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Author Previous Topic: Output for a SD70MAC Topic Next Topic: Using Combustion Turbines for Prime Movers  

PowerEngineer
Engine Wiper

Posted - 04/17/2003 :  8:57:34 PM  Show Profile
I have technical question regarding locomotives using AC generators. Is the alternator typically a synchronous machine or an induction machine? I have always assumed they were induction machines as I have heard many railroad engineers say that the prime mover RPM varies. However, the utility industry uses generators that operate at synchronous speed and power output is changed via torque changes only. Just curious about diesel locomotives.

PowerEngineer

Country: | Posts: 186

AllenGaspers
Engine Wiper

Posted - 05/06/2003 :  4:35:07 PM  Show Profile  Visit AllenGaspers's Homepage
It's an induction machine. Power output is varied by engine rpm, and the amount of amps supplied to the alternator fields. For instance, a SD70 MAC, will start out at about, 350 rpm 10 amps to the fields, and will put out about 6000 watts. At full throttle, and full load, it will be at 905 rpm, 82 amps to the fields, and put out about 2.8 megawatts.
The next train I'm on, I'll jot down some figures from all the throttle settings.



Country: USA | Posts: 191 Go to Top of Page

AllenGaspers
Engine Wiper

Posted - 05/06/2003 :  4:35:07 PM  Show Profile  Visit AllenGaspers's Homepage
It's an induction machine. Power output is varied by engine rpm, and the amount of amps supplied to the alternator fields. For instance, a SD70 MAC, will start out at about, 350 rpm 10 amps to the fields, and will put out about 6000 watts. At full throttle, and full load, it will be at 905 rpm, 82 amps to the fields, and put out about 2.8 megawatts.
The next train I'm on, I'll jot down some figures from all the throttle settings.



Country: USA | Posts: 191 Go to Top of Page

AllenGaspers
Engine Wiper

Posted - 05/06/2003 :  4:43:32 PM  Show Profile  Visit AllenGaspers's Homepage
Powerengineer, I know this is a little off topic, but, I'm tying to understand the differance between a power plant and a locomotive. If a power plant is putting out 690 mega watts, over what time frame is this figured, is this over one revolution, minutes, hours? The power plant that I haul coal to is a synchronous machine, I understand that it has to be to keep the frequency at 60hz, but why do power plants expand the armature to increase power, and locomotives changes field strenght. It would seem to me, that the latter would have fewer moving parts, and less maintance.


Edited by - AllenGaspers on 05/06/2003 4:47:12 PM

Country: USA | Posts: 191 Go to Top of Page

AllenGaspers
Engine Wiper

Posted - 05/06/2003 :  4:43:32 PM  Show Profile  Visit AllenGaspers's Homepage
Powerengineer, I know this is a little off topic, but, I'm tying to understand the differance between a power plant and a locomotive. If a power plant is putting out 690 mega watts, over what time frame is this figured, is this over one revolution, minutes, hours? The power plant that I haul coal to is a synchronous machine, I understand that it has to be to keep the frequency at 60hz, but why do power plants expand the armature to increase power, and locomotives changes field strenght. It would seem to me, that the latter would have fewer moving parts, and less maintance.


Edited by - AllenGaspers on 05/06/2003 4:47:12 PM

Country: USA | Posts: 191 Go to Top of Page

PowerEngineer
Engine Wiper

Posted - 05/07/2003 :  10:36:35 AM  Show Profile
quote:
Originally posted by AllenGaspers

Powerengineer, I know this is a little off topic, but, I'm tying to understand the differance between a power plant and a locomotive. If a power plant is putting out 690 mega watts, over what time frame is this figured, is this over one revolution, minutes, hours? The power plant that I haul coal to is a synchronous machine, I understand that it has to be to keep the frequency at 60hz, but why do power plants expand the armature to increase power, and locomotives changes field strenght. It would seem to me, that the latter would have fewer moving parts, and less maintance.



Hey Allen,

The MW output is instantaneous. The MWh (megawatt-hour) output is over an hour. So, for example, if a plant puts out 690 MW of power for an hour, that would equate to 690 MWh of energy.

It is true that sychronous generators must be utilized to maintain frequency at 60 Hz. In order to increase the electrical output of a synchronous generator, the rotor must be advanced in position with repsect to the rotors of the other generators on the grid. To decrease the electrical output of a synchronous generator, the rotor must retreat in position with respect to the rotors of the other generators on the grid. The reason why this is the case is because the power transfer from a generator to the generator station bus is a function of the difference in phase angles between the internal generator EMF (voltage) and the bus voltage.

Therefore, if power output is to be increased at a specific generating unit, a control signal would generally open up the main steam value on the high pressure turbine and the firing rate would be increased at the boiler. The prime mover (steam turbine) output would then exceed the generator output, and the higher prime mover torque would accelerate the generator above sychronous speed momentarily until the rotor advanced with respect to other generator rotors in the grid enough to increase the electrical output of the generator such that it balances (ignoring losses) the mechancial output of the turbine. Once the electrical output of the generator balances the mechanical output of the turbine, the restraining torque of the generator would equal the torque of the steam turbine and the generator would return to constant, synchronous speed. Reducing power output is the reveserse of increasing power output.

The exciter on the generator may be a DC generator attached to the turbine shaft or a solid state unit, and basically supplies voltage to the rotor winding. Increasing the rotor voltage increases the rotor current which in turn increases the voltage magnitude of the stator windings (the internal EMF). This will result in a slight increase in power output, but generally excitation is used to control the amount of reactive power (VARs) supplied or absorbed by the generating unit.

Some units (for example Nuclear) are operated at constant power outputs and power output is controlled at the plant. Other units are controlled by a central computer system known as an energy management system. The energy management systems contains AGC (automatic generator control) and economic dispatch capabilites that both balance generation with load (including losses) on a real-time basis and adjust generator outputs to the most economical levels based on fuel costs and individual generating unit heat rates (efficiencies).

Governors exist on all generating units to ensure synchronous speed is maintained at all times. Should a large generating unit trip off line, the output of the utility's remaining generating units generally cannot be increased fast enough to avoid a frequency drop. Governors on all generators across the grid will increase the outputs of all generators (owned by all utilities) by maintaining sychronous speed until a utility can increase the output of its own generating units enough to compensate for the lost generating unit. Therefore, in general, governors are a last resort to maintaining frequency across the grid, whereas the energy management system at the central control center is the primary mechanism used to maintain frequency via balancing generator outputs with loads in real time and controlling genrator outputs to minimize fuel costs based on economic dispatch.

Let me know if you have any additional questions.

PowerEngineer



Country: | Posts: 186 Go to Top of Page

PowerEngineer
Engine Wiper

Posted - 05/07/2003 :  10:36:35 AM  Show Profile
quote:
Originally posted by AllenGaspers

Powerengineer, I know this is a little off topic, but, I'm tying to understand the differance between a power plant and a locomotive. If a power plant is putting out 690 mega watts, over what time frame is this figured, is this over one revolution, minutes, hours? The power plant that I haul coal to is a synchronous machine, I understand that it has to be to keep the frequency at 60hz, but why do power plants expand the armature to increase power, and locomotives changes field strenght. It would seem to me, that the latter would have fewer moving parts, and less maintance.



Hey Allen,

The MW output is instantaneous. The MWh (megawatt-hour) output is over an hour. So, for example, if a plant puts out 690 MW of power for an hour, that would equate to 690 MWh of energy.

It is true that sychronous generators must be utilized to maintain frequency at 60 Hz. In order to increase the electrical output of a synchronous generator, the rotor must be advanced in position with repsect to the rotors of the other generators on the grid. To decrease the electrical output of a synchronous generator, the rotor must retreat in position with respect to the rotors of the other generators on the grid. The reason why this is the case is because the power transfer from a generator to the generator station bus is a function of the difference in phase angles between the internal generator EMF (voltage) and the bus voltage.

Therefore, if power output is to be increased at a specific generating unit, a control signal would generally open up the main steam value on the high pressure turbine and the firing rate would be increased at the boiler. The prime mover (steam turbine) output would then exceed the generator output, and the higher prime mover torque would accelerate the generator above sychronous speed momentarily until the rotor advanced with respect to other generator rotors in the grid enough to increase the electrical output of the generator such that it balances (ignoring losses) the mechancial output of the turbine. Once the electrical output of the generator balances the mechanical output of the turbine, the restraining torque of the generator would equal the torque of the steam turbine and the generator would return to constant, synchronous speed. Reducing power output is the reveserse of increasing power output.

The exciter on the generator may be a DC generator attached to the turbine shaft or a solid state unit, and basically supplies voltage to the rotor winding. Increasing the rotor voltage increases the rotor current which in turn increases the voltage magnitude of the stator windings (the internal EMF). This will result in a slight increase in power output, but generally excitation is used to control the amount of reactive power (VARs) supplied or absorbed by the generating unit.

Some units (for example Nuclear) are operated at constant power outputs and power output is controlled at the plant. Other units are controlled by a central computer system known as an energy management system. The energy management systems contains AGC (automatic generator control) and economic dispatch capabilites that both balance generation with load (including losses) on a real-time basis and adjust generator outputs to the most economical levels based on fuel costs and individual generating unit heat rates (efficiencies).

Governors exist on all generating units to ensure synchronous speed is maintained at all times. Should a large generating unit trip off line, the output of the utility's remaining generating units generally cannot be increased fast enough to avoid a frequency drop. Governors on all generators across the grid will increase the outputs of all generators (owned by all utilities) by maintaining sychronous speed until a utility can increase the output of its own generating units enough to compensate for the lost generating unit. Therefore, in general, governors are a last resort to maintaining frequency across the grid, whereas the energy management system at the central control center is the primary mechanism used to maintain frequency via balancing generator outputs with loads in real time and controlling genrator outputs to minimize fuel costs based on economic dispatch.

Let me know if you have any additional questions.

PowerEngineer



Country: | Posts: 186 Go to Top of Page

AllenGaspers
Engine Wiper

Posted - 05/07/2003 :  11:16:49 AM  Show Profile  Visit AllenGaspers's Homepage
Thanks for the response PowerEngineer. Now I understand why they do it this way.
This sounds like an interesting field your in. If you don't mind, could you tell me what your background is. I'm looking at changing careers, and am looking at ideas. I'm leaning towards electrical/electronics. Thanks, Allen.



Country: USA | Posts: 191 Go to Top of Page

AllenGaspers
Engine Wiper

Posted - 05/07/2003 :  11:16:49 AM  Show Profile  Visit AllenGaspers's Homepage
Thanks for the response PowerEngineer. Now I understand why they do it this way.
This sounds like an interesting field your in. If you don't mind, could you tell me what your background is. I'm looking at changing careers, and am looking at ideas. I'm leaning towards electrical/electronics. Thanks, Allen.



Country: USA | Posts: 191 Go to Top of Page

PowerEngineer
Engine Wiper

Posted - 05/07/2003 :  6:21:05 PM  Show Profile
quote:
Originally posted by AllenGaspers

Thanks for the response PowerEngineer. Now I understand why they do it this way.
This sounds like an interesting field your in. If you don't mind, could you tell me what your background is. I'm looking at changing careers, and am looking at ideas. I'm leaning towards electrical/electronics. Thanks, Allen.




Hi Allen,

I am an electrical engineer in the electric utility industry. I spend 12 years with a large utility and five years as a technical consultant to electric utilities. I enjoy the electric utility industry and find many similarities with the railroad industry.

PowerEngineer



Country: | Posts: 186 Go to Top of Page

PowerEngineer
Engine Wiper

Posted - 05/07/2003 :  6:21:05 PM  Show Profile
quote:
Originally posted by AllenGaspers

Thanks for the response PowerEngineer. Now I understand why they do it this way.
This sounds like an interesting field your in. If you don't mind, could you tell me what your background is. I'm looking at changing careers, and am looking at ideas. I'm leaning towards electrical/electronics. Thanks, Allen.




Hi Allen,

I am an electrical engineer in the electric utility industry. I spend 12 years with a large utility and five years as a technical consultant to electric utilities. I enjoy the electric utility industry and find many similarities with the railroad industry.

PowerEngineer



Country: | Posts: 186 Go to Top of Page
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