# Calculating generator RPM

1. Dec 2, 2018

### royp

Hello,
I have a simple question: What is the timer device in a generator? For instance, in order to get the standard frequency of 50Hz, there should be a rotational speed of 3000 rpm. what device measures and ensures this?

Thanks

2. Dec 2, 2018

3. Dec 3, 2018

### Staff: Mentor

@anorlunda what about a standby power generator? It isn't clear to me which the OP was referring to...

4. Dec 3, 2018

5. Dec 3, 2018

### royp

@russ_watters: Sorry I spelt your name wrong in the previous post!

6. Dec 3, 2018

### Staff: Mentor

Ah, an isolated generator. In that case, the speed is controlled by a modern version of this James Watt invention from 1788.

Of course, today it can be done electronically (like the cruise control in some cars), but for simplicity and reliability, mechanical governors based on centrifugal force are still used. If you google "speed governor", you'll find lots of hits.

A key feature is that the governor is a proportional controller. They open the throttle in proportion to some error in the speed. That means that as load changes, speed will also change by a small amount. However if it is not connected to the grid, exactly 50 Hz is not necessary.

7. Dec 3, 2018

### Tom.G

And here is a video along the same lines. (If it doesn't automatically, you may want to skip the first 3 minutes of it.)
Pay attention to the unusual tachometer around the 9 min. mark.

8. Dec 4, 2018

### yungman

I just join in for the fun of it. Seems like it's easier to let the motor run, convert the AC to DC, then use an oscillator to convert the DC back to AC at precisely 50Hz. Converter design is very efficient, might end up easier than to regulate the motor speed for different load. You might be able to buy these off the shelf.

but what do I know.

9. Dec 5, 2018

### sophiecentaur

I seem to remember my Dad telling me about a Synchroscope which was used to compare the phase and frequency difference between a local generator and the grid. This is a bit like an induction motor which will rotate for a frequency difference or stay stationary at an angle corresponding the right phase. The method was to adjust the amount of steam into the turbine until the needle was stationary in the right position and then throw the breaker to connect up. Once connected, the grid and the generator will tend to stay together (obvs the system has to be arranged to be stable), any phase difference can result in power flowing in the wrong direction. Where large distances are involved, the 'tail can wag the dog' and a system can go into oscillation. It's much easier nowadays with predictive computer control.
For links where the two connections cannot reliably be mutually synchronised (like the link between UK and Europe) there is a DC link with autonomous control of frequency on each side.
I saw this f meter in an exhibition in Poole, Dorset, UK. It consists of a set of resonant reeds and used to show the revs of an alternator in the old Poole Power Station. The synchroscope would take over when they got the frequency right I presume.

10. Dec 5, 2018

### Staff: Mentor

It is strictly forbidden to run a home standby generator connected to the power grid (unless there is a special interface box from the utility that provides protections.) Therefore, you should never need to synchronize it.

For permanent backup power installations, they use a transfer switch that assures that the house is either fed from the grid or from the standby generator, but never both.

11. Dec 5, 2018

### dlgoff

12. Dec 6, 2018

### sophiecentaur

I wonder why a unit that appears to be 'panel mounted' (i.e. permanent) would need be suitable for 50 and 60 Hz. The only application I could think of would be on a ship that might be connected to either frequency standard.

13. Dec 6, 2018

### dlgoff

I think it was designed with both so it could be sold in the US (60 Hz grid) or in Europe (50 Hz grid). It accepts a single phase voltage from 100 to 150 VAC.

14. Dec 7, 2018

### Tom.G

As late as the 1960's there were still installations with 25Hz supply. This was in the USA supplied by the hydroelectric plants at Niagara Falls. Most of the building I'm familiar with had been upgraded to 60Hz but the elevator and the motor-gen set for the carbon arc movie projectors were still 25Hz.

Edit:
Some of the incadescent lighting was also on 25Hz. You could see the flicker in your peripheral vision but they appeared steady when looked at directly.

Last edited: Dec 7, 2018
15. Dec 7, 2018

### Klystron

US observation confirmed. Also, worked with power from 50hz sources at ~220 VAC in Asia
Sources [at frequency and with AC output voltage already specified in this thread] simulated available power in Central Europe.

[Some projects provided vacuum cleaners suitable to the power sources. Woe betide the bubba' who attempted to remove the funny AC connector, replace with connector for US mains ("Hey! It's grounded!"), and vacuum his truck.]

16. Dec 7, 2018

### Staff: Mentor

Wow! Is that open to the public? I would love to see it.

17. Dec 7, 2018

### jim hardy

i can vouch it was still there early 70's, still flickering.

I would too.

If you pass this way
there's a quaint little 1910-ish 400kw Westinghouse low head hydro plant that ran until 1970's. Now it's part of the state park at Mammoth Spring.
Has an early Woodward governor.

good trout fishing in that spring fed stream .

old jim

18. Dec 8, 2018

### CWatters

19. Dec 11, 2018

### dlgoff

Been there many times on my way to Batesville.

20. Dec 11, 2018

### Klystron

Nikola Tesla fans might enjoy visiting Westinghouse vintage generating equipment. Legend has Tesla designing many systems for George Westinghouse particularly after falling out with Thomas Edison over patent issues.