Mandatory components: 24v DC supply, No-name PWM speed controller (originally packaged with 24v 250w DC motor)

Existing but replaceable: 24V DC, 14A, 250W, 2750RPM motor.

Need: The most torque I can get at 24V and 15-20A, while remaining under or at 300w. RPMs are not very relevant as I can gear up for rpms as long as I have enough torque to get things moving.

Purpose: Repurpose a 12" wheel scooter drive system for use on a bicycle with 20" wheels, while also improving stop-from-start and uphill performance. Original motor-to-wheel gear ratio of 5.91 will be improved to 11.73 to lessen torque need and to compensate for wheel size difference.

Caveat: I am horrible with the nomenclature of both electronics and motors. My search efforts are restricted by this, and I apologise for my lack of proper terms.

Questions: 1) Can the existing 250W motor be modified to generate more torque at the existing voltage and max Amps?

2) If a new motor is needed, what do I need to look for when these motors do not list torque as a spec? (Even google won't help, since I don't know what to search for)

3) What approach to this would work if my hoped-for approach will not?

  • 2
    \$\begingroup\$ "RPM's don't matter as I can gear up" - no you can't, gearing trades torque for RPM, that's how it works. If you need more torque from your motor, gear down. Otherwise, unless you take the risk and over-drive the motor for starting, you need a different motor. \$\endgroup\$ – John U Jul 22 '16 at 8:19
  • \$\begingroup\$ For clarity: The gearing will have three selectable ratios, the 'best'/'highest' ratio for strting off will be the 11.73 ratio above. The two additional ratios available will be 8.86 and 6.68. This allows the high torque to be delivered to the wheel well at 11.73 and then changing to the lower ratios only when RPM at wheel makes it easy for the motor. Like in a car - you don't start in fourth, you start in first. So again, RPMs don't mater much at the motor IF the torque is high enough. \$\endgroup\$ – 111936 Jul 23 '16 at 0:32

The DC motor parameter you are interested in maximizing is variously called "starting torque" or "stall torque" or "locked rotor torque", or similar. It is the torque (measured in foot-lbs or inch-lbs) created by the motor when the rotor of the motor is prevented from moving. (This is done in a test bench by jamming the motor shaft mechanically and making the torque measurement - e.g. with a torque wrench.)

This "locked rotor" condition is what the motor will experience momentarily when it starts from a completely stopped condition. E.g. when your electric bike is at a standstill and you press the accelerator.

Some DC motor data sheets will state Locked Rotor Torque directly, others leave it to a simple calculation you must perform. In this case you must know the "torque constant" of the motor, which is usually present in the data sheet. The torque constant is a fixed value for a motor which tells you how many ft-lbs of torque the motor produces for each ampere of current passing thru its armature winding. In other words, ft-lbs per amp.

So, if you know the torque constant all you have to do is measure the current flowing thru the motor at stall (e.g. at the instant you apply power to it) and you will know the resultant torque by simple multiplication ( torque constant x amps).

Typically, you will use the same voltage supply for starting the motor that you do for running the motor. In your case this seems to be 24 Volts. So, in most applications the stopped motor will be started by applying the supply voltage directly across the motor's terminals. What is the starting torque in this common condition? Very simple, use Ohm's Law to figure this out. The motor's data sheet will usually list the motor's "internal resistance" or "winding resistance". In a motor like yours it will likely be less than one Ohm. According to Ohm's Law divide the terminal voltage (24 V) by the Winding Resistance and you will get the starting current in Amps. Now, multiply this calculated starting current by the Torque Constant and you get Starting Torque - the torque the motor will produce while the rotor is not moving - exactly what happens for the first instant you apply voltage to the motor before the rotor actually starts to turn. See? It's a pretty straightforward calculation. If you can make a non-intrusive amperage reading in the motor leads (e.g. with a DC current clamp) you can verify the electrical part of the calculation pretty simply.

As a practical matter, typical DC motors will perform closely to the theoretical calculation I described above. The problem comes in actually delivering the full voltage and amperage to the stalled motor. The amperage will be high for a motor like yours. You state "15-20 Amps", but it may be even greater when you do the calculations from the data sheet values. This means you have to use very heavy gauge wire to allow enough voltage to actually reach the motor without significant voltage drop in the supply wires. Plus, you've got to have a voltage supply that can actually deliver that much instantaneous current.

So that, if you are trying to select a DC motor to get the maximum starting torque at the starting condition, you will look for one with the highest Torque Constant and the lowest Winding Resistance. It's that simple.

  • \$\begingroup\$ FiddyOhm - you rule! Excellent explanation of how to find out what I need to find out for any new motor. One hiccup: Most of the motors I have been finding/looking at don't actually have a data sheet accompanying their sale listings. All I get is what's printed on the motor label - RPM, max Amps, wattage, and voltage. \$\endgroup\$ – 111936 Jul 22 '16 at 10:05
  • \$\begingroup\$ That's a common problem these days, especially with CCC components. Unfortunately, in these cases you will have to procure the motor and make the measurements yourself in order to guide your application and circuit design efforts. \$\endgroup\$ – FiddyOhm Jul 22 '16 at 10:11
  • \$\begingroup\$ Oh, also thank you for clueing me in on the appropriate terms to look for regarding all this. The vocabulary is such a challenge for me! \$\endgroup\$ – 111936 Jul 22 '16 at 10:18
  • \$\begingroup\$ Keep this in mind for your future endeavors: The vocabulary of an unfamiliar technology is the place to start the learning process. \$\endgroup\$ – FiddyOhm Jul 22 '16 at 10:31

A DC motor has a torque constant. That means that it will give you some amount of torque per ampere that you supply to the motor.

You can trade RPM for torque with a gearbox, so the torque increases with decreasing RPM while the power for a fixed load stays constant minus gearbox losses.

You can get a different motor with a different torque constant, but bottom line is angular velocity times torque = power, and the power you have available is dependent on your power supply and the losses in your controller.

If you need more torque at the same RPM you need to supply more power.If the motor isn't rated for the additional power/current you need a different motor.

  • \$\begingroup\$ John D. - I have read about windings having influence on torque and rpm. Do you have any input regarding this? \$\endgroup\$ – 111936 Jul 22 '16 at 5:03
  • \$\begingroup\$ Windings do affect torque and RPM, but that's only useful to motor designers. Modifying motors on your own is not the worth the effort. \$\endgroup\$ – jbarlow Jul 22 '16 at 7:23
  • \$\begingroup\$ @jbarlow says "Modifying motors on your own is not the worth the effort." Well, that depends on time available to do it and money available to buy more stuff. If rewinding the one I have would actually provide me with more torque at the existing volts and amps it is not at all something "not worth the effort" - quite the opposite! So - would it do the trick even if you don't think it's 'worth it'? \$\endgroup\$ – 111936 Jul 22 '16 at 10:11
  • \$\begingroup\$ You could rewind the motor with fewer turns of thicker wire for example, but you're not guaranteed to get more torque. That's because it's not just the windings but the overall magnetic circuit that you have to take into account. You could saturate the magnetics or you may not have enough permanent magnet flux to take advantage of the increase. So you could experiment, but without understanding the motor design parameters you can't guarantee anything. \$\endgroup\$ – John D Jul 22 '16 at 11:27
  • \$\begingroup\$ Just getting all of the existing windings on and off a commercially made motor is a serious challenge for a hobbyist, never mind getting to work as well it did before, never mind improving it. \$\endgroup\$ – jbarlow Jul 23 '16 at 4:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.