Principios de generación de voltaje
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Principios de generación de voltaje
- 1. Generación de Potencia y Operación en Paralelo de Generadores OCP Junio 16-20 / 2003
- 3. Fundamentos Del Generador
- 4. Voltage & Current generation Stator Coil Rotor Magnetic field N S
- 5. Wave generation Time Vol tage N S N S N S N S N S
- 6. Three phase generation Time Vol tage N S N S N S
- 8. SR4
- 9. SR4 Generator main stator winding - AC output goes to load main rotor winding - gets DC from rectifiers exciter rotor winding - AC output goes to rectifiers exciter stator winding - gets DC from regulator
- 14. NEMA Standard is 10% Max. Cat Generators are 5% Max . Waveform Deviation volts time
- 15. Harmonics third harmonic (10 %) volts time perfect sine wave (100%) resultant waveform Fifth harmonic
- 16. Harmonics in The Line-To-Line Waveform At Full Load See: Figure 2, Page 3 of LEKX3115 “ Gen Winding Pitch & Harmonics ” % of fundamental voltage
- 17. Slot Pitch Shape of coils Slot Pitch= Coil Throw - 1 Slots/Pole 1 7 5 4 3 2 6 7-1 24/4 = = 1 or full pitch 6-1 24/4 = = 5/6 =.83
- 18. Coil Numbering T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12
- 19. Series Connection High Voltage Low Amps T3 T1 T2 T4 T5 T7 T8 T9 T10 T11 T6 T12 277V 480V T0
- 20. Parallel Connection 7T T8 T9 T10 T11 T12 T1 T4 T3 T6 Low Voltage High Amps T2 T5 T1 T4 T2 T5 T3 T6
- 21. Parallel Connection Low Voltage High Amps T12 T7 T10 T1 T5 T8 T11 T2 T6 T9 T3 240 V 139 V T4 T0
- 22. Delta Connection T7 T10 T8 T11 T12 T9 T6 T3 T4 T1 T5 T2 L1 L2 L3 120 V 120 V 240 V 208V
- 23. 4/6 Wire Generators T3 T1 T2 T4 T5 T6 T0
Notas del editor
- Basics of generators.
- Here is a generator with just two slots in it. The Stator coil wire would go in on the top and come back out at the bottom. The rotor which turns has a magnet with a North and a South Pole. When the magnet rotates, its lines of magnetic flux pass through the stator coil. Those lines of force on the North and South Pole cut the coil or ‘turns’ inducing a voltage into the stator coil wire. If this wire feeds a load then there will be a current as well.
- This is the same picture that we looked at before. When spinning that rotor and depending on where it is, it creates a voltage that goes up, goes to zero and goes down. An AC voltage or a sine wave results.
- The three stator windings, red, blue and green are 120 degrees apart. So when one compares the output waveform the three outputs are 120 degrees apart. This make the output a 3-phase output. This is the typical output of a generator when we say three phase such as 3-phase 480V. Why is 3-phase used? The main reason is an AC winding produces a North Pole and a South Pole that spins inside the air gap. A single-phase winding, produces an alternating North Pole and a South Pole. 3-phase makes one that spins so it always has constant torque. It's always producing a torque in a positive direction. So, a 3-phase motor does not need any capacitors to start it and produces proper rotational torque. Another reason for three phase is to save a little bit on transmitting power. It is not necessary to use quite as heavy of a wire or quite as much copper with a 3-phase system.
- The main output waveform or the ‘base’ waveform is brown. The blue is a 3rd harmonic wave. There are three of these half cycles within half of the base cycle. This harmonic represents about 10% of the base sine wave in amplitude. This would be said to be a 10% third harmonic… this would be a large destructive harmonic. A 10% third harmonic would make the base waveform distort to the shape of the red wave. If an output voltage is monitored with an oscilloscope on a genset with large loads the waveform may appear slightly distorted. The 5th harmonic which is typically in amplitude is also shown. There can be a 7 th as well. The 5th and 7th harmonics produce a lot of heating in three phase electric induction motors and can damage them.
- This information is out of an Engine Division Data sheet. Here is a a comparison of a generator with a 2/3 pitch and generator with a 5/6 pitch looking at the associated ‘line-to-line’ harmonics and percentages. Cat sells mostly 5/6 pitch generators but also sells 2/3 pitched machines too. The 3rd harmonic is pretty low for the 2/3rds pitched machine real low for the 5/6. The 5th and sometimes the 7th can be really high on 2/3 pitched machines and can damage three phase electric motors. When summing the harmonics the 2/3 totals 2.57% and the 5/6 is much lower at 0.84%. The overall or total harmonic distortion’ is much less on the 5/6 machine. Sometimes the 5/6 machine is called optimum pitch for this reason. There are still good reasons to sell the 2/3 machine and that is coming up next.
- This explains how slot pitch is accomplished. Slot pitch is equal to coil throw minus one divided by slots per pole. Coil throw is if we were to take this left edge of this coil and put it in the bottom of slot one and do the same with a lot of other coils and then when we got around to doing the top edge or the right edge, we put the top edge over here in slot seven. We have one through seven, enclosing six teeth. This would make a pitch of 1 or be called ‘full pitch’. Full pitch machines are not used because the waveform tends to be distorted. The reason for spanning as much pole span as possible is to generate as much voltage as possible out of that one turn of the coil. The downside of that is, the end turns will be very long which is expensive. It's good for generators both mechanically and cost wise to try to keep the end turns fairly short. Going from 1 to 6 will make a 5/6, and 1-5 a 2/3 pitch. There is nothing on the tag to show the pitch of the generator set… this information can be found in TMI from the serial number or arrangement number. On really old generators, the information is not in TMI but it can be found or calculated from the rewind data.
- Coil numbering. Generally a 12 wire machine will be numbered in the configuration shown. This is fairly common on competitors generators and also motors. Pictured is a 12 wire machine with a dual voltage winding.
- This produces a high voltage, low current series connection. If line to line is 480 V, what is the line to neutral voltage? It is 277V. The 277V line to neutral is often used to power fluorescent lights where 480V is used for motors. To calculate Line to Neutral Voltage, divide Line to Line voltage of 480 V by 1.732 (square root of 3) = 277 V. If “T0” (Neutral) is not grounded the measurements with a VOM from Ground to T1, T2, and T3 may appear unbalanced showing voltage from 300 to 550V. Therefore, if T0 is not grounded the voltages should be checked from Line to Line or from Line to Neutral (T0).
- Take that same WYE (Y) connected machine parallel the outer windings and a parallel connection takes shape.
- Connect the six neutrals together to complete the wiring and the low voltage connection results. Line to line voltage would now be 240V if it was 480V in series. What is the line to neutral voltage? 139 volts, this voltage is not used very much. Sometimes, mostly in the East Coast the regulator is turned down to lower the line to line to 208V and then the line to neutral is 120V.
- This is a series delta connection. The voltage would be 277V from L1 to L3 but generally, that's used. Again the voltage regulator is typically turned down so L1 to L3 is 240V. Since this is one phase, a center tap will provide six 120V sources and three phase 240V. This allows 120/240V from a single machine. The disadvantage is that the set need to be derated. The two windings on the inverted Vee contribute some to the current but they are limited on the current they can carry and the output of the generator has to be reduced to 50%. This is a large derate but it is done sometimes. A generator rated for 100KW in WYE would only produce 50KW in DELTA. The 208V shown is used sometimes and it is called the “wild leg”.
- This is an example of a 4/6 wire generator. There are four connections brought out, T0, T1, T2, and T3. There are six terminals total. If this were a 48 0V generator for example it could not be rewired for the parallel wye to become a 240V high current output. The advantage of this is that there are fewer windings. This is easier to cool and easier to make the connections in the dog house of the generator. This is seen on 690 frames and larger and may be done for space constraints.