14 • Technical Data
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3.5.1
Preparation for Bypass Contactor
Under normal operating conditions the heat dissipated by an RVS-DN causes heating of the enclosure and
energy losses. The heating and losses can be eliminated by the use of a bypass contactor, which bypasses
the RVS-DN after completion of start-up so that motor current will flow through the bypass contactor.
In models RVS-DN 950A and up, current transformers CT1 and CT2 are mounted outside the RVS-DN and
need to be connected to the RVS-DN by the customer. In this case the customer
MUST NOT ground the
secondaries of current transformers!
Heat dissipation from the RVS-DN is calculated as:
Ploss=3x1.3xI+FAN loss
where:
I
represents
motor current. Note that the motor current during the start process is higher than the motor rated
current.
FAN loss
represents power loss caused by all internal fans (refer to section
10 page 99 for fan loss per
model).
For example, during start of a 820A motor when CURRENT LIMIT is set to 400%, heat dissipation can be
calculated as:
Ploss=3x1.150=12,792Watt
≈
12.8kW
While a 820A motor is running and the motor current is 820A, heat dissipation can be calculated as:
Ploss=3x1.3x820+150=3,198Watt
≈
3.2kW
When a bypass contactor is used this changes the previous calculation to:
Ploss=3x1.3x0+150=150Watt
≈
0.15kW
It is obvious that using a bypass contactor can significantly reduce energy consumption.
Notes:
•
All models from RVS-DN 950A and up, RVS-DN 1000V and RVS-DN 1200V must be implemented with a
bypass contactor (see section
•
Bypass preparation is standard in models RVS-DN 85-170A.
3.5.1.1
Maintaining Current Protection after Bypass Closes
Current flow during soft start and soft stop.
Bypass contactor is open.