Relationship between mechanical friction and electrical resistance in a motor

[jasonbe]

Why is it that when mechanical friction is applied to the rotating part of a motor, the motor offers less electrical resistance?

 

[microtexan]

A little more of an explanation please. How are you determining the electrical resistance of a rotating motor?

 

[blueroomelectronics]

Do you mean why does it draw more current if you apply friction to a running motor?

 

[dknguyen]

First of all, think about why a motor does not short-circuit when you apply a voltage to it. It's resistance is very VERY low and you are applying a voltage across it. Why does it not blow up? (In fact it does if the motor is spinning too slowly and the voltage supply is too high. Worst case for this is when the motor is stalled and not rotating.).

Look at the image attached. It is the internal model of a simple DC motor.

R is the internal resistance you are talking about. V is the voltage supply. X is the winding inductance of the motor. But for your question, we are interested in E. E is the generator voltage (the back EMF) of the motor. E has a polarity that opposes V, and E is higher as the motor spins faster. So you see, the resistance does not become less, but more current flows through a motor spinning more slowly (given the same voltage supply) because a greater voltage appears across the internal resistance.

THe voltage drop across E, X, and R must equal the voltage supply voltage. As the motor spins more slowly, E becomes less and so more voltage must be dropped by X and R. Since X is inductance and only drops the voltage for non-DC frequencies, we'll just talk about R since it is a DC motor (technical details in the motor commutating and switching current direction are hidden in E). So more and more voltage must be dropped across R as the motor slows down because E is reduced and the result is more current flows through R (for a fixed V). At stall (or whenever the motor is spinning slow enough so E is too small), the current flow through the motor approaches I=V/R and you get almost short-circuit.

So instead of saying "applying friction to the motor shaft" you should actually say "when force is applied to the motor shaft to slow it down below no-load/free running speed).

 

[user_88]

Sometimes a motor manufacturer will provide a specification called the locked rotor torque, or stall torque. This is the mechanical holding torque which will reduce the shaft rotation to zero velocity, when the maximum permissible current is applied to the motor circuit.

 

[jasonbe]

I may not have been very clear that I had a resistance meter hooked up to both of the two conductors on the motor. I may have measured amps, which I imagine would be greater. By saying applying mechanical friction I was trying to describe what might be happening when I pressed an object against the shaft, or, as dknguyen said, when I applied a force to the motor shaft to slow it down below no-load/free running speed. dknguyen ‘s response pretty much answered my question. I’m just not that familiar with relations of speed, magnetic fields, and coils. If applying force to a motor generates current, by applying a greater force to the motor shaft – to stop the motor shaft, near the time when the shaft stopped moving, would I be I decreasing the current to a level less than that originally provided by the battery?*

*insert footnote

 

[dknguyen]

dknguyen ‘s response pretty much answered my question. I’m just not that familiar with relations of speed, magnetic fields, and coils. If applying force to a motor generates current, by applying a greater force to the motor shaft – to stop the motor shaft, near the time when the shaft stopped moving, would I be I decreasing the current to a level less than that originally provided by the battery?*

*insert footnote

If you spin the motor in the direction that the voltage supply it is trying to spin it in, then yes, you will assist it (in fact, you will speed it up) producing even MORE back EMF voltage, reducing the voltage that must be dropped across the internal resistance (in fact, it will start acting as a generator and produce current and attempt to back-charge the voltage supply).

If you try to spin the motor in the opposite direction, you will decrease the BEMF and thus it will oppose the voltage supply less causing greater voltage drop across the internal resistance and so theand the current will increase. Applying friction, by definition, would be similar to applying a force trying to spin the motor in the opposite direction.

So it's a matter of what kind of force you are appling to the motor shaft- with the voltage supply or against.

 

[Ubergeek63]

Why is it that when mechanical friction is applied to the rotating part of a motor, the motor offers less electrical resistance?

the resistance never changes, use an ohm meter to measure it.

as you load a motor the current increases to compensate. as the current increases, so does the voltage across the resistance of the armature. as thge speed increases, so does the voltage across the armature winding. And yes, as illogical as it may seem, it is accurate to separate the electrical (resistance) from the electro-mechanical (motor/generator) portions of the motor in this manner. It is, however, still a bit oversimplified if you are doing actual motor control electronics.

Dan

 

[dknguyen]

If he uses a DMM to measure the motor resistance as it's running it might indicate the resistance is changing because the internal Back EMF voltage source is distorting the voltage that the meter is using to measure resistance.

 

[Ubergeek63]

If he uses a DMM to measure the motor resistance as it's running it might indicate the resistance is changing because the internal Back EMF voltage source is distorting the voltage that the meter is using to measure resistance.

Actually i meant when it was disconnected from the circuit. The resistance is a basic motor constant, until you fry the brushes at least.

On the other hand, if he is trying to measure the resistance with an ohm meter while it is running it is possible to fry the meter if it is a high power motor.

Dan

 

[jasonbe]

What types of detection mechanisms use magnetic generators?

 

[blueroomelectronics]

Is this for your gameboard?

 

[jasonbe]

Is this for your gameboard?

I wasn’t thinking that it would be. Have you thought of a way of incorporating the idea?