Category: Featured

Last time, we discussed why the utilities supply 50 or 60 cycle (Hz) power and the aviation industry operates on 400 Hz power.

Obviously, the two systems are not compatible. 400 Hz is 6-2/3 times faster than 60 Hz and 8 times faster than 50 Hz. The speed of motors (and clocks) would be multiplied by the same factor. Also, a few things would just catch fire. However, we need a source of 400 Hz at the airfield so that aircraft can shut down their engines. The engines and internal generators burn fuel, create noise and produce undesired emissions.

The simplest approach would be to build a 400 Hz generator driven by an engine on the ground next to an airplane. But that would consume fuel, create noise and produce undesired emissions just like the aircraft. It would be a solution if no other power was available. However, usually utility power is not far away. The boarding bridge, hangar or other building will be using 50 or 60 Hz utility power.

But, didn’t we just say the two power systems were not compatible?

Yes, we cannot connect the two systems directly but there are ways of converting utility power to 400 Hz aviation power.

One way is to use a motor connected to the utility as the engine to drive a generator producing 400 Hz. This creates what is called a Motor Generator frequency converter, or “MG Set.” Electrical power to the motor generates horsepower out. The horsepower into the generator creates electrical power out. The engineering of the system ensures that the utility 50 or 60 Hz is correct for the motor and that the generator produces the proper voltage and 400 Hz frequency for the aircraft.

Although the MG Set solution is rather simple, it has a couple of drawbacks. First it is mechanical. The rotating parts require continuous lubrication. The air-cooling of the motor and generator create high ambient noise. Finally, because the efficiency of the MG Set is not good for normal and low loads, (50-70%) the operating power expense can be high.

A preferred method of frequency conversion is a Solid-State Frequency Converter (SSFC).

The solid-state design also takes power directly from the utility and converts it to a form acceptable to aircraft and military 400 Hz power systems.

The front end of the SSFC rectifies the utility power and creates a direct current (DC) voltage. DC is a steady voltage that does not have a significant frequency component.

The DC is then switched by power transistors to create an alternating current (AC) waveform at the required frequency of 400 HZ. The efficiency of the SSFC is quite high at all loads; typically approaching 91-94%.

Although both types of frequency converter cost about the same, the solid-state frequency converter design and provide a significantly lower operating cost when compared to MG Sets. Fuel power engine generator sets may be the only choice if utility power is not available.

The idea of making electricity useful for commercial lighting and motors initiated an argument. One side wanted direct current (DC). One wire will always be positive (+) and the other wire is always negative (-).

The other side supported alternating current (AC). In this system, the voltage is constantly moving between positive and negative. The shape of this changing voltage is a sine wave.

The changing AC voltage provided the advantage of using transformers to change the voltage. High voltage at low amps could be sent long distances over small wire, and then transformed to a safe lower voltage for distribution inside a home or factory.

Once the decision was made to use AC, the next question was, how fast should the voltage change? How many cycles per second (Hertz, Hz) should be produced?

As the frequency of the voltage change was made slower, the size and weight of the transformers and generators increased and became more expensive. As the frequency was made faster, more power was lost in the transmission lines, which also increased cost.

The most economical frequency for the power company was around 60 cycles per second. Some countries standardized on 50 cycles per second or Hertz (Hz).

When aviation began using electricity, it was DC power. As AC became more prevalent in aircraft, the primary concern was the size and weight of transformers, motors and power supplies. The idea was proposed to use a higher frequency to make the components lighter, since the length of power transmission was small, the increased power loss would be negligent.

A special generator was designed to create an output of 400 Hz. This allowed a motor which was the size of a watermelon to be replaced by one the size of a one-pound coffee can which could do the same work.

The saving of weight allowed increased cargo capacity and decreased fuel consumption. Power at 400 Hz for aviation was a success and became the standard of modern AC-powered aircraft.

Airports all around the world standardized on the same power system. This included the physical plug and cable as well as the 400 Hz power so that aircraft from anywhere in the world could land and be serviced wherever they landed.
The aviation power system of 400 Hz became one of the first worldwide-adopted standards.