English (GB)
9
The highest permissible setting is the rated maximum current of
the motor.
For motors wound for star-delta starting, the motor-protective
circuit breaker must be set as above, but the maximum setting
must be rated maximum current x 0.58.
The highest permissible startup time for star-delta starting or
autotransformer starting is 2 seconds.
6.2 Lightning protection
The installation can be fitted with a special overvoltage protection
device to protect the motor from voltage surges in the power
supply lines if lightning strikes somewhere in the area. See fig. 8
Fig. 8
Connecting an overvoltage protective device
The overvoltage protective device will not, however, protect the
motor against a direct stroke of lightning.
Connect the overvoltage protective device to the installation as
close as possible to the motor and always in accordance with
local regulations. Ask Grundfos for lightning protective devices.
MS402 motors, however, require no further lightning protection as
they are highly insulated.
6.3 Cable sizing
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 of the motor.
• 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. A cable sizing tool is available on Grundfos Insite at:
https://www.grundfos.com/sp-system/download-sp-app-html.
Fig. 9
Cable sizing tool
The sizing tool provides an 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.
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
to
.
Use the following formula:
I: rated maximum current of the motor.
For star-delta starting, I is equal to the rated maximum current of
the motor x 0.58.
Lx: length of cable converted to a voltage drop of 1 % of the
nominal voltage.
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
Three-phase submersible motor
TM
00
13
5
7
36
05
Submersible motor cables are dimensioned for
submersion in liquid, and will not necessarily have
sufficient cross-section to be in free air.
TM
07
62
5
9
12
20
825045
825017
L1
L2
PE
L3
L
PE
N
Three-phase
Single-phase
Lx =
length of drop cable
permissible voltage drop in %
L =
U
×
Δ
U
I
×
2
×
100
×
L =
U
×
Δ
U
I
×
1.73
×
100
×
L
Length of submersible drop cable [m]
U
Rated voltage [V]
Δ
U
Voltage drop [%]
I
Rated maximum current of the motor [A]
cos
φ
0.9
ρ
Specific resistance: 0.025 [
Ω
mm
2
/m]
q
Cross-section of submersible drop cable [mm
2
]
sin
φ
0.436
Xl
Inductive resistance: 0.078 x 10
-3
[
Ω
/m].
cos
q
---
sin
XI
+
cos
q
---
sin
XI
+
