The concept of "amps drawn" by a motor?

jfitz · Aug 14, 2011

First post and please forgive me if it’s too basic or if it’s in the wrong section. Thanks to all of you for creating such a helpful and informative site.

I have learned a great deal from reading many posts over the last 5 months, but I am embarrassed to admit that I am struggling with the concept of "amps drawn" by a motor. I think I understand how volts can be analogized to pressure in a pipe and that current can be analogized to flow rate through a pipe. From one of Neptronix many informative posts I learned that a "controller limits the current that will come into it. You could hook up a nuclear reactor at full load to your bike and [a 22 amp controller] would still only draw 22 amps." I also have read that a motor "draws" more amps when the motor is dealing with greater resistance, like when you start an ebike without pedaling. .... So should I think of the motor as pulling or sucking the amps out of a battery? But this does not make sense to me because I thought a battery supplies the voltage (pressure) and pushes out the electrons.

jfitz

Arlo1 · Aug 14, 2011

Volts = Pressure
Ohms = Resistance
Amps = Flow
Watts = power

Volts divided by ohms = Amps
Volts divided by amps = Ohms
Amps x Ohms = Volts
Amps x volts = watts
watts/746 = HP
HP x 5252/RPM = Torque in ft/lbs (Torque and HP are always the same at 5252 rpm)

Now the way a electric motor works with a controller is not that hard to understand and most of the guys on the forum are very smart people but some of them forget guys like you and me need it dumbed down a notch to understand.
So if your controller is hooked up to a battery that is 50 volts for instance your motor will want to spin a certain rpm if all the power is given to to the motor this is the KV of the motor so lets say the rpm/volt is 100 (or 100kv) at 50 volts the motor wants to spin 5000 rpm. So in order to be able to travel at lower speeds the controller pulses mosfets (like relays) on and off really fast and it varies the on pulse time to get the rpm/power you need. So say the mosfets were on and of 50% of the time it would want to spin the motor 2500rpm Now when you are traveling with the motor at 2500 rpm with a 50 volt battery and want to accelerate the controller will pulse the mosfets on for a longer time then off. But because of the rpm and the way the whole system works the motor would really only see 25volts but roughly 2 times the amps the controller is flowing out of the batteries, the relationship is related to the rpm/volt of the motor and the amps flowing into the controller.

Basically the whole system works as a buck converter. (google will help explain that)

jfitz · Aug 14, 2011

Arlo1 thanks for your response, but I think I need it dumbed down a few more notches. To paraphrase Venkman in Ghostbusters: Please pretend for a moment that I don't know anything about metallurgy, engineering, or physics, and just tell me what the idea of "draw" means when it is used in electrical terms.

Arlo1 · Aug 14, 2011

OK sorry "amps drawn" is a very wrong way of wording it! A motor can not draw power that's just slang.
The windings in the motor vs the rpm and magnet location determined how much resistance/inductance the motor has. The inductance which is kinda like resistance that changes over time (I have an inductance thread to learn more) and the resistance (ohms) limit how much current (amps) can flow with a given voltage (push)

Just like if your element in your toaster has 10 ohms resistance and you push on it with 100 volts it will flow 10 amps but the toaster doesn't Draw the 10 amps in it actually limits the amps flowing through based on the resistance!

gogo · Aug 14, 2011

For a given (non Amp-limited) electric bike setup, supplying its motor with more voltage will cause its motor to draw more Amps during acceleration (causing a greater rate of acceleration).

The battery "supplies" the voltage, the motor "draws" the amps.

jfitz · Aug 14, 2011

Arlo1 said:
OK sorry "amps drawn" is a very wrong way of wording it! A motor can not draw power that's just slang.
The windings in the motor vs the rpm and magnet location determined how much resistance/inductance the motor has. The inductance which is kinda like resistance that changes over time (I have an inductance thread to learn more) and the resistance (ohms) limit how much current (amps) can flow with a given voltage (push)

Just like if your element in your toaster has 10 ohms resistance and you push on it with 100 volts it will flow 10 amps but the toaster doesn't Draw the 10 amps in it actually limits the amps flowing through based on the resistance!

Thanks, I did not know I was using this phrase incorrectly. I thought I had frequently seen and heard it used in this manner. So the motors do not draw amps. Should I think of them as a water wheel at a mill? The water wheel does not draw or suck the water from a stream, but it will not turn if the amount of the water is not sufficient.

I appreciate the toaster example. I will try to put it in terms of a ebike with a few (hopefully not ridiculous) assumptions: If a 50 volt ebike traveling on a flat road at 20 mph and the motor has 5 ohms resistance the the current flowing to (or through?) the motor would be about 10 amps?

Now if the bike starts climbing a hill and the resistance goes up to 10 ohms then the current flowing through the motor would go down to 5 amp? I think this wrong, but I am not sure why.

jfitz · Aug 14, 2011

gogo said:
For a given (non Amp-limited) electric bike setup, supplying its motor with more voltage will cause its motor to draw more Amps during acceleration (causing a greater rate of acceleration).

The battery "supplies" the voltage, the motor "draws" the amps.

Okay as you use the term "draw" do you mean the motor is pulling or sucking amps out of the battery? In other words, the motor sucks more amps out during acceleration and sucks less out at a crusing speed.

And can you help me reconcile what you wrote with the point Arlo1 made about motors being resistance that limits current flow?

Njay · Aug 14, 2011

Think of a lamp, a 10W bulb connected to a 10V battery. According to Ohm's Law, the current flowing through the circuit (from battery to lamp and back to battery) 10W / 10V = 1A. We can say the bulb pulls or draws 1A from the battery. Now we take the 10W bulb and replace it with a 20W bulb. The current in the circuit is now 2A (20W/10V). We can say the bulb pulls 2A from the battery. This pulls/draws came from the observation that the battery doesn't seem to "decide" what current goes in the circuit, but instead it is the lamp that decides. It is "the (electric) load" that decides, that pulls/draws X amps from the battery. We just don't usually say "this lamps draws X Amps", but that would be as valid as saying the same about a motor. A motor is also an electric load on the battery.

The characteristic that the load (bulb, motor, ...) has that "decides" how much current goes through the circuit, is resistance. For a bulb it is easy to calculate from it's wattage using Ohm's Law; the 10W bulb that pulls 1A when connected at a 10V battery has a resistance of 10V / 1A = 10 Ohm.

In a motor is more complex, because the resistance is not always the same value. It varies depending on motor RPM, and is proportional to it, so, when RPM is low, resistance is also low and more amps flow in the circuit (the motor "draws more amps"), when RPM is high, resistance is also high and therefore less amps flow in the circuit. The lowest resistance a motor can have is when it is stopped, 0 RPM (stalled) - it can be stopped if you lock it's rotor while applying a voltage from a battery. When you go up a hill, the motor does it at lower RPM, therefore it draws more amps (lower resistance) than when you go riding on a flat ground (you go faster on the flat ground - motor spins faster, has more resistance, draws less amps).

I made a series of oversimplifications, but I hope it was the right compromise to be simple to understand.

neptronix · Aug 14, 2011

You can think of amps as torque, and voltage as potential speed.

It is more complicated than that, but that is the most basic explanation i can think of.
If you watch a turnigy watt meter or cycle analyst, you will see your amps be very high at acceleration or hill climbing, and then lower at cruise.

jfitz · Aug 14, 2011

Njay said:
Think of a lamp, a 10W bulb connected to a 10V battery. According to Ohm's Law, the current flowing through the circuit (from battery to lamp and back to battery) 10W / 10V = 1A. We can say the bulb pulls or draws 1A from the battery. Now we take the 10W bulb and replace it with a 20W bulb. The current in the circuit is now 2A (20W/10V). We can say the bulb pulls 2A from the battery. This pulls/draws came from the observation that the battery doesn't seem to "decide" what current goes in the circuit, but instead it is the lamp that decides. It is "the (electric) load" that decides, that pulls/draws X amps from the battery. We just don't usually say "this lamps draws X Amps", but that would be as valid as saying the same about a motor. A motor is also an electric load on the battery.

The characteristic that the load (bulb, motor, ...) has that "decides" how much current goes through the circuit, is resistance. For a bulb it is easy to calculate from it's wattage using Ohm's Law; the 10W bulb that pulls 1A when connected at a 10V battery has a resistance of 10V / 1A = 10 Ohm.

In a motor is more complex, because the resistance is not always the same value. It varies depending on motor RPM, and is proportional to it, so, when RPM is low, resistance is also low and more amps flow in the circuit (the motor "draws more amps"), when RPM is high, resistance is also high and therefore less amps flow in the circuit. The lowest resistance a motor can have is when it is stopped, 0 RPM (stalled) - it can be stopped if you lock it's rotor while applying a voltage from a battery. When you go up a hill, the motor does it at lower RPM, therefore it draws more amps (lower resistance) than when you go riding on a flat ground (you go faster on the flat ground - motor spins faster, has more resistance, draws less amps).

I made a series of oversimplifications, but I hope it was the right compromise to be simple to understand.

Thanks Njay. I appreciate your simplification. You helped quite a bit by informing me that resistance goes down when climbing a hill. If voltage is reletively constant on the ebike then amps= V/i tells me that amps would increase. So then the motor (or other load) draws or pulls out the available amps from a battery that it needs to reach the rpm demanded by the rider. But what about the battery, is it pushing out amps by means of it voltage (pressure)?

Erogo · Aug 14, 2011

I'd be careful remembering something like "the resistance goes down when you climb a hill". If you keep the same speed, that would not be true. When you hit the hill your speed usually drops, and it's because the motor is turning at a lower speed that it has lower resistance. (When it's stopped, the motor's resistance is very close to zero).

I think the most useful comment so far has been that the motor (or other load) determines how much current will flow, given the voltage that is put across it.

The battery's voltage provides a "pressure" into the load, and the resistance of that load determines how much current the pressure will push through. Less resistance to flow means more flow. More resistance to flow means less flow! So it's the load that determines how much current flows from the battery of a certain voltage.

Note also, you said that "amps = V/i". i is usually the abbreviation for current, which is measured in amps. So the formula you gave would normally be "amps = V/r", current is voltage divided by resistance.

The voltage and resistance together determine the amount of current flowing. The motor's pushing, the load is resisting to a certain amount, and the result is the current.

So to talk about a load "drawing" current is, as you noticed, a bit strange, but just one of those conventions that has stuck, despite not necessarily being hyper-accurate.

It does get more complicated when people start to introduce "inductance" etc, but I don't think you need to tackle that until you're comfortable with the first bit, the bit you asked about.

Eric

jfitz · Aug 14, 2011

Eric, thanks for these reminders. I follow about the hill/rpm/resistance/amps relationship, which I only learned today. And I now see my mistake re Ohm's Law. I did mean amps = V/r. Also your right about putting off "inductance."

I am not trying to be hyper-accurate so if the use of the term "draw" in this context is just slang or a shorthand reference, that's helpful to know, and I need to get the correct concept into my head.

When you wrote "The motor's pushing, the load is resisting to a certain amount, and the result is the current." The load that you are referring to is not an electrical load you mean a mechanical load.

Should I think of amps as being pushed or pulled from a battery? Or is there a better way to think of it?

Erogo · Aug 14, 2011

Sorry bro, I made a typo. I meant:

"The battery's pushing, the load is resisting ..."

The battery's voltage is like a pressure, pushing electrons out of it. The load (which in this case is the motor) has a resistance to that pressure of electrons.

(If you take the motor away, and just have a battery on its own, then with no wires between the two ends of the battery there's only air instead, and that has a very high resistance to the flow of electrons.)

The bigger the resistance, (air bigger than motor) the less current can be pushed around by a given voltage.

You can think of current as pushed out of a battery, and depending on how much resistance there is, determines the number of amps of current that flow. But if anyone says to you "the motor's drawing 2 amps", don't get all pedantic on them and tell them that actually the battery's pushing, and the motor's allowing 2 amps. Just nod and smile.

Eric

Arlo1 · Aug 14, 2011

Erogo said:
Sorry bro, I made a typo. I meant:

"The battery's pushing, the load is resisting ..."

The battery's voltage is like a pressure, pushing electrons out of it. The load (which in this case is the motor) has a resistance to that pressure of electrons.

(If you take the motor away, and just have a battery on its own, then with no wires between the two ends of the battery there's only air instead, and that has a very high resistance to the flow of electrons.)

The bigger the resistance, (air bigger than motor) the less current can be pushed around by a given voltage.

You can think of current as pushed out of a battery, and depending on how much resistance there is, determines the number of amps of current that flow. But if anyone says to you "the motor's drawing 2 amps", don't get all pedantic on them and tell them that actually the battery's pushing, and the motor's allowing 2 amps. Just nod and smile.

Eric

Yes this is what he was asking. The motor resists the power and the amount of resistance (Ohms) vs the voltage (push) will allow a number of electrons to flow (amps) So yes the motor does not suck the current out the batteries push it!

ptd · Aug 14, 2011

could you maybe say, instead of draws, that the "circuit" allows, that amount of power to flow through it ? limited by the capacity of the battery, configured (and possibly limited again) by the controller, and then through the motor.

as for increasing and decreasing rpm, and volts and amps. it seems like it's not so much one or the other, but a balancing act by both. at first i thought voltage dictated, or was dictated by, rpm. could it be, that an increase in amps results in an increase in torque, which results in an increase in velocity (rpm), which results in an increase in voltage ? or is it more like an increase in both amperage and voltage simultaneously, results in an increase in rpm ?

so say you start going up a hill. the load is increasing, could you just increase amperage (proportionally to the load) and maintain the same speed ?

and aren't throttles speed based ? so in order to attain a certain speed, does the controller really send an increased voltage to the motor, or does it just keep feeding the most amps it can, until the voltage matches the rpm, or voltage, that the throttle is requesting ?

in the lamp example above, is a dimmer switch kind of an amp based throttle ? or is it more like some kind of proportional response (is this what a potentiometer is ?) ? seems more like the latter, because i can usually put about any size bulb in i want, and get similar results. whereas, if it was power based, you'd think you'd be blowing lower wattage bulbs left and right. so is it that the dimmer itself is just introducing some additional form of resistance ? or is it acting like PWM in a way ? like only allowing a portion of the possible power to get through.

speaking of resistance. are you talking about BEMF ? that, as rpm increase, this back "pressure" increases resistance ?

gogo · Aug 15, 2011

'Tis true, its all formulas with their mathmatical relationships and using terms like "draw" doesn't improve the clarity of the formulas. I was just speaking off the seat of my pants.

Njay · Aug 15, 2011

ptd said:
could you maybe say, instead of draws, that the "circuit" allows, that amount of power to flow through it ? limited by the capacity of the battery, configured (and possibly limited again) by the controller, and then through the motor.

Yes.

ptd said:
so say you start going up a hill. the load is increasing, could you just increase amperage (proportionally to the load) and maintain the same speed ?

That's what happens in the motor, as the load slows it down it starts drawing more current, and torque is proportional to current. So this resistance decrease and respective current increase is just the motor trying to "fight back" the load. There is, however, a limit on the amount of current you can pass through a motor, because as current increases temperature also increases, and at a certain point something will stop working inside the motor.

ptd said:
in the lamp example above, is a dimmer switch kind of an amp based throttle ? or is it more like some kind of proportional response (is this what a potentiometer is ?) ?

The lamp dimmer works roughly as a motor controller, it varies the amount of time that the lamp is on and off, to give the "illusion" of a fractional amount of light.

ptd said:
speaking of resistance. are you talking about BEMF ? that, as rpm increase, this back "pressure" increases resistance ?

Yes, BEMF is like backpressure (is really like a battery, but never reaches the same Volt as the battery powering the motor), and it is BEMF the responsible by the effect of motor resistance going up with RPM.

ptd · Aug 15, 2011

Njay said:
ptd said:

so say you start going up a hill. the load is increasing, could you just increase amperage (proportionally to the load) and maintain the same speed ?

Click to expand...

That's what happens in the motor, as the load slows it down it starts drawing more current, and torque is proportional to current. So this resistance decrease and respective current increase is just the motor trying to "fight back" the load.

i thought that if the the motor slows down, and assuming the throttle position hasn't changed, that the "requested speed" isn't met, and therefore more current is fed to try to achieve (or maintain) this request.

Njay said:
There is, however, a limit on the amount of current you can pass through a motor, because as current increases temperature also increases, and at a certain point something will stop working inside the motor.

i'm also thinking, that as voltage and rpm drop, pwm duty cycle is decreasing, resulting in a) less efficiency (and hence, heat) in the controller, and b) an increase in current, resulting in a squared higher resistance losses in the wire (hence, more heat, in the controller, AND motor??). it would seem to me that temperature only increases when efficiency drops so low that the heat generated is greater than the heat shedding capacity of the motor. and that when those two are exactly equal, that this is the duty rating of the motor, the power level that it can operate at indefinitely. it occurs to me that things will stop working in the controller as well. i'm suprised that it's such a difficult thing to safeguard, directly, as opposed to indirectly, such as temp monitors.

Njay said:
ptd said:

in the lamp example above, is a dimmer switch kind of an amp based throttle ? or is it more like some kind of proportional response (is this what a potentiometer is ?) ?

Click to expand...

The lamp dimmer works roughly as a motor controller, it varies the amount of time that the lamp is on and off, to give the "illusion" of a fractional amount of light.

so, it IS like pwm. my question is, how does the controller know the capacity of the bulb? or is the bulb making a request, and the pwm is saying, you only get x% of what you want? and isn't this like the basis for the amp controlled throttle idea?

REdiculous · Aug 15, 2011

so, it IS like pwm. my question is, how does the controller know the capacity of the bulb? or is the bulb making a request, and the pwm is saying, you only get x% of what you want?

Some dimmers are PWM based, most aren't. You don't need a PWM dimmer for incandescent, halogen and modern CFL. Fluorescent, mercury vapor, solid state and other arc lighting require a PWM dimmer.

Think of an old-school flashlight. When the batteries start to die the voltage drops and the light dims. That's basically what a dimmer does...it changes the RMS voltage (which changes the mean power). Usually dimmers can't go over line voltage and you select your bulb based on the line voltage and the power/brightness requirement.

The dimmer doesn't know the "capacity", which is better thought of as the power-rating, of the bulb. The standard 60w incandescent bulb is resisting about 0.5amp, there is no "request", so if you give it twice the voltage it'll still resist 0.5amp* but it'll allow 120w through, which will probably burn out the filament pretty quickly.

* It may change slightly with temp but we're talking basics.

rhitee05 · Aug 15, 2011

No analogy is perfect, but water in a pipe is a pretty good one. If you hook a garden hose up to the spigot on your house, how much water flows is determined by how much you open the valve (pressure - i.e. voltage), how wide and long the hose is (resistance) and how high the end is (BEMF). There's no "requesting" a certain amount of current, the hose doesn't "draw" current from the spigot. The flow is determined by the water equivalent of Ohm's law (V=IR). The same applies to a motor. The controller functions like the spigot valve, adjusting how much voltage is available to the motor. Ohm's law then determines how much current flows. Saying the motor "draws" so much is common but misleading shorthand.

Njay · Aug 15, 2011

ptd said:
Njay said:

ptd said:

so say you start going up a hill. the load is increasing, could you just increase amperage (proportionally to the load) and maintain the same speed ?

Click to expand...

That's what happens in the motor, as the load slows it down it starts drawing more current, and torque is proportional to current. So this resistance decrease and respective current increase is just the motor trying to "fight back" the load.

Click to expand...

i thought that if the the motor slows down, and assuming the throttle position hasn't changed, that the "requested speed" isn't met, and therefore more current is fed to try to achieve (or maintain) this request.

The typical controller doesn't know at what speed are you going. The basic controller just applies a duty cycle that is proportional do throttle position. Do you understand what this means? No throttle -> 0% duty cycle (= motor disconnected from battery all the time), full throttle -> 100% duty cycle (motor connected to battery all the time, as if no controller is in between), half throttle -> 50% duty cycle (motor is connected to battery only half the time - If the controller's PWM frequency is, say, 1KHz, whose period is 1ms, then this means the motor is connected for 0.5ms, then disconnected for 0.5ms, and so on cyclically).
It is you who, feeling the bike slowing down, pulls more on the throttle trying to maintain speed.

ptd said:
Njay said:

There is, however, a limit on the amount of current you can pass through a motor, because as current increases temperature also increases, and at a certain point something will stop working inside the motor.

Click to expand...

i'm also thinking, that as voltage and rpm drop, pwm duty cycle is decreasing (Njay: only if you loosen the throttle or the controller "feels" too much current passing and limits it by forcing a duty cycle reduction), resulting in a) less efficiency (and hence, heat) in the controller, and b) an increase in current, resulting in a squared higher resistance losses in the wire (hence, more heat, in the controller, AND motor??) (Njay: Since current increases, and losses are proportional to the current ('s square), efficiency drops). it would seem to me that temperature only increases when efficiency drops so low that the heat generated is greater than the heat shedding capacity of the motor (Njay: temperature always increases, because since there is always resistance and current, there are always losses as heat). and that when those two are exactly equal, that this is the duty rating of the motor, the power level that it can operate at indefinitely. (Njay: the motor has an intrinsic heat dissipation rate (depends on how much metal it has, how that metal is exposed to air, the air's temperature, ...); you can let it heat as long as that rate allows it to dissipate the heat without reaching a "critical" temperature at which something will give) it occurs to me that things will stop working in the controller as well. i'm suprised that it's such a difficult thing to safeguard, directly, as opposed to indirectly, such as temp monitors.

Njay said:

The lamp dimmer works roughly as a motor controller, it varies the amount of time that the lamp is on and off, to give the "illusion" of a fractional amount of light.

Click to expand...

so, it IS like pwm. my question is, how does the controller know the capacity of the bulb? or is the bulb making a request, and the pwm is saying, you only get x% of what you want? and isn't this like the basis for the amp controlled throttle idea?

The controller doesn't know, it just lets the battery connect to the motor for intermittent amounts of time (PWM). I mean, a decent controller will measure the current flowing in the circuit ("being drawn" by the motor), so that it can limit it to avoid destruction of the controller and/or motor. This isn't however enough to fully protect the motor; just because a motor can only take 100A max continuously, it doesn't mean that we won't give it more than 100A, but for a limited amount of time. This is very useful; we can, for example, give it 200A or 300A when starting, to have plenty of torque to start moving and accelerating. Once moving, we don't need that much torque anymore.
In the end, what limits "the power" a motor can take, is temperature. A good controller would also measure the motor's temperature and limit its current (forcing a lower duty cycle even if you have your throttle at max) in order to maintain the motor's temperature inside its limits.

liveforphysics · Aug 15, 2011

V = IR

Think of shorting a lightbulb across a battery. Current flows inversely proportional to the resistance of the filament.

What if you want it to average half the current? You flick the switch on and off 50% of the time.

Now imagine when you're flicking it on and off 50% of the time, the inductance of the motor windings keep the current flowing even when the switch opens, because the stored magnetic field starts to collapse, inducing a current as the field lines move. This keeps the current constant and steady even while the switch gets flicked on and off.

That's how it works in laymans terms.

ptd · Aug 15, 2011

Njay said:
The typical controller doesn't know at what speed are you going. The basic controller just applies a duty cycle that is proportional do throttle position. Do you understand what this means? No throttle -> 0% duty cycle (= motor disconnected from battery all the time), full throttle -> 100% duty cycle (motor connected to battery all the time, as if no controller is in between), half throttle -> 50% duty cycle (motor is connected to battery only half the time - If the controller's PWM frequency is, say, 1KHz, whose period is 1ms, then this means the motor is connected for 0.5ms, then disconnected for 0.5ms, and so on cyclically). It is you who, feeling the bike slowing down, pulls more on the throttle trying to maintain speed.

i would instictually disagree with the highlighted above. i get the feeling that there are two instances of pwm going on. firstly, in the controller varying voltage to match the rpm of the motor, and secondly in the case of the controller accounting for a partial throttle condition. it just seems to me that the controller is always accounting for, and maybe averaging, these two variables. i would guess that there is ONE point in time, where the voltage required exactly matches the voltage supplied, while at full throttle, where the controller could be in this passive state.

Njay said:
just because a motor can only take 100A max continuously, it doesn't mean that we won't give it more than 100A, but for a limited amount of time. This is very useful; we can, for example, give it 200A or 300A when starting, to have plenty of torque to start moving and accelerating. Once moving, we don't need that much torque anymore. In the end, what limits "the power" a motor can take, is temperature. A good controller would also measure the motor's temperature and limit its current (forcing a lower duty cycle even if you have your throttle at max) in order to maintain the motor's temperature inside its limits.

i understand that we can feed a motor more than it's rated power, and that this results in the increase in temp. and i can also see now that temp monitoring would be the logical choice, as other variables, like ambient air temp, would come into play. it occurs to me, that introducing pwm because of overheating isn't going to help, might actually hurt, as it results in an additional level of inefficiency, and hence more heat. i'm looking at in the context of whether the throttle should be regulating speed, instead of power, or even motor temp. seems to me it should be something along the lines of variable current limiting, which is why i thought the amp based throttle was interesting.

ptd · Aug 15, 2011

liveforphysics said:
V = IR
What if you want it to average half the current? You flick the switch on and off 50% of the time.

do you mean half the power? i thought flicking the switch like that resulted in half the voltage.

liveforphysics said:
Now imagine when you're flicking it on and off 50% of the time, the inductance of the motor windings keep the current flowing even when the switch opens, because the stored magnetic field starts to collapse, inducing a current as the field lines move. This keeps the current constant and steady even while the switch gets flicked on and off.

constant and steady? straight line? i mean i could see that as an ideal to try to achieve, but isn't there some variance? and is this the basis for torque ripple? and/or harmonics? (both of which are over my head right now, but curious) it's just my understanding that pwm takes x amount of power, reconfigures the voltage and amperage, and outputs x amount of power (minus transformation costs). is this accurate?

Arlo1 · Aug 15, 2011

jfitz. Are all these answers dumbed down enough for you? This is why I tried to help you before these guys who talk the lingo all the time jumped in.
And to make matters worse they get off on tangents that dont even relate to what you want to know. :lol:

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