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Excitation Capacitance Fir Induction Motor Generator Please Help!


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Hi, i need some help,

I am making a Generator, I am in the UK.

i have a diesel engine with more than enough power, i can set this for any rpm, with accuracy.

i have the appropriate belts and pulleys to rotate the motor at above speed.

i have an induction motor:

GEC Alpak induction motor



1450 rpm

21.5 amps

3 phase

50 hz


ins.class f

encl. tefv

brgs D.E. 6309z

N.D.E. 6209z

Diag. 54-67

conn. (triangle)

size d160m


that is all the info from the motor plate, and it has 6 connections under the cover, i havent been able to find much on the net about efficiency rating.

i have the wiring diagrams, and i will have a qualified electrician check it out when i am done.


could anyone help me with calculation for the capacitance i need to self excite? my maths isnt that good!

i will be intending to use it to run single phase tools, and 3 phase machines.

if you need any more info just ask i will provide what i can.

thanks for any help


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Hi Russ, You may find this site helpful http://www.saunalahti.fi/elepal/moottori/gener_e.html .It has a simple circuit diag with the three caps and a way of flashing the generator to ensure residual. Has an example for a 1.5kw with values included. I'm trying to fugure out the calculations myself. I think a phasor diagram will have to be drawn when resistive,capacitive and inductive reactance's are known among other things ....probably, I'm guessing. Anyway this doesn't answer your question but hope this helps. I'll be following this thread.
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hi, a mate came over last night and i think we did it, but i have one question, the info i have tells me how to work out delta wired capacitors, if i want to wire them up in a star i need to use 3 times the value caps? is this right? basically i need 3 caps of 48uf for delta, but 3 caps of 144uf for star, does that seem right to you? it seems a lot to me, i dont want to cook it!,


here is the equation i used.


this comes from, "motors as generators for micro hydro power by nigel smith."

free pdf if you want to google it.


Square root of 3 x volts x full load current = VA


so 1.73205 x 415 x 21.5 = 15454 VA


VA x power factor = W


so, 15454 x 0.86 = 13290 W



square root of (VAxVA - WxW) = VAR



VAR / 3 = ????

???? / V = Amps

in this case Var is 7886, V is 415, so amps required is 6.33 Amps, for excitation,



6.33A / (2pi x cycles x volts)

6.33A / (2pi x 50 x 415)= 48.5 uf or for star x3 = 145 uf



sorry about the mess my laptop wont do symbols, pi is Pi as in 3.14, by the way!





sorry forgot to mention the power factor and efficiency figures i took from the brook compton data sheet included in the pdf, i couldnt find the info for my motor,

and the other thing, i want to wire this up in star, not the delta, as i want the single and three phase outlets on the generator.



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I would be careful with this calculation. What you are doing is determining the capacitance required to give 100% correction and I would be very careful doing this.

At 100% correction, inductive reactance equals capacitive reactance, you have a resonant circuit at the frequency that you are operating at and when resonance occurs, you have a major amplification in voltage if the load impedance is high.


This could result in very high voltages and damage.


I do not believe that the capacitance is super critical, but as the capacitors essentially supply the VARs necessary to flux the iron, the amount of capacitance would be expected to influence the output voltage. If the VARs are too low, the output voltage will be low.


The major problem with this type of approach for a standalone generator, is that it is difficult to regulate the voltage.

Induction generators are good where there is already a supply available and you are augmenting that.


Best regards,


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mark thanks for the input, much appreciated.


firstly though this is a generator born of neccesity not choice, i have the bits i need to do this, i don't have any cash for any other options,


i may be able to run a small single phase supply, to the workshop containing the generator, would that help? if necessary i could give up the 3 phase altogether, if i had to.


my rpm is fixed and with an excess of torque from the engine running the generator, if i adjust the capacitance to achieve the correct voltage, will the voltage not stay the same as the load fluctuates? the load would always be well within the capabilities of the machine as a generator.


i also have a 12v system available, would that be able to generate the excitation current? or do i need to provide the current constantly while in use?


also does the voltage fluctuation matter that much? would their be some way of producing a regulated supply for sensitive equipment, and a rough supply for motors and welders?


i was feeling quite confident, so now i am just throwing out a few ideas see if there is another way of achieving this!



so if i stick with the capacitor bank, is the delta = 3 x 48uf, star = 3 x 144uf sound right to you?, i am just a little concerned with the star setup being 3 times as much, as that is the one i will be using i dont want to make a schoolboy error and fry my only motor! or spend 3 times as much on capacitors as i need to!



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