Secret Of Back EMF

[MarkE]

Hi Mark,

Yes that is the design, I did it in a simple drawing.

Now the question is, do you still think this is not BEMF?

As you ground out the capacitor, the two diodes run in series to positive and negative post of the coil, this shorts out the BEMF when kick back happens.

Like many transformer using dc pulse this is needed to protect the transister from being destroy from BEMF high voltage spikes.

My other question is why would the motor slow down, when I just short out the BEMF?

I believe it's from a negative pulse that pulls the motor magnet field backwards.

Question anyone?

Tom

Tom, you can approximate shorting out the motor by for example putting a really good diode across it.  The BEMF then takes on a low value as it only has to rise enough to make the diode forward conduct.  That gives the motor the highest average torque during the MOSFET off interval.  The higher the voltage that is across the motor:  D1 anode to D3 cathode, IE the greater the BEMF, the faster that current in the motor winding decays, the lower the average torque, and the slower the motor runs against any constant load compared to the "shorted" condition.  If the motor is slowing down when you close the switch, then that means that you are diverting power when the MOSFET is on due to leaky D1 and D3 diodes.

[Dave45]

Tom have you tried to recycle the kick back

[forest]

First,could you resize down images ?

I see that you all state that backEMF during coils magnetizing current (mosfet ON) is the same backEMF which charge capacitor bank.Am I right ?

I , oppositely think it is two different effects. First is due to self-inductance of motor coils , second one is due the LC oscillations between self-inductance of coils and distributed capacitance of wires.Because voltage is rised expotencially in short time it looks like a spike called flyback spike, but in fact it is ringing down oscillation.

What do you think ?

[MarkE]

Forest, if you are addressing me, I use a 72dpi resolution  which is OK for 12 point type and for B size pictures stays below the 1280 pixels that Stefan asks.  The image file sizes are generally well under 100Kbytes.  The alternatives are breaking up the pictures or making them fuzzy.

Because only one of the switches is active, only the magnetizing energy, IE energy stored as I²L_WINDING/2 goes through the D1/capacitor or switch/D3 path.  That energy builds-up when the MOSFET is ON and discharges when the MOSFET turns OFF.

The energy that is stored in the winding capacitance and other parasitics is next to nil compared to the magnetizing energy, and can be safely ignored with no loss in accuracy.  Ringing can be seen when the magnetizing inductance gets completely discharged. Then the very small remaining energy can slosh back and forth between the inductance and capacitance.

[MarkE]

Tom have you tried to recycle the kick back

Dave, in order to recycle the magnetizing energy back into the power supply an active switch is needed on both sides of the motor.