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'[EE] Generator currents'
2011\02\18@072626 by Andrew Wood

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Can i work out how much current a particular generator can create at a given RPM by measuring the resistance accross its terminals, driving it at the RPMs Im interested in and measuring the voltage then just putting the figures into Ohms Law?

Andrew

Sent from  iPhon

2011\02\18@081753 by Sean Breheny

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Hi Andrew,

It depends on what kind of generator, but there are definitely other
significant factors, such as the winding inductance. For example, I
have a brushless permanent magnet motor here which can be spun at up
to 4000 RPM, at which point it generates a phase to phase output
voltage which is a sinusoid with a peak amplitude of 50 volts. The
winding resistance is about 0.1 ohm. However, instead of 500 Amps
output, the most I can get (at ANY RPM) is about 40 Amps, because once
I get to a high enough cycle frequency to get significant voltage, the
inductance has a reactance which is greater than the resistance.

Sean


On Fri, Feb 18, 2011 at 7:25 AM, Andrew Wood <spam_OUTa.j.wTakeThisOuTspamme.com> wrote:
> Can i work out how much current a particular generator can create at a given RPM by measuring the resistance accross its terminals, driving it at the RPMs Im interested in and measuring the voltage then just putting the figures into Ohms Law?
>
> Andrew
>
> Sent from  iPhone
>

2011\02\18@101724 by Andrew Wood

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Hi Sean

Would a DC generator suffer from that to the same extent?


Andrew

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On 18 Feb 2011, at 13:17, Sean Breheny <.....shb7KILLspamspam@spam@cornell.edu> wrote:

{Quote hidden}

>> -

2011\02\18@110659 by Olin Lathrop

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Andrew Wood wrote:
> Can i work out how much current a particular generator can create at
> a given RPM by measuring the resistance accross its terminals,
> driving it at the RPMs Im interested in and measuring the voltage
> then just putting the figures into Ohms Law?

In theory, yes, but in practise there will be things that cause that
calculation to be inaccurate.  The main issue is that simply measuring a
resistance when the generator is not running is not a good way to determine
the true impedence.  It would be better to measure the open circuit voltage
and the voltage under a known load.  That will give you a much better idea
of the true impedence in practise, from which you can get better estimates
of voltage and current under different load and speed conditions.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000

2011\02\18@121326 by RussellMc

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> Can i work out how much current a particular generator can create at a given RPM by measuring the resistance accross its terminals, driving it at the RPMs Im interested in and measuring the voltage then just putting the figures into Ohms Law?

No.
But it does give an effective upper limit.
Maximum power transfer theorum says you get maximum power when Rload =
Rsource. So your peak power occurs at Rload = Rwinding
Power = V.I =  Voc/2  Voc/(2.Rwinding) = Voc^2 / (4.Rwinding).

This is the theoretical maximum power given adqeuate flux, available
mechanical drive, no core saturation and a small host of other
assumptions

2011\02\18@123227 by M.L.

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On Fri, Feb 18, 2011 at 10:15 AM, Andrew Wood <.....a.j.wKILLspamspam.....me.com> wrote:
> Hi Sean
>
> Would a DC generator suffer from that to the same extent?
>
>
> Andrew
>

Any rotating motor is actually an AC machine. A DC motor or generator
has a commutator that switches the terminals at the appropriate point
through a rotation, thus either converting the alternating current to
DC, or vice-versa. If you had current of a single polarity, you would
be unable to produce a changing magnetic field. So no, a DC generator
is going to be subject to the same limitations as an AC motor (or
'generator'.)

-- Martin K

2011\02\22@042351 by Andrew Wood

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Thanks for the replies so far.

Is it true that a generator becomes physically harder to turn the higher the current load?

Presumably in that case a petrol diesel generator must have some way of automatically detecting this and increasing the power to the engine?

Sent from  iPhone

On 18 Feb 2011, at 17:31, "M.L." <EraseMEmspam_OUTspamTakeThisOuTlkeng.net> wrote:

{Quote hidden}

> -

2011\02\22@043335 by Kerry Wentworth

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Absolutely!  You can test this for yourself if you have a DC or stepper motor.  Turn the shaft, then short the windings and try again.  A generator would normally have a governor to keep the output frequency at 50 or 60 Hz.  That would automatically compensate for increased load.

Kerry


Andrew Wood wrote:
> Thanks for the replies so far.
>
> Is it true that a generator becomes physically harder to turn the higher the current load?
>
> Presumably in that case a petrol diesel generator must have some way of automatically detecting this and increasing the power to the engine?
>
>  


-- Internal Virus Database is out-of-date.
Checked by AVG Anti-Virus.
Version: 7.0.289 / Virus Database: 267.11.13 - Release Date: 10/6/05

2011\02\22@053158 by Andrew Wood

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Presumably the governor is on the prime mover rather than the alternator?

What phenomenon is responsible for it becoming harder to turn?

Sent from  iPhone

On 22 Feb 2011, at 09:34, Kerry Wentworth <@spam@kwentworthKILLspamspamskunkworksnh.com> wrote:

{Quote hidden}

> -

2011\02\22@063425 by Isaac Marino Bavaresco

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Em 22/2/2011 07:30, Andrew Wood escreveu:
> Presumably the governor is on the prime mover rather than the alternator?


Exactly. Diesel engines have a fuel injection pump with a governor.
Engines made for electricity generators that need to be lock-stepped to
work together have electronic governors, whilst normal tractor engines
have mechanical governors. Most diesel generators I have seen use normal
tractor engines.


> What phenomenon is responsible for it becoming harder to turn?


It is the same principle  that makes electric motors work, interaction
between magnetic fields in the rotor and stator.

It's not hard to understand that it must be that way, or else we would
be again in the realm of free-energy harvesting.


Isaac

2011\02\22@074602 by Olin Lathrop

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Andrew Wood wrote:
> Is it true that a generator becomes physically harder to turn the
> higher the current load?

Think about it.  If it didn't they'd either be horribly inefficient at low
loads or you'd be getting free energy from somewhere.

> Presumably in that case a petrol diesel generator must have some way
> of automatically detecting this and increasing the power to the
> engine?

Simple gensets usually just have a speed governor and leave it at that.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000

2011\02\22@075005 by Olin Lathrop

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Andrew Wood wrote:
> Presumably the governor is on the prime mover rather than the
> alternator?

What's it matter since they are directly connected, usually on the same
shaft?

> What phenomenon is responsible for it becoming harder to turn?

Magnetic forces from the current.  As current flows, the magnetic field
produced by that current apposes the motion of the magnets in the other part
(rotor/stator) of the generator.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000

2011\02\22@081325 by Isaac Marino Bavaresco

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Em 22/2/2011 09:50, Olin Lathrop escreveu:
> Andrew Wood wrote:
>> Presumably the governor is on the prime mover rather than the
>> alternator?
> What's it matter since they are directly connected, usually on the same
> shaft?


The governor monitors the rotational speed of the shaft, which is indeed
solidly connected to both the engine and the alternator, but of course
the governor must actuate somewhere, and the right place to do so is the
engine's throttle on the fuel injection pump.

2011\02\22@150729 by Herbert Graf

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On Tue, 2011-02-22 at 09:22 +0000, Andrew Wood wrote:
> Thanks for the replies so far.
>
> Is it true that a generator becomes physically harder to turn the higher the current load?

Yes.

> Presumably in that case a petrol diesel generator must have some way of automatically detecting this and increasing the power to the engine?

Yes. Generators have a throttle govener that ensures the correct output
RPM, which translates to the correct output frequency.

You can see the effect whenever switching a heavy load on (or off), the
engine speed drops (or rises) and then settles back at the correct
value.

TTYL


'[EE] Generator currents'
2011\03\09@105633 by Andrew Wood
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Ive been doing some reading up about this and Im a little confused as to whether its caused by an interaction between the magnetic field induced in the rotor pushing against the magnietic field from the stator or whether there is a thrust induced on the rotor itself just from the having current flow through it. Reading about Flemings Left hand rule suggests the latter but if that were so surely transformers and inductor coils would move!?

On 22/02/2011 12:50, Olin Lathrop wrote:
> Andrew Wood wrote:
>> What phenomenon is responsible for it becoming harder to turn?
> Magnetic forces from the current.  As current flows, the magnetic field
> produced by that current apposes the motion of the magnets in the other part
> (rotor/stator) of the generator.
>

2011\03\12@131333 by Sean Breheny

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There are many ways of visualizing it, but the simplest, most
universally-applicable way is to think of the moving electrons and the
force on them due to the Lorentz Force Law F=qVxB where x is a vector
cross product. q is the electron's charge, V is the electron's
velocity, and B is the magnetic field vector at the location of the
electron. To determine the force and torque which one permanent magnet
applies to another, you can either consider them both to be rings of
moving charges and use the Lorentz approach or consider them to be
magnetic dipoles and use the equations for the force and torque on a
magnetic moment in a magnetic field.

Sean


On Wed, Mar 9, 2011 at 10:56 AM, Andrew Wood <KILLspama.j.wKILLspamspamme.com> wrote:
{Quote hidden}

>

2011\03\12@135930 by Isaac Marino Bavaresco

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Em 9/3/2011 12:56, Andrew Wood escreveu:
> Ive been doing some reading up about this and Im a little confused as to
> whether its caused by an interaction between the magnetic field induced
> in the rotor pushing against the magnietic field from the stator or
> whether there is a thrust induced on the rotor itself just from the
> having current flow through it. Reading about Flemings Left hand rule
> suggests the latter but if that were so surely transformers and inductor
> coils would move!?


Yes, transformer and inductor coils want to move. They only don't
usually move because they are mechanically constrained.
But if a coil turn subjected to AC is loose, it may vibrate and suffer
mechanical fatigue or the enamel may wear due to the brushing against
other neighboring turns and create a short-circuit.

Isaac

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