5. Checking of motor and cable
1. Supply voltage
T
M
00
1
3
71
35
97
Measure the voltage between the phases by means of a
voltmeter.
Connect the voltmeter to the terminals at the mains
connection.
The voltage should,
when the motor is loaded
, be within ±5% of
the rated voltage. The motor may burn if there are larger variations
in voltage.
If the voltage is constantly too high or too low, the motor must be
replaced by one corresponding to the supply voltage.
Large variations in voltage indicate poor electricity supply, and the
pump should be stopped until the defect has been found.
Resetting of the motor starter may be necessary.
2. Current consumption
T
M
00
13
7
2
35
9
7
Measure the current of each phase while the pump is
operating at a constant discharge pressure (if possible
at the capacity where the motor is most heavily loaded).
The motor full-load current appears on the motor
nameplate.
The difference between the current of the phase with the highest
amp consumption and the one with the lowest amp consumption
must not exceed 10% of the lowest amp consumption.
If so, or if the current exceeds the full-load current, check the
following possible faults:
•
Too high operating pressure which may overload the motor.
•
Too high speed which may overload the motor.
•
Damaged pump which may overload the motor.
•
The motor windings are short-circuited or partly disjointed.
•
Too high or too low supply voltage.
•
Poor connection in leads. Weak cables.
Points 3 and 4: Measurement not needed if supply voltage and current consumption are normal.
3. Winding resistance
T
M
00
13
73
35
97
Disconnect the phase leads in the terminal box.
Measure the winding resistance as shown on the
drawing.
The highest value must not exceed the lowest value by more than
5%.
If the deviation is higher, and the supply cable is OK, the motor
should be overhauled.
4. Insulation resistance
T
M
00
13
74
35
97
Disconnect the phase leads in the terminal box.
Measure the insulation resistance from each phase to
earth (frame).
(Make sure that the earth connection is made carefully.)
The insulation resistance for a new, cleaned or repaired motor
must be approx. 10 M
Ω
measured to earth
.
For a given motor the critical insulation resistance (R
crit
) can be
calculated as follows:
R
crit
= U
N
[kV] x 0.5 [M
Ω/
kV
] .
If the measured insulation resistance is lower than R
crit
, the motor
must be overhauled.
