# Using back EMF to charge a capacitor

#### PGIX

Joined Aug 30, 2022
10
Hello all,

I’ve been looking online and have seen in places using the back EMF of a coil to charge a capacitor, using circuits similar to what is seen below.
My question is of energy. I understand that the energy in the coils back EMF is stored energy from when the coil was energised. Firstly, if I charge capacitor the will there be any change to back EMF or supply current? e.g., more current drawn initially, magnitude of the back EMF increase, back EMF duration increases, magnetic field changes .... Also, a question on efficiency, say we have a completely lossless system where 10J goes through the coil initially and 2.5J returns through the back EMF the rest of the energy is work done by the coils magnetic field. If I can capture this 2.5J and use it would, i then have a 100% efficient system (lossless case)?

Please don’t get me wrong in not proposing free energy or anything of the like, what I’m wondering is if you could boost the efficiency of a coil which experiences large back EMF by capturing stored energy.

Thank you to anyone who can shed some light on the matter.

#### Ian0

Joined Aug 7, 2020
9,815
What you have drawn is a flyback converter, which is a very common circuit. It can be made with efficiencies >90%, though 80% is common due to the voltage drop across the diode.
The back emf from the inductor is limited by the voltage already across the capacitor.
It is normally drawn like this:

#### DickCappels

Joined Aug 21, 2008
10,175
Your drawing took a moment to recognize.

Above is a more conventional representation.

The efficiency can be very high, usually in the range of about 70 to 90%. What you have drawn is often referred to as a boost or to older people, flyback converter. It is very popular at low power, maybe less than 50 watts. Under 25 is probably more common. For higher efficiency some sort of "forward converter" is used. Where in the boost converter the rectifier conducts when the active switch is off, a forward converter, in some cases a boost converter (variation) the rectifier conducts while the active switch is on. This saves the energy from being stored then released from the inductor, and energy to be transferred more directly resulting in lower overall losses.

#### PGIX

Joined Aug 30, 2022
10
Your drawing took a moment to recognize.
View attachment 275101
Above is a more conventional representation.

The efficiency can be very high, usually in the range of about 70 to 90%. What you have drawn is often referred to as a boost or to older people, flyback converter. It is very popular at low power, maybe less than 50 watts. Under 25 is probably more common. For higher efficiency some sort of "forward converter" is used. Where in the boost converter the rectifier conducts when the active switch is off, a forward converter, in some cases a boost converter (variation) the rectifier conducts while the active switch is on. This saves the energy from being stored then released from the inductor, and energy to be transferred more directly resulting in lower overall losses.
If you connected this topology to the coil of a motor could you boost the efficency of a motor. Effectivly the inductor is used for its magnetic feild. would capturing its energy have any effect on that?

#### PGIX

Joined Aug 30, 2022
10
What you have drawn is a flyback converter, which is a very common circuit. It can be made with efficiencies >90%, though 80% is common due to the voltage drop across the diode.
The back emf from the inductor is limited by the voltage already across the capacitor.
It is normally drawn like this:
View attachment 275102

How is the back EFM be effected?

#### Ian0

Joined Aug 7, 2020
9,815
How is the back EFM be effected?
Because the back EMF appears across the diode and the capacitor. The current which continues to flow in the inductor (which gives rise to the back emf) flows into the capacitor instead.

(and no, it won’t boost the efficiency of a motor)

#### DickCappels

Joined Aug 21, 2008
10,175
If you connected this topology to the coil of a motor could you boost the efficency of a motor. Effectivly the inductor is used for its magnetic feild. would capturing its energy have any effect on that?
Not really. The inductors are designed to keep most of the magnetic force inside the core, not only for efficiency, but because in many cases the radiation causes interference with other equipment.

#### PGIX

Joined Aug 30, 2022
10
Because the back EMF appears across the diode and the capacitor. The current which continues to flow in the inductor (which gives rise to the back emf) flows into the capacitor instead.

(and no, it won’t boost the efficiency of a motor)
Am I understanding correctly that if I captured this energy to say charge batteries the motor just drawns more current from the get go. Basically, I dont actually think its possible but i need to explian why. I know that any enrgy captured will come from some where but at the same time ther energy criculates the system without the harvesting scheme. so why cant i collect it and get some energy back?

#### PGIX

Joined Aug 30, 2022
10
Not really. The inductors are designed to keep most of the magnetic force inside the core, not only for efficiency, but because in many cases the radiation causes interference with other equipment.
I dont understand how this relates. A motors coils generates a back EMF. Why cant I collect this current and for aa little extra efficency.

#### Ian0

Joined Aug 7, 2020
9,815
Firstly, we need to establish what sort of motor you are thinking about.Let’s assume that it is a simple DC permanent magnet motor.
In that case, it will only be driven by a switching MOSFET if it is being speed controlled.
if it is being driven by a switching MOSFET, it will already be equipped with a flyback diode.
That diode will already be catching the back EMF and limiting it to 0.6V so that current can continue to flow for longer in the motor windings.
Don’t forget the the flyback current starts at the value that was flowing in the windings, and reduces to zero at a rate of dI/dt=V/L, where V is the flyback voltage.
The lower the flyback voltage, the longer the current keeps flowing.

#### Alec_t

Joined Sep 17, 2013
14,313
Am I understanding correctly that if I captured this energy to say charge batteries the motor just drawns more current from the get go.
All the energy comes from the power supply. If you use some for charging a battery then either less is availble for the motor (if the supply energy is constant) or more has to come from the supply.
A motors coils generates a back EMF. Why cant I collect this current and for aa little extra efficency.
This is done all the time in electric vehicles. When the vehicle is coasting or braking its motor is acting as a generator, converting mechanical inertial energy into electrical energy which is fed back to the main battery.

#### PGIX

Joined Aug 30, 2022
10
Firstly, we need to establish what sort of motor you are thinking about.Let’s assume that it is a simple DC permanent magnet motor.
In that case, it will only be driven by a switching MOSFET if it is being speed controlled.
if it is being driven by a switching MOSFET, it will already be equipped with a flyback diode.
That diode will already be catching the back EMF and limiting it to 0.6V so that current can continue to flow for longer in the motor windings.
The motors a bit weird so ill try to keep it simple, but yes its effectivly a BLDC motor. When the phase turns off the phase continues to draw currnet due to a large back EMF. This current contains about a 3rd the energy which went in. So if i can collect this energy i could get a boost to efficency right. But in reality this takes energy somewere away right?

#### PGIX

Joined Aug 30, 2022
10
All the energy comes from the power supply. If you use some for charging a battery then either less is availble for the motor (if the supply energy is constant) or more has to come from the supply.

This is done all the time in electric vehicles. When the vehicle is coasting or braking its motor is acting as a generator, converting mechanical inertial energy into electrical energy which is fed back to the main battery.
Isnt collectiung energy form breaking collecting the energy from the rotor via its back emf. When you turn of a coil it also produceds a back efm, what about that?

#### Ian0

Joined Aug 7, 2020
9,815
The motors a bit weird so ill try to keep it simple, but yes its effectivly a BLDC motor. When the phase turns off the phase continues to draw currnet due to a large back EMF. This current contains about a 3rd the energy which went in. So if i can collect this energy i could get a boost to efficency right. But in reality this takes energy somewere away right?
Torque is proportional to current. The current causing the back emf is already flowing in the motor windings causing torque.
if you captured it and took it elsewhere, it would reduce the motor torque.

#### PGIX

Joined Aug 30, 2022
10
Torque is proportional to current. The current causing the back emf is already flowing in the motor windings causing torque.
if you captured it and took it elsewhere, it would reduce the motor torque.
The coils turn off when the feild need to be denergised to prevent a counter ratation torque. so would collecting this current be fine?

#### Ian0

Joined Aug 7, 2020
9,815
The coils turn off when the feild need to be denergised to prevent a counter ratation torque. so would collecting this current be fine?
What makes you think that whoever designed the electronic commutation for the motor didn’t do so in the most efficient manner possible?

#### PGIX

Joined Aug 30, 2022
10
What makes you think that whoever designed the electronic commutation for the motor didn’t do so in the most efficient manner possible?
I have measured it the coils continues to draw current far longer than what would be optimal. Its worth say this motor is a prototype. I unfortunatley cannot speak to the inventor as he has passed away. On the subject, even if comutation is perfect, during back emf charge passes through the coil but if i collect this charge it reduces motor torque? or is it more that the capaitor charges reducing the current through the coil? Finnally the current of the back EMF dose not come form the source it comes from the coil right. I know that that energy is still a portion of energy supplied by the DC supply but during back EMF there isnt current out the supply?

#### DickCappels

Joined Aug 21, 2008
10,175
One way to find out is to do the experiment. We are not trying to stop you from trying your idea but instead are trying to give you the benefit of our individual insights into the problem.

#### PGIX

Joined Aug 30, 2022
10
One way to find out is to do the experiment. We are not trying to stop you from trying your idea but instead are trying to give you the benefit of our individual insights into the problem.