Engineering Speed of a DC generator is proportional to the generated voltage?

[ajskelly]

Homework Statement

If the speed of a DC generator is reduced by half, will this reduce the Voltage and the EMF by half as speed is directly proportional to EMF? Resistance is staying constant,

Relevant Equations

E=V+IaRa

I'm confused as I'm aware speed is directly proportional to the EMF, but is it proportional to the voltage as well? If the speed of a DC generator is halved and everything else is remaining constant the EMF will have so will the output voltage half

 

[cnh1995]

If the speed of a DC generator is halved and everything else is remaining constant the EMF will have so will the output voltage half

Yes, provided it is a separately excited generator.

 

[ajskelly]

How about a shunt wound generator, for example...

the output voltage is 200V
the Ia is 50A
the Ra is 0.03ohms

E=V+IaRa
E=200+(50X0.03)
E=201.5

The speed of the generator is now reduced by half. As the speed of the generator is directly proportional to the EMF, half the speed will mean half the EMF so...

E/2 = 201.5/2 = 100.75

Now if I wanted to find the current which would have changed, can I use

E=V+IaRa

so can I say,

100.75= 100+(IaX0.03)

100 being the 100V that was found by dividing the orignal voltage by 2 due to the speed being halved.

Hope this makes sense!

Thanks for your help

 

[cnh1995]

As the speed of the generator is directly proportional to the EMF

I'm aware speed is directly proportional to the EMF

I prefer 'emf is directly proportional to the speed'.

Also, emf is proportional to the magnetic flux produced by the generator's field winding.
This means emf is directly proportional to the product of field flux and rotor speed.
As the terminal voltage changes, the field flux also changes.
Hence, in case of shunt generator, you will need to do some math considering field winding resistance to find the terminal voltage.

 

[ajskelly]

Sorry I forgot to thank you cnh1995 for your assistance,
Cheers!