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Re: Locomotive electrical systems
Author: SP5103
Date: 08-01-2013 - 09:30
Trying to keep it as basic as possible, but it is still a hard subject to condense -
For each throttle position, a certain amount of horsepower is developed by the diesel engine and converted to electricity by the main generator or traction alternator. Horsepower can be converted directly to the electrical measure of power which is watts (or kilowatts if you prefer). Current (amps) x voltage = watts = hp.
Voltage can be thought of as the electrical pressure. If the voltage exceeds the insulating ability of the equipment or cabling, it will arc through the insulation or across the air gap. Current or amps is the flow. If the amps are too high, it generates too much heat and things will start to melt.
Ohms' Law states that current, voltage and resistance are all in proportion to each other. Increasing the voltage or decreasing the resistance will allow more current to flow. The main generator (or traction alternator), traction motors, switch gear and cabling all have maximum working values for both current (amps) and voltage.
At a stop or in the lower speed range of their design, traction motors have fairly low resistance. Because resistance is low, the current is high so the ammeter will show high amperage which is the amount of force or drawbar pull being created. Since amperage is high and only a fixed amount of watts/hp is are inputted, the voltage will be low.
As traction motor speed increases, the magnetic fields of the armature increasingly have to cut through the magnetic fields created by the field coils which increases the counter electro motive force (cemf). This increases the electrical resistance of the traction motor which in turn reduces the amount of current (amps) that can flow through them, and as a result of the lower current flow the voltage increases since the amount of watts (hp input) has remained steady.
Due to cemf, as speed increases the amps drop and the voltage rises. This causes two potential issues - First, the voltage might increase to a value higher than the nominal rating of the equipment and cabling. Second, the lower amps mean the drawbar pull is lower. At some point, the rising voltage will cause the electrical system to arc, or the cemf will become so high as to balance against not allowing the train speed to increase.
To compensate, the electrical system must do something to lower the traction motor resistance (cemf) to allow more current flow and lower the voltage to stay within working limits. There are several ways of doing this:
One option is to change how the traction motors are connected to the main generator or traction alternator. For example, a typical EMD D77 is rated at 1050 amps. If you connect 6 of them in parallel, then at full power you need 6300 amps. The AR10 traction alternator is good for 4200 amps, which just happens to be 4 D77 traction motors in parallel (as in a GP38AC/39/40,etc), but the AR10 does not have enough capacity for 6 in parallel at full power. So on a SD38AC/40/45, the traction motors are wired in series in groups of two which form three strings connected in parallel (series-parallel). At maximum power at low speed, each traction motor will be getting one-half the voltage (since they are paired in series) and one-third of the AR10 current output since there are three strings in parallel). The AR10 in this case is limited by the black box excitation system to about 3200 amps since the traction motors can only use 1050 each (3 parallel strings x 1050 = 3150). This allows the locomotive to develop its maximum potential of drawbar pull at low speeds within the current and voltage ratings of its design.
As speed increases, so does the cemf and system voltage. When voltage approaches maximum, the forward transition relay will pick up triggering forward transition. The generator excitation is interrupted, and after a short time delay to allow the voltage to decay, the switchgear changes from series-parallel to parallel, and then excitation is restored. With all 6 motors now in parallel, each traction motor will receive full voltage and one-sixth (or one-fourth on a GP) the current. This allows the locomotive to operate at a higher speed at the maximum power available.
Should the speed decrease, there will be a corresponding increase in amperage and decrease in voltage. Before the amperage draw becomes excessive, the backwards transition relay will drop out due to the lower voltage and cause the system to go from parallel back to series-parallel. (Most but not all locomotives have automatic backward transition. Early EMD switchers do not, and the over voltage relay will cycle dropping out the excitation which shows as a repeated wheel slip.)
Another option is field shunting, where a portion of the current is connected in parallel around the traction motor field. This reduces the magnetic field of the traction motor field, lowering the cemf, allowing more current to flow, and lowering the voltage. Some locomotives also use field shunting in the main generator, and there can be multiple steps of field shunting by connecting various resistance across the fields. The transition arrangement and speed that the events occur vary widely with locomotive design.
More modern designs by EMD and GE eliminated the traction motor connection changes by redesigning the traction alternator with two sections that operate in series or parallel. Many transition relays are designed to take the current flow into consideration, while some transition systems are triggered by the speed of the locomotive (since the theoretical voltage and amperage can be calculated depending on speed and transition).
"Transition" varies with locomotive design. Early EMD switchers start in full series, and then change to series-parallel. As an option, there can be an additional one or two steps of field shunting to allow high speed operation. Later EMD switchers with D32 main generators are in permanent series-parallel with the option of one step of field shunting. It is interesting that while a SW1500 was in permanent series-parallel, a GP15-1 uses series-parallel to parallel transition as do most GP units before the AR10. Those GP units with the AR10 are in full parallel. Low horsepower SD units are series-parallel, though high horsepower SD units use series-parallel to parallel. Those with alternator transition are full parallel.
On the SDP45 being discussed, my guess is that the light trains should not have taxed the system too bad and still allowed decent acceleration without the normal loss of drawbar pull during the few seconds needed to affect transition. Starting in full parallel would not have been the fastest unless there were some additional excitation changes made. I wonder if another modification allowed the full AR10 output of 4200 amps (700 amps per motor max) instead of the normal 3200 amps while starting in series-parallel?
For each throttle position, a certain amount of horsepower is developed by the diesel engine and converted to electricity by the main generator or traction alternator. Horsepower can be converted directly to the electrical measure of power which is watts (or kilowatts if you prefer). Current (amps) x voltage = watts = hp.
Voltage can be thought of as the electrical pressure. If the voltage exceeds the insulating ability of the equipment or cabling, it will arc through the insulation or across the air gap. Current or amps is the flow. If the amps are too high, it generates too much heat and things will start to melt.
Ohms' Law states that current, voltage and resistance are all in proportion to each other. Increasing the voltage or decreasing the resistance will allow more current to flow. The main generator (or traction alternator), traction motors, switch gear and cabling all have maximum working values for both current (amps) and voltage.
At a stop or in the lower speed range of their design, traction motors have fairly low resistance. Because resistance is low, the current is high so the ammeter will show high amperage which is the amount of force or drawbar pull being created. Since amperage is high and only a fixed amount of watts/hp is are inputted, the voltage will be low.
As traction motor speed increases, the magnetic fields of the armature increasingly have to cut through the magnetic fields created by the field coils which increases the counter electro motive force (cemf). This increases the electrical resistance of the traction motor which in turn reduces the amount of current (amps) that can flow through them, and as a result of the lower current flow the voltage increases since the amount of watts (hp input) has remained steady.
Due to cemf, as speed increases the amps drop and the voltage rises. This causes two potential issues - First, the voltage might increase to a value higher than the nominal rating of the equipment and cabling. Second, the lower amps mean the drawbar pull is lower. At some point, the rising voltage will cause the electrical system to arc, or the cemf will become so high as to balance against not allowing the train speed to increase.
To compensate, the electrical system must do something to lower the traction motor resistance (cemf) to allow more current flow and lower the voltage to stay within working limits. There are several ways of doing this:
One option is to change how the traction motors are connected to the main generator or traction alternator. For example, a typical EMD D77 is rated at 1050 amps. If you connect 6 of them in parallel, then at full power you need 6300 amps. The AR10 traction alternator is good for 4200 amps, which just happens to be 4 D77 traction motors in parallel (as in a GP38AC/39/40,etc), but the AR10 does not have enough capacity for 6 in parallel at full power. So on a SD38AC/40/45, the traction motors are wired in series in groups of two which form three strings connected in parallel (series-parallel). At maximum power at low speed, each traction motor will be getting one-half the voltage (since they are paired in series) and one-third of the AR10 current output since there are three strings in parallel). The AR10 in this case is limited by the black box excitation system to about 3200 amps since the traction motors can only use 1050 each (3 parallel strings x 1050 = 3150). This allows the locomotive to develop its maximum potential of drawbar pull at low speeds within the current and voltage ratings of its design.
As speed increases, so does the cemf and system voltage. When voltage approaches maximum, the forward transition relay will pick up triggering forward transition. The generator excitation is interrupted, and after a short time delay to allow the voltage to decay, the switchgear changes from series-parallel to parallel, and then excitation is restored. With all 6 motors now in parallel, each traction motor will receive full voltage and one-sixth (or one-fourth on a GP) the current. This allows the locomotive to operate at a higher speed at the maximum power available.
Should the speed decrease, there will be a corresponding increase in amperage and decrease in voltage. Before the amperage draw becomes excessive, the backwards transition relay will drop out due to the lower voltage and cause the system to go from parallel back to series-parallel. (Most but not all locomotives have automatic backward transition. Early EMD switchers do not, and the over voltage relay will cycle dropping out the excitation which shows as a repeated wheel slip.)
Another option is field shunting, where a portion of the current is connected in parallel around the traction motor field. This reduces the magnetic field of the traction motor field, lowering the cemf, allowing more current to flow, and lowering the voltage. Some locomotives also use field shunting in the main generator, and there can be multiple steps of field shunting by connecting various resistance across the fields. The transition arrangement and speed that the events occur vary widely with locomotive design.
More modern designs by EMD and GE eliminated the traction motor connection changes by redesigning the traction alternator with two sections that operate in series or parallel. Many transition relays are designed to take the current flow into consideration, while some transition systems are triggered by the speed of the locomotive (since the theoretical voltage and amperage can be calculated depending on speed and transition).
"Transition" varies with locomotive design. Early EMD switchers start in full series, and then change to series-parallel. As an option, there can be an additional one or two steps of field shunting to allow high speed operation. Later EMD switchers with D32 main generators are in permanent series-parallel with the option of one step of field shunting. It is interesting that while a SW1500 was in permanent series-parallel, a GP15-1 uses series-parallel to parallel transition as do most GP units before the AR10. Those GP units with the AR10 are in full parallel. Low horsepower SD units are series-parallel, though high horsepower SD units use series-parallel to parallel. Those with alternator transition are full parallel.
On the SDP45 being discussed, my guess is that the light trains should not have taxed the system too bad and still allowed decent acceleration without the normal loss of drawbar pull during the few seconds needed to affect transition. Starting in full parallel would not have been the fastest unless there were some additional excitation changes made. I wonder if another modification allowed the full AR10 output of 4200 amps (700 amps per motor max) instead of the normal 3200 amps while starting in series-parallel?
