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Unread 11-16-2002, 03:59 PM
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What happens / why do motors stall?

I've been reading some of the posts in this board. Quite a few mentioned that when motors stall, they draw much more current and heat up. At some point, I realized i don't really understand why this happens. So long story short, can someone explain the physics behind why motors stall and then draw a lot more current?

I tried searching, but as I said, many posts talk about motors stalling and drawing current, but after skimming just the first page of results, I didn't really find an explanation of the physics behind this.
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Unread 11-16-2002, 04:43 PM
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An easy way to think about a motor is as two parts in series: a back EMF and a winding resistance. The winding resistance is what you read on an ohmmeter if you put it across the motor leads - for something like the FP motors, it's very low, around 0.1 Ohms or so, and essentially doesn't change with respect to the motor's velocity. The other part of the model, the back EMF, is basically just another voltage source in series with the resistor. It is proportional to the speed of the motor, so at no load, when the motor is hauling, the back EMF is very close to 12V, but at stall, the back EMF drops to 0V.

This means that at no load, the current through the motor is equal to (12V supply - almost 12V back EMF)/ 0.1 Ohms

or, in other words, not very much, because the back EMF is almost equal to the supply voltage.

But, at stall, the back EMF drops to 0, so the 12V from the battery is sunk completely across the motor winding resistance, which gives a current on the order of 120 Amps.


That's how the math goes, but as for the physical origin of the back EMF, that comes from the spinning motor wanting to act like a generator. When you drive a motor, either electrically or by spinning it with your hands, it causes a coil of wire to move through a magnetic field. Whenever that happens, it causes a current in the wire, which manifests itself as back EMF. The winding resistance, on the other hand, simply comes from the resistance of the wire in the coils itself.
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Unread 11-17-2002, 12:32 AM
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Alright, that explains it. Now that I've thought about it (as well as read up on a few HowStuffWorks articles ), I understand it.

Only thing I don't get is how does the back EMF cancel out the incoming voltage. I get how the generator-effect of the motor produces it, but I guess I'm not exactly sure on what it is and where it goes electically.
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Unread 11-17-2002, 09:18 AM
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Quote:
I tried searching, but as I said, many posts talk about motors stalling and drawing current, but after skimming just the first page of results, I didn't really find an explanation of the physics behind this.
It gets quite confusing especially if you haven't taken physics yet. There are formulas that also help explain this and are useful unfournatly I don't have the book that I got the from on me.
Quote:
I get how the generator-effect of the motor produces it, but I guess I'm not exactly sure on what it is and where it goes electically.
Ahh It goes back into the wires. Where else would it go?? Get a voltmeter and attach it to a motor. Turn the motor's shaft by hand. You should get a voltage reading.
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Unread 11-17-2002, 09:38 AM
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Quote:
Originally posted by wysiswyg

It gets quite confusing especially if you haven't taken physics yet. There are formulas that also help explain this and are useful unfournatly I don't have the book that I got the from on me.
Aye, I'm taking ap physics right now - we just didn't get into the electricity unit yet.

Quote:
Originally posted by wysiswyg
Ahh It goes back into the wires. Where else would it go?? Get a voltmeter and attach it to a motor. Turn the motor's shaft by hand. You should get a voltage reading.
Yeah, I understand that it goes back into the wites, but when it was said the current is (12V supply - back EMF)/ 0.1 Ohms, how does the back EMF cancel out the 12V power supply? Is the back EMF traveling the opposite way of the incoming voltage? If so, I thought that electricity flows towards ground, and so wouldn't the back EMF be not traveling towards ground? Basically I don't understand how the back EMF flows.
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Unread 11-17-2002, 01:20 PM
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At stall, a motor has no back EMF... The back EMF is caused by the turning of the rotor. This is exactly why it does heat up so much, because when there is no back EMF, the current is only limited by the internal restance of the wires. In other words, the motor is simply a resistor when stalled. Again the back EMF is caused by the rotating rotor. When the rotor spins fast enough that the induced back EMF is equal to the forward EMF, the motor has reached it's "no load speed" or "free speed".

In stall, the amount of heat generated is the power applied to the motor (voltage * current) since ALL this power is wasted since the mechanical power produced is zero (torque * ang. velocity = 0).
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Unread 11-17-2002, 05:04 PM
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Thats nice but would someone show the formulas. That should help alot.
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Unread 11-17-2002, 05:26 PM
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Quote:
Originally posted by wysiswyg
Thats nice but would someone show the formulas. That should help alot.
745.7 watts per horsepower, just for reference (conversions).

P (power) = I (current) * V (voltage)

Ohm's Law:
V (voltage) = I (current) * R (resistance)

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Unread 11-17-2002, 05:31 PM
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There is more than that. :edit:
I found them oo well it's a bit disjointed. Im quoting it.
Quote:
It turns electrical current into mechanical torque at the output shaft. Torque = current times Kt, the torque constant. You can also model a motor as a resistor, plus a generator. The generator voltage, or 'back emf' is related to the speed - the maximum or no load speed occurs when the back emf is equal to the applied voltage (minus a little for friction). So this can be described as Back emf = rpm x Kv, the speed constant - or, max speed = Voltage x Kv. If Kt is in units of in-oz/amp, and Kv is rpm/v, then Kt x Kv = 1352
Of course if you really want to be confused read my topic about the fisher price motors. Someone posted so many equations about motors it will make your head spin.
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Unread 11-17-2002, 05:58 PM
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Thanks for all this, but a lot of it I got from the "Power, speed, and torque... AGH" thread. My question still stands - exactly why does the back EMF sink the input voltage? Is EMF a voltage that travels the opposite direction the incoming voltage travels, or is it just a force that reduces the pressure of the incoming voltage? If the latter, then I don't see the relationship between this force and the voltage. I basically need some help on the physics of the EMF.
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Unread 11-17-2002, 06:18 PM
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I think that you're problems come from a misunderstanding voltage. Voltage doesn't flow. It's best to think of voltage as a potential. Current flows from higher potential to lower potential. Voltages can be though of as either positive or negative. It all depends on which lead you treat as the positive side (this is the polarity; if you switch which lead you consider positive, it's equivalent to multiplying the voltage by -1).

The other important thing to remember is Kirchoff's Voltage Law which says that in a complete loop, the sum of the voltages will be zero. In this case, you add the battery power supply of 12 V, the voltage drop due to the internal resistance of the motor, and the voltage drop due to the back EMF. Or:
Vs - Vr - Vemf = 0

Now, Vs is constant at 12 V. Vemf is dependent on the speed of the motor. Vr is dependent on the current, I, and the resistance, R. R is constant. And Vr = I * R.
Through this, you can see that as the back EMF increases, Vr will decrease and therefore I will decrease. As EMF decreases, Vr will increase and therefore I will increase. Because the back EMF is directly proportional to the speed of the motor, the current is inversely proportional to the speed of the motor. Hence, at stall, the current through the motor is very high.

This relates to power through P= V * I. It's important to take the power across the entire system and not just the back EMF or the resistance. In this case, V is constant at 12 V. Therefore, power is dependent on the current. This means that as the speed of the motor decreases, power increases. Or, the power is inversely proportional to the speed of the motor.

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Unread 11-17-2002, 06:23 PM
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Of course I tend to not think about why but just understands that it happens. But now that I think of it I'm getting a nagging feeling that I'll have to know this stuff for ap physics.
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Unread 11-17-2002, 06:52 PM
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Quote:
Originally posted by SuperDanman
Thanks for all this, but a lot of it I got from the "Power, speed, and torque... AGH" thread. My question still stands - exactly why does the back EMF sink the input voltage? Is EMF a voltage that travels the opposite direction the incoming voltage travels, or is it just a force that reduces the pressure of the incoming voltage? If the latter, then I don't see the relationship between this force and the voltage. I basically need some help on the physics of the EMF.
The easiest way for me to think of it is as follows:

Current (I) = Voltage(V)/Resistance(R)

using this formual, we can get the following:

I = (Vbattery - Kv*Ang)/(Rbattery + Rmotor)

where:
I = current
Vbattery = battery voltage
Kv = motor's velocity constant - (rad/s)/V
Ang= motor's angular-velocity (rad/s)
Rbattery = battery's internal resistance + wiring or speed controller resistance
Rmotor = motor's armature resistance

from this, we can see that since the battery voltage will be 12V, once the RPM reaches a certain speed the top part of the equation, the (Vbattery - Kv*Ang) will be negative. This will make the current negative. the sign of the current tells which way the current is flowing. If the current is negative it means the motor is generating more voltage than the battery, which causes the current flow to reverse direction and the battery will start to be charged, instead of drained.


Thats the only way i understand it, the EMF stuff was jibberish to me too

Tom
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Unread 11-17-2002, 07:03 PM
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Alright, Matt explained the part of this whole thing I didn't get. Thankies. (That's my one downfall - when I don't understand something, I keep on bugging people until someone explains it ). And Tom, thanks for your basically-sum-it-up formula.
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Unread 11-18-2002, 01:45 PM
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Alright,
There has been a lot of discussion so far as to what happens when the motor stalls and why current is so high and much of it is true but...
As with any system there are a lot of variables, but at any one point in time, one of the variables maximizes and takes over above all else. When the motor is turning, the commutator is moving across the brushes such that the load is constantly changing, the windings in which current is flowing is changing and the heat developed is spread across these windings.(even the motors in the servos have three windings.) When the motor is at rest,(i.e. no current flowing) at least two windings are in contact with the brushes so that the motor can get started. (Open one up and look at the motor construction.) When the motor is at stall and the speed controller is supplying full battery to the motor, the load resistance is half the average load, therefore twice the current, (in only two windings instead of averaged over all the windings) and there is some heat developed by a principle called eddy current in the motor armature. Because current is at a maximum, the brushes are getting hot and maybe arcing, and since the motor is not turning, the brushes can't give up their heat to other parts of the commutator. Since the motor is not turning, what little effect the fan had on heat is reduced to zero. As most of you know by now, as heat rises so also do the losses in wire, contacts, etc., adding more heat to the system. So you can see that stalling a motor at full current can cause some rapid heat rises within the motor. The motors we are given are wound with a varnish coated wire. (That's the red color of wire when you look inside.) At a certain temperature, that varnish begins to break down, and even boils off the wire. With no insulation, the wires begin to short together(lower resistance, higher current) and eventually short to the armature of the motor. That burning smell from a bad motor is the varnish and if things got bad enough, the burning lubricant in the bearings.
At other than stall, some of these other variables have more of an effect, like back EMF. BTW it is back EMF that you use to brake. By causing the motor to act as a generator and shorting the output of the motor in "brake mode" on the speed controller, the back EMF is turned into mechanical force that opposes the force of the moving robot.
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