some kind of reduction gearing to get sufficient torque at low speeds like 15rpm (although without knowing the load it will be hard to determine how much)

1. the pole number should not really matter too much. what does matter is the torque constant of the motor and the motor voltage.
2. If you want the simplest solution possible I would just buy a DRV10987 EVM(http://www.ti.com/tool/DRV10987EVM) to drive your motor. It will take care of all the more detailed stuff like controlling the motor commutation. it is also sensorless so you dont need a motor with sensors. It takes a analog voltage input as its speed input (0-3.3V) so you might be able to find a pedal that is a pot and use that as the input. 50W should be enough for your application too. Then all you need to worry about is to find a 24V hobby motor that is about 50W. Aslo you shouldne need gears.