What happens when alternator load is increased

In summary, when load is increased on a synchronous machine, the armature current increases. This extra current comes from the armature's self inductance and the field current must be increased to maintain a constant voltage.
  • #1
jaus tail
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So I'm studying synchronous machines and what happens when load is increased.

In case of alternator(synchronous generator) if the load is increased, like more lamp is added in parallel,
then the armature current would increase. But where does this extra current come from?

The rotor cannot go faster because:
there is locking between rotor magnetic field and armature rotating magnetic field.
If the rotor accelerates, then the armature rotating magnetic field also accelerates, that means the frequency of output current changes. that can't happen as bus is assumed to be at constant voltage and constant frequency?

What exactly happens and where does this extra load current come from?

In case of sync motor when the shaft is loaded, the resultant air gap flux is behind leads the field flux. So when the shaft is loaded(load torque increased), the speed reduces and the air gap flux is a bit more in phase with field flux. Since the rotor has to move at constant speed, the air gap flux has to go back to initial position(leading field flux), so it draws more armature current.

Armature flux leads the air gap flux. Air gap flux is phasor addition of armature flux and field flux.

If armature flux is increased, air gap flux goes back to initial position.

What actually happens when alternator load is increased? And if the rotor does speed up how does it know it has to speed up?
 
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  • #2
You add more alternators. You bring up more power stations.
 
  • #3
Last edited:
  • #4
If load current increases, then 2 things are needed to maintain constant voltage.
1) Fuel consumption must increase.
2) Field current must increase.

To elaborate:
1) If the alternator is regulated for constant voltage, which I am assuming, & the load current increases, it is also true that the power has increased. An alternator transduced mechanical power to electric power. Since electric power has increased, the mechanical power inputted must increase. So more fuel must be burned to keep steady voltage.
2) The armature (usually the stator) has a large self inductance. The increased current will result in a larger voltage drop across said inductance, which reduces the terminal voltage. To compensate, the filed current must be raised so that the generated open circuit voltage increases, such that after the inductive drop is accounted for, the terminal voltage is at the target value.

Did I help? BR.

Claude
 
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Likes jim hardy
  • #6
Thanks for replying.

Okay so I looked at the phasors.
As per them, when load current increases the resultant flux(air gap flux) tends to slip back.
So the field mmf rises to pull the resultant flux(air gap flux) back towards it, back to original place.

But how does the field current know it has to rise? The field flux(by rotor only) of sync motors are independently controlled. So how does it vary as per the load? I mean it can't be instantaneous, right?

How does field current know it has to change and by how much does it change? How does it calculate?

So if i remove the load, the armature current will go zero, then the rotor will accelerate, right? Then how will it come back to sync speed since synchronous motor has to run at sync speed.

As per the phasor diagrams in the link: https://www.physicsforums.com/threads/synchronous-generator-excitation.829603/#post-5215184
the rotor is first pushed ahead and this causes the armature current to flow to bring the resultant flux back to the original place.

But what if the load current is increased without increasing mechanical power manually? I mean how does the rotor know it has to accelerate. And where does the extra input power come from to supply to additional load?

One explanation could be that increased load current could send load current more lagging to voltage. Like power factor goes low and low, so resultant phasor addition of armature flux and field flux is again back where it was.
ugenetic3_zps0b47bca3.gif


Like at first alternator is not loaded. When it is loaded lightly, it's like leading armature current will flow, so the leading current is like 90 degrees leading the terminal voltage, so the resultant flux is back where it was. now when machine is loaded, the armature current rises but it starts lagging. Like it moves clockwise. As machine is more and more loaded, the armature current is in phase with terminal voltage and then starts lagging with more armature current.

Not sure if this really happens. Any insight would be helpful?
 
  • #7
jaus tail said:
But how does the field current know it has to rise?

There is some mechanism present that senses the voltage drop and applies more current. This is called the voltage regulator.

BoB
 
  • #8
Ah...okay. like some feedback element.
 
  • #9
jaus tail said:
Like at first alternator is not loaded. When it is loaded lightly, it's like leading armature current will flow, so the leading current is like 90 degrees leading the terminal voltage, so the resultant flux is back where it was. now when machine is loaded, the armature current rises but it starts lagging. Like it moves clockwise. As machine is more and more loaded, the armature current is in phase with terminal voltage and then starts lagging with more armature current.

Not sure if this really happens. Any insight would be helpful?

You must decide what you will vary and what you will hold constant in your thought experiment. I stated those conditions on my drawings
Here's keeping both field current and terminal volts constant , just applying torque. That's not unrealistic for a machine tied solidly to a stout bus(infinite bus is a useful concept)

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Observe "Condition 2"
fixed terminal volts determines radius of circle
note armature amps and armature mmf have same magnitude and direction.
To restore unity power factor at that load you (or rbelli's voltage regulator) must raise field amps.

You'll have to think one step at a time.
 

FAQ: What happens when alternator load is increased

1. What is an alternator load?

An alternator load refers to the amount of electrical current that is being drawn from the alternator, which is responsible for generating electricity to power the car's electrical systems.

2. How does increasing the alternator load affect a car?

Increasing the alternator load can put additional strain on the car's engine, as it has to work harder to supply enough power to the alternator. This can result in decreased engine performance and fuel efficiency.

3. What are the consequences of a high alternator load?

A high alternator load can lead to a drained battery, as the alternator may not be able to keep up with the demand for electricity. It can also cause the alternator to overheat, potentially damaging it and other electrical components in the car.

4. How can I reduce the alternator load?

One way to reduce the alternator load is by turning off non-essential electrical components, such as the air conditioning or radio, when they are not in use. It is also important to regularly maintain the car's electrical system and alternator to ensure optimal performance.

5. Can a high alternator load be dangerous?

In extreme cases, a high alternator load can be dangerous as it can cause the car's electrical system to malfunction and potentially lead to a breakdown. It is important to monitor and manage the alternator load to avoid potential hazards.

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