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Comment by collin128 | Hacker News Reader

Not an engineer but I used to sell generators. I believe constant rpm eng/alternator is pretty common practice. From memory and a quick Google search, 1800 rpm produces 60hz and 1500 produces 50hz in newer alternators. I'm unfamiliar with these engine/alternator combos so they likely operate slightly differently.

Most power generating units operate at a relatively fixed rpm for longevity and will ramp a little during heavy load.

Source: https://www.generatorsource.com/Generator_Frequency_Conversi...

Also pulled up an old spec sheet for a Cummins and it had the same rpm/Hz specs.

Link to PDF: https://www.google.com/url?sa=t&source=web&rct=j&url=https:/...

Edit: added link to spec sheet

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Diesel engines for power generation will have a governor control that sets the throttle based on the load to maintain whatever rpm is required for 60 Hz (constant speed control). The load in this scenario is uncontrolled and the genset simply follows it.

A diesel locomotive's engine controls are about producing the right amount of power to match the operator's throttle control. Electrical frequency at the generator (alternator) terminals is irrelevant since that output is being rectified to DC anyways. This is great for a train, because it means you can have full power at any speed - the speed of your train and the speed of your engine are completely decoupled from each other. But if you now connect this normally variable frequency AC output directly to 60 Hz loads, you will need to figure out how to set the throttle to best maintain something close to 60 Hz and your power output will be limited.

If you buy a large engine for a generator the manufacture will have one pin on the ECU for the 50/60hz switch which just changed the RPM. Generators are not enough of a market for the manufactures to make their own engines (unless they have other divisions that also need an engine for something else) that meet emissions, but they are large enough a market overall that companies that make engines want to get in on the extra profits they can by selling to the market (they get to spread the cost of emissions development across more engines), and the 50/60hz RPM needs are only a few hours to code/test on top of all the other work they are doing anyway.

There may be minimum quantities in your engine order, though the contract will provide for spare parts which might get you a single replacement many years after the generator is manufactured (if the rest of the engine is still available)

Older/basic generators would run at fixed frequency to generate the correct frequency. I believe the voltage is regulated by regulating the field strength in the rotor. Modern generators have inverters that handle the output and regulates the 60Hz/110V or 50Hz/220V etc. In this case the RPM is controlled by the wattage required (with a min and max RPM). Motor -> alternator -> inverter -> load

It depends on the size.

The larger the generator, the more likely you have a constant-RPM setup.

True, but that is changing. Inverters are not very expensive anymore, provide cleaner power, and can match the frequency of the something else allowing you to put several generators in parallel and disconnected one for maintenance. They also allow running lower RPM which at lighter loads can save fuel/money. (A constant rpm generator often uses more fuel at 50% load than at 80% load!)

Inverter generators can also sell power back to the grid. To pay for the fuel you need more than the typical retail price of electric, so this is only worth it if you have a need for a backup generator anyway. However if you have this need talk to the power company, they often will give you a discount because when there are load issues (See Texas last week), and the ability to add power to the grid in this time will be something they are interested in.

Today most large generators are still constant RPM, but I don't expect that to last.

Why would a constant-RPM generator not be able to feed into a grid?

Also, you can run them in parallel without problems, as long as you keep the governors from oscillating. But proper PID tuning and sensing architecture should turn that into a non-issue.

Larger engines btw. just disable cylinders for part-load operation.

They can, and do: if the governor is good enough. Overall it is easier to use an inverter, but there are several ways to solve the problem.

Cylinder disabling has a place too.

1800 rpm 60hz / 1500rpm 50hz given a four pole alternator.

One of the reasons old stationary engines (in Europe) in the pre-governator time ran at a fixed speed of 1500 RPMs

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