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IOM Manual: IOM-0000-CST
Series CST: Submersible Turbine
01/01/2018 - Edition 1A
MEASURING RESISTANCE BE-
TWEEN LEADS (MOTOR WIND-
ING RESISTANCE)
The general condition of motor windings can be deter- mined
by measuring the resistance of the motor windings (i.e. the re-
sistance between the motor leads) and comparing the measured
resistance with values given in the motor manual. The resistance
is measured with an ohm-meter and the value is stated in ohms.
The basic procedure for measuring motor winding resistance is
given below.
Turn off the power and disconnect the leads to be tested from the
panel.
Failure to turn off the power will damage the meter and can
cause serious or fatal shock.
Failure to disconnect the leads can result in false readings.
Set the meter selector knob to “Ax 1 “. Clip the meter leads
together and ad-just the meter to zero.
Unclip the leads and attach them. See FIGURE 6.
Resistance measured between the motor leads prior to splicing
the drop cable to the motor leads should be within the mo-
tor winding resistance limits specified in the motor manual.
Resistance measured between the drop cable leads after splicing
the drop cable to the motor leads will indicate the resistance of
the drop cable plus the motor windings. The motor winding re-
sistance is obtained by the formula below. The calculated value
should be within the limits specified in the motor manual.
Motor winding resistance reading measured at drop cable from
FIGURE 6.
A higher winding resistance than shown in the motor manual
indicates a possible burned (open) winding, an open cable, a
loose connection or the wrong motor (different HP or voltage
than readings being referenced).
A considerably lower winding resistance than shown in the
motor manual indicates a possible shorted (burned together)
winding or the wrong motor.
Unequal resistance between the windings on a three phase mo-
tor indicates a burned winding or a faulty connection.
OHMS
ZERO
OHMS
OHMS
ZERO
OHMS
DROP CABLE
OR
MOTOR LEADS
DROP CABLE
OR
MOTOR LEADS
SINGLE PHASE, 2 WIRE - MEASURE BETWEEN TWO WIRES
SINGLE PHASE, 3 WIRE - MEASURE BETWEEN BLACK AND
THREE PHASE - MEASURE BETWEEN EACH PAIR, AB, AC
YELLOW AND THE MEASURE
BETWEEN RED AND YELLOW
GROUND
SET SELECTOR
KNOB AT
R x 100K
SET SELECTOR
KNOB AT R x 1
R x 100K
R x 1
R x 10
R x 100
R x 1000
R x 10K
10
20
25
5
2
0
R x 100K
R x 1
R x 10
R x 100
R x 1000
R x 10K
The values below are for copper conductors. If aluminum con-
ductor drop cable is used, the resistance will be higher for each
foot of cable of the same size. To determine the actual resis-
tance of aluminum drop cable, divide the ohm readings from
this chart by 0.61. This chart shows total resistance of cable
from control box to motor and back.
FIGURE 6 – Measuring Winding Resistance
C. CURRENT UNBALANCE TEST
For three phase units, THE CURRENT UNBALANCE
BETWEEN LEGS OF THE POWER SUPPLY SHOULD
NOT EXCEED 5%. Current unbalance is determined by mea-
suring the amperage in each of the three legs and then calculat-
ing the percent current unbalance using the formula below. This
calculation must be performed using each of the three hookups
shown.
THE HOOKUP THAT RESULTS IN THE LOWEST PER-
CENT CURRENT UNBALANCE SHOULD BE USED
FOR THE FINAL CONNECTION OF THE POWER
LEADS. This procedure is commonly known as “rolling the
leads”. To prevent changing the motor rotation, be careful to
follow the hookups shown below very carefully. A worksheet
and sample calculation are given. See FIGURE 7.
