Mecc Alte AVR S.R.7/2 Automatic Voltage Regulator (Replacement)

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Meccalte S.R.7/2 AVR a negative feedback system, in which the constantly maintained variable is the alternator voltage output. This voltage, after adequate conditioning, is compared with a signal that is stabilised in voltage, temperature and age. The result is an error signal which, when opportunely amplified and compensated, produces a control level that enters an impulse width modulator.
The output of this modulator pulses an SCR with continuous excitation voltage produces in conjunction with, undergoing electromagnetic conversion by generator operation, transforms into generator output voltage. The value of this size is controlled by the “sensing” block.

Error amplifier

Mecc Alte S.R.7/2 Voltage Regulator is a confrontation signal that is highly voltage-,temperature- and age-stabilised, and which is generated inside the electronic regulator. The “VOLT” trimmer positioned on the regulator can modify its value. Each reference signal variation corresponds to an output voltage variation.

Impulse width modulator

This part of the electronic regulator transforms the control signal into respective impulses that dirve the power SCRs, so that the value of the generator continuous excitation voltage is changed.

Sensing

In this part of the circuit the alternator output voltage is suitably conditioned and applied to an AC/DC converter, the output signal of which is compared to the reference signal.

Protection block

Figure 1 clearly shows that the part of the electronic regulator called the protection block is made up of two parts:

a) low frequency protection

b) overload protection.

Low frequency protection

The frequency-voltage converter is a fundamental part of the protection block. When the frequency of the output voltage falls below the value of a given threshold, the output signal of this block produces a control signal which lowers the output voltage of the generator, in order to stop over excitation (a typical example is the lowering of the speed for the preheating of the prime mover).

As can be seen from figure 3, when the output voltage frequency of the generator falls by more than 10% of the nominal value, the output voltage reduces in line with the frequency (-7,6V/Hz).