Does this load somehow decrease the resistance of the armature causing it to draw more current? Is it due to the torque the load will give to the motor?
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asked Oct 8, 2017 at 9:09
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\$\begingroup\$ Look up "Back EMF" \$\endgroup\$– Kevin WhiteCommented Oct 8, 2017 at 10:02
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1\$\begingroup\$ The speed is decreased, the BEMF voltage is decreased, too. So you have to feed more current: \$V=I\cdot R +V_{BEMF}\$ \$\endgroup\$– Marko BuršičCommented Oct 8, 2017 at 10:07
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\$\begingroup\$ If you're wondering where "back EMF" comes from, remember the motor is also a generator. If you turned the shaft by hand (or water or wind power) you'd just call it EMF. Slow down, decrease EMF, it cancels out less of the power source, hence more V across the same armature R. \$\endgroup\$– user16324Commented Oct 8, 2017 at 10:44
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\$\begingroup\$ What they said: Motor acts as an alternator and makes voltage to oppose drive voltage. Steady state occurs when Vapplied - Vback_EMF x I motor balances load power. When load is increased motor speed drops so BEMF drops so Vsupply-Vbemf is larger so current and power increase until stability is reached. \$\endgroup\$– Russell McMahon ♦Commented Oct 8, 2017 at 11:12
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1 Answer
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From Wikipedia's Counter-electromotive force.
The term back electromotive force is also commonly used to refer to the voltage that occurs in electric motors where there is relative motion between the armature and the magnetic field produced by the motor's field coils, thus also acting as a generator while running as a motor.
Simple version: When a motor is accelerated from zero speed it also acts as a generator.
This voltage is in series with and opposes the original applied voltage and is called "back-electromotive force" (by Lenz's law).
The generator voltage opposes the driving voltage.
With a lower overall voltage across motor's internal resistance as the motor turns faster, the current flowing into the motor decreases.
The faster the motor spins the less current it draws.
Back to your question:
Why does armature current increase when load on the DC motor increase?
When the load increases the motor slows down. This reduces the counter EMF. This allows a higher current to flow.
Does this load somehow decrease the resistance of the armature causing it to draw more current? Is it due to the torque the load will give to the motor?
No, but it looks like it.
simulate this circuit – Schematic created using CircuitLab
- In Figure 1 (a) the motor is stalled so there is 1 Ω across the 12 V supply and 12 A will flow.
- In (b) the motor is running close to half-speed and the back EMF is 5 V. This means that there is 12 - 5 = 7 V available to drive through the motor's 1 Ω resistance so current falls to 7 A.
- In (c) the motor is running close to full speed and the back EMF is 10 V. That means there is only 2 V available and current falls to 2 A.
answered Oct 8, 2017 at 15:13
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\$\begingroup\$ does the armature current increase if I increase the supply voltage? I'm kind of confused here because a supply voltage increase would cause an increase in rpm which would cause an increase in back emf. An increase in back emf would then lead to a decrease in armature current. However the back emf would've never increased if the speed wasn't increased. And the speed was increased because the supply voltage was increased. So could you please explain this to me? \$\endgroup\$ Commented Dec 11, 2019 at 7:30
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\$\begingroup\$ Re-run my calculations with BAT1 = 24 V. What happens the current and the back-EMF? How fast (approximately, relative to the 12 V situation) would the motor have to go to reduce the current to 2 A again? \$\endgroup\$ Commented Dec 11, 2019 at 10:58