dc generator, urgent or I am toast

[skyman68]

mayday!!!
i have an exam monday 2nd and I have conflicting feedbacks.i need help asap.

here the 2 questions:

assume that a cst speed dc generator provides a cste output voltage.if the electrical load increase , the voltage regulator will:

a.maintain the intensity of the excitation current constant
b.increase the intensity of the excitation current
c.decraese the intensity of the excitation current


a is te correct answer in my feedback

now, a second question, seem the same with a wrong answer

if the load increase on a cste speed generator what does the voltage regulator do:

a.increase field excitation
b.decrease filed excitation
c.remain the same

a is the correct answer

so, when u incrase the load, do the generator increase the field, or does it maintain the current.???

[M.85]

Skyman,

Call Oxford aviation,tell dina(the secretary) you have a french friend(ex oat student)who told you you could call them and would like to ask a question to Mike Allen about a atpl problem..
Most likely he will answer your question..if mike not in ..ask for steve chesser..he knows everything about anything..

Bets luck,

M.85

[Mr Magoo]

The field excitation has to be INCREASED to maintain generator output voltage, ie you push more current through the field windings to increase the magnetic flux so that the armature will then give a higher output, thus maintaining generator DC output constant.

:ok:

[skyman68]

Ok, thank you , I see...

I resume:field excitation increases to maintain output current constant (when load increases).

they will not toast me!

.

[Mr Magoo]

By george I think he's got it, (almost,) the generator output VOLTAGE is maintained at a constant value, current drawn can vary as you select/deselect services. For example you want to maintain a CONSTANT 24V bus VOLTAGE irrespective of whether you're only drawing a couple of Amps or lots of Amps - perhaps turning ovens on to heat the SLF's dinner!

Anyway make sure you know your resistors and capacitors in series and parallel and also know all about secondary batteries and a few logic gates - I'm sure you'll be fine mate
:ok:

[Send Clowns]

M.85 the ever-helpful Dina no longer works for OATS. She's one of ours now. If you ask her now, she'd put you through to me.

Note the DC generator is not itself a constant-speed device, just the question has the speed constant (engine RPM unchanged). The extra current drawn would tend to make the output voltage dip, except that this dip causes the voltage regulator to increase the excitation current to maintain output voltage.

[Alex Whittingham]

My understanding is that the output voltage is a function of field strength and rotational speed. If the rotational speed is the same then the task of the voltage regulator must be to keep the field strength constant to achieve a constant voltage output.

In theory field windings can be in series or in parallel with the loads. In this context 'loads' are things you can switch on, they are always arranged in parallel in the circuit so the more loads you switch on the lower the overall circuit resistance and the greater the current that flows (I = V ÷ R)

If the field windings are in series with the loads it means that, as you switch more things on, the current also increases through the field windings and output voltage increases. Not good.

If the field windings are in parallel with the loads then, as you switch more things on, there is a greater tendency for electrons to flow through the loads and less tendency for them to flow through the field coils, the current in the field coils drops and output voltage reduces. Not good.

If you carefully put some of the field windings in series and some in parallel you can arrange it so that output voltage stays roughly constant when you switch things on.

The key word here is 'roughly'. To get more accurate voltage control you tweak it so there is a slight tendency for output voltage to fall as loads increase, in order to do this you put slightly more of the field windings in parallel.

Now we have a situation where, as loads increase, the excitation current in the field coils falls slightly, this reduces output voltage slightly. The job of the voltage regulator is to adjust (reduce) the resistance of the parallel circuit with the field coils in it so that current flow through the coils is restored to its previous level and output voltage restored.

Thus the full sequence should be:

Loads increase, excitation current falls, voltage output falls, voltage regulator increases field current, voltage increases to previous level.

.....and you could answer this (a) or (b) depending on which stage of the process you thought the examiner was asking about. Acting on 'information received' we believe the 'proper' answer is (b).

Any AEOs or electrical engineers out there do pitch in and correct me if I've got this wrong.