series dc motor to generator 2

[jmarsha]

Question.
If I'm using a series dc motor to move my car, will it turn into a generator when the car is going down hill? And which way will the current flow ? The same way as when it was a motor? I would apply a load other than trying to charge the batteries directly.
Thanks in advance. John

 

[jbartos]

It depends on a type of the dc motor. Some dc motors will supply dc current, become generators, if the shaft speed exceeds its rated speed, approximately, and the current will flow in the opposite direction. A dynamic breaking resistor may be used instead of battery charging.

 

[jmarsha]

Great..
So far this is good. Please help me through this. " if the shaft speed exceeds its rated speed," If I'm coasting down hill and reduce the voltage to the motor would this cause it to generate? Also is there a problem with brush location when this happens?
"It depends on a type of the dc motor." I was hoping to use a series dc motor with no interpoles.
Talk to me about this " dynamic breaking resistor "

Thanks John

 

[don01]

jmarsha,
I suggest to get some info on dynamic breaking have a look at articles by train buffs. Also research hoists and cranes.
I will take some quotes from an Australia mag called "Silicon Chip" Sept '88 issue
The article is part 11 of "The Evolution of Electric Railways " & talks about Brisbane Aust. Electrification.
" ... driven by 8 ASEA 480V 310amp DC traction motors... connected in series pairs across the 110V dc. .....the dynamic brake acts by varying the current to the field windings of the traction motors while a heavy duty 1.29 ohm brking resistor is connected across the armatures......Because this regeneration process depends on motor armature speed the braking control system must sense train speed and apply more field current to the motors as the train slows down... below 40kmh this decelaration rate cannot be provided by dynamic breaking alone as this would demand too much field current. Below 40 kmh air brakes steadily take over.."

As i said this is from a magazine not my brain, and it does not have any formulas but I hope it has some value and helps you with the hunt
I am not sure if these are series or compound motors. Other articles reference using the transmission lines as the braking load. For trains using the same supply etc

regards
Don

 

[jmarsha]

I'm not to anxious to throw this energy away, as with braking. I would like to catch it and put it back in the batteries or a fly wheel or pump water up on the roof. I also want to stay with this series DC motor for starting torque reasons. The theory I'm having trouble with is the downhill spinning of this motor with reduced or 0 input voltage, will it start to generate current in the opposite direction? The residual flux. The brush location. The left hand vs. the right hand rule. I think I understand but sure would like to hear someone else explain it.
A test engineer having trouble with E=IR.
John

 

[jbartos]

Suggestion: Reference:
1. Smeaton R. W. "Switchgear and Control Handbook," Second Edition, McGraw-Hill, 1987
Reference 1, page 24-18, Par. 12 "Reversing Dynamic Lowering Hoist Control" suggests to connect the series motor as a "shunt" motor with some useful details and References.

 

[xnuke]

Basics:
To produce motoring action, it requires that a current-carrying conductor be present in a magnetic field.
To produce generating action, a conductor, a magnetic field, and relative motion between the two are required.

Explanation:
There are two voltages present in the DC motor which affect the amount of current through the armature and its direction: the terminal voltage applied (Vt) and the counterelectromotive force developed in the armature coil (CEMF).

In the normal case of motoring, no CEMF is initially present to oppose Vt and a large current flows through the low-resistance armature creating a magnetic field which interacts with the stator's magnetic field to produce motor action. This causes the armature to rotate since the requirements for motoring action are met. The direction of rotation can be determined by the right hand rule for motors, where the thumb points in the direction of conventional current flow, the index finger points in the direction of the magnetic field from north to south pole, and the direction of motion is given by the middle finger (provided all three fingers mentioned are at 90 degrees to each other).

Since the armature (a conductor) is now rotating in the stator's magnetic field, there is relative motion between the two, and the requirements for generator action are met. This is what produces the CEMF. The direction of the CEMF can be determined by the right hand rule for generators where the thumb points in the direction of relative motion, the index finger points in the direction of the magnetic field from north to south pole, and the direction of the positive pole of the CEMF is given by the middle finger (provided all three fingers mentioned are at 90 degrees to each other). The armature CEMF thus opposes the Vt, which is Lenz's Law, and limits the current flow during normal operation. The magnitude of the CEMF is determined by Faraday's Law, which states that the voltage generated is proportional to the rate at which the conductor cuts the magnetic lines of flux.

While motoring, Vt will always exceed CEMF at constant shaft load and speed. If the speed increases due to the load going down (as is the case when going down hill without any brakes applied), the rate at which the armature (conductor) cuts the magnetic lines of flux goes up, causing CEMF to go up. Now CEMF > Vt, producing a generating condition and current flows in the exact opposite direction that it was flowing while motoring, that is, back toward the positive pole of your battery (or other voltage source). As this occurs, the kinetic energy is converted into electrical and the DC machine speed slows.

I would think the energy of the generating current may be stored for later use, though I would not recommend recharging your battery with it directly, as problems may result due to the fact the current generated would be difficult to control and could overcharge your battery, producing gaseous hydrogen which may reach explosive levels. I would store the energy in a capacitor bank or moderate it somehow before charging the battery with it in a controlled fashion.

The dump trucks I drove in an iron mine during the summers while I attended college used this principle of converting the wheel motors into generators, called dynamic braking, to lower the speed of these huge trucks to a point where a friction brake could safely be applied without burning out the brake pads too frequently. The energy created by dynamic braking was radiated as heat through huge resistor banks rather than stored for reuse. You could see the grid packages glowing red at night sometimes on the trucks that had to go downhill carrying heavy loads frequently. I alway thought this was rather wasteful.

Best of luck with your understanding of this feature of electromagnetism.

 

[jmarsha]

OK
In the armature windings the phase difference between Cemf and Vt is 180 degrees. How does the storage capacity of a capacitor compare to that of a battery? I have seem some work on this and I will research.. Thanks again. John

 

[jbartos]

Suggestion to Xnuke February 24:
It appears that the battery would be overcharged, from an energy standpoint, in case that one started with the fully charged battery and drove downhill, or when the downhill trip was enormously long or with a very heavy load carried by the vehicle and the battery was only partially discharged. In that case, one needs some dynamic braking and one will waste some energy. Apparently, there will always be situations and trips which will waste a noticeable amount of energy. The battery recharging will work efficiently in some instances only, unless the battery is vastly oversized. The capacitor storage is probably less practical since the capacitors tend to be voluminous in comparison with other energy storage options. Clearly, one would have to produce something valuable and marketable while driving through situations that require a lot of dynamic braking.