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Acrogen is dedicated to the design and manufacture of Variable Mains Alternating Current switch mode power supplies and controllers. They are called VMACs for convenience.

There are three ways of controlling mains power. These are burst control, phase control and wave control. These three approaches are discussed more fully below.

Until now, only the first two ways, burst control and phase control, were easy to do using mechanical contacts, thyristors or triacs. The use of a VMAC now allows the third control technique, wave control, to be easily implemented.

Wave control has positive advantages with current and impending legislation though at present it is relatively expensive. However, for certain types of loads, VMAC units and the use of wave control offer an effective solution. They also open up new control opportunities that were either impossible or very difficult to do.

It is Acrogens' view that VMAC type units allied to wave control will become the dominant energy control technology in the long term, especially as the cost of semiconductors becomes relatively cheaper and the adoption of ever more stringent legislation makes the other technologies more and more expensive to bring into line.


This is the usual technique adopted where mains is used to operate something like a resistive heater. The power is switched on and off repeatedly, say, every 15 to 30 seconds. The ration of 'on' to 'off' dictates the average power supplied to the load. Simple circuitry is involved and it is relatively cheap to implement.

The main objection, which is getting quite serious, is impending 'flicker' legislation. The electricity suppliers also do not like this kind of load switching as it can cause difficulties in providing a 'within tolerance' voltage supply.


This is the technique normally used to vary the mains when controlling a load like a light bulb. The power is switched on, (or off), at some point every half cycle. The percentage of the half cycle that is 'on' dictates the amount of power supplied to the load. More complicated circuitry is required with a medium cost burden.

The main objection, perhaps even more serious than above, is the appalling power factor that is imparted by the load current, especially at half power setting. The current utilities are not going to accept this for much longer, especially if a viable alternative comes along.


This technique may be used to control virtually any type of load. The power applied by the VMAC type unit is the same waveshape as the incoming mains (sinusoidal) but just at a reduced amplitude (voltage). It is the most complicated circuitry and the highest cost. However, it provides the perfect answer to flicker legislation and power factor legislation. The power drawn from the mains and supplied to the load is smooth and progressive.

In fact, wave control already is used in a very crude way by either variable resistors (rheostats) where up to one quarter of the full output power is lost in the resistance. It is also done using variable auto transformers which give quite reasonable efficiencies but are difficult and slow to control, plus they are prone to brush wear problems and track damage when left at one setting for long periods of time. Finally, 'linear amplifier' power supply units will provide good wave control but with the penalties of very poor efficiencies and exceptionally high cost.


VMACs can perform wave control in two different ways. Each method has certain advantages.


The A.C. mains is converted directly to the desired value by switching transistors in a circuit similar to part of a switch mode power supply. Because of its direct conversion, the overall efficiency is in the region of 97-98%. The neutral connection is maintained between input and output. Input and output frequency has to be the same and the input and output voltage phase angle is the same as well. However, due to its 4 quadrant operation, the current can flow in either direction.


In this arrangement, the A.C. mains is converted to D.C. via an isolating transformer and then the D.C. is converted to A.C. again. The advantage of this approach is that the output frequency as well as the amplitude can be varied. The output can also be isolated and floating with respect to the input. The disadvantages of indirect conversion are lower efficiency, 85-90% at best, and also much higher circuit costs.


The following problems occur with burst and phase control. Wave control implemented by a VMAC unit offers effective control and overcomes or eliminates them all.

·  Mechanical Shock and Noise - Indicative of stress in capacitors and inductors especially when phase controlled. It often leads to premature failure of components and control equipment.

·  Electrical Stress - Sudden application of mains means that the load has up to the peak mains potential applied in a few micro seconds. This high dv/dt stress may also be accompanied by a large di/dt as well.

·  Excessive Dissipation - Non linear waveforms may cause load components to experience excessive or abnormal current values that they do not have under steady state sinusoidal operation.

·  Mains Surges - These are applied to the load with phase and burst control. Wave control can limit these.

·  Inrush Current - Soft start is required here. This is easy with wave control.

·  Current Limiting and Shorts - Thyristor type switching has to wait for current to go to zero before switching off. VMAC units have current detection continuously operating. They will disconnect within 20 microseconds or so.