It is often said that stepper motors are inefficient but how inefficient are they compared to a brushed-motor. I could just build something and measure it but it would be nice to know beforehand whether it's a fool's errand.
Say I have a stepper motor. I want to attach it to a pulley and lift a 0.5kg weight up 1.2m. There will be a gearbox - I'm allowed to choose the ratio. I'm allowed to choose the pulley diameter. The speed of the lift can be whatever you like so long as it takes less than one hour. Let's ignore the efficiency of the gearbox for the moment - that can be estimated.
The lift is 0.5 * 1.2 * 9.81 = 5.9 joules. How much electrical energy must be given to the motor?
Here are the spec's of the stepper motor
- Rated voltage: 5 V DC
- Rotor stride angle: 5.625°
- Shaft stride angle: 0.088°
- Number of phases: 4
- Current: 40mA
- In-traction Torque >34.3mN.m(120Hz)
- DC resistance 50?±7%(25?)
- Friction torque: 0.12 N m
- Phase inductance: 0.003 H
- Pull in torque: 0.06 N m
- Frequency: 100 Hz
- Idle In-traction Frequency > 600Hz
- Idle Out-traction Frequency > 1000Hz
- Friction torque 600-1200 gf.cm
- Pull in torque 300 gf.cm
- Coil: Unipolar 5 lead coil
- Model 28BYJ-48
This motor has a 1:64 gearbox. I could buy a motor with a different gearbox. We'll assume I use a ULN2003 driver board and can drive it at whatever frequency I choose. Ignore the efficiency of the ULN2003, it's easy enough to calculate.
Let's do the sums for a brushed motor. A JS-30, 7rpm, 5V motor run off 3 AA cells raises the load in 2.4min and takes around 45mA. Joules equals volts times amps times time so:
- 4.5 * 144 * 0.045 = 29 joules
- 5.9/29 gives 20% efficiency
Given the numbers in the stepper motor datasheet, How many joules would it take?