Why PMG system is no longer necessary?

Supply to electronic regulator: From pmg or auxiliary winding?

The pmg system provides energy supply to the electronic regulator by means of a small alternator with permanent magnets. This unit is usually located towards the rear of the alternator. Mecc Alte ensures the power supplying to the electronic regulator by means of an auxiliary winding buried in the main winding. It follows a brief description of the differences and the common features of those two systems.

Weight and size.

For the same power, a machine with an auxiliary winding is lighter and smaller than a machine with PMG. In fact the auxiliary winding has a negligible weight and its inclusion in the main stator does not have any effect on the size of the machine which therefore remains unchanged. The PMG (consisting of a permanent magnet rotor and a stator) has however a certain weight and its inclusion has a significant effect on both the weight and the length of the machine.

Voltage supply of the system.

The power output from the pmg is not affected by the power generated by the alternator or by distorted loads. The output from an auxiliary winding slightly is. This voltage variation must be considered when designing the electronic regulator (AVR). If the AVR is properly designed, these effects can be completely compensated. Therefore, from the user point of view, there is not any difference between the two systems. Due to a careful design of the electronics, MeccAlte generators with auxiliary winding system can meet exactly the same performances of a pmg-fitted generator during power transients or with distorted load.

Transient power of the system

The pmg system can only provide a limited transient power compared with the auxiliary winding system. This means that the pmg has got a limited starting capability on electrical motors, that require a higher than nominal current to start up. The auxiliary winding system has got a much higher motor starting capability. In a pmg the excitation power is limited to the size of the pmg, while in the Mecc Alte system the excitation power is theoretically equal to the machine power, which is considerably higher than the power needed for excitation.

Short circuit capacity.

Both systems are able to maintain a correct supply to the electronic regulator even in the event of a short circuit within the main stator. In this situation the sensing on the voltage of the electronic regulator is zeroed and the guaranteed short circuit current is greater in both cases than 3 times the nominal current. The Mecc Alte system could also provide higher short circuit current values without compromising the weight, the size, or the cost of the alternator; however this is not possible with the pmg system.

Conclusion

The two systems are more or less equivalent from the performance point of view. The pmg system has only a theoretical advantage in applications with distorted loads; actually the Mecc Alte auxiliary winding system is able to provide the same performances with distorted loads and transient power as the pmg system by means of a specifically designed electronic regulator. The Mecc Alte system certainly has advantages in terms of the motor starting capability and the excitation power available, as well as in size and costs.