10.2 Start Initiation Checks - Emerson AMPGARD RVSS Instruction Booklet

AMPGARD RVSS
Reduced Voltage Soft-Starter
User Manual
Gate driver boards to SCR gates
The Gate Driver Boards receive fiber optic signals from the Fan-out
Boards, convert them from light into voltage signals and pass them
on to the SCR gate circuits.
SCR heatsink PTC thermistors to Gate Driver Boards
SCR heatsink temperatures are measured by thermistors mounted
on the heat sinks. The thermistor signals are passed to the Gate
Driver Boards where they are converted from voltage to light signals,
passed on to the Fan-out Boards and transmitted to the I/O Board
for processing.

10.2 Start initiation checks

When a start is initiated and before the start sequence commences,
AMPGARD RVSS executes an extensive self-check process. This
process covers the power electronics.
On start initiation, the Pre-Start Check sequence checks each SCR
truck SCR. If a fault in any element is detected (shorted SCR, load
side single line to ground, load side phase to phase short, high
di/dt due to load side long shielded cable, surge capacitor in circuit
or PFCC connected, or phase open), the power poles must be
examined to identify the failed component. See section 2.2.4.
10.2.1 Troubleshooting power electronics faults
10.2.1.1 SCRs shorted check
A repetitive Pre-Start Check fault will indicate at least one fault in
the SCR power poles. If that occurs, power down the unit and
wait 15 minutes for the power pole snubber board capacitors to
discharge.
Extract the SCR truck.
Once the SCR truck is out of the RVSS cabinet, use a digital
multimeter to perform resistance checks as follows.
SCRs on heat sinks
Perform a resistance measurement for each power pole in either of
two ways (see Figure 119):
From rear input (upper) stab cluster to inner long heat sink, (both polarities), or
•
From rear output (lower) stab cluster to the inner heat sink (both polarities).
•
Depending upon which test method is used, acceptable
measurement values will be as follows. Any value outside the
ranges shown (in particular, very low resistance) will indicate a failed
SCR.
Either stab to inner heat sink - 500 kilo ohms to 2.5 megohms
•
If either measurement is outside of this range or measures as a short circuit, that phase
leg assembly likely needs to be replaced.
Instruction Booklet IB020003EN
Figure 119. SCR truck rear power pole test points
Gate driver board SCR gate connections
To test the SCR gate to cathode circuits, for each SCR under test,
disconnect the gate (white) and cathode (red) wires.
Test the gate to cathode resistance with a digital multimeter.
Acceptable values are 8 ohms to 11 ohms.
When the failed power pole has been identified, remove and replace
the subject phase leg assembly.
10.2.1.2 Load side single line to ground
With the unit powered down, check the load side output
connections for grounds.
10.2.1.3 Load side phase to phase short
With the unit powered down, check the load side output
connections for phase to phase shorts. Disconnect the motor leads
to avoid reading the low resistance of the motor. Test the output of
the truck phase to phase. Test the disconnected motor terminals
phase to ground.
10.2.1.4 High di/dt due to cable lengths
Load shielded cables have distributed capacitance whose
capacitance value depends on the cable, number of cables per
phase and length of each cable run.
This capacitance may present an effective low impedance load to
the RVSS if the sum of cable lengths per phase exceeds 1000 feet.
This could result from a single cable per phase of greater than 1000
feet, or two parallel cables per phase whose total length is 1000 feet
or more. Such a load could cause high rates of current change during
a start that could result in SCR damage.
High cable capacitance can be mitigated by installing series
inductors in the output circuits. These inductors can be included in
the RVSS design if it is known at order time that there will be long
cable lengths, and how long they will be.
With power off, check the length and configuration of the cables
between the RVSS and its motor(s). If the length sum per phase
exceeds 1000 feet, contact the factory. Do not attempt to start the
RVSS.
10.2.1.5 High di/dt due to surge capacitors
Surge capacitors are often installed in AC motor terminal boxes, to
protect from surges in voltage.. These capacitors do not compromise
across the line starters, but will cause failure of SCR powered soft
starters. Check the output circuit from the RVSS output terminals
Effective January 2021
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