Installation
27
Medium UPM
8
Inrush current
All electronic pumps contain electronic units that must
be protected by filters including capacitors and ECM
pumps frequency converters with AC/DC rectifiers
containing capacitors to equalize the waves. This is
not the case in most asynchronous pumps.
Fig. 40 Rectification of VAC voltage to DC voltage
The load of electronically commutated motors (ECM)
behaves as a capacitive load and not as a motor load
like in a standard pump. At start, the capacitor is
unloaded. Hereby the amplitude of the current peak
depends on the grid impedance, until the capacitor is
charged. The faster the capacitor is charged, the
higher amplitude, and the faster the pump can be
started. After this period of time, the current will drop to
the rated current.
Definition: Inrush current is the current peak charging
the capacitors in the electronics when the supply
voltage is connected.
Note: When discussing measurements, it is important
to refer to the same method. Since 2007, Grundfos
uses the IEC 61000-3-3 Annex B method for
measuring inrush current.
The inrush current peak charges the bulk capacitor to
325 VDC as fast as the power grid allows. That shows
that inrush current is not only depending on the
integrated electronics but as well on the impedance of
the grid.
There is a "VDMA Einheitsblatt 24225" - "Wet runner
circulating pumps - Specification for the measurement
of the inrush current".
If you use a relay to switch the power supply of the
pump, you risk excessive wear on the relay contact
surface.
To avoid such problems there are different external
and internal solutions.
External solutions in the controller of the
appliance unit
• Specific relays with silver tin oxide (AgSnO
2
) inrush
relay contacts.
• Switching at ZERO crossing.
• Standby operation - pump only switches via the
PWM signal.
Internal solutions in the pump
• NTC resistor in the power input circuit or power
cable (passive)
Inrush current limitation by NTC resistor
At startup the operating temperature of the pump
including the NTC resistor is cold. In this situation the
NTC resistor has a high resistance and is able to limit
the inrush current to the specified inrush level of the
chosen variant. During operation the operating
temperature of the pump including NTC resistor is hot.
There is no inrush current but the NTC resistance
decreases so that the loss is limited.
Fig. 41 Inrush current limitation
TM06 0822 1014
TM06 0819 1014
Mains
EMC-filter Rectifier Bulk capacitor
TM07 1965 2418
At restart, the operator must ensure that
the NTC resistor has been cooled down
so that efficient operation is guaranteed.
Normally, it takes 1 minute to cool down
the resistor.
When the power supply to the pump is
switched on and off via an external relay,
you must ensure that the contact
material of the relay is able to handle
higher inrush currents.
The inrush current is measured on a
flicker network according to IEC 61000-
3-3:1994 + A1, + A2, Annex B. See also
"VDMA Einheitsblatt 24225".
2
+
L
N
PCB
C1
C2
C27
C4
NTC1
VDR1
NTC5
C39
C6
D3
L4
+
-
~
~
43
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