Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Comparing the powers of electric motors 2

Demonstration

Comparing the output power of two motors.

 

Apparatus and materials

Small electric motors of different powers, 2

Line shaft unit

Knife switch

Mass, 1 kg

Power supply, LV

Rubber band or driving belt

Cord

Stopwatch & metre rule

Health & Safety and Technical notes


The person controlling the switch must stop the motor before the load reaches the line shaft. (If the load spins round the shaft, the string may break and the load go flying.)
 
In all activities where loads may fall on toes, precautions (such as using cardboard boxes full of waste material) should be taken.

The motors suggested are part of the Malvern Energy Conversion Kit, now obtainable from Beecroft & Partners Ltd


Procedure


a Clamp the two electric motors to the bench on either side of the line shaft unit, so that the driving belt can be connected to either of them. The line shaft is used for lifting the load of 1 kg.

b Link one of the motors to the line shaft and connect it to the power supply.
 
c Measure the time taken to raise the load from the floor to the bench.
 
d Repeat for the second motor.

Small electric motors

Teaching notes


1 The same amount of work has to be done in both cases. The same amount of energy will be transferred from the power supply to gravitational potential energy in the raised weight, but the faster motor is the more powerful.

Labels on the motors may indicate the horse-power or wattage. The motor does not always supply this power, (the maximum recommended). Instead the motor adjusts itself to the load. If more power than that is demanded the motor will overheat and perhaps burn out. Its efficiency is also likely to be lower.
 
2 It may be worth indicating that animals, including humans, adjust to the load as well; watch how slowly someone staggers when they have to lift something heavy. Safety rules try to prevent people hurting themselves. (Injuries to backs may not manifest themselves for many years.) 

This experiment was safety-checked in January 2006

 

Related guidance


Power and energy

 

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Comparing the powers of electric motors 1