Why do more magnets affect how long a homopolar motor spins?

[Julie]

Hi All,

My daughter did a science experiment on homopolar motors. The only variable was the number of magnets on each motor. We found out through the experiment that the more magnets we attached to the motor, the longer it spun before the battery died. I'm assuming it has to do with the magnetic force causing it to spin more efficiently. I've found all sorts of info on why the motor spins but nothing to explain our findings. Can anyone help or point us in the right direction? Thanks so much!

 

[Dale]

I would guess that the resistance is increased leading to a lower current and therefore a slower discharge.

 

[Julie]

I would guess that the resistance is increased leading to a lower current and therefore a slower discharge.

Thank you Dale!

 

[sophiecentaur]

Back emf would be greater with more magnets?

 

[CWatters]

+1

When you connect a battery to the motor it typically accelerates until the back emf is equal to the battery voltage. The stronger the magnetic field the greater the back emf at a given rpm. So increasing the field reduces the rpm. Reduced rpm reduces the output power. Reduced output power means reduced input power and longer battery run time.

Aside: Some of this is counter intuitive. You might expect stronger magnets to make a more powerful motor but its not quite that simple. Normally if you design a motor you want to achieve a specific rpm/volt so when increasing the strength of the magnets you would normally also reduce the number of turns to keep the rpm/volt the same. Reducing the number of turns reduces the winding resistance and hence reduces I^2R losses making the motor more efficient. That in turn makes the motor more powerful for a given input power. Or it reduces the input power for a given output power.

 

[Julie]

Back emf would be greater with more magnets?

Thank you!

 

[Julie]

+1

When you connect a battery to the motor it typically accelerates until the back emf is equal to the battery voltage. The stronger the magnetic field the greater the back emf at a given rpm. So increasing the field reduces the rpm. Reduced rpm reduces the output power. Reduced output power means reduced input power and longer battery run time.

Aside: Some of this is counter intuitive. You might expect stronger magnets to make a more powerful motor but its not quite that simple. Normally if you design a motor you want to achieve a specific rpm/volt so when increasing the strength of the magnets you would normally also reduce the number of turns to keep the rpm/volt the same. Reducing the number of turns reduces the winding resistance and hence reduces I^2R losses making the motor more efficient. That in turn makes the motor more powerful for a given input power. Or it reduces the input power for a given output power.

Thank you so much for the explanation!