Author Topic: Motor BEMF  (Read 17632 times)

cadstarsucks

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Motor BEMF
« on: September 01, 2007, 02:36:42 PM »
BEMF is a characteristic of a motor that always exists and is proportional to motor speed.  In the case of a Golden HBS-36 is it approximately 0.12V/RPM. 

Industrial BDC motor controllers sense motor speed by measuring the BEMF when the switch is off and correct for the drop across the rotor resistance by sensing the motor current.

In the Golden HBS-36 it is the same story accept you are looking at BLDC so the BEMF is three phase AC and the RPM is 3600RPM divided by the number of motor poles.

As an aside, how is the startup from a full stop with a heavy passenger?  Is it "rough" (motor cogging)?  If so than a better controller could make it smooth.  Of course anything costs more - even if there is no excuse other than to line the manufacturers pockets.

When under load the voltage developed across the winding resistance appears in series with the BEMF, hence the ability to add a little extra drive voltage at high loads to compensate.

Dan

mustangman

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Re: Motor BEMF
« Reply #1 on: September 01, 2007, 05:26:30 PM »
  So, If I am correct, the current controller is not exactly what you would prefer as a professional EE. OK , design or redesign a new one/

Dave

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Re: Motor BEMF
« Reply #2 on: September 01, 2007, 07:24:01 PM »
How about a layman's explanation of BEMF? I have a good electronics foundation -- Air Force trained for communications maintenance -- but a lot of this theoretical stuff goes right over my head.

Also, if someone (you?!) came up with a good design for the ideal controller and made it open source, couldn't hobbyists like ourselves buy the components and put it together ourselves? Making PCB's isn't that difficult; after that comes the soldering, which is far from rocket science... The thing that is needed is a good design to start from.

(let me know if this is too far off topic. Don't want anyone getting all huffy over that, do we now.) ;)

myelectricbike

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Re: Motor BEMF
« Reply #3 on: September 01, 2007, 08:06:21 PM »
First Dave you have to understand what electromotive force is. Think of it this way. when a conductor is in the presence of a magnetic field things do not get interesting until either a current passes through the conductor or the magnet is moved. In the case of the magnet moving a current is induced in the conductor and in the case of a current flowing through the conductor the magnet is moved.

These two inverse relations are a phenomenon we call the electromotive force. And it may help to clarify your understanding to note that "back" EMF is more commonly known as Counter EMF, or CEMF. (...better stop here. Don't want to confuse anyone by getting too technical.)  ;)
« Last Edit: September 02, 2007, 07:53:46 AM by myelectricbike »

cadstarsucks

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Re: Motor BEMF
« Reply #4 on: September 01, 2007, 08:48:01 PM »
How about a layman's explanation of BEMF? I have a good electronics foundation -- Air Force trained for communications maintenance -- but a lot of this theoretical stuff goes right over my head.

Also, if someone (you?!) came up with a good design for the ideal controller and made it open source, couldn't hobbyists like ourselves buy the components and put it together ourselves? Making PCB's isn't that difficult; after that comes the soldering, which is far from rocket science... The thing that is needed is a good design to start from.

(let me know if this is too far off topic. Don't want anyone getting all huffy over that, do we now.) ;)
LOL  I would unfortunately I would need permission as what I consider the best technique is not:

http://www.tinaja.com/magsn01.asp

It ends with a 3 phase power bridge such as what is in a every controller.  But it starts with a string of numbers in a micro or even just a memory chip... The string of numbers has the side benefits of reducing the harmonics in the motor and making the low speed torque smoother.

The halls tells the micro where in the cycle the motor is to maintain sync and how fast it is going.

The micro controls the bridge.

In the case of a higher rail voltage than the motor rating the micro can apply a lower voltage sine simply by changing to a lower amplitude data stream for lower speeds to maintain the current draw to reasonable levels-20A in the case of an HBS-36.

One thing I have not been able to dredge up for some reason is what frequency these motors are being driven at. :(

BEMF... Back ElectroMotive Force is the opposite of the motor effect - the generator.  Just as when current goes through a conductor that is in a magnetic field the conductor is pushed (as in a motor) when a conductor is moved through a magnetic field a voltage is produced that will push current through an external circuit.  That voltage is proportional to the rate at which the conductor is passing through magnetic flux and hence proportional to RPM in the motor or generator.  A car regulator regulates by reducing the field current in the alternator which reduces the flux which reduces the output voltage at a given motor RPM.

Dan

cadstarsucks

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Re: Motor BEMF
« Reply #5 on: September 01, 2007, 09:08:13 PM »
First Dave you have to understand what electromotive force is. Think of it this way. when a conductor is in the presence of a magnetic field things do not get interesting until either a current passes through the conductor or the magnet is moved. In the case of the magnet moving a current is induced in the conductor and in the case of a current flowing through the conductor the magnet is moved.

These two inverse relations are a phenomenon we call the electromotive force. (...better stop here. Don't want to confuse anyone by getting too technical.)  ;)
Wait a minute.  You admit that a motor and a generator are equal but opposite effects, and I presume you would admit that when you draw current from a generator you drop voltage on it's internal resistance...but you won't admit that there is a drop between the "motor" and it's internal resistance?!?

Dan

myelectricbike

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Re: Motor BEMF
« Reply #6 on: September 01, 2007, 09:53:08 PM »
Its not a matter of "admission." You seem to have a mind set, possibly from another forum or from your work or home environment, that there is some kind of automatic and competitive ying/yang here, when there is not. This forum is not intended to be confrontational but rather cooperative. We are not here on the basis of controversy but rather we are here on the basis of assistance and cooperation. In that regard this is all I can say to assist you in adapting your attitude to the intent and rightful purpose of this forum. Please keep this in mind if the intent of your remarks is to merit response from me and other owners who are here to give and to get help from each other rather than to argue.

Aside from this I am having difficulty with your purpose here and what you are trying to say. (Dave, on the other hand, understands exactly what it is you are trying to say. ;D)

We know that Counter EMF is proportional to motor speed and can be used to measure RPM but with far more circuit complexity than when using a Hall sensor.

As for internal resistance I am not sure exactly to what you are referring to although you may be referring to inherent capacitance or the charge stored as an induction current in the motor winding when power is removed, which requires induction dumping before Counter EMF is stable in proportion to RPM.

Under no load conditions inherent capacitance or the charge stored as an induction current in the motor winding is small and dumping occurs on the order of milliseconds.

Under load, however, the current through the windings is much greater than under no load condition and a much larger inherent capacitance or charge stored as an induction current in the motor windings is developed which takes much longer to dump before Counter EMF becomes stable, with the initial spike also going deeper into the negative region.

But again I am not really sure if this is what you are referring to by internal resistance and even so what point you would be trying to make or what question you are trying to ask.
« Last Edit: September 02, 2007, 10:50:23 AM by myelectricbike »

Dave

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Re: Motor BEMF
« Reply #7 on: September 02, 2007, 09:12:30 AM »
... Aside from this I, like Dave, am having difficulty with your purpose here and what you are trying to say.

... But again I am not really sure if this is what you are refering to by internal resistance and even so what point you would be trying to make or what question you are trying to ask.

Hold up there pardner; I have no difficulty with Dan's "purpose here". I'm just trying to better understand how these things work from someone who has a solid understanding of how they work. You speak for yourself. Please don't lump me in with your issues.
« Last Edit: September 02, 2007, 09:18:51 AM by Dave »

myelectricbike

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Re: Motor BEMF
« Reply #8 on: September 02, 2007, 09:29:42 AM »
Well thanks there Dave. You finally told us what your purpose for being here is. Thanks for letting me know my inclusion accomplished something. For all we knew your purpose in posting hear was to promote Crystalytes or LaFrees.   ;D
« Last Edit: September 02, 2007, 09:40:32 AM by myelectricbike »

Dave

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Re: Motor BEMF
« Reply #9 on: September 02, 2007, 10:04:40 AM »
Well thanks there Dave. You finally told us what your purpose for being here is. Thanks for letting me know my inclusion accomplished something. For all we knew your purpose in posting hear was to promote Crystalytes or LaFrees.   ;D

The only thing you've accomplished is solidifying your position as Official Golden Motor Troll.

myelectricbike

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Re: Motor BEMF
« Reply #10 on: September 02, 2007, 10:23:23 AM »
Sorry you feel that way Dave, but I was here long before you which makes you the troll, now doesn't it? :(
« Last Edit: September 02, 2007, 10:41:37 AM by myelectricbike »

cadstarsucks

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Re: Motor BEMF
« Reply #11 on: September 02, 2007, 11:38:58 AM »
Its not a matter of "admission." You seem to have a mind set, possibly from another forum or from your work or home environment, that there is some kind of automatic and competitive ying/yang here, when there is not. This forum is not intended to be confrontational but rather cooperative. We are not here on the basis of controversy but rather we are here on the basis of assistance and cooperation. In that regard this is all I can say to assist you in adapting your attitude to the intent and rightful purpose of this forum. Please keep this in mind if the intent of your remarks is to merit response from me and other owners who are here to give and to get help from each other rather than to argue.
Actually I like to but but get very frustrated when misunderstanding is the rule of the day.
Quote
Aside from this I am having difficulty with your purpose here and what you are trying to say. (Dave, on the other hand, understands exactly what it is you are trying to say. ;D)

We know that Counter EMF is proportional to motor speed and can be used to measure RPM but with far more circuit complexity than when using a Hall sensor.

As for internal resistance I am not sure exactly to what you are referring to although you may be referring to inherent capacitance or the charge stored as an induction current in the motor winding when power is removed, which requires induction dumping before Counter EMF is stable in proportion to RPM.
Sorry I used the wrong term, my bad.  Counter EMF is there regardless of if you can see it or not.  It is the motor voltage exclusive of losses and in an electronic simulation would be electrical energy converted into mechanical energy which would then be subject to friction, rolling and wind resistance losses.
Quote
Under no load conditions inherent capacitance or the charge stored as an induction current in the motor winding is small and dumping occurs on the order of milliseconds.

Under load, however, the current through the windings is much greater than under no load condition and a much larger inherent capacitance or charge stored as an induction current in the motor windings is developed which takes much longer to dump before Counter EMF becomes stable, with the initial spike also going deeper into the negative region.

But again I am not really sure if this is what you are referring to by internal resistance and even so what point you would be trying to make or what question you are trying to ask.
What I am saying is that CEMF is always there and is always stable with motor RPM, even if you can not see or use it.  It is the CEMF that sets no load speed in a BDC motor and the winding resistance that lowers the speed with load.  The motor accelerates until the CEMF equals the applied voltage minus the current times the winding resistance at which time the system stabilizes.

In a pure electronics simulation the motor is represented by a battery whose voltage is directly proportional to motor speed in series with the winding resistance, inductance and capacitance.  The motor speed in turn would be effected by the mechanical, feedback and control systems which makes for some very messy and often undefined math.  (you can make some measurements and assumptions but over the life of the system, component wear changes things)

Dan

myelectricbike

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Re: Motor BEMF
« Reply #12 on: September 02, 2007, 12:49:47 PM »
...
What I am saying is that CEMF is always there and is always stable with motor RPM, even if you can not see or use it. 

Granted... is aways there... but not granted that it is always stable but rather that upon removing applied power to the motor there is a spike in the negative direction and a quick return to a stable CEMF under no load condition and a more pronouced negative spike and longer time for retun to stable CEMF under a condition of load.

Quote

 It is the CEMF that sets no load speed in a BDC motor and the winding resistance that lowers the speed with load. 

Surely your reference to "winding resistence" then means "inherent capacitance or the charge stored as an induction current in the motor winding." Please correct me if this interpretaion of what you mean is incorrect.

Quote

The motor accelerates until the CEMF equals the applied voltage minus the current times the winding resistance at which time the system stabilizes.

Humm... CEMF=Va-I(?) If indeed you mean "inherent capacitance or the charge stored as an induction current in the motor winding" then this can actually be determined by running the motor under load then switching if off and measureing the amount and time of recovery of the induction dump between the initial dump spike and stable CEMF voltage.

Quote

In a pure electronics simulation the motor is represented by a battery whose voltage is directly proportional to motor speed in series with the winding resistance, inductance and capacitance.  The motor speed in turn would be effected by the mechanical, feedback and control systems which makes for some very messy and often undefined math.  (you can make some measurements and assumptions but over the life of the system, component wear changes things)



An ideal battery (zero internal resistance) of 36 volts, which is supplying power to an external load resistance of 2.592 ohms will produce a current of 13.89 amps and a power of 500 watts. If the battery has an internal resistance of .01 ohms then with this load the terminal voltage of the battery will drop to 35.86 volts. The output current will drop to 13.84 amps and the output power will be 496.16 watts.
« Last Edit: September 02, 2007, 01:24:21 PM by myelectricbike »

cadstarsucks

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Re: Motor BEMF
« Reply #13 on: September 02, 2007, 01:26:40 PM »
An ideal battery (zero internal resistance) of 36 volts, which is supplying power to an external load resistance of 2.592 ohms will produce a current of 13.89 = A and a power of 500 watts. If the battery has an internal resistance of .01 ohms then with this load the terminal voltage of the battery will drop to 35.86 volts. The output current will drop to 13.84 amps and the output power will be 496.16 watts.
Well skipping to the end of your verbose and erroneous post ...  V=IR, it does not matter if it is the internal RESISTANCE of a battery or a motor. 

Therefore if the speed of the HBS-36 motor drops from 308RPM to 216RPM we get that it's internal resistance is (36V (the curve test voltage) x (1-216RPM/308RPM)) (the voltage across internal resistance) / 20.5A (the full load test current) = 0.5 ohms

As a proof you go back the other way - P=I²R ...  20.5A²0.5=210W which coincidentally is just about the difference between the curve input power of 730W minus the curve output power of 509W.  Where then is MY error?

Dan
« Last Edit: September 02, 2007, 01:41:46 PM by cadstarsucks »

myelectricbike

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Re: Motor BEMF
« Reply #14 on: September 02, 2007, 01:47:36 PM »
Confrontational again. That is your error.

If the battery pack has an internal resistance of .5 ohms then with the external load of 2.592 ohms the terminal voltage of the battery will drop to 30.18 volts. The output current will drop to 11.64 amps and the output power will be 351.37 watts.  Measuring the internal resistance of a battery by applying an external load and then measuring the voltage drop is how you determine whether your battery has failed which simply measuring voltage across the terminals in a no load situation does not reveal. Also voltage measurements must be taken over time since a bad cell or other anomaly may take some time to show.

Another issue...

...
As a proof you go back the other way -

One never tries to prove the validity of a method or formula by going "back the other way." Going back the other way is reserved to check arithmetic only and not the validity of the formula or method.

Also, you have provided any proof to contradict anything I've said.


« Last Edit: September 02, 2007, 02:44:06 PM by myelectricbike »