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DC motor undervoltage (continued)

Tha fios agaibh

Aug 11, 2014
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Chris got me thinking about motors in another post
Chris;
We are talking about AC loads here driven by a transformer so the DC point is sort of moot.

Since you brought it up, ...I've always consider all motors it to be more inductive than resistive for the simple fact that the motor windings create magnetic fields where energy is stored.

Wouldn't a DC motor still draw more current (creating heat) trying to drive/maintain the same load at a lower voltage?
John
I agree that a DC motor will draw less current with less voltage especially without a significant load on it.
But if the voltage and current go down, so does the power and the amount of torque developed.
If the torque demanded of the motor remains the same, then wouldn't the current increase when the voltage drops?
 

Martaine2005

May 12, 2015
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Hi John,
That should be fairly easy to reproduce.
My 12v DC Minicraft tools have speed control. Putting the tool under load whilst reducing the voltage increases the current as the motor stalls.
Same applies to my AC power tools. Have the speed at full and cutting / loading the tool whilst decreasing the speed, really loads up the motor and will stall if I left it. And the current shot up.
I didn't leave it like that for long for obvious reasons..
Same thing with my 18v tools.

Martin
 

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
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I'm not worth a crap in the mornings so I'm just lurking for now. Thus far I don't see anything arguable but after reading some of what I posted in the other thread I fully expect rebuttals. Some of the rebuttals will probably be posted by myself! :)

Chris
 

Minder

Apr 24, 2015
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If the voltage is not reduced then the torque (current) goes up depending on load, and can eventually exceed the Continuous Torque rating of the motor, once you exceed this then the peak torque range is entered and if not limited in time or the load reduced, it usually results in a burnt motor.
So the two options are reduce voltage resulting in reduced torque or reduce the load.
If the motor load keeps the range in the continuous torque range, then it will continue operating without visible problems.
The maximum torque of a DC motor is usually at zero RPM and will stay fairly flat or gradually drop over the RPM range.
M.
 

Tha fios agaibh

Aug 11, 2014
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If the voltage is not reduced then the torque (current) goes up depending on load
So if voltage goes down with the same load applied, the current goes up. Right?

I realize something like a DC fan motor without much of a load would draw less current because less power would be used.

Putting the tool under load whilst reducing the voltage increases the current as the motor stalls.

Thanks Martin, that's as I expected. Obviously a stalled motor would do so but I'm talking about what a constant heavy load would do.

Chris; I'm not looking for a rebuttal, rather a clarification of the subject of running motors undervoltage.
Another point of contention you seemed to have, was my statement (quoted in post #1) about a motor being more inductive than resistive.
You believe it to be more resistive than inductive?

John
 

Martaine2005

May 12, 2015
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LOL,
Chris, where's ya gloves, where's ya gloves? (Chelsea Tune).
I think I know the answer! (that's a first).
I deleted it though. Because I am more than likely wrong!!
:(

EDIT:. The real reason is because he wants you! No third party invited...:D
Martin
 
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Minder

Apr 24, 2015
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So if voltage goes down with the same load applied, the current goes up. Right?

John
If the voltage is reduced for a given load then yes the current can go up.
Consider what limits the current to the motor, example, in the case of a 100vdc motor, if 100 v is applied immediately, the current is limited by the armature resistance, which can be a few ohms, the resultant high current (torque) cause rapid acceleration, the motor then also becomes a generator and creates a voltage which opposes the applied voltage, this generated voltage increase with rpm until the rated rpm of the motor at 100 vdc is reached, assuming an off-loaded motor, the opposing voltage is almost equal to the applied voltage, at which point the current is minimal.
Any load on the motor causes a reduction in rpm, which results in a lower generated voltage and hence increases the current.
Torque increases at a cost of rpm.
M.
 

Tha fios agaibh

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Thanks Minder, your "generated voltage"
Explanation of EMF makes it easy to understand.
 

CDRIVE

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Thanks Minder, your "generated voltage"
Explanation of EMF makes it easy to understand.
OK, it's mea culpa time. In the thread that spawned this topic I made the bold statement that DC brushed motors are predominantly resitive but my tests prove otherwise.

I was too damn lazy to dig out a small motor and do bench tests for proof, so I attempted to spice it. Tina is my spice of choice but their DC motor model leaves much to be desired. It's modeled akin to a resistor. This would tell me damn near nothing. At that point I fired up my copy of Picaxe VSM, which is an (interactive mode only) version of ISIS. Their DC motor model is far more robust than Tina's. Here's an image of the test circuit where I set the Load Max/Torque (%) property to 100% to stall the motor.
upload_2015-10-31_9-19-25.png
This next image was snapped after resetting the Load Max/Torque (%) to 50% and waiting for the motor to reach max RPM.
upload_2015-10-31_9-28-20.png
The result indicate that when stalled or immediately when starting the power source sees a purely resistive load. As the motor RPM increases the motor produces back EMF do to generator action. When it reaches full RPM the motor's load current is almost exactly halved.

I should also note that the term Back EMF is not related to the back emf that's experienced when breaking a circuit to an inductor. It's the same polarity as the supply voltage of the motor. The generator action of the motor produces reduction in input current much like a battery as it's charged.

Here's an image of the motor properties..
upload_2015-10-31_9-52-34.png

And here's an explanation of how it's modeled..
upload_2015-10-31_9-54-4.png
upload_2015-10-31_9-55-27.png
While playing with this model I messed with the Coil Inductance property but I haven't concluded how or what results change as a result. I think this is because Picaxe VSM only runs in live interactive mode. If I had the full blown (EXPENSIVE) version of ISIS/Proteus I would be able to generate transient plots and see much more.

Cheers,
Chris
 
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