Motor Starting
An overload capacity equivalent to 250% full load impedance at zero p.f. can be sustained for 10 seconds.
Short Circuit
A current forcing unit must be fitted at the time of manufacture to provide a sustained short circuit current for discrimination of
protective devices. Following the initial short circuit current envelope determined by the machine reactances/time constants, the
standard machine does not support a sustained short circuit current.
Voltage Build Up
The exciter design is such that voltage build-up is achieved from residual levels only.
Standby Ratings
These are in accordance with NEMA MG 1 - 22.84.
Parallel Operation (SC Machines Only)
The standard machine is fitted with a fully interconnected damper winding and the standard phases sequence is U-V-W when rotating
clockwise viewed from the drive end. When paralleling Stamford a.c. generators the neutrals can be connected. When paralleling
dissimilar machines it is recommended that the neutrals are not connected as differences in waveform may result in harmonic
currents. For types of parallel operation refer to Section - Six Accessories.
AUTOMATIC VOLTAGE REGULATOR TYPES
PC (Series 6)
The A.V.R. on this frame designation is two phase sensed and controls the voltage within close limits under all
load conditions providing high performance and the flexibility needed for most generator applications. The
external wiring has been kept simple and comprises 3 push-on-type connections. By keeping the wiring simple
no optional electrical accessories are available.
SC (Series 4)
Two types of A.V.R. can be fitted on this frame designation depending upon the application and performance
required:-
Type 1. The standard A.V.R. is two phase sensed and controls the voltage within close limits under all load
conditions, providing high performance and flexibility needed for most generator applications. The design is such
that a wide range of electrical accessories can be added to enhance the Specification.
Type 2. This comprises a special A.V.R. and three phase sensing unit for special applications and derives its
sensing voltage from all 3 phases of the machine. All generators when an unbalanced load is applied, will result
in differences between phase to phase and phase to neutral voltages depending upon the degree of unbalanced
load. It is normally accepted that if unbalanced loading can be limited to within 30% the impact on the machine
regulation will be marginal on both two phase and three phase sensed A.V.R.'s. If, for any reason, unbalanced
loading occurs above this level the overall regulation will exceed the normal specified limits. On a two phase
sensed A.V.R., the phase directly linked with the A.V.R. sensing, will be maintained to close regulating limits but
the other phase will float outside the normal regulation limits. By incorporating a three phase sensed A.V.R.
overall regulation will not improve but the overall balance around a set point will improve. For example, if the
overall voltage regulation is 5% on a two phase sensed A.V.R. the regulation could be all one way either +5% or
-5% on one of the phases and close to 0% on the other phases. On the three phase sensed A.V.R. although
the overall regulation would still be 5% this would tend to float around the nominal setting to give a regulation of
+2% between all phases.
A second advantage of three phase sensing is that it provides a more stable reference when operating on thyrsitor or rectifier loads
which, due to the high harmonic content in the current waveform distort the generator waveform. The recommendation is that where
high thyrsitor or rectifier loads are used, the best results will be obtained with a three phase sensed A.V.R.
NOTE: A range of electrical accessories is available for both two phase and three phase sensed A.V.R.'s see Section
6 for details.
MARINE MACHINES (SC TYPE ONLY)
In order to meet the requirements of marine classifying societies certain modifications to the standard machine are required. From
the electrical performance point of view the standard machine is suitable for marine use providing a current forcing unit is fitted to
give short circuit maintenance for discrimination of protective devices. From the mechanical point of view it is necessary to
incorporate a shaft manufactured from approved material and to add drip-proofing features to cater for the inclination of the machine
in the form of louvered covers on the air outlets The requirements of most main classifying societies can be met in this way.
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