How to verify the reliability of torque rating of DC motors given on online sites?

I have to decide the motors to be used for my robotics project, but while searching for the DC motors I have found out a rather unusual fact that I can find several motors who provide the same torque while there is a significant difference in weights. I am curious to know why?

For eg. I can find 12 kg cm torque motor weighing just 125 gm whereas I can also find 17.5 kg cm torque motor weighing 600 gm. I really don't understand where does the difference lie? And is it reliable to use the lower torque motors.

• Generally, higher torque = higher armature current = larger diameter conductors and larger/lower reluctance magnetic circuit = heavier – Chu Nov 28 '15 at 8:55
• And what might be the full speeds of those two motors? – Andy aka Nov 28 '15 at 11:40
• Torque * speed = power. So you can at least do a sanity check on their figures. If these are geared motors then they can achieve high torque in small size, if you can accept low speed. As for the reliability of the lower torque motor ... how can we tell ? Only you know what load it's driving. – Brian Drummond Nov 28 '15 at 11:45

It's all about efficiency. Basically if you put more winding copper into the motor it will be able to produce a given amount of torque more efficiently than one with a small amount of windings. However more windings will also make the motor much larger physically. Which of these is important to you really depends on the application. For example, an air conditioner motor will have more windings because efficiency is more important than weight, while a model aircraft motor will have less windings and trade efficiency for weight.

In terms of reliability the main issue is heating in the motor. The smaller motor will simply get a lot hotter and also have less mass to absorb the heat. If you don't deal with this heat properly then it could fail.

The other aspect is the motor RPM. If you can run a motor faster it is generally more efficient. However at some point the motor will either fly apart, or core losses will start to dominate, reducing efficiency again. In a well designed motor these points will all converge at the max operating speed (hopefully with a bit of safety margin on the flying apart RPM).

In terms of assessing this information you'll need to look at the rpm/voltage or torque/current constant and the winding resistance. Using these two numbers you can work out the efficiency of your system and decide what sort of efficiency/size tradeoff is appropriate for your application.

• It has nothing to do about efficiency, motor run faster because the windings have less turns - smaller back EMF. – Marko Buršič Nov 28 '15 at 18:52
• @MarkoBuršič - It is the total volume of copper that sets the I^2R losses, and hence efficiency. Thicker wire with less turns just has more I and less R compared to thinner wire with more turns, but the motor performance characteristics are not changed. – Jon Nov 29 '15 at 11:58
• The window space in the iron gaps have constrained dimension, you can place many turns of thin wire (high rpm, low torque) or less turns of thick wire (low rpm, high torque). No space is left in gaps, now where did you find the information you are reffering at? – Marko Buršič Nov 29 '15 at 14:32
• Changing the winding size only affects the operating voltage. It does not change the maximum torque that can be produced by the motor. – Jon Nov 29 '15 at 17:18

Power is in proprotion of cube of dimensions, as well its weigth. For example we have a motor with dimensions A x A x A with power of P1 = 1W, then what will be the power rating of similar motor with dimensions 2A x 2A x 2A? The answer is P1*2^3 = P1*8. Why is so? Because the weight of machine is growing with x^3 of its dimensions (more steel, more copper), because the volume grows.
Power is related to torque and speed, P = M * omega, therefore the same motor can have large torque, but small final speed or high speed and small torque. You should look both parameters: speed and torque.

Motor Torque ratings are a factor of its electrical and electro-magnetic construction. Its weight has nothing to do with its Torque. These two numbers are not mutually exclusive. While some heavier motors may have larger torque to weight ratios, there is no single rule of thumb to gauge them by. You cannot verify the torque by weight alone.