5.4 Cable sizing
Submersible motor cables are
dimensioned for submersion in the
Caution
liquid, and will not necessarily have
sufficient cross-section to be in air.
Make sure that the submersible drop cable can
withstand permanent submersion in the actual liquid
and at the actual temperature.
The cross-section (q) of the cable must meet the
following requirements:
•
The submersible drop cable must be sized to the
rated maximum current (I
•
The cross-section must be sufficient to make a
voltage drop over the cable acceptable.
Grundfos supplies submersible drop cables for a
wide range of installations. For correct cable sizing,
Grundfos offers a cable sizing tool on the USB stick
supplied with the motor.
Fig. 8
Cable sizing tool
The sizing tool provides a accurate calculation of the
voltage drop at a given cross-section on the basis of
the following parameters:
•
cable length
•
operating voltage
•
full-load current
•
power factor
•
ambient temperature.
The voltage drop can be calculated both for direct-
on-line and star-delta starting.
In order to minimise operating losses, the cable
cross-section may be increased. This is only cost-
efficient if the borehole provides the necessary
space, and if the operating time of the pump is long.
The cable sizing tool also provides a power loss
calculator that shows the potential savings of an
increased cross-section.
) of the motor.
n
As an alternative to the cable sizing tool, select the
cross-section on the basis of the current values of
the given cables.
The cross-section of the submersible drop cable
must be large enough to meet the voltage quality
requirements specified in section
Determine the voltage drop for the cross-section of
the submersible drop cable by means of the
diagrams on pages
459
and 460.
Use the following formula:
I = Rated maximum current of the motor.
For star-delta starting, I = rated maximum current of
the motor x 0.58.
Lx = Length of cable converted to a voltage drop of
1 % of the nominal voltage.
length of drop cable
Lx =
permissible voltage drop in %
q = Cross-section of submersible drop cable.
Draw a straight line between the actual I-value and
the Lx-value. Where the line intersects the q-axis,
select the cross-section that lies right above the
intersection.
The diagrams are made on the basis of the formulas:
Single-phase submersible motor
U × ΔU
L =
cos
I × 2 × 100 ×
Three-phase submersible motor
U × ΔU
L =
cos
I × 1.73 × 100 ×
L
= Length of submersible drop cable [m]
U
= Rated voltage [V]
ΔU
= Voltage drop [%]
I
= Rated maximum current of the motor [A]
cos φ = 0.9
ρ
= Specific resistance: 0.02 [Ωmm
q
= Cross-section of submersible drop cable
2
[mm
]
sin φ = 0.436
Xl
= Inductive resistance: 0.078 x 10
5.1
General.
sin
XI
-- -
+
q
sin
XI
-- -
+
q
2
/m]
-3
[Ω/m].
13