# How to measure the back emf of a DC motor?

Discussion in 'General Electronics Chat' started by Pratik_S, Apr 14, 2015.

1. ### Pratik_S Thread Starter Member

Apr 14, 2015
49
2
Hey,
I have implemented the circuit attached along on a breadboard. i have used BC547 npn transistor and 4007 diode a 12V battery (power supply) and a 12V , 60 RPM , DC motor as an inductive load. My main aim is to calculate the back emf that the motor generates and after that i want to verify if it is being reduced or eliminated by adding a freewheeling diode in the circuit as shown. If any one out here could explain me in simple terms would be of great help. Thanks in advance!

Experiments that i have tried are:-
1. I removed the freewheeling diode and checked the voltage across the motor on the DMM by turning it ON and OFF expecting that while disconnecting the battery terminals i would get a negative voltage value for few seconds till the motor stops rotating. But i didn't see such a response. Is this the right method?

2. Then i tried the other method :-
As soon as the coil in the motor starts rotating , a back emf will be induced in it due to the flux that it cuts , and this will tend to reduce the current through it,
let the supply emf be 'E'
the bck emf be 'e'
Resistance of the coil be 'R' and
the current be 'I'
then, I=[E-e]/R ...(i)
since e is directly proportional to angular speed 'w'
as there is increase in angular speed the current will decrease
SO i measured the resistance of the motor by puuting an ammeter and a voltmeter as shown in the second attachment.
After the results seen on the ammeter i calculated the Resistance , by V=IR ;
and then substituted these values of I,R and E in the formula (i) still the result i got was e=0 with the freewheeling diode and without the freewheeling diode in the circuit.

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Jul 31, 2013
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The back emf of a running motor will never exceed the supply voltage, I would think of it as internally changing the impedance of the motor only. Externally the supply voltage will always be across the motor terminals and the diode will always be reverse biased and so will never conduct.
As shown in the diagram it will perform the same function as a snubber diode in a static inductive circuit (ie a relay ) and could protect the transistor from momentary reverse voltage spikes on opening the switch (transistor) I don't know if rotating motors behave like that though. As always ..I might be wrong but its kicked off the conversation..
I would add that it's unlikely you would see any reading for the back emf on stopping the motor with a DMM because they are too slow in resolving the voltage (especially auto-ranging ones). An analogue meter might show a flick but a storage scope would be the way to see it...
Also...can we assume it is a plain ordinary dc motor that acts as a generator when turned off..If it's a brushless dc motor with built in drive circuitry there may be no back voltage to observe (?)
Answering the first question, whatever the formulas show...yes the diode will limit any short reverse voltage spike or longer generator voltage as the motor slows, to .6v. The motor will also slow down faster..

Last edited: Apr 14, 2015
3. ### Pratik_S Thread Starter Member

Apr 14, 2015
49
2
@cornishlad Yes as you said the back emf wont exceed the supply voltage.. and thanks for starting the conversation.. i need a more specific answer that will solve my query.

4. ### cmartinez AAC Fanatic!

Jan 17, 2007
3,698
2,792
Take a look at this article. It explains in detail what back EMF is and how you can measure it.

JohnInTX and #12 like this.
5. ### #12 Expert

Nov 30, 2010
16,704
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Good job, @cmartinez
Some questions just can't be answered in 30 minutes of typing. Finding the website that tells the whole story is much better than saying, "Google it yourself". My Google-fu isn't all that good, so I appreciate your effort.

6. ### cmartinez AAC Fanatic!

Jan 17, 2007
3,698
2,792
Yeah well, thanks... I once got my knuckles rapped by a moderator in this site for giving away the answer to a question too easily... It was then that I understood that this site is for people to learn electronics and not just given the answers to everything they ask... It's the same educational approach that I've taken with my own kids all their lives, and so far it's worked wonderfully (my 18-year old has just been granted a scholarship by my country's most prestigious university) ... the system is simple: first do your homework, then I'll help you with any doubts you may have.

7. ### RRITESH KAKKAR Senior Member

Jun 29, 2010
2,831
89
Hello,
i have never tried this in deep calculation.
but yes i remember very long days back.
There was a transformer, i was just testing to get bass in speaker connected with radio audio board.
so, i just noticed that the spark with sudden shock comes in wires this was in school days.
anyway, after years back in engg days i was building home made ups which connecting high amp source and toutchingf the wires at input with Mosfet or BJT was giving a shock.
it was great test, never do this.
anyway, just use college Oscilloscope/CRO to see/ watch the feed on it, Multimeter will not work here!

8. ### JDT Well-Known Member

Feb 12, 2009
658
85
The back EMF of a relay coil and the "back EMF" of a brushed motor are not really the same thing.

With a relay, there is an inductive effect where the inductance wants to keep the current flowing. This means that when the drive is removed, the voltage instantly REVERSES and, if not limited, can reach a high value.

With a motor, when the drive is removed, the motor continues to turn due to its mechanical inertia. The motor then operates as a generator, producing a voltage with the SAME POLARITY as the original drive voltage. This generated voltage is proportional to the speed of the motor and so can be used as a way of measuring the speed.

Often, the motor speed is controlled by pulse-width-modulating the supply. During the time when the drive is off, the GENERATED voltage can be sampled to measure the actual motor speed. This can be fed back to the controller to get a constant speed with changing load.

Of course, a motor is also inductive. So at the instant of the switch-off there will be a negative spike that will quickly go back to the positive generated voltage. So a back-EMF catching diode is still needed.

Jul 18, 2013
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There is a condition where it will occur this is in a large overhauling load where the motor rpm will decay slower than the applied voltage.
Max.

Jul 31, 2013
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Glad you put that right..ie The back emf is the same polarity as the supply. I was thinking that then wrote something silly ! So please ignore "or longer generator voltage as the motor slows" in #2

11. ### PackratKing Well-Known Member

Jul 13, 2008
850
216
A DMM, hitched across the motor terminals, will measure supplied DC voltage...

The action of the brushes / commutator, will impress a back AC emf onto the circuit... that can be measured by switching the DMM to AC... What I haven't done yet, is find out if that represents actual voltage, or useable current.
It sure has the ability, to generate highly annoying QRM in radio...

Jul 18, 2013
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This is brush noise due to arcing.
Max.

13. ### atferrari AAC Fanatic!

Jan 6, 2004
2,670
784
It seems I have to re read the article several times (after all, electric motors rank VERY high in my stumbling blocks list). (Maybe at the very top...)

If I say: stop applying DC for a brief moment and measure the voltage at the same terminals where that DC is being applied, what you get is BEMF, am I right?

Not pesimistic but I cannot believe it is that simple. As my ex insisted in saying, I was always wrong. Why not now?

14. ### cmartinez AAC Fanatic!

Jan 17, 2007
3,698
2,792
It is that simple... DC motors are also generators, and (the way I understand it) when they reach unloaded top speed it's when both fields (forward and backward) reach a balance... otherwise they'd keep on spinning faster and faster, on to infinity...
Would the veterans in this forum please step in and correct me if I've said something stupid? thanks...

15. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
Worked for me. I was building a run-on starter detector for one of my airplanes. If the Bendix Starter Drive hangs up, the engine back-drives the starter (Permanent magnet field) motor, turning it into a generator. To test this, I took a starter, connected it to a 12V battery, spun it up, and then broke the battery connection and watched the BEMF as the motor coasted down.

Jul 18, 2013
10,859
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DC motor shunt or series?
The popular Universal motor and starter motors in general are series motors, these essentially operate in the runaway condition and can, if unloaded run up to the point where they disintegrate.
One reason why you should not run an automotive starter on the bench, it winds up pretty high. (Unless PM field of course)
This is the main reason that DC motor drives with wound field motors, have a field loss detection.
One question that used to be in many formal exams was, If a DC shunt wound motor is running and the power to the field is removed, What occurs?
1/ The motor slows and stops.
2/ The motor speeds up to destruction.
3/ The motor carries on running normally.
Max.

17. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
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I tested both series-wound and permanent magnet starter motors because both are used in airplanes. I was very careful not to overspeed an unloaded starter on the bench during testing.

There is a big difference in the BEMF of a permanent magnet motor vs a series-wound one. In PM motors, the BEMF was several volts, while with a series motor, it was a fraction of a Volt due to some stray magnetism in the poles. I made a circuit that lights a LED to indicate a run-on starter with either type of motor...

Apr 14, 2015
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19. ### Pratik_S Thread Starter Member

Apr 14, 2015
49
2
No access to CRO's finding a way to measure it on a DMM. Thanks.

RRITESH KAKKAR likes this.
20. ### Pratik_S Thread Starter Member

Apr 14, 2015
49
2
Hey i tried this didn't work, can you please elaborate about the connections?