# Requesting Advice For Selecting And Sourcing A "Medium Torque" Motor

For a prototype project I'm building, I need to select a motor. I'm not familiar with different types of motors, so I have been trying to do some research. However, most of the stuff I have been able to find has been about low-power motors (e.g. handheld devices, models, etc) motors or about high-power motors (e.g. car winches). I haven't been able to find much information (or sources) for medium-power ("human scale") motors and I've been getting confused by the different types (e.g. AC vs DC, series wound, permanent magnet, etc) and the different trade-offs for these within this power range.

Essentially, my key requirements are:

1. To pull a cable in (and let it out) against a force of up to around 2 kN at speeds of up to around 0.5 m/s.
2. Within that force range, the line speed should NOT vary with the applied force.
3. To control the direction (in/out), speed of the motor and the position of the cable with a computer.

The machine will be fixed in place, so it can be mains powered. The total distance the cable will move will only be a few meters. The machine can also be quite large, so the cable could be spooled from a drum/wheel with any radius up to about a max of 0.5 m (I was considering using a bicycle wheel). So the max torque needed of the motor would be something like 1 kNm (with a 0.5 m drum/wheel) to 200 Nm (with a 10 cm drum/wheel).

If anyone has an advice to offer on what type of motor would be suitable for this application and where (ideally in Europe) it could be sourced from, I would be most grateful.

• This sounds like a close match for a car winch. If you need constant speed vs load and you want to do ti with a minimum power (> 1 kW) motor per Neil's answer, you'll need a speed control loop in the motor controller.. Also, letting out against that force, the motor will generate power : you need to absorb that power either regeneratively or in a braking resistor, and consider a mechanical brake for safety.
– user16324
Aug 3, 2020 at 10:33

When choosing a motor, thinking about torque first is not the way to go. As you have pointed out in your question, you can vary the torque simply by varying the size of the winding drum. You need to worry first about the power of the motor. This gives you the weight, cost and power supply requirements. Then an appropriate gearing will give you the right speed and torque.

If you want to pull 2 kN at 0.5 m/s, then you need 1 kW mechanical output, which will require a 1.5 to 2 kW electrical input.

The line speed should not vary with the applied force? All motors, except synchronous and stepper motors, will vary speed when under different force loadings, but you're unlikely to be using one of those(#). Permanent magnet and shunt-wound motors will change a little, series wound motors will change a lot. Either type of motor can use feedback to control the speed to any given degree of constancy.

If you are going to control the position of the cable with a computer, then you will first need to sense the position of the cable. There are lots of different ways, including counting the turns of the drum with an angle sensor, or measuring the distance from crane to hook with an ultrasonic sensor. Different ways have different accuracies, and different ease-of-use, or requirements for setting a zero. Anything that uses motor or drum position is subject to cable winding errors on the drum. What position accuracy do you want, ±1 mm, ±1 cm, ±10 cm? The amswer will make a big difference to what techniques you can use. Ultimately, the position feedback can be used with your computer to control the motor speed and position.

(#) A BLDC motor can be used in stepper-motor mode or with a full position controller, and this would give you direct control over its speed and position. You might find one powerful enough.

• That's a really good point about searching by power, thanks. Searching for BLDC motors in this power range brings up a lot apparently designed for electric skateboards (e.g. banggood.com/de/…) which presumably I could then somehow gear down to the required speeds. Although, sourcing an appropriate gear set seems even more challenging than sourcing the motor.
– Rich
Aug 3, 2020 at 15:37
• @Rich 2.8 kW in that size, frightening! If you want position control, then ideally you'd use a hoverboard motor, not a skateboard motor, that is, a motor with sensors. Aug 3, 2020 at 16:29

As Neil_UK has answered, you can't get a "uniform speed" motor. What you can (and must) do is to measure the cable speed and adjust the motor appropriately.

This might be as simple as taking a small generator, putting a (rubber) pulley on it, then fixing the generator so that movement of the cable causes the pulley to rotate. Your computer would then read the voltage out of the generator and close a velocity loop to keep the voltage (and the cable speed) constant.

A more common sensor would be a optical encoder. This will put out logic-level pulses at a rate which is proportional to cable speed, and is easy for the computer to read. Then the control loop will attempt to keep the interval between encoder pulses constant, rather than keeping a generator voltage constant, but it works out to be the same thing in the end.

If you want to get classical about this stuff, you don't need a computer to control your speed. A quite simple op amp will do the basic job - but you then need to produce a driver which will go from a few volts and mA of op amp to kW of motor drive. Note, though, that this will also be true if you use a computer.

• Having a separate sensor for the cable seems very reasonable, especially since I also want to measure and react to the tension in the cable. So now I "just" need to source a suitable motor and/or motor/gearbox combination, which I guess is the main thrust of my question.
– Rich
Aug 3, 2020 at 15:43