The diesel generator sets (diesel generators) currently adopted by mainstream data centers are generally equipped with a brushless excitation system of permanent magnet generator (PMG) plus exciter, that is, it including both PMG and AC exciter. This structure is called two-stage brushless excitation or PMG excitation system, and it is a standard configuration for modern high-performance diesel engines.
I. Typical Configuration: PMG plus AC exciter (two-stage brushless excitation)
The excitation system of modern data center generators is usually composed of the following three parts:
1.1. Permanent Magnet Generator (PMG)
o It is installed on the main shaft and rotates at the same speed as the main rotor.
o The rotor is a permanent magnet, and the stator outputs three-phase alternating current.
o It is not affected by the voltage fluctuations at the main generator terminal and provides a stable and reliable initial power supply.
2.2. Exciter
o It is also a rotary motor, and its stator winding is powered by PMG.
o The armature of the o rotor rotates along with the axis, generating high-frequency alternating current.
o The output is rectified into direct current by a rotating rectifier (diode bridge).
3.3. Rotating rectifier + main generator rotor excitation winding
o The rectified DC is directly supplied to the rotor excitation winding of the main generator.
o No brushes, no slip rings, achieving brushless excitation.
o control core:
Automatic Voltage Regulator (AVR) : Receives feedback on the output voltage of the main generator and regulates
The conduction Angle (or equivalent control signal) of the thyristor between the PMG and the stator of the exciter, thereby precisely controlling the magnitude of the excitation current.
II. Why do data centers need to use both PMG and exciter generators simultaneously?
If only PMG is used for direct rectification to the main rotor? The theory is feasible, but it has fatal flaws:
o Rapid strong excitation (forced excitation) of AVR cannot be achieved;
o The output voltage of PMG is fixed, making it difficult to dynamically adjust the excitation current.
o In the event of a short circuit or sudden load increase, it is unable to provide a sufficiently large instantaneous excitation current (insufficient peak voltage).
At present, mainstream brands such as Caterpillar, Cummins, MTU, Volvo Penta, Weichai, Yuchai, etc., all have the following standard diesel engine models for data centers (such as C32, QSK60, 16V4000, etc.)
o PMG permanent magnet pair exciter
o AC main exciter + rotary rectifier
o Digital AVRs (such as Basler, Woodward, ComAp)