Regeneration space theory in 4-quadrant regions of operation of DC motor 1

[mawad]

Dear All,
My background is mechanical and I was looking to understand the theory of regenerative braking in DC motors. I found this article online

http://www.pmdcorp.com/news/articles/html/Regenerative_Braking_Deep_Dive.html

In Figure 3, it shows that in braking quadrants 2 and 4 there is regeneration space which you can regenerate energy within this space only and you need to apply voltage outside this space in quadrant 2 and 3 to brake or slow down the load.

I am wondering if there is textbook explain these issues and their theories. I only find this article and I looked for many textbook, but I couldn't find detail explaination of this point. Can you recommend me a textbook. I need urgently.

Thanks in adavance

 

[mawad]

Many thanks waross and cswilson.
@cswilson
Just to clarify,

In the first case I = (V - E) / R = (-10 - 20) / 5 = -6 amps, E>V and opposite signs and the electrical power will be positive(V*I) while in the second case I = (10 - 20) / 5 = -2 amps, E>V and same sign, but electrical power is negative (V*I). I understand that both case 1 and 2 are in braking mode as E>V, but one has negative power and other positive power. Could I consider the regeneration happens only with the negative electrical power case.

 

[cswilson]

No, that is absolutely not the case! (But it is a very common misconception.)

Consider the motor as a "black box" and just look at what happens electrically at the terminals. If V and I are of the same sign, you are transferring electrical power into the motor, converting electrical power into mechanical power. This is called "motoring".

If V and I are of opposite signs you are transferring electrical power out of the motor, converting mechanical power into electrical power. This is called "generating".

To confirm, consider the mechanical system. If the applied torque and velocity are of opposite signs, the motor is being decelerated by the electrical system. The motor is losing kinetic energy, and that energy must go into the electrical system. This is called "generating", and it occurs regardless of whether E and V are of the same, or different, signs.

If you read Skogs' initial reply above, the attempt in the diagram you showed to make a distinction between the two cases you cite is one of the key things he did not like about the diagram.

 

[waross]

Another way to view this is be looking at the armature current. In the real world the brush-gear is somewhat non-linear so for the sake of simplicity we will neglect the voltage drop across the brushes for now.
The simplified armature current will be determined by the DIFFERENCE between the applied voltage and the back EMF.
Consider a motor driven at a speed such that the back EMF equals the applied voltage. If the motor is allowed to slow down, the back EMF will decrease and the difference in voltages will cause a current to flow. The slower the motor, the greater the current and the greater the torque trying to accelerate the motor.
If the motor is driven over-speed, the back EMF will exceed the applied voltage and the machine will be generating, or regenerating if you prefer.
The difference between the applied voltage and the back EMF determines the armature current.
The armature current, and the relative directions of the current and applied voltage determines the quadrant of operation.
The difference between the applied voltage and the back EMF is relatively small for all normal modes of operation. (Save for starting. The same relationship holds but the voltage differences and the current may be many times normal during starting.)
The difference in speed between full current motoring and full current regeneration is relatively small.
There is a relationship between voltage difference and the speed range between motoring and re-generating.
Work on an understanding of how the difference between back EMF and applied voltage affects motor action or re-generation, at all speeds and in both directions..
Then try to express those relationships in a four quadrant graph.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mawad]

Many thanks, for your answers. I have another question. In hybrid vehicles during regenerative braking, the motor is used as generator and there is no applied voltage and will work in two braking quadrants and in this case EMF=-I(Ra+Rcircuit)where Ra the armature resistance and Rcircuit is the regenerative circuit resistance, am I correct! If so, the motor/generator in this case can't brake high loads as it will be limited to the vehicle speed and the circuit resistance. Also, the braking level will be controlled based on the regenerative circuit resistance. If that correct, this explains to me why the mechanical brake used with regenerative braking to dissipate the high torque and regenerate the rest. I will be grateful if you tell me what I understand is correct or not.

 

[waross]

Look up regeneration with VFDs. Many hybrid vehicles are using induction motors. As the frequency is lowered on an induction motor turning at a constant speed, the current will drop. As the frequency drops below the frequency associated with the motor synchronous speed the motor will go into regeneration and the motor will regenerate. An induction motor rated at 1760 RPM at 60 Hz. will generally develop full rated torque down to about 40 RPM. When greater braking force is needed the friction brakes will assist the regeneration.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cswilson]

mawad:

You say, "In hybrid vehicles during regenerative braking, the motor is used as generator and there is no applied voltage."

This is not correct. Hybrid and electric vehicle drives are a lot more sophisticated than that. As you step on the brake pedal, it will apply a voltage somewhat less than the motor's back EMF to create an opposing current (and therefore braking torque) that is approximately proportional to the amount you have depressed the pedal.

The limitation on the current is how fast the battery pack (possibly supplemented by a capacitor bank) can absorb this regenerated current. Generally, the controls are set up so that as the system gets close to this limit, the mechanical brakes are engaged as well.

It's easiest to understand this principle with DC motors. As waross points out, these vehicles do not use DC motors. Most hybrids use permanent magnet AC motors; most pure electrics use AC induction motors. With these, you also have issues of AC phase angle, and with induction motors of slip frequency, but the general principle of voltage difference (now considered as RMS values) still holds.

 

[waross]

With regenerating induction motors, the frequency of the applied voltage has a lot to do with motoring and/or regenerating. The voltage has more to do with reactive current and power factor. It gets complicated.
Spend some time browsing this forum with an emphasis on VFDs. There is a lot of good stuff regarding induction motors regenerating through a four quadrant VFD.

Bill
--------------------
"Why not the best?"
Jimmy Carter