|
Post by electrosteam on Apr 18, 2010 11:31:37 GMT
Any ideas on sizing a brake resistor for a battery operated loco ?
Resistor dissipated energy would come from the track speed (8 km/h) kinetic energy of the loco, two car batteries, two cars and 4 adults on level track. I am considering: - 300 W 24 V dc motor, - motor is 16 A full load current, - select brake resistor value 1 ohm, - resistor constructed from 10 x 0.1 ohm 5 W wire wound in series, - rated resistor current is 7 A, - peak resistor current will be 24 A, - resistor at ambient when brake applied at full speed by relay contact.
Comments welcomed on the possible results.
Happy machining, John.
|
|
steam4ian
Elder Statesman
One good turn deserves another
Posts: 2,069
|
Post by steam4ian on Apr 18, 2010 13:08:53 GMT
G'day John
Too late in the evening to do the full maths, Your resistor will dissipate 50W hence your 7 amps rated current.
At full speed your braking will try and lose 576 watts in the resistor and at half speed 144 watts.
My gut feeling is that the resistors will not be beefy enough. A single resistive step will not allow much control over the braking. You need something that will hold your speed on a grade.
You would be better off getting getting some high power resistors through RS Components or Farnell. These resistors come mounted in an aluminium heat sink.
BTW, you can now get regenerative controllers that will put the energy back into the battery. A new loco at my club has such a controller.
Regards Ian
|
|
jackrae
Elder Statesman
Posts: 1,333
|
Post by jackrae on Apr 18, 2010 19:42:59 GMT
At 8kph I think that you will find you braking effect quite small and decreases exponetially as speed reduces. I would suggest (sounds of horror screams from others) that you instal a heavy duty relay and, when braking is required, simply use it to shorten out the motor terminals. The commutator and winding resistances will control the current to safe levels. If the effect is too strong you will feel the wheels momentarily stall, but I believe this very unlikely. On our club loco we have a 4QD unit and although it has regen braking, it's effect is not extreme and an analysis of current backflow to the batteries suggest relatively low current. The attached file (I hope) shows the engine amps during a running day. The negative values are the regenerative braking back-feed jack Attachments:
|
|
|
Post by peterseager on Apr 18, 2010 22:29:34 GMT
My Hymek has a Parkside Controller capable of regen braking. When I built my remote console I ignored the regen braking. You had to turn a pot to apply it then transfer to the hand-brake on the driving trolley to bring the train to a stand at the right point. All a bit rushed. Slowing the train on the throttle control and a final touch on the hand brake seems adequate. The Hymek slows very quickly if the throttle is closed quickly. If you have a long gradient and a heavy train maybe electronic braking would be more useful?
Peter
|
|
steam4ian
Elder Statesman
One good turn deserves another
Posts: 2,069
|
Post by steam4ian on Apr 18, 2010 22:49:23 GMT
G'day
Me again. I support Peter regarding the regenerative controller. As I said last post the club loco has such a controller and you don't need the friction brake except for the last part of the braking, and then not much.
Jack's comment about effectiveness at 8kph really depends on the gearing, at your full speed the motor would have 24V at the armanture. I would NOT short the armature as Jack suggested, all the heat is dssipated in the armature which is not good. If the motor has a PM type field you risk demagnetising it.
If your track is level I concur with Peter's comments about too many things to operate in what is really a short space of time, whilst regenerative braking will work, dynamic braking fades off with falling speed. In full size dynamic braking is used for controlling trains on long grades, in our sizes I think that is also an appropriate use.
I could do the maths given more info on train weight, maximum gradient and speed you want to hold the train at. If your track is flat it isn't worth the effort.
Regards Ian
|
|
|
Post by electrosteam on Apr 19, 2010 0:36:14 GMT
Thanks all for the comments. As ever very useful.
I suspect the motor will only put out 20V, or less, in braking because the motor will initially be in overload and it is not designed as a generator. My reading of the published literature says the 5W wire-wound resistor will take 5x rating for 5 seconds. This means about 16A held for the full 5 seconds. My proposal could put out 20A initially, but quickly drop off.
The loco has standard Simplex wheels and bearings with siderods and there is a double reduction rubber belt drive - all adds to a significant frictional load.
The loco demonstrates incredible braking when hand pushed backwards from stationary against the 20 A back emf protection diode. It really is difficult to get it to move backwards by hand when required.
The reheostatic braking will be set up to be capable of staying applied with the loco key control off. The park brake will then be the friction plus the one ohm resistor, and the 20A diode in the backwards direction. I do not think it will move away on our flat club track.
Note that hand pushing can always be achieved in the direction that the reverser is set - just select the direction before pushing.
The loco will be tested late this week - results to be posted. If the resistor gets too hot, I will just add some more in series.
Happy machining, John.
|
|
jackrae
Elder Statesman
Posts: 1,333
|
Post by jackrae on Apr 20, 2010 18:45:21 GMT
John,
Irrespective of what value resistor you have for braking or reverse diodes, dynamic braking is just that, Dynamic. You will get no braking effect from the motor unless the loco is actually moving. It cannot therefore be considered as a "parking" brake.
Since my idea of using a dead short isn't finding too much favour, you might want to consider the use of 24volt truck bulbs as braking resistors. They are high wattage, capable of running hot and give some degree of visual feedback. In past project work, I've found the use of such bulbs a cheap and easy way of substituting for high wattage, low value resistors - in the good old days of valve radios, bulbs were used as ballast resistors in some circuit designs, they were called baratters then (unsure of spelling). The purist might say they have an undesirable temperature characteristic for use as brakes, but model engineering isn't a pure science. jack
|
|
|
Post by alanstepney on Apr 20, 2010 19:06:17 GMT
Barretters. And I still have a few of them in my collection.
|
|
isc
Statesman
Posts: 708
|
Post by isc on Apr 21, 2010 12:20:08 GMT
Elements for old bar heaters(you can still get them here), you can put a clip on the outside of the wire and use that as an adjustable connection. isc
|
|
|
Post by ausdan on Apr 24, 2010 2:52:16 GMT
More of a general comment really: I was...then some 20yrs have pass... going to build a controller out of 1970 ? series of ME , as I had receive a petrol. electric loco in pieces, that had some parts that were built from the said series.
Basically there were two 6" x 1 1/2" drums, with a copper tracks and a bunch of contact/fingers that would re route the power and put the motor in different configurations of in series and parallel etc.
Braking wise, it had a large hand wound 1/8 wire wrapped along a length of narrow 1" board 8" long of about 12 coils, there where notches in the board that kept the coils apart, I assume this was some type of dynamic braking.
Obviously with the luxury of petrol engine couple to a 32v generator, you did not have to worry about efficiency and pro longing battery life.
Come 25yr later, I have just completed collecting pieces for the electrical drive line consisting of 4 x 24v 300w motors, 6.5hp petrol and 24v alternator , using the modified alternator as the variable generator/power source.
Danny
|
|
|
Post by peterseager on Apr 24, 2010 20:02:11 GMT
ME 1970 it is. Issue 3402, 2 Oct 1970 thru 3405, 20 Nov 2007. This was one of several sets of articles by the author entitled "Diesel Electric Traction for Model Engineers".
For 5"G, he uses 3 yds of 10 SWG nickel-chrome wire wound as described above by Danny and having a total resistance of 1 ohm. He warns it can reach 200-300 degrees C. There is a separate load for each pair of motors and each pair of motors is wired in parallel.
The speed and brake are one control with an off position between the functions. When the 1st brake position is selected, the motors are connected in their braking configuration using the 1 ohm braking resistor. The second position selects a centre tap giving 1/2 ohm and the third position short circuits each pair of motors.
The author used series wound motors and cross connected the fields and armatures of each pair of motors to balance the heat generated in the motors.
I hope this makes sense, there is a lot in the articles.
Peter
|
|
|
Post by peterseager on Jun 8, 2010 20:00:23 GMT
John
Any results to report yet?
Peter
|
|
|
Post by electrosteam on Jun 14, 2010 8:34:31 GMT
Peter, The loco was first trialled this weekend. Three adults plus a small child on two passenger cars. Good acceleration (2 x 300 W, 24V from 2 x auto batteries) but the speed is a bit lacking. The gearing gives 5 km/h, not the 5 mph required (my mistake). The rheostatic brakes are 10 x 0.1 ohm 5 W wire-wound resistors in series on each motor. At full speed, brake application causes the belts to slip on the toothed pulleys. The motors are rated 16A, so 24 A of braking is probably a bit severe. For the few tests attempted, the resistors were barely warm. We will re-build with 1.5 ohm, or larger, and report back later. We had the track to ourselves, so after the first tentative circuit clearing twigs, bark and leaves we let her rip. Then, after lunch, a fast (?) circuit brought us off the rails as the loco attempted to leap over a newly fallen branch. The result is a shorted Mosfet - I think I hit the reverser when I dived for the control switch. John. Attachments:
|
|
isc
Statesman
Posts: 708
|
Post by isc on Jun 14, 2010 11:17:51 GMT
Perhaps you need some form of interlock on the reverse? isc
|
|
|
Post by peterseager on Jun 14, 2010 13:49:51 GMT
John Thanks for that report. Look forward the hearing about the next trial.
If you use resistors in series it will make it easy to tap in to provide a multi position brake control which I feel will be more gradeable.
Peter
|
|
|
Post by electrosteam on Jun 15, 2010 12:30:07 GMT
Peter, Thanks for the idea of a gradeable brake controller.
I selected a series string of resistors to give me a positive indication of a resistor failure but, as you suggest, it provides an opportunity to fit extra levels ( one or more) of application. Something to consider.
isc, Yes, an interlock or position the reverser well away from the control keyswitch. The physical re-positioning is easier than the interlock, so that is what we will do. The interlock can be included in the next loco development (a RhB HGe2/4). I would consider a voltage sensitive interlock, any suggestions for an alternative ?
Happy machining, John.
|
|
steam4ian
Elder Statesman
One good turn deserves another
Posts: 2,069
|
Post by steam4ian on Jun 16, 2010 2:03:40 GMT
G'day John
I agree, your braking is "probably a bit severe" if it slips the toothed belt/s.
Remember, in full size dynamic braking is only used on long grades.
I suggest you buy yourself another two dozen 0.1 ohm resistors and double up each resistor set. You really want dynamic braking when decending a grade. I can think of two grades on my home track where it would be handy.
BTW, we have a new club battery loco with a regenerative controller. It is not my preference because it means you just set the pot to get the speed. It would be more fun if there was some kind of notched controller, must work out a circuit.
Regards Ian
|
|
|
Post by peterseager on Jun 16, 2010 17:14:43 GMT
Thats the basis of the Hymek interlock. A transistor base is connected to the speed control pot via a resistor and the emmitter to 0V. When the speed pot is over 0.5 v the transistor switches on and triggers a couple of D type flip-flops that store the direction control setting. Hence the direction command to the motor controller can only change when the speed pot moves from zero. A similar circuit is included in the Parkside hand control but I have added more logic in my remote console to control the sound system for engine startup and stopping.
Peter
|
|
|
Post by peterseager on Jun 17, 2010 21:04:29 GMT
John Another thought on the direction switch, I had the same trouble with a toggle switch getting knocked, but without serious problems because of the interlock. I replaced the switch with a rotary switch. Solved the problem and it is actually prototypical. I should add that it a three-way switch with a middle Off/Engine Only position. Hence the two D-types because of the three states. With the sound system it is useful because you can demonstrate the engine sound without moving off and is also a safety feature in that when Off is selected you can't knock the throttle and start the train during loading.
Peter
|
|