Shaft Current and Bearing Insulation
Irregularities in the SG magnetic
circuit lead to a small axial flux that links the shaft. A parasitic
current occurs in the shaft, bearings, and machine frame, unless the
bearings are insulated from stator core or from rotor shaft.
The presence of pulse-width modulator
(PWM) static converters in the stator (or rotor) of SG augments this
phenomenon. The pertinent testing is performed with the machine at no
load and rated voltage. The voltage between shaft ends is measured
with a high impedance voltmeter.
The same current flows through the
bearing radially to the stator frame. The presence of voltage across
bearing oil film (in uninsulated bearings) is also an indication of
the shaft voltage.
If insulated bearings are used, their
effectiveness is checked by shorting the insulation and observing an
increased shaft voltage. Shaft voltage above a few volts, with
insulated bearings, is considered unacceptable due to bearing in-time
damage.
Generally, grounded brushes in shaft
ends are necessary to prevent it.
Phase Sequence
Phase sequencing is required for
securing given rotation direction or for correct phasing of a
generator prepared for power bus connection. As known, phase
sequencing can be reversed by interchanging any two armature (stator)
terminals.
There are a few procedures used to
check phase sequence:
• With a phase-sequence indicator (or
induction machine)
• With a neon-lamp phase-sequence
indicator (Figure 8.1a and Figure 8.1b)
• With the lamp method (Figure 8.1b)
When the SG no-load voltage sequence is
1–2–3 (clockwise), the neon lamp 1 will glow, while for the 1–3–2
sequence, the neon lamp 2 will glow. The test switch is open during
these checks.
The apparatus works correctly if, when
the test switch is closed, both lamps glow with the same intensity
(Figure 8.1a).
FIGURE 8.1 Phase-sequence indicators:
(a) independent (1–2–3 or 1–3–2) and (b) relative to power
grid.
With four voltage transformers and four
lamps (Figure 8.1b), the relative sequence of SG phases to power grid
is checked. For direct voltage sequence, all four lamps brighten and
dim simultaneously. For the opposite sequence, the two groups of
lamps brighten and dim one after the other.
