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Why doesn't current go down as well as up?

Old 07-13-2010, 08:34 PM
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pd1
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Default Why doesn't current go down as well as up?

Does current really stay the same when the throttle is reduced ?

There is a lot of talk about ESC's as only being switches and when the throttle is reduced the current remains the same, only on and off some.
The more the pulses are off, the lower the current "reading".
But it is claimed that the current remains at full. Just on and off.

Work can be measured in watts. 750 watts equals one horsepower.

We can get watts by multiplying voltage (measured) and multiplying it by current (amps, measured)
Or you can use a wattmeter and see watts directly on the screen.

Watts go up or down directly to the work being done.

If you install a larger prop, the current will go up if everything else stays the same.

If you increase voltage, the motor will spin faster doing more work and draw more current.

If you now just reduce the throttle, you are doing less work.

Please explain to me how a motor doing less work, spinning slower, at a lower RPM, will draw the same current than when it was spinning faster doing more work?

Current is based on the work the motor is doing irrespective of the ESC.

The ESC acts as a switch, it allows the current to flow from the battery to the motor at whatever the motor demands. That demand is based on the work it is doing.


Voltage remains constant but actual current must drop.

Please show me how this is wrong.
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Old 07-13-2010, 08:59 PM
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The current that your meter measures is the AVERAGE current. Your ESC is actually doing two different switching operations superimposed on one another.

First, your ESC is doing what we call "commutation". Commutation involves switching the current through the different phases of the motor in order to keep up with (actually to lead) the motor rotation. The speed at which this occurs is a function of the motor RPM and requires that the ESC continually "sense" the position of the motor in order to know when to switch the current to a new phase.

The second switching operation your ESC does is called "pulse width modulation" (PWM) and involves switching the current to the motor on and off at a very rapid rate, usually several thousand times per second and generally much faster than the commutation rate. When the ESC switches "ON" the current tries to flow at the maximum rate and thus maximum possible stress to the battery, ESC and motor. When the ESC switches off, your battery, ESC and motor are coasting with very little stress. The ESC sets the ratio of ON time to OFF time (PWM) based on your throttle setting.

Lets take as an example a 222 Watt peak power system such as an 11.1 volt battery delivering 20 amps when running at full throttle. At 1/2 throttle the PWM turns the MAXIMUM CURRENT (20 amps) on 50% of the time. This gives you an AVERAGE current of 20 x 50% = 10 amps. 11.1 volts x 10 amps = 111 watts. You motor, only working 1/2 the time can't keep the propeller spinning at top speed so it slows down. The ESC, continually sensing the position of the motor slows down the commutation rate to stay "in sync" with the motor, but it does NOT change the PWM ratio. Average power (both horsepower and watts) goes down, but the peak current remains high (and might actually go a little higher).

Does that help?
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Old 07-13-2010, 09:19 PM
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The other thing to consider is that the ESC is fitted with capacitors that smooth the voltage. Although the PWM curcuit 'switches' on and off producing voltage 'spikes and troughs' the capacitors damp out the pulsation and deliver a steady voltage that is the average of all the 'offs' and 'ons' that the switching curcuit produces....

The ESC does not directly control current, it controls voltage. the current drawn is a result of the voltage delivered from the ESC and the impedance of the motor, which in itself is dependant on motor specification and load. Current does in fact go up or down depending on throttle position and motor load.

Steve
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Old 07-13-2010, 09:39 PM
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As already said AVERAGE current does indeed go up and down with throttle.

But the output FETs in an ESC are not linear devices they are switches. The changing AVERAGE is obtained by switching the current for varying percentages e.g. if 20A is on permanently the average current is 20A, if the 20A current is on 50% and off 50% the measured average is 10A. The average voltage AT THE MOTOR also varies in the same way. It's only when we measure these values, as we normally do, at the INPUT to the ESC that we see a constant voltage and a variable current.

BTW JetPlaneFlyer sort of has the idea but the wrong way round. The capacitors effectively store the input current so the the ESC can output high current pulses without these pulses being reflected all the way back to the battery. So the battery can deliver a steady current into the caps and they are responsible for coping with the PWM pulses.

Steve
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Old 07-13-2010, 10:20 PM
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I think I understood what all the previous posters said, but maybe I could summarize it. This ought to tell me if I have understood it myself too!

The motor never sees anything but full (11.1v) voltage, and draws the full current that its physical properties determine at that voltage. In order to make it spin at anything but top speed, we turn the power on and off very quickly.

I find it hard to make a useful analogy, but maybe it's like pushing a bowling ball by hitting it with a hammer? The more often you strike, the faster it goes
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Old 07-13-2010, 10:41 PM
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Originally Posted by mesh View Post
I think I understood what all the previous posters said, but maybe I could summarize it. This ought to tell me if I have understood it myself too!

The motor never sees anything but full (11.1v) voltage, and draws the full current that its physical properties determine at that voltage. In order to make it spin at anything but top speed, we turn the power on and off very quickly.
That's pretty much true. The ESC switches on and off the full voltage although the current doesn't actually rise and fall instantaneously. This is due to the "inductance" of the motor. Though our motors tend to have very low inductance so that, at the PWM frequencies our ESC's use, the current easily rises to maximum for even the shortest of "ON" times. Likewise the current continues to flow for a moment after the ESC switches "OFF" by going through some diodes which are part of the ESC drive circuitry.
Originally Posted by mesh View Post
I find it hard to make a useful analogy, but maybe it's like pushing a bowling ball by hitting it with a hammer? The more often you strike, the faster it goes
The bowling ball analogy only sort of works. The ESC hits the motor at the same (PWM) rate all the time. What changes is the size of the hammer. Wider pulse width equals bigger hammer (more energy transferred with each pulse/blow).
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Old 07-13-2010, 11:11 PM
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I see that the voltage remains the same and the capability of passing full current through the ESC is still there.

BUT

If the motor is only turning the prop at an RPM that is much less than at full throttle, then the motor is doing less work.

If it's doing less work, why would the demand for current be the same as at full throttle?

This is going to be a hard one for me to wrap my brain around.

Paul
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Old 07-13-2010, 11:33 PM
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This has helped me alot, I am an electrician with a degree in electronics, you never stop learning!
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Old 07-13-2010, 11:53 PM
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Originally Posted by pd1 View Post
I see that the voltage remains the same and the capability of passing full current through the ESC is still there.

BUT

If the motor is only turning the prop at an RPM that is much less than at full throttle, then the motor is doing less work.

If it's doing less work, why would the demand for current be the same as at full throttle?

This is going to be a hard one for me to wrap my brain around.

Paul
Ok, I think I'm getting an understanding of your confusion.

There are three main items that determine the instantaneous current flow in our motor circuits. The voltage of the battery, the total resistance of the current path, and the speed at which the motor is spinning. Lets look at each of these...

Once we've selected a battery the voltage is pretty much set. It's a little higher at full charge, a little lower near full discharge, but doesn't vary all that much. Especially if we're trying to be nice to it.

The resistance, and to a small degree the reactance (mostly inductance, but relatively small) of the current path. This is made up of internal battery resistance (the reason voltage drops a bit under load), wire resistance, ESC FET resistance, and motor winding resistance. Each of these resistances is in the range of milli-ohms in our equipment. Since these resistances are in series, they all contribute to limiting the current. We can compute the PEAK current by using Ohm's law ( I = V/R ) by plugging in the battery voltage and the total of all the resistance in the current path. Usually the resistance of the motor winding is the largest single component of the total resistance and dominates the equation. I've already discussed how the inductance modifies this slightly.

Finally the motor RPM... this actually has an inverse effect on current. The higher the RPM, the lower the current. This is because of the "back-EMF" of the motor. A spinning motor acts like a generator, even when it's running as a motor. As the motor spins faster it produces a higher voltage (Kv = RPM's / volt, tells us how much). This "generated" voltage offsets some of the battery voltage, thus the "V" in our Ohm's law equation effectively goes down. We can directly observe this effect with in in-flight data logger. If you keep the throttle position constant the motor will draw more current in a climb, and MUCH LESS in a dive. The ESC is still switching ON and OFF with the same ratio because we haven't moved the throttle.

In a dive the motor is actually doing LESS WORK even though it is spinning at a higher RPM. The "work" of a spinning motor = Torque x RPM. In a dive the RPM goes up but the torque goes way down. In a climb the torque goes way up and the RPM goes down a little, thus MORE work.

You're still confusing instantaneous current with average current. The instantaneous current is not a function of the WORK the motor is doing. It is a function of the characteristics of the components and the motor RPM as just discussed above. The instantaneous current actually goes UP at lower RPM's. But because of the PWM switching reducing the AVERAGE current to a lower value, the motor has less available electrical power from which to make mechanical power, so it spins the prop at a lower RPM, assuming static flight conditions.
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Old 07-14-2010, 09:22 AM
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Originally Posted by pd1 View Post
I see that the voltage remains the same and the capability of passing full current through the ESC is still there.

BUT

If the motor is only turning the prop at an RPM that is much less than at full throttle, then the motor is doing less work.

If it's doing less work, why would the demand for current be the same as at full throttle?
You're still confusing the average current, which is what is related to the motor load/speed, with the peak currents switched by the ESC.

When you throttle down, the ESC doesn't provide any lower instantaneous current it provides the same current but for a lower proportion of the time. E.g. for half throttle you might have pulses of 40A for half the time on, half off (20A average) then when you lower the throttle that changes to 40A on for only 20% of the time (8A average). The system effectively smooths the short 40A pulses out into an average current of 8A. What you measure coming out of the battery is 8A but what the components inside the ESC see is still the 40A pulses.

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Old 07-14-2010, 12:57 PM
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The analogies are helping me to visualize the procedure.
The problem is in any analogy there is always another that points to a different conclusion.

In a full sized fixed wing plane with a fixed pitch prop, if you lower the nose from level flight, the rpm will go up.
Nothing to do with partial charging, just the wind is now helping to turn the prop.
So poor analogy, but still a valid idea.

I'm still trying to visualize the full amperage across the timing pulse.
My hang up seems that I can't see where the call for "X" current is coming from.
The battery and ESC don't initiate the amount of current draw.
If the motor isn't spinning fast, I can't see it requiring a large current draw.
Not yet anyway.

Thanks.

Paul
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Old 07-14-2010, 01:15 PM
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Originally Posted by pd1 View Post
The analogies are helping me to visualize the procedure.
The problem is in any analogy there is always another that points to a different conclusion.

In a full sized fixed wing plane with a fixed pitch prop, if you lower the nose from level flight, the rpm will go up.
Nothing to do with partial charging, just the wind is now helping to turn the prop.
So poor analogy, but still a valid idea.

I'm still trying to visualize the full amperage across the timing pulse.
My hang up seems that I can't see where the call for "X" current is coming from.
The battery and ESC don't initiate the amount of current draw.
If the motor isn't spinning fast, I can't see it requiring a large current draw.
Not yet anyway.

Thanks.

Paul
The key issue is what you said above about the battery and ESC not initiating the current draw. The motor and prop do that. By providing the power in pulses you are simply cutting the amount of total power delivery the motor and prop can have. You are, in effect, starving the motor and prop of power. The motor and prop respond to this by slowing down. They want more power but can't have it so they run in a "crippled condition" to use whatever power they can have. That's a simplistic explanation but hopefully graphic enough to help.
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Old 07-14-2010, 02:10 PM
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Paul,

Perhaps a better way to think of this is that the battery and ESC "push" the current to the motor. The major limiting factor on the current is the resistance of the wire in the motor. If the motor doesn't turn, then ALL of the power turns into heat in the motor, and to a lesser extent in the ESC, battery, and lead wires. A stalled (and therefore delivering 0 horsepower) ideal (zero resistance) motor would theoretically DEMAND infinite current! For a non-ideal motor the current is limited only by the wire resistance, regardless of whether the motor is turning or not. The motor saves itself from destruction by shedding energy in the form of mechanical work so that all the energy doesn't turn into heat in the motor windings and fry the motor. Also, as described in a previous post, the "back-EMF" of the spinning motor effectively reduces the voltage, thus reducing the peak current in each pulse.

Assuming that the current is being delivered to the proper "phase" the motor will try to rotate. When it does the ESC "senses" this and supplies the current to the next "phase" to keep the motor moving in the same direction and accelerate the rotation. This is commutation. Conversely, by applying the current to the "wrong" phase we can create a brake effect, trying to turn the motor in the direction opposite to how it is rotating, thus slowing it down.

The PWM "chopping" of the current limits the total energy pushed at the motor (but it does NOT limit the peak current of each ON pulse). This is important when the motor is stopped or moving slowly because otherwise the average current would go very high, the motor windings would overheat, the insulation would fail, and the magic smoke would escape.

Each ON pulse gives the motor a little push. Some of that push turns into mechanical energy, and some into heat inside the motor. To achieve maximum efficiency the ESC must deliver the pushes at just the right moment. This is the challenge of commutation. But the loss due to resistance is ALWAYS there. The instantaneous loss is I squared R. Where I is the instantaneous current and R is the winding resistance. The total resistive loss is the mathematical integral of the instantaneous loss over time.
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Old 07-14-2010, 02:26 PM
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Originally Posted by pd1 View Post
The battery and ESC don't initiate the amount of current draw.
If the motor isn't spinning fast, I can't see it requiring a large current draw.
But you've got the sequence of actions in the wrong order. Remember the motor doesn't just magically think to itself "lower throttle" and decide to go slower, it is the ESC "asking" the motor to spin slower. The ESC controls the motor (that's why it's called an Electronic Speed Control).

And the ESC is not physically capable of simply limiting the current, because it can only switch full on or full off....it can't be partially on to let a little current through. So the way it signals the motor is by delivering the normal current but only for part of the time. The motor then "sees" that as a lower average current and so turns slower to match its speed to whatever current the ESC is willing to deliver.

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Old 07-14-2010, 02:38 PM
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Originally Posted by MustangMan View Post
Paul,

Perhaps a better way to think of this is that the battery and ESC "push" the current to the motor..
No, exactly the opposite is true. The motor and prop pull the current from the battery. The battery supplies what is demanded from the motor and prop up to its design limits. It is like a fuel tank. If nothing draws fuel, nothing happens.

The major limiting factor is not in the motor at all but in the battery. The motor will draw what it needs as long as the battery and ESC can supply it.

Motor resistance isn't even the major issue with the motor. It is a very minor issue. The major issue is the work it does. It is designed to turn at a given rpm. The more air resistance encountered by the prop, the more work it has to do. The more work it has to do, the more current it wants to draw. If it tries to turn too much prop, then obviously things will overheat when it tries to draw more current than it is designed to handle and the motor will damage itself assuming the battery and ESC can keep up with the demand. If they can't keep up with the demand, then they will overheat and damage themselves.

It all starts with a spinning prop. It all ends with the battery delivering the power. The battery "pushes" nothing at all. The ESC just acts like the fuei injection system or carburetor in an automobile engine. It supplies power as we direct it to. The engine responds by running at whatever rpm it can with the amount of power we let it have.
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Old 07-14-2010, 05:44 PM
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I think I'm seeing the light. My problem is I am trying to make sense of an electrical problem by using physics.

If I looked at the pulse on an oscilloscope, The amplitude would be the voltage and the pulse width would be the current.
Shortening the pulse width will slow the motor down without reducing the voltage.

The motor will still try to do the same work, but is basically strangled by the ESC into going slower and doing less work.
The current can be the same, just less duration.

So the work per nano second is the same wide open or partial throttle. Just the total work is lower due to shorter pulses.

Paul
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Old 07-14-2010, 05:59 PM
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Originally Posted by pd1 View Post

So the work per nano second is the same wide open or partial throttle. Just the total work is lower due to shorter pulses.

Paul
Yes, on average. The motor wants to get to its design rpm's. How close it gets to it depends on how much power we let it have. The pulse frequency and duration determine how much power it can have.

The physics of it is the same as with an automobile engine. The only difference is that the engine doesn't pulse. We simply change the amount of air fuel mixture and the engine gets as far as it can to its design rpm given the amount of air/fuel available to it.

The purpose of the pulse modulation is to improve efficiency. You can imagine what would happen if we simply tried to sink the current away resistively.
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Old 07-14-2010, 06:38 PM
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That's pretty much it. One minor point, the width of the pulse on the 'scope is the time duration of the pulse, not amperage. The amperage essentially rises in direct proportion to the voltage.

I develop my understanding by thinking about the physics of electrical systems. Under initial conditions (motor stopped), the motor looks to the ESC like a small coil of relatively short wire. The inductance is pretty small so at the relatively low frequencies at which our ESC's do their PWM, we can mostly ignore reactive (inductance) effects. Thus the motor looks like a resistor made from a 3' piece of small wire. If you try to put 11 or 12 volts across that wire a lot of current will flow and the wire will get very hot. An ideal motor would have wire with 0 Ohms resistance. Real motors use copper which is a very good, but not perfect, conductor. As the motor begins to spin it causes the magnetic field of the permanent magnets to induce a voltage into the windings. An ideal motor spinning at an RPM that matched the Kv x applied (battery) voltage would exactly match the applied (battery) voltage and the current would be zero. If an external source of mechanical energy spins the motor faster than this speed, then the motor generates a higher voltage than the applied voltage and would actually feed current back into the battery.

Our "sensorless" brushless ESC's actually depend on this generated "back-EMF" to provide the position information they need in order to determine when to switch the current to a different phase winding. This, of course, leads to the problem of what to do when the motor is not turning at all, and thus there is no "back-EMF" to detect. The standard approach is the ASSUME the motor is turning at some low RPM, set the PWM ratio to a low value (to limit the average current), and simple begin sequencing the phases without any motor position feedback. After a few 10's of seconds the motor will hopefully begin spinning and we can start looking for the position feedback and synchronize the phase sequencing to it. If no feedback is present then STOP applying power to the motor, rest a second or two, and start again (or wait until the throttle is lowered below shutoff and brought up again). You can see an ESC doing this if you simply hold the prop in a fixed position and tell the ESC to start the motor. Most ESC's will try, give up, try, give up, etc.
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Old 07-14-2010, 06:49 PM
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Victory Pete
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Fascinating! Please continue!
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Old 07-14-2010, 09:52 PM
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I think I have it.

I don't know why I envisioned the width of the pulse as anything but time.
Voltage can be the amplitude and the current is free to go to what ever instantaneous demand the motor calls for.

Current can vary, but not limited by the total work.

Makes more sense now, thanks everyone.

Paul
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Old 07-14-2010, 11:00 PM
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Originally Posted by pd1 View Post
I think I have it.

I don't know why I envisioned the width of the pulse as anything but time.
Voltage can be the amplitude and the current is free to go to what ever instantaneous demand the motor calls for.

Current can vary, but not limited by the total work.

Makes more sense now, thanks everyone.

Paul
The width of the pulse is time. It is the time - the width - and the frequency of the pulses that determines the amount of power available on average. The amplitude of the current will be whatever the prop and motor are drawing. In this type of circuit, we don't adjust the amplitude. We only adjust the time and frequency. The load determines the amplitude. It tries to get all the current it wants full time. We provide it only part time through the pulse modulation. If we adjusted the amplitude, we would have to sink off the unused current as heat.

I wonder if you are thinking of the current as a sine wave as it would appear in your home electrical service. It is not. It is the classic square wave. On and off. Black and white. Full current and no current. The wave forms will have flat tops and vertical sides on your scope. It is an AC circuit but not the same kind that powers your home appliances.
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Old 07-14-2010, 11:04 PM
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Has anyone viewed this on an actual O scope? I have a Tektronics 2246 and would love to have a look. Is the waveform after the ESC different than before the ESC? I would think they would be different.
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Old 07-14-2010, 11:04 PM
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Now you can wrap your head around the next situation. As you know engines start a minimum torque at rest and increase torque as rpm's increase. Electric motors are exactly the opposite. They have maximum torque at rest and the torque declines as rpm's increase. You can noodle that one over for a while.
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Old 07-14-2010, 11:07 PM
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Originally Posted by Victory Pete View Post
Has anyone viewed this on an actual O scope? I have a Tektronics 2246 and would love to have a look. Is the waveform after the ESC different than before the ESC? I would think they would be different.
VP
Before the ESC there is no wave form. it is DC. It would show as a flat line on the scope. In order to view the square wave simply put your probes across any two of the motor leads.
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Old 07-14-2010, 11:12 PM
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Originally Posted by fmw View Post
Before the ESC there is no wave form. it is DC. It would show as a flat line on the scope. In order to view the square wave simply put your probes across any two of the leads between the ESC and motor.
I will set up the scope and and have a look, I will try to take some pictures.
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