AC Generator Theory

A 2-pole motor rotates once every cycle, so at 50Hz, 3000rpm.

A 3-pole rotates once every 1.5 cycles, so at 50Hz, 2000rpm.

A 4-pole rotates once every 2 cycles, so at 50Hz, 1500rpm.

In practice, the spin speeds are a gnat's slower than this, as there is always slippage.

There is no such thing as a 3 pole generator!!!! Magnetic poles are always in pairs, there is no such thing as a magnetic monopole. Thus, for every "North" pole, there is a "South" pole.

Most generators are synchronous machines so that there is always a fixed relation between frequency and rotation RPM.

It is important to realize that most generators are three phase, so when we talk about poles, it is important to understand that a "2 pole per phase" generator spins at 3600 RPM if 60 Hz and 3000 RPM if 50 Hz. A "4 pole per phase" generator spins at 1800 RPM if 60 Hz or 1500 RPM if 50 Hz. A "6 pole per phase" generator spins at 1200 RPM if 60 Hz and 1000 RPM if 50 Hz.

A 6 pole per phase, three phase generator, will have a total of 18 poles.

There are things called asynchronous generators that are essentially induction motors that are connected to a grid and run at a speed above synchronous speed. However, asynchronous generators produce "real" power (KWH) but consume "reactive or imaginary" power (KVAR). Consequently, asynchronous generators cannot produce any electrical power unless they are connected in parallel with a synchronous generator that supplies the reactive power component.

I am very interested in using asychronous motors as generators in stand alone applications but need to drive other motors so capacitor exitation is not very workable how can I find out what size synchronous generator I would need to supply reactive power. I am working the 15 - 75 KW 3 phase range

I can not believe the answers you've been given: Guys

Freq = (# of Poles x RPM) / 120

a 4 pole generator rotating at 1800 RPM will have a frequnecy of 60 Hz

It assumes a supply frequency of 60Hz whereas 50Hz is also widespread, believably.

f=pn/120

frequency = hertz = cycles/sec

p = no.of poles

n = rpm of the alternator

It is common to have 4 pole alternators running at 1500 rpm with 4 poles to delivery 50 Hz.

The equation given earlier is correct, the rpm given for 50 hz is also correct, the fact that there is no such thing as a 3 pole machine is also correct.

f = (poles x rpm)/120

total electrical degrees = 180 x Poles

the relationship is fundamental

1. 1 full cycle of AC is developed for each pair of magnetic poles: cycle/2poles

2. There are a fixed number of poles in a full circle: poles/rev

3. There must be an even number of poles

Thanks everyone!! very helpful indeed!

to be honest i need an engine to produce 60 Hz and i received an offer that claims that the particular engine can produce 60 Hz with 1500 rpm (and 4 poles). I knew that the rpm should be 1800 but i wanted to make sure.

thanks again!

Yes there is a relation between frequency and rpm ( revolution per minute) of the ac generator. Its formula is f=2*pi*n/60 where:

f is frequency in hert, and n is speed in rpm.

If we consider the case of an AC generator , normally we used to take the synchronous speed i.e the speed of the maganetic field rotating.

The synchronous speed is given by the formula

N(rpm)= 120*freq/no of poles

If we consider a 2pole motor, operating at 50Hz , then it will rotate at 3000rpm. The poles of a generator will be in pairs only.

In the case of power factor correction we used to run synchronous motor in over excited condition. We used to call it as synchronous condenser, where it will generate the reactive VARs required. But I think it,s not related to the question asked for.........

The basic formula is freq = (number of poles x rpm) divided by 120

example: a large commercial aircraft engine such as the Pratt & Whitney JT9D-7 uses a 6 pole AC generator which turns at 8000 rpm's to develop the 400HZ that a Boeing 747-100/200 uses. How ever the Integrated generator drives on the 747-400's uses a 4 pole IDG which turns at 12000 rpm's in order to produce the same 400HZ.

No previous comentator has thought to mention that RPM generally relates to the prime mover. Although this is generally directly coupled to the alternator there are still belt driven models about where the pulley ratio has to be taken into account. (Onan still produce and sell one such). Ther is also the "inverter" type generators (i believe honda amongst others make these) where the gen rpm is varied to accomodate the load but the frequency is held constant by the switched mode power supply type inverter included in the set. That is to say the user has a variable speed set with a constant frequency output.

one comment stated that asynchronous gens cannot produce power without being coupled to a synchronous source. That is incorrect. these are in regular use and are currently available. ( see Fischer Panda gens).