Wikipedia:Reference desk/Archives/Science/2017 November 18

= November 18 =

Back EMF in DC Motors
Most materials that I have read about the dc motors mostly point out the back emf generated in in the armature windings. Does this mean that no back emf is generated in the field coils? — Preceding unsigned comment added by Adenola87 (talk • contribs) 07:45, 18 November 2017 (UTC)
 * Back emf, also known as Counter-electromotive force, in a motor armature winding is voltage produced by relative motion between the armature and the magnetic field produced by the motor's field coils. (This voltage opposes the applied voltage, or allows the motor to act as a generator). Stationary field coils that carry a constant current produce a constant field so there is no back emf of this kind on them, only Ohm's law and ohmic heating apply to the DC current and voltage here. A transient change in field coil current may be detected when the motor speed or load changes. This is due to transformer coupling to the changing armature current and is not a back emf effect. Blooteuth (talk) 16:03, 18 November 2017 (UTC)
 * Incidentally, that article isn't particularly good. See Brushed DC electric motor for a more comprehensive treatement of the subject. Tevildo (talk) 19:25, 18 November 2017 (UTC)


 * When the DC motor is running, there is a back emf in the armature windings, since the current through them is interrupted by the commutator. There are usually small-value capacitors fitted across the commutator brush terminals to reduce or suppress this back emf, which causes excessive sparking, brush wear and radio interference if not suppressed. The current in the armature windings is not interrupted while the motor is running, so there's no back emf in them, but they do experience a back emf when the motor is switched off, since this is a 100% change of current. At all times when the motor is running, there is however, electromagnetic induction into the field windings caused by the magnetic field in the nearby rotating armature. "Back emf" is reserved for self-induced voltage in a coil when the current through it is changed abruptly - in almost all practical applications it is used when referring to the complete removal of current through the coil. "Induced emf" or simply "induction" is used to refer to the voltage induced in a coil by an external magnetic field whose strength is changing, as developed in the field windings already discussed. This magnetic field may be from a simple magnet moving nearby, or another nearby coil which is in motion with direct current flowing in it or a stationary coil with alternating current flowing in it. This latter AC application is the principle on which the transformer is based. Akld guy (talk) 01:46, 19 November 2017 (UTC)
 * The article Counter-electromotive force explains that the expression can apply equally validly either to voltage caused by relative motion of the armature in the surrounding magnetic field (my response) or to self-induced voltage that opposes a change in current through an inductance (Akid guy's response). @Tevildo thank you for linking to Brushed DC electric motor which is a good article. Blooteuth (talk) 14:43, 19 November 2017 (UTC)
 * Nevertheless, I would caution against using "back emf" for anything except a self-induced voltage in a single coil when the current through it is changed abruptly. It's a term that's almost exclusively used to refer to the undesirable very high voltage developed across a coil when the current is turned off, such as the several hundred volts that can be developed across a relay winding at the moment of switch-off when the relay was operating at a much lower voltage, such as 12 volts. The term would never be used to describe the voltage induced in the secondary of a transformer, for example. Its very name ("back") implies that the voltage is in the reverse polarity to that of the original operating voltage, and this is indeed the case in a single coil back emf situation. In the armature-to-field scenario, the induced voltage is reverse-polarity and aiding-polarity as the armature rotates toward and away from the field, so "back emf" doesn't seem to be a correct term. Akld guy (talk) 21:30, 19 November 2017 (UTC)
 * @Akld guy Let's be aware that the two usages of "back emf" are equally valid because they describe the same phenomenon of a magnetic field and a conductor in relative motion. The difference is which one is stationary. In my response it is the field that is stationary. In your response it is the conductor (coil) that can be stationary while its own-produced field collapses around it. Since the OP asks about motors (presumably not employed as generators) the armature Counter-electromotive force is indeed a nett "back emf" that opposes the driving voltage, except at start-up or when the motor is stalled. With a commutated armature each winding moves through a virtually constant field strength during the small rotation angle when it is in circuit. Blooteuth (talk) 13:06, 20 November 2017 (UTC)
 * Please don't refer to me as Akid guy. If you look at my page you'll see why I'm named AKLD_GUY. I'm not going to argue with you as I've said all I wish to say and we will not agree. Akld guy (talk) 20:24, 20 November 2017 (UTC)
 * I apologize for misreading your name and have fixed my mistake. You have an issue of disagreement with the article Counter-electromotive force which is the result of many editor's work. You should try engaging constructively with them at Talk:Counter-electromotive force. If obduracy prevents understanding that a back emf arises from an armature's movement through the magnetic field, the obdurate one may have difficulty explaining why the current drawn by a DC motor decreases with increasing r.p.m. Blooteuth (talk) 10:46, 21 November 2017 (UTC)

Easier to convert to natural gas: diesel or otto?
What motor is easier to convert to natural gas? --Hofhof (talk) 11:35, 18 November 2017 (UTC)
 * Otto engines are very easy to adapt for running on gas. Converted Vehicles usually even have a simple switch for gasoline or gas which you can savely switch while you drive. Diesel can not be converted completely because Diesel engines usually work with self ignition on pressure max, but there are systems where a gas tank is added to lower the use of diesel. In theory that is. I never saw any Diesel-Gas mixture engines besides the military ones, that basically make the engines capable to run on anything, from crude oil to gas.
 * I actually owned a car with an Otto engine and a gasoline- aswell as a gas-tank. Worked great and saved me some thousands cash for fuel during the time i owned it. The only flaw was the installed 90l gas tank that took away half of the trunk space but i never needed all the trunk space anyway. --Kharon (talk) 13:50, 18 November 2017 (UTC)
 * Your car didn't run on natural gas. Andy Dingley (talk) 20:33, 18 November 2017 (UTC)
 * Potayto, potahto. Its a mixture of carbone fuel gases in both cases. It doesnt make a difference for an otto engine: See Natural_gas_vehicle. --Kharon (talk) 00:55, 19 November 2017 (UTC)
 * But still, your car didn't run on natural gas. Nor would your car have been able to store a useful quantity of natural gas (as it can't be liquefied by simply pressurising it, it would require substantial refrigeration). Andy Dingley (talk) 01:01, 19 November 2017 (UTC)
 * Yes and there are about 5,000 (genetical different!) potato varieties worldwide. You need a different tank for LPG than for CNG, so what? The question was about combustion engines, not about gas storage tanks. --Kharon (talk) 01:12, 19 November 2017 (UTC)
 * " You need a different tank for LPG than for CNG, so what? "
 * The "so what" is that there's no way to make a useful CNG tank that will fit in a car. Andy Dingley (talk) 11:26, 19 November 2017 (UTC)
 * Really? This seems eminently practical to me ;-) Alansplodge (talk) 19:39, 19 November 2017 (UTC)


 * One could just skim wp for "CNG" and find a whole list of vehicles. Even a tiny car can be CNG fitted.--TMCk (talk) 19:53, 19 November 2017 (UTC)
 * The Fiat Panda is a good example of how difficult it is to make this work. The TwinAir engine was designed for bifuel CNG from the outset, but the provision of car size gas cylinders is still such a problem (energy / volume is only something like a sixth of petrol). The Panda has to triple its volume fuel capacity to manage this, which involved raising the suspension (the LPG duel fuel version doesn't need this) to make space. There's also a serious price premium for the Natural Power - something like 20%. Andy Dingley (talk) 21:50, 19 November 2017 (UTC)


 * Are you seriously still claiming "...there's no way to make a useful CNG tank that will fit in a car..."???--TMCk (talk) 22:28, 19 November 2017 (UTC)
 * ... the difficulty is in fitting the passengers and luggage around the tank.  D b f i r s   22:49, 19 November 2017 (UTC)


 * Difficult but not impossible as claimed. Gosh, batteries are taking up more space and they managed to make it work just as they managed to make CNG work.--TMCk (talk) 22:55, 19 November 2017 (UTC) And no, no trunk space used in case of the Panda.--TMCk (talk) 22:59, 19 November 2017 (UTC)
 * Not for a conversion, no. And this is a question about conversions.
 * The Panda tanks needed to squeeze in tankage that's twice the size of the existing petrol tank (and the petrol tank is still needed), which needed major rearrangement and a suspension lift beneath. They also cost a couple of thousand, just for the tanks, and they give the car likely limited lifetime of only 10 years (current pressure vessel regulations require their replacement then, and neither inspection nor economic replacement seem practical for a 10 year old car).
 * This new car design is right on the edge of what's practical - and conversions aren't. Andy Dingley (talk) 23:19, 19 November 2017 (UTC)


 * Obvious link: octane rating. Natural gas (i.e. methane) has a RON of 120, making it even more knock-resistant to engine knocking than high-grade gasoline, and even worse than that at diesel combustion (where you need the injected mass to auto-ignite as fast as possible).
 * Now, of course there are incovenients to running on gaseous fuels - fuel tank size can be a problem as hinted above, for instance. But those are not really a diesel vs. gasoline thing. Tigraan Click here to contact me 14:26, 18 November 2017 (UTC)


 * Courtesy link: Otto engine -- Are there currently any vehicles using this? 2606:A000:4C0C:E200:C9A:4B44:2E28:1611 (talk) 19:20, 18 November 2017 (UTC)
 * It is the normal 4-stroke petrol engine! -- Q Chris (talk) 10:53, 20 November 2017 (UTC)
 * Lol, that's not clear from the article -- it only shows old-timey machines. 2606:A000:4C0C:E200:E958:86E3:541F:E7F1 (talk) 04:39, 21 November 2017 (UTC)
 * Just to clarify, the Otto engine was the first to use the Otto cycle. Tevildo (talk) 07:01, 21 November 2017 (UTC)
 * Thanks. I took the liberty of adding that to article (here). — [dynamic IP]:2606:A000:4C0C:E200:E958:86E3:541F:E7F1 (talk) 17:12, 21 November 2017 (UTC) ... note: my edit was reverted; anybodiy care to clarify this in the article?


 * In practical terms, neither. Conversions are possible, but they're somewhere between "Well I wouldn't start from here if I were you" and "There's not much left of the Ship of Theseus by the time you've finished". As such natural gas engines require either gas pipelines to supply them, or heavy pressurised storage tanks, then these are large engines and so they're built from industrial or truck diesel engines - but they're built from scratch as gas engines, and although the cylinder blocks might start out the same, there's little in common when they're finished. Most of the larger ones (for electricity generation, rather than mechanical output) are gas turbines on the Brayton cycle, not piston engines. Larger ones are CCGT where the gas turbine exhaust has its heat recovered by a steam boiler and turbine (something like half of the UK's electricity is coming from these at present ).
 * What you may be thinking of instead are the far more common autogas conversions of spark ignition petrol Otto engines to run on LPG (propane), rather than Natural Gas (methane). This is easier to store (it liquefies at a manageable low pressure), so the dense liquid stores in a similar volume to a petrol tank, often replacing the spare wheel storage space. It's also very easy to convert the engine for its use. A typical 1990s-onward installation uses an extra set of fuel injectors, one per cylinder drilled into the inlet manifold before the inlet valves. The control of these is then taken from the existing (and highly developed) manufacturer's own petrol injection system, by using the petrol injector timing and multiplying that by an adjustable factor, set during conversion.  The complex mapping of ignition timing and fuel volume from all of the engine parameters has already been done by the car maker - the gas injection just runs at a fixed multiple of this. The engine is started and warmed through on petrol, then switches automatically.  Such conversions are popular in Europe (my Volvo 940 has such a conversion) where petrol is heavily taxed but Autogas LPG is far cheaper (half the volume price). Filling stations are available at the "one per small town" and every motorway or large road service station level. Andy Dingley (talk) 20:31, 18 November 2017 (UTC)
 * Natural gas vehicle seems to state that vehicles capable of running on either natural gas or gasoline are not unheard of. Here in Southern California it seems just about all the public transit buses were modified to run on CNG in the 2000s. Granted, I don't know what's involved in that. --47.138.163.207 (talk) 09:29, 20 November 2017 (UTC)
 * Several things are needed. Mostly a damned good reason for doing so. In Europe, and a few US cities, that reason is the reduction of diesel particulate pollution from idling buses. That justifies the costs for bus replacement. Buses are large, expensive anyway, and can justify the cost of the on-board tankage.  A further problem is that of refilling them - this is slow (several hours) and requires dedicated filling bays (not just one shared pump) with protection for the equipment and cheap overnight electricity. That's far easier to do in a well organised bus garage than in a domestic flat with an ad hoc parking space outside. Andy Dingley (talk) 10:41, 20 November 2017 (UTC)


 * Isuzu sell them commercially. https://www.isuzu.com.au/truck-range/cng-trucks/ as do Freightliner . Approximately 7% of Australian truck sales are CNG. My impression is that they are diesel based. Greglocock (talk) 06:20, 19 November 2017 (UTC)
 * ... fitted to the same type of vehicle as diesel engines, but the combustion is much more like that in a petrol (gasoline) engine.   D b f i r s   22:24, 19 November 2017 (UTC)