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Re: Someone please explain shifting gears in a diesel
Author: SP5103
Date: 12-16-2010 - 11:31
I disagree somewhat with their description.
At a given throttle position, a diesel-electric locomotive's diesel engine will create a certain amount of horsepower. This can be converted into kilowatts, a measure of electrical work. (hp x 700 = kw; the figure 700 corrects for generator efficiency.) The Voltage x Amperage (current) = Kilowatts. D-E locomotives are not like your house current or portable generator; the voltage is not constant but varies. When stopped, the their is very little voltage, so therefor the amperage is high. As the traction motors start to turn, the magnetic fields of the armature and the fields cross each other which creates an electrical back pressure (emf) which cause the voltage to rise. The faster the traction motors turn, the higher the voltage due to increasing back emf. Assuming the same throttle position resulting in the same hp and corresponding kw; as the speed increases, the voltage increases and the amperage decreases; if the speed slows, the voltage decreasess and amperage increases. The amperage or current is what determines the amount of pull a locomotive creates. Too much sustained amperage creates heat, which eventually destroys the cabling, generator or traction motors. If the voltage limit is exceeded, it arcs through the insulation and shorts it out.
Their are design limitations as to how much voltage or current a traction motor or generator can succesfully operate at. As a result, the connections between the generator and traction motors determine how the voltage and current are divided. Remember Ohms law? If you put four traction motors in series, each motor will recieve full amperage but only a quater of the voltage - good for low speed pulling but may not be capable of higher speeds. If the traction motors are all wired in parallel, then each motor receives full voltage but only 1/4 or 1/6 the current (depending on 4 or 6 motors) - good for higher speeds, but the amperage limit will quickly be exceeded at low speeds. In between, you have series-parallel - where pairs of motors are wired in series, and the pairs are in parallel. This means each motor will get 1/2 the voltage, and either 1/2 (4 motor) or a 1/3 (6 motor) of the current.
Regardless of the motor connection arrangement - if the back emf gets to high it will actually balance with the voltage - which means the electrical system can't propel you any faster. Some locomotives employ traction motor field shunting. A contactor closes bypassing a portion of the traction motor fields through resistors. The armature remains at full strength, but the reduced field lowers the back emf allowing the loco to accelerate. This motor field shunting is part of "transition", and depending on the loco may have not have any or multiple steps of shunting.
At some speed, you reach the limit of the design voltage of the equipment despite any field shunting in use. (The maximum voltage is anywhere between 250v and 1000v+ depending on the size and age of the equipment, 600v is a nominal figure only.) To allow the loco to go faster, the next step is to change the traction motor connections to allow them to use the higher voltage. The loco will drop excitation, allow the electrical power to dissapate, change the motor connections, and then reapply power. This is the "shifting" that you notice on older locomotives, because you are hearing the engine drop its load to allow for the connection change. The steps of field shunting are not noticeable trackside, but if you are in the cab you might notice it if you know what to look for.
Since transition is a function of voltage which is related to speed, depending on the loco either a voltage relay or a relay connected to a speedometer drive will trigger transition. (Very ealry locomotives had a selector handle which the engineer had to properly position manually depending on speed.) Transition on a locomotive will vary by model, and sometimes even by loco. The smallest GEs with a generator connected to only one traction motor still has one step of motor field shunt. A GP35 had the most complicated transition system imaginable - something like 18 or more steps of transition (IIRC). They start out in series-parallel, have several steps of traction motor field and generator field shunting; then change to full parallel motor connections; then several more steps of traction motor field and generator field shunting. This is the reason both EMD, GE and Alco had to develop traction alternators for use above 2500 hp - it was getting too complicated to keep the main generators within operating tolerances.
EMD switchers are a special case in their own right. Most of them are wired in full series to start, and then change to series-parallel. Some of the newer designs use only series-parallel. One or two steps of field shunting to allow higher speeds was actually an option, even on the MP15. The other issue is that most EMD switchers only have forward transition. As the speed slows, the engineer must either momentarily close the throttle or change a selector to allow the connections to change back to series.
A GE 70 ton by comparison goes from series-parallel, to parallel, to paralles-shunt. The most common D-E DC loco arragement seems to be series-parallel to parallel with various steps of shunting on older locomotives; newer EMD DC locomotives are full parallel for 4-motor units; series-parallel to parallel for 6 motor units - no field shunting - due to improvements in motor and traction alternator designs. Add dynamics, and you get a whole new mess of switchgear since DC dynamics are a series type connection.
If forward transition fails, or if you do not have the full range of transition equipment, you can only go so fast before the back emf balances the voltage. If backwards transition fails, you might damage or destroy the main generator or traction motors due to overloading.
At a given throttle position, a diesel-electric locomotive's diesel engine will create a certain amount of horsepower. This can be converted into kilowatts, a measure of electrical work. (hp x 700 = kw; the figure 700 corrects for generator efficiency.) The Voltage x Amperage (current) = Kilowatts. D-E locomotives are not like your house current or portable generator; the voltage is not constant but varies. When stopped, the their is very little voltage, so therefor the amperage is high. As the traction motors start to turn, the magnetic fields of the armature and the fields cross each other which creates an electrical back pressure (emf) which cause the voltage to rise. The faster the traction motors turn, the higher the voltage due to increasing back emf. Assuming the same throttle position resulting in the same hp and corresponding kw; as the speed increases, the voltage increases and the amperage decreases; if the speed slows, the voltage decreasess and amperage increases. The amperage or current is what determines the amount of pull a locomotive creates. Too much sustained amperage creates heat, which eventually destroys the cabling, generator or traction motors. If the voltage limit is exceeded, it arcs through the insulation and shorts it out.
Their are design limitations as to how much voltage or current a traction motor or generator can succesfully operate at. As a result, the connections between the generator and traction motors determine how the voltage and current are divided. Remember Ohms law? If you put four traction motors in series, each motor will recieve full amperage but only a quater of the voltage - good for low speed pulling but may not be capable of higher speeds. If the traction motors are all wired in parallel, then each motor receives full voltage but only 1/4 or 1/6 the current (depending on 4 or 6 motors) - good for higher speeds, but the amperage limit will quickly be exceeded at low speeds. In between, you have series-parallel - where pairs of motors are wired in series, and the pairs are in parallel. This means each motor will get 1/2 the voltage, and either 1/2 (4 motor) or a 1/3 (6 motor) of the current.
Regardless of the motor connection arrangement - if the back emf gets to high it will actually balance with the voltage - which means the electrical system can't propel you any faster. Some locomotives employ traction motor field shunting. A contactor closes bypassing a portion of the traction motor fields through resistors. The armature remains at full strength, but the reduced field lowers the back emf allowing the loco to accelerate. This motor field shunting is part of "transition", and depending on the loco may have not have any or multiple steps of shunting.
At some speed, you reach the limit of the design voltage of the equipment despite any field shunting in use. (The maximum voltage is anywhere between 250v and 1000v+ depending on the size and age of the equipment, 600v is a nominal figure only.) To allow the loco to go faster, the next step is to change the traction motor connections to allow them to use the higher voltage. The loco will drop excitation, allow the electrical power to dissapate, change the motor connections, and then reapply power. This is the "shifting" that you notice on older locomotives, because you are hearing the engine drop its load to allow for the connection change. The steps of field shunting are not noticeable trackside, but if you are in the cab you might notice it if you know what to look for.
Since transition is a function of voltage which is related to speed, depending on the loco either a voltage relay or a relay connected to a speedometer drive will trigger transition. (Very ealry locomotives had a selector handle which the engineer had to properly position manually depending on speed.) Transition on a locomotive will vary by model, and sometimes even by loco. The smallest GEs with a generator connected to only one traction motor still has one step of motor field shunt. A GP35 had the most complicated transition system imaginable - something like 18 or more steps of transition (IIRC). They start out in series-parallel, have several steps of traction motor field and generator field shunting; then change to full parallel motor connections; then several more steps of traction motor field and generator field shunting. This is the reason both EMD, GE and Alco had to develop traction alternators for use above 2500 hp - it was getting too complicated to keep the main generators within operating tolerances.
EMD switchers are a special case in their own right. Most of them are wired in full series to start, and then change to series-parallel. Some of the newer designs use only series-parallel. One or two steps of field shunting to allow higher speeds was actually an option, even on the MP15. The other issue is that most EMD switchers only have forward transition. As the speed slows, the engineer must either momentarily close the throttle or change a selector to allow the connections to change back to series.
A GE 70 ton by comparison goes from series-parallel, to parallel, to paralles-shunt. The most common D-E DC loco arragement seems to be series-parallel to parallel with various steps of shunting on older locomotives; newer EMD DC locomotives are full parallel for 4-motor units; series-parallel to parallel for 6 motor units - no field shunting - due to improvements in motor and traction alternator designs. Add dynamics, and you get a whole new mess of switchgear since DC dynamics are a series type connection.
If forward transition fails, or if you do not have the full range of transition equipment, you can only go so fast before the back emf balances the voltage. If backwards transition fails, you might damage or destroy the main generator or traction motors due to overloading.
