Danfoss VLT Series Design Manual page 118

6. Application Examples
6.1.11. Fixed Variable Speed Pump Wiring Diagram
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6.1.12. Lead Pump Alternation Wiring Diagram
Every pump must be connected to two con-
tactors (K1/K2 and K3/K4) with a mechanical
interlock. Thermal relays or other motor pro-
tection devices must be applied according to
local regulation and/or individual demands.
6.1.13. Cascade Controller Wiring Diagram
The wiring diagram shows an example with
the built in BASIC cascade controller with one
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variable speed pump (lead) and two fixed
speed pumps, a 4-20 mA transmitter and Sys-
tem Safety Interlock.
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MG.11.B2.02 - VLT is a registered Danfoss trademark
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VLT HVAC Drive Design Guide
RELAY 1 and RELAY 2 are the built in
relays in the frequency converter.
When all relays are de-energized,
the first built in relay to be energized
will cut in the contactor correspond-
ing to the pump controlled by the
relay. E.g. RELAY 1 cuts in contactor
K1, which becomes the lead pump.
K1 blocks for K2 via the mechanical
interlock preventing mains to be con-
nected to the output of the frequen-
cy converter (via K1).
Auxiliary break contact on K1 pre-
vents K3 to cut in.
RELAY 2 controls contactor K4 for
on/off control of the fixed speed
pump.
At alternation both relays de-energi-
zes and now RELAY 2 will be ener-
gized as the first relay.