Secret Of Back EMF

[tinman]

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 ?

No-the only one that claims this is Tommy.

What charges the caps is the collapsing magnetic field around the inductor,when the mosfet becomes open. When you have a low resistive load placed on the inductive kickback,the magnetic field collapses slowly(motor bogs down),when you have a high resistive load,the magnetic field collapses much more quickly(caps have high internal resistance)-motor dosnt bog down so much.

The simple SSG pulse motor is a good learning tool,and you can see all this happen on a scope when you place different resistive loads on the kickback output.

[TommeyLReed]

Hi TinMan,

Please explain where the high voltage comes from?

If this is a simple load on the motor then why would the caps reach 50v-100v?

Tom

[tinman]

Hi TinMan,

Please explain where the high voltage comes from?

If this is a simple load on the motor then why would the caps reach 50v-100v?

Tom

Hi Tom
I am glad you asked,as the answer will show you why it isnt the backEMF you are collecting. The back EMF voltage is always less than the supplied voltage to the coil(coils) in the motor. I think you were supplying 19 volts from your power supply?,so the back EMF voltage would be less than that. This means that the 50-100 volts in your cap could not be from the back EMF.

When you power up an inductor(coil),a magnetic field is built around the inductor(this im sure you know).When the power flowing into the inductor is abruptly disconected(mosfet opens),the magnetic field around the inductor collapses.The speed that this field collapses is directly related to the load applied to the inductive output-(in your case,caps),and the voltage produced by the inductive kickback is directly related on how fast that field can collap's. So if you have a very low resistive load on the kickback output(say 10 ohms),then the maximum voltage reached may only be say 12 volts across that 10 ohm's-but the current will be high and flow for longer period of time, due to the magnetic field collapsing slower. If you have say a 100 ohm load on the inductive kickback output,then your voltage across that 100 ohm load may be 70 volts,but the current flow will be lower,and for a shorter period of time,because the magnetic field collapses faster.

As your caps have a high resistance value in your setup,you can achieve a high voltage from the inductive kickback.If you place a 10 ohm resistor across your cap's,the voltage will drop,but the current will rise,and your motor will bog down.
Look at your inductive kickback as a generator hooked to a motor-the only difference in your case ,is that the generator and motor are all one device. But as you load the generator with heaver loaed's,you will see your motor bog down-just as in a nomal generator/motor system.

If you like,i can take the time to build a pulse motor,and make a video showing you on a scope how all this happens.

[MarkE]

Hi TinMan,

Please explain where the high voltage comes from?

If this is a simple load on the motor then why would the caps reach 50v-100v?

Tom

Tom the energy that builds up in the motor magnetizing inductance has to be dissipated.  The capacitor alone stores energy with little dissipation.  Voltage builds up until something gives: the capacitor and/or the diodes so that the magnetizing energy gets dissipated each cycle.

[MarkE]

Hi Tom
I am glad you asked,as the answer will show you why it isnt the backEMF you are collecting. The back EMF voltage is always less than the supplied voltage to the coil(coils) in the motor. I think you were supplying 19 volts from your power supply?,so the back EMF voltage would be less than that. This means that the 50-100 volts in your cap could not be from the back EMF.

When you power up an inductor(coil),a magnetic field is built around the inductor(this im sure you know).When the power flowing into the inductor is abruptly disconected(mosfet opens),the magnetic field around the inductor collapses.The speed that this field collapses is directly related to the load applied to the inductive output-(in your case,caps),and the voltage produced by the inductive kickback is directly related on how fast that field can collap's. So if you have a very low resistive load on the kickback output(say 10 ohms),then the maximum voltage reached may only be say 12 volts across that 10 ohm's-but the current will be high and flow for longer period of time, due to the magnetic field collapsing slower. If you have say a 100 ohm load on the inductive kickback output,then your voltage across that 100 ohm load may be 70 volts,but the current flow will be lower,and for a shorter period of time,because the magnetic field collapses faster.

As your caps have a high resistance value in your setup,you can achieve a high voltage from the inductive kickback.If you place a 10 ohm resistor across your cap's,the voltage will drop,but the current will rise,and your motor will bog down.
Look at your inductive kickback as a generator hooked to a motor-the only difference in your case ,is that the generator and motor are all one device. But as you load the generator with heaver loaed's,you will see your motor bog down-just as in a nomal generator/motor system.

If you like,i can take the time to build a pulse motor,and make a video showing you on a scope how all this happens.

The generator BEMF is always less than the supply voltage.  The motor winding BEMF is a flyback voltage limited only by the external circuit and the breakdown voltage within the motor.  Recirculating the motor winding energy speeds the motor up by supplying more average current and therefore torque.  As there is no specific DC current path provided for the winding energy, the capacitor charges up until something starts leaking badly.