SECTION THREE
OPERATION OF THE MACHINE
OPERATION
Fig. 1. Block Diagram of Excitation System
The block diagram above indicates the main electrical components and basic interconnections.
The control system is based on the main stator winding providing excitation power via the automatic voltage regulator
(A.V.R.) to the main exciter. Residual magnetism of the machine is utilized via a circuit within the A.V.R. to provide a
positive voltage build up. A reference signal is fed into the A.V.R. from the main stator through a high source
impedance which is rectified and smoothed to maintain the voltage regulation to within fine limits. The main exciter
output is fed into the main rotor windings via a rotating 3 phase bridge rectifier unit incorporating a surge suppressor to
protect the diodes against voltage transients during heavy load surges, i.e. short circuits or out of phase paralleling.
A frequency sensitive circuit is incorporated into the A.V.R. which functions only when the shaft speed falls below
approximately 90% of the rated speed. The voltage then reduces proportionally to any further reduction in speed, thus
assisting the prime mover to recover after the application of high starting torque motor loads or high kilowatt load
changes. This also provides low speed protection to the main rotor by ensuring the excitation does not exceed the safe
operating level at all speeds.
PERFORMANCE
Voltage regulation
Voltage regulation is maintained within the limits of ±2% from no load to full load including cold to hot variations at any
power factor between 0.8 lagging and unity and inclusive of a speed variation of 41/2%. If Current Sensing Kit supplied
(for improved regulation of ±1% ) see Section Six accessories.
Waveform Distortion, TH F and TI F Factors
The total distortion of the voltage waveform with open circuit between phases or between phases and neutral is in the
order of 2%. On a 3 phase balanced harmonic-free load the total distortion is in the order of 3.5%. Machines are
designed to have a THF better than 2% and a TIF better than 50.
A 2/3 pitch factor is standard on all stator windings to eliminate 3rd, 9th, 15th ..................harmonic voltages on the line-
neutral waveform.
Response
Rapid response to transient load changes is a significant feature, upon application of full load at a power factor of 0.8
lagging, the output voltage recovers to within 3% of the steady state value in 0.25 seconds. The machines comply with
the transient conditions laid down by BS 4999 Part 40 voltage grades VR2 11 to VR2.23 as standard Check with factory
for compliance with grades VR2.31 to VR2.23.
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