Sum of torque

[dieter]

EOW,


Steppermotors is a good idea to get a very low RPM. But usually they are driven by synthesized Sine waves / Phases. If you can control these shapes PWM wise, then it may be possible to reduce the power consumption.


Otherwise, my experience with stepper motors tells me, the slower they are rotating, the hotter they get (because of unneccessary  DC currents), and heat means loss here.


Also, the rotation is rather vibrating, not very smooth. Not sure if this is a problem.


The SMC800 control card does in fact allow the shaping of the 2 phases, but only in a 2 bit resolution. Like:  0,1,2,3,2,1,0.. not really a smooth sine wave. And that is used to control -12 to +12 volts. I don't know if this card is still manufactured. There may be other, similar ones.


BR

[EOW]

I watched a lot of specifications of stepper motors and I think it's not a good solution. The easiest it's to build a small device and turn it at 750 RPM or more. It's not necessary to have an efficiency greater than 1, if there is no torque on the support (it can be test with w=0 for the support) and if the efficiency is greater with the support in rotation I think it could be ok. For example, if I have an efficiency of 0.2 for the device when the support don't turn, the efficiency must increase of 5/3*0.2=0.33 (if ratio of bigpulley/smallpulley = 3/1 and if the motor turns at 750RPM). Like that a simple test can be done with a drill and 2 dc motor (any efficiency could be ok). The efficiency must increase like the angular velocity of the support (linear law).

The general formula for the extra power is :

Power = F(R1w - R2w)

with:

R1 the radius of the big pulley
R2 the radius of the small pulley
F the force on the belt, it's Pm/(R1*wm), with wm the angular velocity of the motor and Pm the power used by the motor
w the angular velocity of the support

So the extra power is:

Power = Pm*(R1w - R2w) / (R1 * wm)

The efficiency of the device is :

Eff = (Power * ηg*ηb + Pm*ηg*ηb*ηm) / Pm = (Pm*(R1w - R2w) / (R1 * wm) * ηg*ηb + Pg*ηg*ηb*ηm) / Pm = (R1w - R2w) / (R1 * wm) *ηg*ηb + ηg*ηb*ηm

With:

Pm the power consumed by the motor
Pg the power recover by the generator
ηb the efficiency of the belt
ηm the efficiency of the motor
ηg the efficiency of the generator

I don't take in account friction and lost by Joule effect in electric circuit.

For example, with Pm=20W, w=750RPM, ηm=0.6, ηg=0.6, ηb=0.95, R1=3, R2=0.5, wm=4000 RPM, the efficiency is 0.43, without the support the efficiency is 0.34. With a motor with 750 RPM, the efficiency move up to 0.81
If ηm = ηg = ηb = 1, then the efficiency is 1.15 with 4000 RPM and 1.83 with 750 RPM for the motor

[EOW]

With the same support, the small pulley don't turn at 6w but at 4w, so the sum of energy is 0.

[dieter]

Sum of energy is zero? You mean energy gain is zero, llike in efficiency = 1.0 - losses?


BR

[EOW]

Yes, efficiency=1 (I recover that I lost).

I study this new idea. Without the support3, the efficiency is 1. I will calculate all energies for find the global efficiency.

The motor needs -9FRwt
Torque F6/F9 gives 2R*2w*Ft=4FRwt
Torque F5/F8 gives 4R*2w*Ft=8FRwt
The friction (brake from the pulley2 to support3) gives 3FRwt

The sum is +6FRwt