Secret Of Back EMF

[TommeyLReed]

Hi Tinman,

I built many motors and even have 19hp DC motor. BEMF it created due to starting a motor under load. When motor speed is full speed, less if any BEMF is created.

The reason speed controller fail is the increase of BEMF when motor are at the stall point.

I can also prove this with a transformer load using DC pulse. This way you just might understand how BEMF really works.

This is what BEMF does using a coil to recover BEMF. This motor is being power by BEMF only!

https://www.youtube.com/watch?v=2xLwBaub4lc&list=UUp3mD3EJromKns3YpglKdpA


Tom.

[TommeyLReed]

Hello all,

If you think Back EMF is not something special, look what happens when you use a transformer and DC pulse.

https://www.youtube.com/watch?v=Ty5CLgjmd4c

Tom

[MarkE]

Tom, Tinman is correct.  Take a small DC motor such as from a toy.  Measure the winding resistance with a DMM.  Then hook the motor up to a power supply with a current sense.  Lock the rotor.  turn on the power supply to a low voltage and measure the current.  It will be V/R.  Unlock the rotor and as the motor comes up to speed the current goes down.  The net current through the motor is:  I=(V_SUPPLY - w*K_GENERATOR)/R_WINDING.  Since the motor torque is:  I*K_TORQUE = (V_SUPPLY - w*K_GENERATOR)/R_WINDING*K_TORQUE. 

[TommeyLReed]

Hi Mark,

Clearly you don't understand my test, I know that. I was showing the effect of the diode when it cancels out the BEMF by grounding it..

As I place the diode in the motor, the BEMF is shorted out, this increase load while slowing down the motor a little.

When I take the diode out and feed the bemf into the capacitors the motor increasee with speed and load drops, that was my point of this test.

This test show the grounding out BEMF with a diode on the +/- create a greater load then without it.

That was my point on the video!

Tom.

[MarkE]

Hi Mark,

Clearly you don't understand my test, I know that. I was showing the effect of the diode when it cancels out the BEMF by grounding it..

As I place the diode in the motor, the BEMF is shorted out, this increase load while slowing down the motor a little.

When I take the diode out and feed the bemf into the capacitors the motor increasee with speed and load drops, that was my point of this test.

This test show the grounding out BEMF with a diode on the +/- create a greater load then without it.

That was my point on the video!

Tom.

Tom, there are two sources of BEMF in a motor:  BEMF induced by motion which is the generator BEMF, (flux changes across the conductors), and BEMF caused by changes in the current path to the motor windings.  When you switch your MOSFET ON or OFF the latter is at work.  When the MOSFET turns ON the current in the motor windings does not change instantly.  The motor winding inductance produces a BEMF in response to applied voltage changes that decays with time.  The current changes from its previous value following an inverse exponential towards a limit value of the difference between the power supply voltage and generator BEMF divided by the winding and circuit resistance.  When the MOSFET switches OFF inductor BEMF develops to maintain the winding current.  When it builds high enough your diode conducts and curent continues into your capacitor and whatever you have connected across it.  If the voltage on the cathode side of your diode is less than the generator BEMF, then current builds-up towards a limiting value of: (w*K_GENERATOR-V_DIODE-V_CAPACITOR)/R_CIRCUIT.  When you remove the short across the capacitor, the capacitor ultimately charges to the point that it does not load the generator BEMF.