Backup power generators for electronic and inductive consumers

28.03.2023

The choose of a suitable backup power generator should be made conscientiously, since an unsuitable generator not only endangers the intended use, but can also destroy the electrical consumers and is itself endangered.

Electronic power consumers and power consumers with electronic control of different types often have a different form of power consumption than the voltage, which must be taken into account when developing the backup power supply.

Here are some examples of voltage (yellow) and current draw (green) when operating from the DSO grid:

light

 Charger for mobile phones

BluRay receiver

power supply laptop

TV or monitor without PFC

device with mixed power supply

All of the electronic power consumers listed above have no built-in power factor correction and consume only part of the voltage wave.

These are mostly small power consumers with a power of up to 75W. Devices with power higher than 75W must already have built-in power factor correction. For example, a large TV has a peak current draw in standby mode, after switching on its power supply becomes completely active and the current draw of such a TV looks very different. The same happens with a desktop PC.

Examples:

large TV with power factor correction

TV with partial power factor correction

desktop PC with different loads on the internal power supply depending on the activity

To compare the current draw of a light bulb and a side grinder:

A special case are tools with electronic power control (with power controller):

approx. 50% power

100% power

Difference of the form between voltage and current can lead to undesirable effects. A distinction must be made between conventional generators and inverter generators.

The current in conventional generators is taken from the alternator winding, which is an inductance in itself, and becomes part of the circuit once the load is connected:

The alternator winding is not to be compared with the secondary winding of the transformer as the two are excited differently and the primary winding of the transformer is connected to the DSO grid with a low internal resistance.

For comparison of voltage (yellow) and current (green) when operating a drill with power control via phase cut control from the public grid and from a conventional generator:

A clear deformation of the voltage can be seen in the case with the conventional generator. The voltage regulator only regulates the active voltage, but does not control the voltage form. Overvoltage can occur in unloaded parts of the voltage wave, although the active voltage remains within limits.

The voltage in the maxima of the sine wave increases to 325V at the effective voltage of 230V. In the event of an uneven load on the voltage wave, voltage peaks of 400V and more can occur, which can damage other power consumers located in the same circuit. For example, the electronics in the circuit, LED lamps, etc. can be destroyed.

Phase cut control is not only used for tools, but also for other household appliances such as vacuum cleaners, washing machines, heat pumps, etc.

Phase cut control is also used in soft starters that regulate the starting current of the electric motors.

The electricity in inverter generators is generated electronically. The energy is first stored in capacitors, which are charged by the alternator via a controllable rectifier. The DC voltage of the capacitors is converted into the AC voltage by the B2 bridge circuit:

For comparison of voltage (yellow) and current (green) when operating a drill with power control via phase cut control from the public grid and from a generator with inverter technology:

It can be seen that the inverter generator can keep the voltage form better than a conventional generator and the maximum amplitude of the voltage within a voltage wave remains within the permitted range.

The peculiarity of electronic loads or loads with electronic power control is that they consume only part of the voltage wave.

Phase cut control uses thyristors which only close when voltage comes to 0V and the current through them tracks the voltage after the thyristors open. This causes different distribution of energy within the voltage wave and causes the associated processes. The voltage at the unloaded part of the voltage wave can rise so high that it can damage other current consumers present in the circuit.

Another type would be the electronic power consumers with switching power supplies without built-in power factor correction.

These have a rectifier and capacitors, whose energy is replenished at the moment when the amplitude of the voltage wave reaches a higher value than the voltage of the capacitors. This leads to a pulsed decrease in current.

Conventional generators (inductivity as a current source) and inverter generators (capacitor as a current source) behave differently with regard to current consumers with pulsed current consumption. Capacitor is able to let the current rise and fall much faster than a coil (inductance). The rising edge of the current pulse comes up much more slowly in the conventional generator and the falling edge causes the transient processes that arise from the energy stored in the coil (E=LI²/2).

These transient processes are caused by the release of the energy stored in the coil and represent harmonics that can reach high amplitudes if the current consumption by the connected consumers approaches zero.

The impulse current draw is common for electronic devices with power supplies without power factor correction. These devices are capable of generating harmonics, but are themselves affected by them and in some cases even destroyed.

There is also some distortion of the voltage wave caused by uneven loading within the wave.

Inverter generators have different characteristics because the capacitor works differently than an inductor and reacts differently to changing loads and current fluctuations:

The rising edge of the current pulse looks completely different with the same load and there are no transient processes after the current pulse.

Inverter generators are therefore much more suitable for sensitive electronic devices than conventional generators (also with AVR).

The voltage wave produced by the inverter generator is also able to better maintain the voltage form.
And what about the public grid?

This is how the current draw looks like with the same electronic load from the public grid:

It can be seen that in the public grid, the peaks of the sine wave have a certain deformation caused by a large number of electronic power consumers.

Should one understand the above measurement results in such a way that conventional generators are not at all suitable for modern electronic consumer? The answer is NO!

Conventional generators can still be used as a power source, but their properties and the properties of the power consumers to be supplied must be taken into account when planning the backup power supply.

The loads are generally divided into linear (ohmic) and non-linear.

The ohmic load present in the circuit is able to dampen the transient processes and harmonics in such a way that they are no longer dangerous for sensitive electronics. Ohmic load loads the parts of the voltage wave that are not loaded by the electronics alone, offers a way out for the energy stored in the coil by current pulses and thereby dampens the harmonics.

In a dormitory, electronic devices with a pulse-like power consumption without power factor correction are primarily small power consumers with a power of up to 75W. Total power of such devices in a house is about 300-400W, and a resistive load of about 100-200W (a pair of light bulbs), as a rule, is able to stabilize their operation by damping transient processes. In the case of higher pulsed power, separate solutions must be found that are precisely tailored to the respective case.

Inverter generators usually do not require these protective measures and are therefore a better backup power source for sensitive electricity consumers. However, such generators often have a lower output and are more susceptible to feedback power and induction currents from connected power consumers.

As a rule, inductive consumers of electricity with motors have a starting current, which, depending on the design, can be 3 to 6 times higher than the rated current:

Grinding machine without electronic control (start-up and normal operation)

In the case of power supply from an inverter generator or power station, the output voltage may collapse because they have electronic overload protection that can react to the instantaneous value of the current:

The voltage collapses while the current through the connected load reaches the maximum allowable value. The energy stored in the inductive load (E=LI²/2) causes self-induction, which may also damage the generator's inverter module.

It is very important when operating power consumers with motors from an inverter generator that the required starting power does not exceed the maximum power of the generator, otherwise its inverter module may be damaged.

In such a case, a resistive load present in the circuit could divert some of the return currents and thereby protect the generator to a certain extent. If the inductive load is to be alone in the circuit, the voltage peaks caused by self-induction can reach too high a value and damage the electronics of the generator.


The electricity consumption of a house usually has a complicated character, since each active device contributes to the total electricity collection.

Here is an example of the power draw of a house with running LED lighting, computer, monitor; Telephone system, satellite system, refrigerator, etc.

Here is another special case, the washing machine with the motor running, the speed of which is regulated by phase cut control:

You can see that there is another part with a clear inductive behavior.

Several active current consumers can to some extent balance each other by loading different parts of the voltage wave and avoiding "dangerous" fall with unloaded parts.

The power factor is decisive for the entire system. In a normal household, this is approx. 0.7 - 0.8 and is difficult for a normal consumer to assess. The generator not only has to cover the active power, but also the entire reactive power, which is why it is recommended not to operate the generator with more than 80% of its nominal power.

The resistive current consumers and current consumers with built-in power factor correction present in the circuit play an important role and stabilize the whole system.

Here is the current draw of the same household with the kettle on (left) and with the washing machine (right) on while the water is being heated:

The generator as a power source and the power consumers to be supplied form a closed system whose elements affect each other and it is extremely important to analyze the power consumers to be supplied when selecting a suitable generator.

The generator for backup power of the house should be chosen taking into account its characteristics and the characteristics of consumers, since the wrong choice of generator can harm both consumers and the generator itself.

The generator for backup power supply should be chosen taking into account its characteristics and the characteristics of electrical consumers, since the wrong choice of generator can harm both consumers and the generator itself. Könner & Söhnen gives only general recommendations for the use of its generators.


Disclaimer:

These instructions can only be taken as a recommendation, are illustrative and must be adapted to the exact local circumstances and conditions during installation. The installation itself should be carried out in compliance with all standards and regulations. We take no responsibility for the wrong installations and their consequences.