Load on an electric motor vs. current draw

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Discussion Overview

The discussion centers on the relationship between the load on an electric motor and its current draw, exploring concepts from classical physics and experimental observations. It includes theoretical considerations, practical implications, and calculations related to back electromotive force (back EMF) in electric motors.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • One participant notes that classical physics predicts a current draw based on resistance and voltage, but experiments show that actual current draw is significantly lower when the motor is spinning.
  • Another participant explains that back EMF generated by the spinning coils reduces the effective voltage across the motor, leading to lower current draw unless the motor is blocked.
  • A question is raised about calculating the actual current flow by considering back EMF and supply voltage, indicating interest in the mathematical relationship.
  • A formula for calculating back EMF is provided, relating it to applied voltage, current, and motor resistance.

Areas of Agreement / Disagreement

Participants generally agree on the role of back EMF in affecting current draw, but the discussion includes questions about the calculations and implications, indicating some uncertainty in the details.

Contextual Notes

The discussion does not resolve the complexities of calculating back EMF or the implications of varying loads on current draw, leaving some assumptions and dependencies on definitions unaddressed.

Packocrayons
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According to classical physics, if I take an electric motor with a resistance of 1 ohm, and run 10 volts through it, 10 amps of current will run through it. But through experiment, a lot less current will run through the motor, an average brushless motor with a resistance of less than a milliohm will draw only a few amps when spinning, even at high voltages.
At the same time, if I take the same motor, run the same voltage through it, and put a load on the motor, it will draw a lot more current.
Does this have something to do with the presence of permanent magnets near the coils or is there another phenomenon going on here?
 
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Exactly that. The spinning coils inside the magnet will cause a back emf that reduces the effective voltage on the coil. The full current only flows if the motor is blocked. A load will cause slower spinning, less back emf, and more current drawn. It has to be like this, because there is energy transmitted to the load and this energy has to come from the electrical power supply.
 
So if you generated the back-emf voltage, and subtracted it from the supply voltage, you would be able to find the actual current flow?
What's the easiest way to calculate that voltage?
 
Back EMF (DC) = Applied Voltage - (Current through the motor x Resistance of the motor)
 
That is much simpler than I made it, thanks.
 

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