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chapter discard.fm Page 10 Tuesday, June 7, 2011 9:29 PM
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How to measure
Use a volt meter (set to proper scale) to
measure the voltage at the pump termi-
nal box or starter.
On single-phase units, measure between
power leads L1 and L2 (or L1 and N for 115
volt units). On three-phase units, meas-
ure between:
• Power leads L1 and L2
• Power leads L2 and L3
• Power leads L3 and L1
What it means
When the motor is under load, the
voltage should be within ± 10% of the
nameplate voltage. Larger voltage vari-
ation may cause winding damage.
Large variations in the voltage indicate
a poor electrical supply and the pump
should not be operated until these vari-
ations have been corrected.
If the voltage constantly remains high or
low, the motor should be changed to the
correct supply voltage.
Supply voltage
How to measure
Use an ammeter (set on the proper
scale) to measure the current on each
power lead at the terminal box or
starter. See the motor nameplate for
amp draw information.
Current should be measured when the
pump is operating at constant discharge
pressure.
What it means
If the amp draw exceeds the listed serv-
ice factor amps (SFA) or if the current
imbalance is greater than 5% between
each leg on three-phase units, check the
following:
1. Burned contacts on the motor starter.
2. Loose terminals in starter or terminal
box or possible wire defect.
3. Too high or too low supply voltage.
4. Motor windings are shorted or
grounded. Check winding and insula-
tion resistances.
5. Pump is damaged causing a motor
overload.
Current measurement
How to measure
Turn off power and disconnect the
supply power leads in the pump
terminal box. Using an ohm or mega
ohm meter, set the scale selector to
R x 100K and zero adjust the meter.
Measure and record the resistance
between each of the terminals and
ground.
What it means
Motors of all HP, voltage, phase and
cycle duties have the same value of
insulation resistance. Resistance values
for new motors must exceed 1,000,000
ohms. If they do not, the motor should
be repaired or replaced.
Insulation resistance
Troubleshooting
WHEN WORKING WITH ELECTRICAL CIRCUITS, USE CAUTION
TO AVOID ELECTRICAL SHOCK.
It is recommended that rub-
ber gloves and boots be worn, and metal terminal boxes
and motors be properly grounded before any work is done.
WARNING: Failure to ground the pump may result in
serious electrical shock.
Preliminary tests
