Why does a DC motor have a restricted speed?

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Discussion Overview

The discussion centers on the factors that limit the speed of a DC motor, exploring various theories and concepts related to back EMF, structural integrity, and the relationship between motor operation and generator behavior. Participants examine both theoretical and practical aspects of motor speed limitations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that structural integrity and vibrations due to centripetal acceleration are primary limits to motor speed.
  • Others argue that back EMF is a significant limiting factor, as it increases with speed, reducing current and torque until friction losses balance the motor speed.
  • A participant notes that a DC motor operates similarly to a DC generator, suggesting that the maximum speed correlates with the voltage generated when the motor is turned.
  • Concerns are raised about the ability to inject current into the coil at high speeds, with some suggesting that the inductive properties of the coil complicate this process.
  • There is a discussion about whether it is possible to attain an EMF greater than the supply voltage, with some participants expressing skepticism about this idea.
  • One participant mentions that mechanically spinning the motor faster than normal could allow it to generate a higher voltage than usual.
  • Another participant introduces the concept of using a step-up transformer as an alternative to achieve higher voltages.
  • Some participants challenge earlier claims about back EMF and its effects on different types of motors, indicating a lack of consensus on the applicability of certain principles across motor types.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the factors that limit DC motor speed, particularly concerning the roles of back EMF, structural integrity, and operational behavior as a generator. The discussion remains unresolved with no clear consensus on the primary limiting factors.

Contextual Notes

Participants highlight the complexity of the interactions between voltage, current, and motor speed, with unresolved questions about the assumptions made regarding inductance and the timing of current application. The discussion also reflects varying levels of expertise among participants, which may influence the interpretations of motor behavior.

  • #31
leafy said:
Unload motor require current to overcome friction. Loaded motor require current for the load. We assumed ideal, no friction, so the only requirement is current for the load.
Even without friction there are still electromagnetic losses, but OK let’s imagine all of that is somehow recaptured. So now you have a motor that you rev up and then just let it spin. If there is no current through it then it does nothing but spin. What is the point of having it there?
 
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  • #32
Dale said:
Even without friction there are still electromagnetic losses, but OK let’s imagine all of that is somehow recaptured. So now you have a motor that you rev up and then just let it spin. If there is no current through it then it does nothing but spin. What is the point of having it there?
But there is current through it. Each time the switch close, current flow to speed up the wheel. Each time current flow to the 24V, curren flow slow down the wheel. Each cycle, there is a speed up and slow down. The net is zero current with is steady state.
 
  • #33
I know that you think, or want, this problem to be about motors. But, you are missing some fundamental understanding about energy storage and transfer with magnetics. I strongly suggest you set aside the motor part and study a boost converter. It is the simplest circuit configuration that does this sort of thing. Understand the voltage and current waveforms in this simpler case first. Then move on to a DC motor, if you want.
 
  • #34
I think solid state is harder.
 
  • #35
leafy said:
But there is current through it.
If there is current through the motor then the current through the output is not equal to the current through the input. Also, if you are switching back and forth there is no reason that the “forth” current would equal the “back” current.
 
  • #36
Net current = 0
 
  • #37
leafy said:
Net current = 0
What is the “net current”?
 
  • #38
leafy said:
Hm... you might have read the diode wrong.
Well that's a bit of egg on my face then. That's what I get for not brushing up on my EE101. My apologies.
 
  • #39
leafy said:
Unload motor require current to overcome friction. Loaded motor require current for the load. We assumed ideal, no friction, so the only requirement is current for the load.
The portion of rotation that charges the batteries will put a mechanical load on the motor, slowing it down. The battery will need to supply current after this to speed it back up. You have created a (bad) motor generator, which is not new as has been discussed.
It is much more difficult than you imagine to create new stuff
Linus Pauling was once asked how he had so many good ideas. He said "I just have a lot of ideas and I get rid of the bad ones" One needs to be able to do both of these efficiently to be successful. With respect, you need some more work on the second part.

/
 
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  • #40
leafy said:
But there is current through it. Each time the switch close, current flow to speed up the wheel. Each time current flow to the 24V, curren flow slow down the wheel. Each cycle, there is a speed up and slow down. The net is zero current with is steady state.
Thread is closed temporarily for Moderation...
 
  • #41
leafy said:
Each time the switch close, current flow to speed up the wheel. Each time current flow to the 24V, curren flow slow down the wheel. Each cycle, there is a speed up and slow down. The net is zero current with is steady state.
I believe that by “net is zero current” you mean that the current speeding up the motor with the 12 V supply is the same as the current slowing down the motor with the 24 V recharging. That is simply false, there is nothing that would require it from a circuits standpoint and, as you noticed, the conservation of energy forbids it.

As perpetual motion machines are not permitted to be discussed here, this thread will remain closed.
 
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