Load on an electric motor vs. current draw

In summary, according to classical physics, an electric motor with a resistance of 1 ohm running at 10 volts should have 10 amps of current. However, in experiment, a brushless motor with a resistance of less than a milliohm draws only a few amps even at high voltages. This is due to back emf created by the spinning coils inside the permanent magnets, which reduces the effective voltage on the coil. Putting a load on the motor will cause slower spinning, less back emf, and more current drawn. To calculate the back-emf voltage, one can use the formula Back EMF (DC) = Applied Voltage - (Current through the motor x Resistance of the motor).
  • #1
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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  • #2
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.
 
  • #3
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?
 
  • #4
Back EMF (DC) = Applied Voltage - (Current through the motor x Resistance of the motor)
 
  • #5
That is much simpler than I made it, thanks.
 

Related to Load on an electric motor vs. current draw

1. What is the relationship between load on an electric motor and its current draw?

The load on an electric motor and its current draw are directly proportional. This means that as the load on the motor increases, the current draw also increases.

2. What factors affect the load on an electric motor?

The load on an electric motor can be affected by a variety of factors, including the type of load (e.g. constant or variable), the speed of the motor, the efficiency of the motor, and the environmental conditions (e.g. temperature).

3. How does the current draw of an electric motor affect its performance?

The current draw of an electric motor is a measure of the amount of electricity required to power the motor. It directly affects the performance of the motor, as a higher current draw can lead to increased energy consumption, heat generation, and potential damage to the motor.

4. What is the difference between load on an electric motor and torque?

Load on an electric motor refers to the external force or resistance that the motor must overcome to perform its intended function. Torque, on the other hand, is a measure of the motor's rotational force. While load affects the current draw of the motor, torque affects its speed and power.

5. How can I calculate the load and current draw of an electric motor?

The load and current draw of an electric motor can be calculated using the formula: Load = Torque x Speed / Motor Efficiency and Current Draw = Load / Supply Voltage. These calculations will vary depending on the specific motor and its operating conditions.

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