Electric vehicles > Electric bikes
Secret Of Back EMF
TommeyLReed:
Hi All,
I throught it would be a great test to show some basic back EMF test on this pulse motor. This is free extra energy while you add input energy.
https://www.youtube.com/watch?v=6BnHNdMzW2I
Tom
MarkE:
Tommey that's a nice video, but the conclusions are incorrect. The energy recovered from the motor BEMF through the rectifiers into the capacitor bank all came from your power supply. The motor operates as a generator that you tap through your diode / capacitor network when the MOSFET is off. When you shorted the capacitors the generator worked against the small resistance of the diodes and the wires, slowing down a lot each MOSFET off interval. That's why the motor ran slowly. When you removed the short, the capacitors charged until a new equilibrium point was reached that balanced the power supply output energy each cycle against the electrical and mechanical losses. Instead of heating the wires as with the short, much more of the power from the power supply went into mechanical work by spinning the motor faster against the friction in the system. If you put a resistor or some other circuit that does work across the capacitor bank, then for the same power supply settings and MOSFET duty-cycle, the motor will slow down.
An experiment that you can do that I think is educational would be to put a fixed value power resistor, RSERIES, between the power supply and the motor. Set the power supply to some fixed voltage with the current limit greater than: ILIMIT >= VSUPPLY/RSERIES, and then vary the load across your capacitor, keeping track of the load resistance, load voltage, and motor speed. See what load value gives the most total electrical plus mechanical power.
TommeyLReed:
Hi Mark,
First of all there is no PWM involve in this design.
This works only off the RPM with a optical transistor and 4 incoder on the disk at every 90deg.
The change in rpms is do to the fact BEMF is not running back to the coil.
The fee energy is coming from the BEMF of power input coil.
The motor speed up while the BEMF is collected, it slow down when shorted out and increase load.
Tom
MarkE:
--- Quote from: TommeyLReed on August 13, 2014, 01:30:36 AM ---Hi Mark,
First of all there is no PWM involve in this design.
This works only off the RPM with a optical transistor and 4 incoder on the disk at every 90deg.
--- End quote ---
In the video you showed a board connected to an optical interrupter switch that drives a MOSFET that chops the motor current. And from your description above, the MSOFET chops the power supply connection to the motor. You are just using the rotation angle instead of a fixed frequency to drive the chopping / PWM. Once the motor is going some relatively constant speed the MOSFET turns on and off at regular intervals and is for most intent and purpose acts same as a fixed frequency PWM chopper.
--- Quote ---
The change in rpms is do to the fact BEMF is not running back to the coil.
--- End quote ---
Actually, that is backwards. The motor runs slowly when a lot of the energy built-up during the MOSFET ON time gets dumped into the external circuit during the MOSFET OFF time. The motor runs its fastest when there is no external circuit other than the power supply. Every last bit of energy that leaves the motor during a given MOSFET OFF interval gets replenished during the MOSFET ON time.
--- Quote ---
The fee energy is coming from the BEMF of power input coil.
--- End quote ---
If you measure carefully, you will find that there isn't any free energy. All of the energy that can be accounted for in the external circuit and work performed by the motor spinning against the friction load and then some can be shown to come from the power supply.
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The motor speed up while the BEMF is collected, it slow down when shorted out and increase load.
--- End quote ---
Absolutely, that is correct for the reasons stated above.
--- Quote ---
Tom
--- End quote ---
tinman:
@ Tommey
First you need to know that it is not the backEMF you are collecting in your cap's-you are collecting the inductive kick back from the inductors within the motor when the mosfet opens.
BackEMF(counter EMF) dose the opposite to what you state,in that backEMF is what brings the motors current draw down-not raise it as you suggest. Back EMF is the voltage, or electromotive force, that pushes against the current which induces it.If there is no back EMF,the current draw will be very high.The more back EMF you have,the lower the current draw is
To explain a little better-Lets say you are supplying your motor with 10 volts @ 1 amp,and the voltage going to the inductors in your motor is very close to 10 volts. The backEMF (once the motor is up to running speed)may be say 7 volts.This means that the voltage across the inductors is only 3 volt's.So it is taking 10 volts @ 1 amp to maintain that 3 volt's across the inductors of the motor. If there was no back EMF,then the voltage across the same inductors will be very close to 10 volt's,and it would take a lot more amp's(current) to maintain that 10 volts across the inductors.A simple way to see this is when you first start an electric motor,and there is very little backEMF.You will see a high current draw(start up current)-same as when you place an electric motor under load.
So back EMF is needed to bring current draw down-it dosnt raise the current draw of an electric motor.
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