Motor resistance value isn't correct

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Discussion Overview

The discussion revolves around the unexpected resistance readings from an electric motor, specifically the discrepancy between calculated resistance based on current draw and the resistance measured with an ohmmeter. Participants explore concepts related to motor behavior, including back EMF and the implications of motor stalling.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant notes that the motor draws 0.12 A from a 2.76 V source, suggesting a resistance of 16.23 Ohms, but the ohmmeter reads 1.2 Ohms, which seems contradictory.
  • Another participant emphasizes that a motor is not a simple resistor and does not adhere to Ohm's law in the same way, introducing the concept of back EMF.
  • It is mentioned that the motor must be running to draw only 0.12 A at 2.76 V, and that stalling the motor could lead to much higher current draw.
  • One participant speculates that the brief spike in resistance reading could be due to improper contact of the probes on the motor terminals.
  • Another participant reiterates the calculated resistance and discusses the implications of back EMF, suggesting that if the 1.2 Ohm reading were accurate, the motor would draw an implausible 2.3 A when stalled.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of the resistance readings and the behavior of the motor, with no consensus reached on the underlying reasons for the discrepancies in measurements.

Contextual Notes

Participants reference concepts such as back EMF and the behavior of motors under different conditions, but the discussion does not resolve the specific reasons for the conflicting resistance measurements.

Flyingwing12
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Motor resistance value isn't correct...

I have an electric motor that registers a .12 A draw off of a 2.76V source. This obviously indicates to you and me a resistance of 16.23 Ohms.

The problem is my Ohmmeter function on the MM. It reads out 1.2 between the motor leads, and this arithmetically can't be so.

There was a split second when the ohmmeter spiked to about 16.3 Ohms, but only for a split second, and then fell back to 1.2 Ohms.

Any advice to as why the meter is showing this impossible reading? ( OK, not impossible, but you get the idea.)
 
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A motor is not a resistor and therefore it does not follow the V = RI.

Google "back emf"
 


The motor needs to be running for it to take only 0.12A at 2.76V. When it is stalled, it can pass much more current (and can burn out).
 


uart said:
A motor is not a resistor and therefore it does not follow the V = RI.

Google "back emf"

Ohhh. I see. So the reading I am getting is commutator touching the coils of wire?

I am a spring chicken when it comes to electronics. I am currently taking an AC/DC course at my technical school. So excuse my Newbie sense of direction.
 


I have an electric motor that registers a .12 A draw off of a 2.76V source. This obviously indicates to you and me a resistance of 16.23 Ohms.

I get 23 ohms for this.

Anyway, the motor generates "back EMF" when it is turning and this opposes the supply voltage to reduce the supply current.

If the figure of 1.2 ohms was correct, the motor would draw 2.3 amps when it was not turning. ( 2.76 volts / 1.2 ohms = 2.3 amps). This is why it is not a good idea to stall an electric motor.
 


Hmm, I think the reason the reading spiked to 16.3 ohm for a brief period is your probes were not touching the terminals properly. It may be caused by a jerk while taking reading.
 


vk6kro said:
I have an electric motor that registers a .12 A draw off of a 2.76V source. This obviously indicates to you and me a resistance of 16.23 Ohms.

I get 23 ohms for this.

Anyway, the motor generates "back EMF" when it is turning and this opposes the supply voltage to reduce the supply current.

If the figure of 1.2 ohms was correct, the motor would draw 2.3 amps when it was not turning. ( 2.76 volts / 1.2 ohms = 2.3 amps). This is why it is not a good idea to stall an electric motor.

I meant a current of .170 A
 

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