Internal Voltage vs. No-Load Voltage in Generators

you would want to test a generator across any load that can be connected to it, from zero to its full rated load, to be sure its produces a stable voltage across that entire range.(typically plus or minus 5%)

Yes, the internal voltage and the no-load voltage condition are essentially the same thing. You are trying to identify the Thevenin equivalent circuit for your generator. The output impedance will change with the inclusion of any internal feedback of your generator to maintain a constant output voltage. This test is performed at power levels far below the maximum output power level because the Thevenin equivalent is only valid for a linear circuit. Once the Thevenin equivalent has been established, the threshold where non-linear output power can now be found. The advantage of this approach is that it does not matter which part of your generator is becoming non-linear.

Hi redfred,

Thanks for your answer! Regarding this sentence, ¨This test is performed at power levels far below the maximum output power level because the Thevenin equivalent is only valid for a linear circuit.

My expected no-load/internal voltage was calculated as 610V, whereas in the performed test, it is less, around 560V or 570... Is this difference due to the non-linear components?

Thanks a lot!

Best regards,

No, if the test is performed properly. Meaning that the only electric load your generator output is seeing is your voltmeter and the internal wiring of the generator. Your low voltage is less than a 10% drop in expected output voltage. This maybe normal depending on the design of this generator.

Some questions:-

1. Are you talking AC or DC generator? I do not think you ever told Us!
2. If AC, was your design calculation for a mean value over half cycle or rms?
3. Is your measured value r.m.s.? Or is it a mean rectified value meter calibrated for r.m.s assuming a perfect sine wave, which is the usual case for less expensive DVM or moving coil meters?
4. Have you checked the waveform or harmonic content of your generator output?
   

Note that :-

1. Ratio of r.m.s./mean magnitude is about 1.1 for sine wave. You have that size of error.
2. 10% third harmonic voltage can give about 3% error on a mean measuring meter.
3. 10% third harmonic voltage is only 1% of fundamental power, so has little effect if meter reads true r.m.s.

Try thinking of it this way. Voltage in a generator is like pressure in a hose. When there is no flow (i.e. current) the pressure will build up to match the source. That is your theoretical maximum pressure (or generator/transformer voltage). When you allow the water to flow, the line pressure will drop. The ability of the source to maintain the pressure, while water is flowing, is called regulation.

Same thing, different terms.

Hi Jorge,

"...the internal voltage in the generator is the same as no-load voltage, isnt it?...", yes it is, but remember that it is a mathematical artifice that is used to model the behavior of a generator. Once load is put on the machine, this voltage "source" needs to be adjusted (or not) according to the type of analysis being done.

"...the purpose of testing the generator with no-load voltage..." is to provide a starting point for understanding how the machine behaves under varying conditions, it also needs to be tested to understand how the non-linear part of the machine, the rotor and stator iron, affects the model.

There are two different types of models, linear and non-linear, and two loading conditions, unloaded and loaded, for a total of 4 different conditions. To fully and accurately characterize a machine it is necessary to run test under all the combined conditions. That's where the saturation curve and the generator capability curve come from, they account for the change in the no-load internal voltage as the machine goes from no-load to fully loaded (and everything in between) conditions.

As explained by other replies, the simplest theory of a direct-current generator represents it as a perfect fixed voltage source in series with a fixed value resistor.

Think of a practical basic generator, such as a brushed commutator dynamo, which was standard on motor cars, before the alternator/rectifier machine was used.

The fixed field winding is connected in parallel with the output.

If the output voltage falls due to resistance of rotor winding and brushes, then the voltage to the field winding is reduced. This reduces the strength of the field - which reduces the voltage induced in the rotor.

So with any amount of load, the internal voltage behind the output resistance is less than with zero current and is not equal to the no-load voltage.

Similarly, slip in the drive belt with increased load reduces generator speed and reduces the e.m.f. induced in the windings and, consequently, the output voltage.

The usefulness of a no-load test is that it gives a measure of the machine as a starting point to which the effects of speed, resistance and field can be added.

A generator requires minimum power to drive it at no-load, which means that a test variable-speed motor to drive it can be small and affordable.

A basic test is to measure no-load voltage at rated speed/ field voltage/field current - if this is not correct then the machine has a fault (or your design was wrong). Varying field current at fixed speed gives a characteristic curve.

If an AC generator is considered, then the inductance of the machine windings and the phase of the load current relative to the e.m.f. have to be included - but here again the no-load test is the starting point for measurement of the machine characteristic numbers.