What it shows
Selecting a motor for a robot or pump needs its torque-speed curve and efficiency. A homemade rope brake loads a small motor with known torque, while voltage, current and rpm are measured to plot the characteristics and show the back EMF effect.
The science
A spinning armature in a magnetic field generates an opposing voltage called back EMF, proportional to speed. Armature current is (supply minus back EMF) divided by armature resistance, so current is highest at stall and lowest at no load. Torque is proportional to current. The rope brake gives torque as (force difference x pulley radius), mechanical output is torque x angular speed, and efficiency is output over input.
Materials
Cost: under Rs 1,500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A 12 V DC motor with a 3 cm pulley on its shaft
- A cord, two kitchen spring balances (0 to 1 kg) or two 100 g weight sets
- A 12 V battery or sealed adapter
- Two multimeters
- An IR tachometer made from a sensor and Arduino, or a phone app tachometer
- A wooden stand with clamps
- A ruler and thermometer
Safety
For this project: Use a battery or sealed adapter only. Keep hair, clothing and fingers away from the pulley. The motor and cord will get hot, so keep each run short. An adult must supervise.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
10 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Fix the motor firmly to the stand and pass the cord over the pulley with a spring balance on each end.
Measure the pulley radius and the armature resistance by gripping the shaft and measuring V and I at a very low voltage of 1 V.
Run the motor unloaded at 12 V and record voltage, current and rpm.
Increase the brake by tightening the cord a little at a time and record both spring balance readings, the current and the rpm.
Calculate torque = (F1 minus F2) x radius x 9.81 for readings in kg.
Continue for about eight load steps up to a current that the motor can safely carry (keep within its rated current) and stop immediately if it smells hot.
Compute back EMF = supply minus I x armature resistance at each point and plot it against rpm.
Calculate mechanical output power, electrical input power and efficiency for each point.
Plot torque against speed (a straight falling line) and efficiency against torque.
Write the report and mark the best operating region of the motor.
What you should see
The torque-speed graph is a falling straight line, the back-EMF graph a rising straight line, and efficiency peaks at about 20 to 50 percent of stall torque, typically at 40 to 70 percent efficiency for small motors.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is back EMF?
A voltage produced by the rotating armature that opposes the supply voltage.
2Why is starting current highest?
At zero speed there is no back EMF, so current is limited only by armature resistance.
3Where is efficiency highest?
At moderate load, not at no-load or stall, where output power is near zero.
4Why do geared motors give more torque?
The gear train trades speed for torque while power stays nearly the same.
Take it further
Repeat at 6 V and 9 V supply and show that the whole line moves down in speed with the same slope.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Robotics
Motors, Gears and Motor Drivers
Make things move: DC motors, servos, steppers and how to drive them
Hardware & repair
Using a Digital Multimeter
Measure voltage, current, resistance and continuity safely