Hertz

[Russell Ensslin]

So I'm told that here in America we use 60 Hertz per second.
That would mean that there is some giant armature somewhere spinning around 60 times every second.
Or perhaps the armature has three magnates on it spinning twenty times per second.
I don't believe that's even physically possible, Is it?
Is that what happens?

 

[mbrooke]

So I'm told that here in America we use 60 Hertz per second.
That would mean that there is some giant armature somewhere spinning around 60 times every second.
Or perhaps the armature has three magnates on it spinning twenty times per second.
I don't believe that's even physically possible, Is it?
Is that what happens?

Generator Frequency (f) = Number of revolutions per minute of the engine (N) * Number of magnetic poles (P) / 120
Conversely, P = 120*f/N

 

[zbang]

Wwweeerrrllllll.....

Hertz is cycles per second, so we say just "60 Hertz".

If there are only two poles in the generator then yes, 60 RPS or 3600 RPM. But when you start to add poles the rotation speed can slow down. The turbines at Hoover Dam spin at 180 RPM (40 poles), quite easy for an armature/rotor that weights around 400 tons. Other generators will have different rotations based on the number of poles.

Take a look at Alternator.

 

[ActionDave]

Take a look at Alternator.

Pay particular attention to the sidebar in that article. The first AC power plant they are talking about is right in my back yard.

 

[big john]

Generator Frequency (f) = Number of revolutions per minute of the engine (N) * Number of magnetic poles (P) / 120

Yep, the way I always remember it is that for a given stator winding the positive and negative pole have to pass by to make a complete cycle.

 

[JFletcher]

So I'm told that here in America we use 60 Hertz per second.
That would mean that there is some giant armature somewhere spinning around 60 times every second.
Or perhaps the armature has three magnates on it spinning twenty times per second.
I don't believe that's even physically possible, Is it?
Is that what happens?

60Hz is 3600 x/min, or what your average Briggs and Stratton engine spins. Formula 1 engines reach 20,000 RPM. Turbochargers spin at over 100,000 RPM. That's 1667+ spins per second. and if you think that only small, extremely well balanced things can spin that fast w/o self destructing, the fastest spinning neutron star has a frequency of 716HZ (716 spins per second). It's probably been chugging along like that for millions of years.

If the Earth were as dense as a neutron star, it would be the size of a golf ball. and we spin only once per day. Imagine if we were spinning 86,400 seconds/day x 716 revolutions/second, or 86,862,400 times as fast as we are now!!!

The slowest genset Ive personally seen is a 20cyl GE 710 that idled at 286RPM and pumped out about 2MW at 900RPM (4500 HP engine). Yeah, it's hard to think of that much copper and steel spinning around, but really, 900RPM is slow.

On the other extreme are the Wartsilla marine engines; the 14 cylinder one makes over 100,000 HP at just over 100 RPM.

http://o.aolcdn.com/dims-global/dim...com/media/2011/07/worlds-largest-diesel-3.jpg

if you look to the left, you'll see steps down into the crankcase.

 

[junkhound]

Back in the 60's when SI nomenclature came into being, a wag put a sign on the bike rack outside the eng. lab:

2Hz-sec parking ONLY

 

[Besoeker]

So I'm told that here in America we use 60 Hertz per second.
That would mean that there is some giant armature somewhere spinning around 60 times every second.
Or perhaps the armature has three magnates on it spinning twenty times per second.
I don't believe that's even physically possible, Is it?
Is that what happens?

Some numbers for you.
Steam turbines drive electric generators or, more accurately, alternators. These are often two pole machines running at 3600 rpm for 60Hz countries. So, not only is it possible, it is commonly done.

I live in the UK and it's 50Hz here so the alternators run at 3000rpm. Drax, the largest power station in UK has six alternators, each 660MW making the total capacity of the power station just a gnat's shy of 4,000MW.

At the other end of the scale, the company I used to work for made electric motors used mainly in the machine tools industry. These were some tens of kW, 55kW (75HP) was one of the more common sizes.They were mostly rated at 20,000 rpm. So, not only is the 3600 rpm possible, it is not by any means a limitation.

 

[Russell Ensslin]

So if this thing is 6 feet across and spinning at 100 revolutions per second; it's radius is 3 feet times two pie. Isn't that20,000 ft per second? 1/9 of light speed?
I don't believe it


Sent from my iPhone using Tapatalk

 

[iwire]

So if this thing is 6 feet across and spinning at 100 revolutions per second; it's radius is 3 feet times two pie. Isn't that20,000 ft per second? 1/9 of light speed?
I don't believe it

I don't believe it either.

You need to look up the speed of light in feet per second, you are off by many magnitudes.


( A bit of a hint, the speed of light is about 186,000 miles per second)

 

[GoldDigger]

So if this thing is 6 feet across and spinning at 100 revolutions per second; it's radius is 3 feet times two pie. Isn't that20,000 ft per second? 1/9 of light speed?
I don't believe it


Sent from my iPhone using Tapatalk

Another small problem: 20 x 100 is 2000 not 20000.

 

[Besoeker]

So if this thing is 6 feet across and spinning at 100 revolutions per second; it's radius is 3 feet times two pie. Isn't that20,000 ft per second? 1/9 of light speed?
I don't believe it

If the thing is 6ft across (daimeter) then the radius isn't 3 feet times two pie. It's 3 feet. At 100 revolutions per second that would 1885 fps.
In any case, I don't know where you got the 100 revolutions per second. Synchronous speed for a two pole machine, the fewest number of poles you can have, is 3600 rpm on your 60 Hz supply. Which is 60 revolutions per second.

Also, note iwire's point about the speed of light.

You seem to have got into a total muddle with the numbers.

 

[JFletcher]

So if this thing is 6 feet across and spinning at 100 revolutions per second; it's radius is 3 feet times two pie. Isn't that20,000 ft per second? 1/9 of light speed?
I don't believe it


Sent from my iPhone using Tapatalk

RPM is revolutions per minute. 100RPM is so slow that if the engine were clear, you would have no problem following everything; you could even watch the flame front propagate.

The speed of light is 186,000 miles per second. A typical race engine only moves the pistons around 5,000 feet/minute, or ~1/200,000th the distance in 60x the time.

In other words, a Formula One engine moves about 12 million times slower than the speed of light. That huge diesel I linked moves 2000x slower still, or 24 billion times slower than light speed.

Hope you're not trying to reach the stars any time soon lol.

 

[K8MHZ]

So if this thing is 6 feet across and spinning at 100 revolutions per second; it's radius is 3 feet times two pie. Isn't that20,000 ft per second? 1/9 of light speed?
I don't believe it

The rim velocity of a 6 foot diameter generator at 60 Hz would be 1130.4 fps, or 770 miles per hour.

Compare that to a street bike engine. 13,500 rpm with a 58mm stroke, approx. crank lobe radius 30mm or about 1.25 inches.

Rim velocity = 8831 fps, or 6,021 mph.

 

[GoldDigger]

If the thing is 6ft across (daimeter) then the radius isn't 3 feet times two pie. It's 3 feet. At 100 revolutions per second that would 1885 fps.

Russell wrote "radius" where he meant to say "circumference". To get the linear speed of a point on the outside of the rotor you need to multiply by the circumference not the radius when you are using revolutions per second for the speed.
If you measured speed in radians per second then the 2 pi factor would not be used.

 

[drcampbell]

The peak piston speed of the Wartsila-Sulzer RTA96-C turbocharged two-stroke Diesel engine is in the neighborhood of 2000 feet/minute.
(2.5-meter stroke, 102 rev/min -- see http://www.centralgagenerator.com/index_htm_files/BIG 2STROKE DIESEL.pdf )

The tip speed of a 6-foot-diameter, 3600-rev/min turbine is in the neighborhood of 70,000 feet/minute, which is nowhere near 1/9 Warp speed.
In fact, it is (of necessity) less than Mach 1.

 

[mpoulton]

The rim velocity of a 6 foot diameter generator at 60 Hz would be 1130.4 fps, or 770 miles per hour.

Compare that to a street bike engine. 13,500 rpm with a 58mm stroke, approx. crank lobe radius 30mm or about 1.25 inches.

Rim velocity = 8831 fps, or 6,021 mph.

The 8831 number is actually feet per minute - missed the conversion from RPM to rev/s. So the rim velocity is only about 100MPH. Still pretty fast.

 

[K8MHZ]

The 8831 number is actually feet per minute - missed the conversion from RPM to rev/s. So the rim velocity is only about 100MPH. Still pretty fast.

Oops....you are right.

 

[Besoeker]

Russell wrote "radius" where he meant to say "circumference".

It's what he should have said.

To get the linear speed of a point on the outside of the rotor you need to multiply by the circumference not the radius when you are using revolutions per second for the speed.

Which is what I did to arrive at the 1885fps.

If you measured speed in radians per second then the 2 pi factor would not be used.

And you'd ω, the term used in SI.

P=Tω
is so much simpler than HP =550ft-lbf/sec or 33000ft-lbf/min

Not sure how the Greek symbol will show but I'll give it a try.........

 

[zbang]

In fact, it is (of necessity) less than Mach 1.

And to further muddy the waters and drift away from the OP's question- because a mach number relates to the speed of sound in a medium, it will vary. In dry air at 70 deg(f), it's about 1125 ft/sec but in water at 70 deg(f) it's about 4870 ft/sec. In super-heated steam (300 psi, 500deg(f) superheat), the speed of sound is about 1775 ft/sec .