Science 7–10 · Year 9
Efficiency of an electric motor lifting a load
Physical sciences — Energy, content group Law of conservation of energy (NSW Stage 5 focus area)
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The idea
Only part of the electrical energy supplied to a motor becomes gravitational potential energy of the load; the rest becomes thermal energy in the windings and bearings, and the ratio is the efficiency.
What you need
- small geared DC motor, 3 to 6 V, whose output shaft turns at about 200 rpm at 3 V, with a 20 mm diameter cotton reel or pulley on that shaft, mounted on a clamp, 1
- thread 1.5 m, 1
- 50 g, 100 g and 200 g masses with hooks, 1 each
- DC power supply 0 to 6 V, 1
- digital multimeters as ammeter and voltmeter, 2
- switch and 4 mm leads, 1 set
- stopwatch, 1
- metre rule, 1
How to do it
- Mount the motor at the bench edge with the thread wound on its reel and the 100 g mass hanging over the edge. Connect the supply, switch, ammeter and motor in series with the voltmeter across the motor.
- Set the supply to 3.0 V. Mark a 0.50 m lift on the rule.
- Close the switch and start the stopwatch together; stop both when the mass has risen 0.50 m. Record the average current and voltage during the lift and the time. Three trials.
- Calculate energy in = V x I x t and useful energy out = m x g x h; efficiency = out / in x 100 per cent.
- Repeat with 200 g and then 50 g and compare efficiencies.
- Touch the motor case after a minute of running and note where the missing energy went.
What you should see
For 0.100 kg lifted 0.50 m the useful energy is 0.49 J. A 20 mm reel turning at 190 to 200 rpm winds the thread in at about 0.20 m/s, so the 0.50 m lift takes about 2.5 s; an ungeared hobby motor runs at thousands of rpm at 3.0 V and cannot hold that lift speed, and the 9.8 mN m this load asks of a 10 mm reel radius is at or beyond the stall torque of many small motors. If the motor draws 0.4 A at 3.0 V for 2.5 s it takes in 3.00 J, so the efficiency is 16 per cent. Efficiency changes with load: it is low for a very light load, rises as the load increases, and falls toward zero as the load approaches the point where the motor stalls. The case is warm after a minute of running. The learner knows it worked when energy out is always less than energy in.
What changes
- What you change
- load mass (kg) or supply voltage (V)
- What you measure
- efficiency (%)
- What you keep the same
- same motor and reel
- same 0.50 m lift
- meters read at the same moment mid-lift
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- A motor converts all the electrical energy into movement.
- Efficiency is a fixed property of a motor whatever the load.
- The lost energy is destroyed.
Safety card
Hazards
- falling mass if the thread slips
- hot motor case
- thread wrapping around fingers
Controls
- keep hands clear of the reel while running
- clamp the motor firmly
- limit runs to 30 seconds
Note
Heat or electrical energy is involved. Complete the school's risk assessment for the activity before the lesson, using CSIS 1.7 (Risk assessment – a pre-requisite for risk control) from the department's Chemical Safety in Schools package (2021 Technical Update), which the NSW Department of Education Science safety and compliance page names for risk assessment advice.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC5-EGY-01SC5-WS-01SC5-WS-05SC5-WS-06
- Australian Curriculum v9AC9S9U05AC9S9I03AC9S9I04AC9S9I06
Sources
The pages the author read to write this activity.