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Then I stand corrected, obviously learning wrong in EE school, or in my head have since tied "BEMF" to the field-collapse noise, not having used the learned material since. Some lab time was spent looking at traces and learning about suppression techniques since my studies at the time was mostly in RF, not motors. Some of that Wiki page material was a real flashback to my textbooks of 50 years ago.We were playing with crude digital circuits that used the frequency of the spikes as a tachometer, versus measuring the BEMF during the SCR cutoff intervals. Given this was in the '60s I obviously went off half-cocked on the notion that with the magnitudes of advances since then, the MRR control circuits of the past decade are doing the same thing. It seemed obvious. At least to me. But whadda I know?
Back EMFI recommend you read the High Frequency Motor Drive page (CLICK HERE) before you read this page. That page provides a bit of background knowledge for what is discussed here.Back EMF (called BEMF) sensing decoders use the motor's intrinsic ability to function as a generator to sense what speed the motor is running.When you are running at less than full speed, the decoder only has the power turned on to the motor part time (roughly the percentage of full speed that you are running).The decoder senses the BEMF generated at less than full speed and adjusts the following pulse lengths to compensate for changes in the motor speed.Note that nothing in this relates in any way to how fast the pulses are being applied (i. e. whether the decoder is normal or high frequency - with or without Dither).The response time and memory of the BEMF system determines whether it is an "auto goose" to help the loco past stiction (motor starting frition), a binding mechanism or tight track work issues (short time constant) or "cruise control" to keep the train running at the same speed uphill and down.One BEMF item which is NOT frequently discussed is the number of bits of resolution. Lenz, as an example, uses a 12 bit system, which will allow the decoder to sense speed variations as small as 0.02%. Other manufacturers may be as coarse as 8 bits (0.3%). The more bits, the smoother the control.Oct 14, 2013
1) BEMF SHORT ANSWERBEMF is like cruse control for an electric motor. BEMF refers to a voltage generated by the motor that can, under certain circumstances, be measured by the decoder. That voltage tells the decoder what rotating speed the motor is going (think speedometer). With the decoder now knowing the current speed of the motor, the decoder can compare it to the speed requested by the throttle. If the speed does not match, the decoder then changes the level of power applied to the motor to correct the speed. The loop of measuring the speed and then adjusting the speed occurs continuously giving you precise regulated motor speed control under wide motor load and speed conditions.
You used SCRs to power DC electric motors? Was the power supply delivering unfiltered rectified DC voltage? ...
Yep EMF is basically exhibited as voltage, and Back EMF is voltage generated in the reverse direction.Yes, it is caused by the collapse of the magnetic field when you remove power from a winding (as power is applied to the next winding), it's not really the motor acting as a generator, a generator would imply the conversion of force into electricity, i.e. turning the shaft. It is clearly related to the inductance of the windings of the motor, since that inductance affects the BEMF as the magnetic field collapses, more inductance more voltage (also the more rapid the collapse more voltage, etc. etc)
I read it. It's correct, but a bit flawed in a couple of places, a bit misleading in a few spots. It could use a more detailed explanation, but for the purposes of his article, the simplifications he made get the point across. Your last statement is not a quote from that document, and in this case, with motors with commutators, your last statement is incorrect.I don't want to argue with you, and won't.. find an engineer with a EE or a person with a degree in Physics and he'll tell you what I am telling you.Greg
Regarding the "back EMF" discussion that seems to be turing towards argument: I think you guys are using the same words with different interpretations of the application. I do NOT have my engineering degrees in EE, but have run across BEMF in more than one context during my career. I understand and agree with what Peteski is saying about looking at the circuit while the decoder is not providing voltage to see what voltage the "motor" is producing as a generator. And, I understand the concepts of spikes from brushes making and breaking circuits, which could be used in logic to make decisions about applyhing power or actuating sounds.But, I have also seen applications of BEMF where there are no spinning motors, just pulsed coils being placed near metalic objects for the purpose of detecting holes in those objects (voids and cracks). This is looking for signals from magnetic field collapses.I have also often seen cases, many involving things other than BEMF, where two capable people from different technical disciplines are miscommunicating because they are using the same words while trying to discuss what are two different things that they are assuming are the same thing, based on the differing definitions of the same jargon between disciplines. Another application of "a little knowledge is a dangerous thing." Since I did not look-up the previous discussion that involved "holes," I don't know if that is what is happening in that thread.In the context here, it seems that there is some speculation involved about how various manufacturers do, or maybe could apply various principles to various phenomena to achieve more realistic operation for our models. Without definitive statements from the actual designers about their own products, it seems that there is room for some speculative discussion among us.
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