The op is using a BC547 to switch the motor. 100ma max collector current. That would imply the motor is tiny and is working through a gearbox to get 60rpm. So..if it is a permanent magnet motor capable of generating dc when spinning, there would appear to be very little inertia and and also friction from the box when 12v drive is removed. I can imagine such a motor stopping very quickly giving little time to measure the BEMF. and it's steadily ramping down so good luck trying to get anything sensible from a DMM

 

Hey i tried this didn't work, can you please elaborate about the connections?

Sorry Pratik but if you ask me, please don't. I am the less qualified here to suggest anything about this. What I wrote there was my surprised provisory conclusion about a subject that has been always close to a mistery and hard to grasp.

I am following your thread with real interest. BTW, Max IS the guy who.

 

Trying to get in a lab and use their scope for the same. Thanks all for your valuable inputs. @cornishlad Thanks for clearing my concepts.

 

One way of detecting the operating voltage if unknown of a DC motor is to back feed it at the rated rpm and read the resultant DC.
Max.

 

One way of detecting the operating voltage if unknown of a DC motor is to back feed it at the rated rpm and read the resultant DC.
Max.

You mean to actually spin it and measure its output voltage?

 

I'm sure Max does mean that..But a bit tricky with your gearbox motor especially if there is a worm gear in there....

 

You mean to actually spin it and measure its output voltage?

Yes, or conversely if you know the maximum/rated rpm, apply a known fixed DCV and measure the rpm, then extrapolate the max applied voltage from this from the plate rpm.
If on a G.B. then remove the motor first maybe!
Max.

 

I guess my post was too subtle... Connect it to DC, spin it up, and break the connection to the power supply leaving the scope probe connected to the motor. As you disconnect the power supply, the motor voltage will instantly drop to its BEMF voltage which will decay as the motor spins down...

 

But that has pretty much been common fact that a free running (no load) PM motor, whether DC or 3 phase will generate a voltage who's level will be equal to the applied voltage that will cause an equivalent rpm. (minus a few losses).
Max.

 

Hey i tried by connecting this circuit on the DSO (Digital Storage Oscilloscope) . Didn't see any sudden or instantaneous voltage drop after removing the supply voltage. Some 10mv fluctuations and this drop as seen in the attached file once the supply voltage is taken off.

 

If you have a gear box attached, especially if high ratio, the rpm will drop drastically when power is removed due to the braking action of the G.B.
Remember a reverse acting diode will only come into play for any voltage that is a reverse of the applied or self generating voltage.
Is this a purely academic exercise?
Max.

 

If you have a gear box attached, especially if high ratio, the rpm will drop drastically when power is removed due to the braking action of the G.B.
Remember a reverse acting diode will only come into play for any voltage that is a reverse of the applied or self generating voltage.
Is this a purely academic exercise?
Max.

The diode is been put to eliminate the reverse current . This is actually a research exercise in which this is my task , how to measure and hence what is to be done to eliminate it.

 

What is the anticipated source of the reverse current?
Max.

 

What is the anticipated source of the reverse current?
Max.

May be the inductance in the motor .

 

The DSO picture in post #30 shows the decay of the BEMF perfectly. The decay looks like a RC time constant. The motor has stopped by the time the voltage is zero.

 

May be the inductance in the motor .

Your display shows only the resultant decay of the generated voltage, there is no reverse polarity induced voltage shown to be present.
Max.

 

The polarity of the BEMF is the same polarity of the applied voltage. Think about it: when the motor is running, the BEMF polarity subtracts from the applied voltage so as to reduce the running current of the motor. An instant after you suddenly disconnect the applied voltage, the BEMF is still there because the shaft is spinning. As the motor spins down, the BEMF decays to zero. The motor is acting as a tachometer.

 

@MaxHeadRoom[/USER] I also came across this method of coupling two identical dc motors , connecting one motor to the supply voltage and the other should be connected to the voltmeter. The voltage we get on the voltmeter is the Back emf of the motor. My question here is how to couple the motor? I tried by coupling their shafts but didn't see any output on the DVM. What coupling method should be used . Is this the correct method?

 

This is what is meant in post#24 as back feeding, if these are P.M. motors you should see an output from the driven motor.
DC for brushed, or AC for BLDC etc.
If both motors are identical, the output of the driven should almost equal the applied voltage of the driver.
Max.