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.
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?