The Harmonics-1000 Amplifier

Common voltage wave-shapes from public power mains are far away from this specification. In most cases the voltage wave-shape is "flat topped", caused by the wide spread of equipment with rectifiers.

The high demand on the maximum voltage sine-wave distortion is for good reason:
Linear loads draw the current during the full period of the sine-wave while nonlinear loads like rectifiers draw their current only during a short time around the voltage maximums. This sharp current pulses near the voltage peaks with a high content of harmonics are very sensitive on the voltage wave-shape in this part.

The following two measurements on the same load, one with a pure sine-wave from the Harmonics-1000 amplifier and the other with a "flat topped" sine-wave straight from the public mains, demonstrate the drastic difference.

An amplifier for this application must be able to generate a 230V sine-wave at a permanent current of 16A. The low distortion requirements demand a high quality amplifier better than many high quality HIFI audio amplifier. A few years ago this was only possible with linear amplifiers. In the meantime also switched amplifiers can fulfill the requirements with the benefit of a much better efficiency of over 90% compared to the 50% of the linear amplifier. This is a big difference at a power of around 4kVA

Today almost all amplifiers for power mains generation are switched amplifiers. Beside the advantages of the switched amplifiers there are also some disadvantages:
Over-current and over-voltage protection has to be very sophisticated. If you want to drive loads with high inrush currents or with voltage feedback this protection circuits often shut down the amplifier and disables testing. Some amplifier cause also much high frequency noise on the input side (mains side) of the amplifier which may disturb other equipment. The nonlinear switched amplifiers like to ring together with some nonlinear loads.
In all this aspects the linear amplifiers are still better but unfortunately also much more expensive.

Both, the linear and the nonlinear amplifiers has a rectifier at the input which converts the sinusoidal input mains voltage into a positive and a negative dc-voltage. The dc-voltages define the maximum output-swing. (about 350V or 230Vrms ) All parts are designed to cover the full output power of 4kVA plus the losses.

The above described amplifier can generate any wave-shape within the dc-voltages and his bandwidth. But in the given application we need only an output wave-shape which is close to the mains input voltage wave-shape.

The Harmonics-1000 amplifier has dc-voltages of only 33V. Because this is more than 10 times smaller than for a full swing version it can be built as a linear amplifier with all his advantages.
The output of the amplifier is floating on the input power mains voltage. Such an amplifier is able to generate any voltage wave-shape within the wave-shape of the two dc-voltages.

Measurements and 5 years of field experience with the Harmonics-1000 have shown that this voltage band is enough to cover most industrial mains voltage qualities. Beside the 33V Range there is also an 66V range with a reduced current capability of 8A. Furthermore a series up/down transformer (voltage booster) corrects nominal voltage differences between in- and output. All these settings are automatically managed by the amplifier controller.

Beside the much smaller design which results in lower space, lower losses and lower costs which gives you an linear amplifier for the price of a switched one, there is also another advantage which is important in the given application:

Inrush current:
The protection circuit for such an amplifier can be done in an easy way. In case of an over-current (>about 60A) or over-voltage the floating amplifier is short circuited via a solid crowbar circuit so that the Harmonics-1000 output is directly connected to the power mains input. This not only protects the amplifier from damage it also allows to deliver high inrush currents to the connected load, only limited by the capabilities of the public power mains. (for a 230V/16A mains typically up to 400A for some ms or up to 60A for some periods). For the short duration of such high inrush currents it is not important to maintain the proper voltage sine-wave at the output because it is not part of the test, but the amplifier must often be able to deliver such a current to start-up the device properly. Some other amplifier simply switch-off if the over-current protection trips.

Voltage monitoring:
During harmonic current measurement also the voltage on the EUT is permanently analyzed. If any of the voltage harmonics is too high the system reports an "Out of Spec" message. So you have all the time full control over the proper working of the amplifier.

Disadvantages:
We don't want to say nothing about the disadvantages: A full swing amplifier can also generate a 60Hz sine-wave out of a 50Hz mains or vice versa while the Harmonics-1000 not can. But the advantages of lower price and less problem with troublesome loads makes the Harmonics-1000 anyway to the better choice.