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I have a 5.5 kW motor that I'm using for testing. I tried using it to lift a 15lb weight but it struggled. I reduced the amount of rope on the spool (there was a lot and then it was able to lift it at about 1ft per second. However, it was taking in 100 amps to do so!

The setup is drawn in a link below, it's like this:

The motor is attached to a spool that has rope, the rope goes about 2-3 meters up into a pulley and comes back down. The rope is then tied to a 15lb dumbbell. The spool diameter is 50mm (25mm radius) and the motor is rated for 10Nm of torque.

The rope effectively makes a bigger spool when wrapped, which comes to about 120mm diameter when the motor couldn't lift the weight. I left most of the rope off to retest when it finally lifted it.

Now, using the formulas and what I understood from here: Understanding required torque for a motor lifting a weight

I calculated that if I wanted to move 15lb (6.8kg) at 2m/s:

6.8 * 9.8m/$s^2$ = 66.64N

6.8 * 2m/$s^2$ = 13.6N

Torque Needed = (66.64 + 13.6) * 0.025 = 80.24N * 0.025 = 2Nm (The 0.025 is the spool radius of 25mm converted to meters)

The battery output was 44 V and the motor took 100 amps to move the weight, totaling to 4400W. I calculated that moving this weight should take less than 200W. The motor is rated for 10Nm of torque, yet according to the formula it only needs 2Nm! I may hook up a stick to the motor and use a scale to directly measure it's torque output.

Is there something wrong in my setup? Do these formulas not apply the way I'm using them? Any insight would be helpful, thanks!

The motor is connected to one end of the spool. From end to end, the spool is about 80mm long.

After Testing:

Just did some testing. With very little rope on the spool, it can match gravity and barely pick it up at around 60amps. As it takes more amperage, it starts accelerating the weight. It can just barely start picking up the weight with all the rope around the spool.

However, even with the spool filled to the brim, with 120mm diameter, the math says:

(66.64 + 13.6) * 0.025 = 80.24N * 0.06 = 4.8Nm

That's still only half of what the 10Nm specification of the motor!

Definitely how much rope is on the spool is affecting the torque needed. However, does the distance the rope is from the motor (whether at the same end or the opposite end of the spool as opposed to the motor) play a role? Is there a way to incorporate the distance of the rope from the motor into the calculations?

Would supporting the spool at the other end help with this?

Setup Drawing

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  • $\begingroup$ it was not entirely clear to me the following a) is the dumpbell accelerating throughout its travel? Or does it reach its maximum in a few milliseconds? b) does the motor work up to a point and then get stuck (when the diameter of the spool with the rope becomes 120mm)? Finally, is it possible to post a picture of the spool attached to the motor and the setup? $\endgroup$
    – NMech ♦
    Jan 5 at 13:41
  • $\begingroup$ also could you provide an estimate of the mass per per (or weight per m) for the rope? $\endgroup$
    – NMech ♦
    Jan 5 at 13:44
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    $\begingroup$ Sounds like you need some gearing. $\endgroup$
    – Solar Mike
    Jan 5 at 14:05
  • $\begingroup$ And what is the motor rated voltage? $\endgroup$
    – Transistor
    Jan 5 at 21:45
  • $\begingroup$ The rated voltage is for a 12S battery which is what I'm using. Up to 50v, but it was charged to nominal voltage around 44v. The rope is very very light, the whole thing wouldn't even be a couple pounds. The weight only starts moving around full amperage draw and doesn't seem to accelerate. However, I'll be testing it again today to make sure. I'll edit this into the question. @NMech $\endgroup$
    – zapshe
    Jan 5 at 23:37

1 Answer 1

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I directly measured the torque of this motor. It seems that while rated for 10Nm, it's only able to output around 4Nm. This nearly perfectly correlates well to the equations.

It seems that while the site claims 10Nm, they are also selling 3 different versions of the motor. All 3 versions are given the same spec sheet with the only difference being their KV ratings. I have the highest KV rated motor, meaning the lowest KV rated motor is likely the one able to produce the 10Nm of torque specified. It's very disappointing that they would slap the 10Nm label on all 3 versions when it's clearly not true and are all rated for 5kW.

Everyone's help was appreciated. It's good to know that my methodology was not flawed.

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