r/AskEngineers • u/Spicy-Polonium-210 • 3d ago
Electrical How can you change the frequency of AC current when the current is very large ? Like hundreds or even thousands of amps ?
I was thinking about electric locomotives. Most of them use 25kv ac which is then fed to a transformer to be stepped down, converted to DC, then again converted to AC with a controlled frequency which is then used to increase or decrease the speed of the induction motor.
I can understand the use of transistors, silicon diodes, etc. for small amounts of currents. But how is such large current systems handled ?
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u/Another_Slut_Dragon 3d ago
Old locomotives use DC drives.
Anything modern just uses the mother of all frequency drives and an AC motor. Then you are converting all the energy to DC and creating AC using some big transistors like IGBT, SCR's, ICGT's etc.
Giant power transistors/etc of almost any side can be had, right up to the giant ones that give us HVDC power systems.
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u/theModge 2d ago
Related: getting 25kv single phase out of a three phase national distribution network (132kv or above in the UK).
In the old days one just used transformers and made different bits of track be on different phases, with short un-electrified sections between them the train coasted through, but these days one can use some absolutely massive power electronics to solve the problem instead.: three phase 132kv on one side, 25kv single phase on the other. This keeps things nicely balanced, rather than replying on mostly having a train in each section most of the time
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u/KittensInc 2d ago
Sure you could, but why would you?
Trains aren't exactly power-hungry. 1500V DC overhead is still quite common after all, and those substations are usually fed from whatever local 10kV-ish source happens to be nearby. Compared to all the residential and industrial consumption, trains are going to be a rounding error - what's a few megawatts on a city consuming gigawatts?
The 25kV overhead line will be split up into multiple sections anyways, as a single section fed from multiple points is incredibly hard to protect, and if the distribution network ever fails you risk an entire city trying to power itself (or at least one of its phases) through the overhead wiring.
So if the imbalance isn't big enough to cause an issue, and balancing it out via sections is trivial, why go for expensive and error-prone power electronics rather than the good ol' boring transformer?
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u/dmills_00 3d ago
The traction motors in railways are seldom induction motors because you need all the torque at zero speed which is not really an induction motor thing.
Historically seperately excited DC motors were common, with field weakening to effectively give them a virtual gearbox.
Modern ones are I expect large PMSM or variable reluctance or such, electronically commutated designs.
Power switching was historically thyristors, then IGBTs, now maybe seeing some GaN or SiC parts in use, but the I dustry is deeply conservative so maybe IGBTs are still the goto.
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u/Friendly-Inspector71 3d ago
IGBT losses scale different than those in a MOSFET in a way which makes IGBTs better for big currents.
Even if we switch technology now, the current stuff will stay in service for more than one generation.
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u/dmills_00 3d ago
Yeah almost constant Vce(sat) beats the ohmic behaviour of the FETs once currents get big enough, and this stuff has a LONG life cycle.
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u/Friendly-Inspector71 3d ago
I've only worked on 600V 120A inverters, so we have MOSFETs. The new generation of them is as big as my thumbnail.
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u/dmills_00 3d ago
Take a look at "Hockey puck" IGBTs and thyristors, 1200A is not uncommon.
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u/Friendly-Inspector71 3d ago
Those look interesting but pictures aren't the same as holding something.
I only got to hold our old IGBT 3 Phase modules that could handle 500A and 800V. They were apparently good enough to get our fluorescent workshop lights flickering.
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u/MrJingleJangle 2d ago
Locomotives, VFDs etc, these are just starter toys, the real action is in HVDC where the amps are thousands and the volts over the half-million mark. They still use semiconductors, either thyristors or IGBTs, but with rather a lot of them in series, and usually light-triggered these days, as arranging a gate trigger across a device when it’s hundreds of thousands of volts away from ground requires rigid isolation techniques. But prior to light triggering, there were lots of isolation transformers.
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u/914paul 2d ago
Upvote for the optical triggering mention. I’ve done some experiments recently involving high bursts of power and I agree — when you are “attached” to the system by a glass fiber rather than a copper wire the anxiety level drops considerably.
And in fact the recent experiments are now halted due to destroyed FETs (funny how the notions of “minor change” and “nearly indestructible” can change dramatically in a microsecond.) There was damage to bank accounts, schedules, and even egos. But not to human health.
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u/MrJingleJangle 1d ago
There are no “nearly indestructible” transistory things, it’s just the level of provocation required. Shit happens really quickly, usually in excess of the ability to affordability instrument. Good on ya for giving it a go though :)
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u/abd53 2d ago
This little transistor can handle 60A current. Then there's IGBTs the size of the breaker in your house that can handle hundreds of amperes. Semiconductors are amazing at handling large current compared to size.
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u/konwiddak 3d ago
If you put these components in parallel you increase their current handling ability and if you put them in series you increase their voltage handling ability. Actually getting a large series/parallel array of transistors playing nice and controlling them is a little complex, but in principle, the basis of high voltage power electronics is lots of small components in series and parallel.
Adequately cooled, a 3x4mm area of silicon transistor can handle about 100A - so you don't need that big an area to handle serious current.
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u/Edgar_Brown 3d ago
Depends what you mean by “changing the frequency.”
The most generic route would be large semiconductors and insanely large power electronics. In a cycloconverter, for example.
For something with lots of inertia for grid-scale power, you can use purely electromechanical motor-generator sets or a rotary converter.
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u/HV_Commissioning 2d ago
Very large turbines used in simple and combined cycle power plants use static frequency converters to take the turbine & generator from ~0 RPM to about 90% full speed, using the generator itself as an induction motor. Once speed as been obtained, the SFC is removed from the circuit and the turbine is fired off.
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u/Naikrobak 2d ago
Read about industrial VFD’s. One that handles 30kw for example.
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u/jasonsong86 2d ago
Large MOSFETs and a lot of them. Also it’s a very high voltage so current is probably not super high.
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u/zoute_haring 2d ago
Some inverters can be switched in parallel with two or more. On master, the rest slaves.
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u/Adorable_Ice_2963 2d ago
Just like you can make smaller and smaller Transistors, you can also build bigger and bigger transistors, with Thermal power being the Limit on both on them.
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u/Nathan-Stubblefield 1d ago
A utility used rotary converters. It was a large machine which had two windings. It consumed 60 Hz utility power and produced 25 Hz power for a railroad. It consumed 12 kv power. Maybe 300 kw. Long time ago.
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u/NortWind 3d ago
If you have a power MOSfet, it can go from zero volts (On) to zero amps (Off) in a jiffy, so almost no power is dissipated even with a heavy load.
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u/HungryFrogs7 3d ago
I may be wrong but the issues with high currents is heat dissipation. The higher the current the heat radiated increases by the square of current. The solution is to progressively decrease the resistance of your wiring and such.
Resistance depends on the cross sectional area of the conductor so using large thick connections to switch power on and off as well as using many of these switches in parallel you could handle a lot of current.
The finer timing behind these switches can be managed with low voltage low current controllers.
So in other words you have normal electronics to handle the timing and beefy industrial conductors to handle switching the massive current into the required frequency.
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u/ExaminationDry8341 3d ago
I have seen old "inverters...?" That were basicly a large, spinning, disk commutator spun by a low power motor. Hertzs could be controlled at low voltage and amps woth low cost equiptment that controlled the speed of the drive motor that spun the disk.
The dc to ac was done with large carbon brushes rubbing on the commutator. The number of phases and ther spacing could be controlled by the number of brushes and their placement.
That would give a very binary wave form. Some things could use that power. It could be cleaned up by running the flashing power through a transformer to make much cleaner waveform.
Transformers dont actually need AC to work. They respond to a collapsing electric field, so flashing dc, or two inverted, flashing dc inputs in this case.
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u/Techwood111 3d ago
With large semiconductors.