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VK3XX > INFO 05.07.02 21:00l 113 Lines 4761 Bytes #999 (0) @ WW
BID : BD0940VK3XX
Read: DB0FHN GUEST
Subj: Re: VK3ABK > Tesla's motors
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Sent: 020704/0730Z @:VK3FRS.#MEL.VIC.AUS.OC #:25178 [Kilsyth] $:BD0940VK3XX
From: VK3XX@VK3FRS.#MEL.VIC.AUS.OC
To : INFO@WW
I hope this may be of general interest too!
Warren referred to induction motors and posed questions about rotational
speeds. So here we go.
First a table of synchronous speeds, ie. the speeds at wich AC machinery
runs.
Basically it all comes down to the number of magnetic poles for which the
machine is designed.
Poles Speed on 50 Hz mains Speed on 60 HZ mains
rev/min rev/min
2 3000 3600
4 1500 1800
6 1000 900
this goes on for machines up to 10 poles or more.
So you would see, Warren, that for a generator to generate 60 Hz and
running at 2400 rev/min it would need to have three poles. But such a
design is impossible! Sorry to take issue with you!
Large Utility turbine generators in your country are two pole ( except for
hydro generation where 10 pole speeds are more likely) and run at 3600
rpm. In mine they are 3000 rpm for 50 Hz.
You raised induction motors running at 1700 rpm and that certainly is
typical in the USA, They are 4 pole motors and that is the most common
type. The difference between 1700 and 1800 is that an induction motor is
not a synchronous machine. According to its mechanical load it will run
below synchronous speed down to its maximum "slip" of a few percent after
which it will stall, and without overcurrent protection it will burn out!
So does that amswer your particular question?
This leads me to why 60 Hz machinery is more likely to fail when fed with
50 Hz mains. Here I am talking particularly about motors and transformers.
I will take the induction motor. The magnetic ciruit of an induction motor
can be simulated by a resistor and an inductor in parallel. When the
voltage is applied the portion of magnetising current in the resistor
doesnt know whether it is 50 Hz or 60 HZ. But that going through the
inductor sees a lower reactance so for the same applied voltage the
current through it is greater on 50 Hz than on 60 HZ so the net
magnetising current is greater.
Similarly the load current is simulated by an inductor (the magnetic
leakage inductance)in series with a variable resistance ( effectively the
mechanical load). With a lower leakage reactance at 50 Hz and a given
range of speed slip ( the resistor) the current drawn for a given
mechanical load is also more. This means that the motor is less efficient
and the heating is greater than on 60 Hz. The cooling fan also is running
slower on the motor so the heat dissipation is less and the motor
eventually suffers an insulation failure and burns out! It wont develop
the rated mechanical output either without stalling. This assumes equal
rated voltages apply too.
A similar analogy exists for the transformer when use on a lower
frequency.
Modern computer designs have very little margin for such "maloperation".
On the other hand 50 Hz machines run on a 60 Hz supply go a lot faster and
suffer from mechanical overspeed failures.
So does that help? Again my disclaimer of this being very superficial.
Going on from that "What are the advantages of 50 or 60 Hz mains?"
Mention has been made of 400 Hz distribution in aircraft for which the
main reason is weight.
So clearly 50 Hz requires heavier machinery than 60 Hz in general terms
but it also doesnt have to rotate so fast. With modern material properties
it is easy to design either for 50 or 60 Hz for running at any synchronous
speed.
Now I havent mentioned generators. If you design a 4 pole generator for a
50 Hz system (typically a small diesel genset) and run the set at 1800
rev/min it will generate 60 Hz AC output at 20 % greater output. 60 Hz
generators will probably overheat run at 1500 rpm. It is also running at
its overspeed test frequency so no margin for centrifugal stresses.
So like all engineering there is a compromise and choice of frequency
becomes an economic decision favouring the higher frequencies.
As a final wrap up no-one has mentioned 33 and two thirds Hertz systems!
I thing this was the first AC system frequency used in Europe early last
century by the Swiss railways in particular. Series DC variable speed
motors used on trains will run on AC supplies but the commutation of them
only works at low frequencies. I think some rail systems still use them.
Lighting however flickered as someone mentioned so they just multiplied
the frequency by three and got to 50 Hz. Hence its future use for all
lighting in Europe. By this time the USA had come of the dark ages thanks
to Mr Edison so I guess 60 Hz was quickly settled upon to keep transformer
and rotating machinery metal down to mimimum weight!
Cheers to all and
73 - GORDON, VK3XX @ VK3FRS
Message timed: 17:25 AEST on 04 Jul 2002
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