Danfoss VLT Series Design Manual page 132

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 contactors (K1/K2 and K3/K4) with a mechanical interlock.
Thermal relays or other motor protection devices must be applied according to local regulation
and/or individual demands.
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RELAY 1 and RELAY 2 are the built-in relays in the adjustable frequency drive.
When all relays are de-energized, the first built-in relay to be energized will cut in the
contactor corresponding to the pump controlled by the relay. For example, RELAY 1 cuts
in contactor K1, which then becomes the lead pump.
K1 blocks K2 via the mechanical interlock, preventing line power to be connected to the
output of the adjustable frequency drive (via K1).
Auxiliary break contact on K1 prevents K3 from cutting in.
MG.11.B4.22 - VLT ® is a registered Danfoss trademark.
VLT ® HVAC Drive Design Guide