Quote:
Originally Posted by johnmyster
Um, yeah, ohm's law agrees with me.
You're saying that heat is directly proportional to current, which means you're thinking of a motor as a resistive device. Cool. Ohm's law works great for resistive devices. Actually, it works perfect.
Well, ohm's law says that for your resistive motor, when I decrease voltage, current decreases. V=I*R. So current is directly proportional to voltage. Voltage drops. Current drops. Ohm's law, right? Yup. I checked it alright. There you go.
You're thinking of a motor as a constant power device. Thus, whatever it can't get in terms of voltage, it makes up in current. However, your very assumption of a resistive motor contradicts this. Resistive and constant power are two different things. But then again you contradict yourself, because you say that power will fall when current and voltage decrease. So does current drop, or does it increase? Which is it? Pick a side here.
Motors are somewhat resistive and somewhat inductive by nature. However, for DC motors, it's safe to say the inductive contribution falls out.
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R drops with voltage and current increases DC motors can NOT be looked at as pure resistive device. Inductive resistance (back EMF) plays a big part as it opposes the current.
Newton's laws say a force applied to (a DC motor in this case) should continue to accelerate. When we put current through a DC motor a torque is induced on the motor shaft. The only thing opposing this torque is some small amount of bearing friction. Yet, the shaft does NOT continually accelerate (as Newton suggests would happen if an unbalanced force is applied to a system). The motor quickly reaches some steady speed at which it will continually operate. The phenomenon that keeps the motor from continually accelerating is the second electromagnetic principle mentioned above. Because the coil is moving in an external magnetic field, there is an induced voltage in the coil. This voltage polarity is such that it opposes the battery and, hence, reduces the current through the coil. In reality, the current going through the motor will only be enough to generate enough torque to overcome friction and the inertial load on the shaft...that is if the motor is strong enough to turn the shaft at all. If we apply such a heavy load to the shaft that the motor cannot turn it, we say the motor is "stalled." Imposing such conditions on a motor for any length of time is a sure way to ruin a motor. A DC motor will "try" to stay at it's constant power, conditions may or may not permit it.
So you have "R" which is made up of the (back EMF) plus the pure resistive part that equal the total resistance. If conditions cause "V" to drop then back EMF also drops which is a part of "R", So if "V" and "R" have dropped now ohms law tells us "I" has increased (proportionately.)
With increased current let look at the pure resistive part of "R". P=R*I^2 Now there is more power going to the resistive part of "R" which is where the heat comes in.
Now the part I said your loosing voltage and current I wasn't talk about the motor but the source (battery) not being able to supply both and it's effect on the motor. (note: if you take a lead acid bat that low you can damage it)
It's to bad that magnetic barrier is there like the speed of light barrier, other wise Newton's law might let a 12 volt battery speed you past the sound barrier
