User Guide

Application guidelines
19AB237986441643en-US0701
Winding resistance is the resistance between
indicated terminal pins at 77°F (resistance value
+/- 7%).
Winding resistance is generally low and it
requires adapted tools for precise measurement.
Use a digital ohm-meter, a “4 wires” method and
measure under stabilised ambient temperature.
Winding resistance varies strongly with winding
temperature ; if the compressor is stabilised at a
dierent value than 77°F, the measured resistance
must be corrected with following formula:
a + t
amb
R
amb
= R
77°F
a + t
77°F
t
77°F
: reference temperature = 77°F
t
amb
: temperature during measurement (°F)
R
77°F
: winding resistance at 77°F
R
amb
: winding resistance at t
amb
Coecient a = 234.5
The MMT is dened for compressors without
their own motor protection. This MMT value
is the maximum at which the compressor can
be operated in transient conditions and out of
the application envelope. The tripping current
of external overcurrent protection (thermal
overload relay or circuit breaker not provided
with compressor) must never exceed the MMT
value.
The max. operating current is the current when
the compressors operates at maximum load
conditions and 10% below the highest value of its
nominal voltage (59°F evaporating temperature
and 154.4°F condensing temperature).
Max Oper. A can be used to select cables and
contactors.
In normal operation, the compressor current
consumption is always less than the Max Oper. A
value.
The MCC is the current at which the motor
protection trips under maximum load and
low voltage conditions. This MCC value is the
maximum at which the compressor can be
operated in transient conditions and out of
the application envelope. Above this value, the
internal motor protection or external electronic
module will cut-out the compressor to protect
the motor.
Winding resistance
MMT (Max Must Trip current)
Max. operating Current
MCC (Maximum Continuous
Current)
Danfoss MCI soft-start
controller
Compressor model
Soft start reference
ambient max. 104°F
Soft start reference
ambient max. 131°F
SM / SZ 084
MCI 15C
MCI 15C
SM / SZ 090
SM / SZ 100
MCI 25C
SM / SZ 110
SM / SZ 120
MCI 25C MCI 25C*SM 112-124 - SM/SZ147
SM / SZ 161 - 148
SM / SZ175-185
SY240-300-380
MCI 50CM *
* By-pass contactor (K1) required.
The inrush current for the Danfoss scroll
compressors with motor code 4 (400V / 3 / 50Hz
or 460V / 3 / 60Hz) can be reduced using the
Danfoss digitally-controlled MCI compressor soft
starter. MCI soft starters are designed to reduce
the starting current of 3-phase AC motors; MCI
soft starters can reduce the in-rush current by
up to 40%, thereby eliminating the detrimental
eects of high starting torque surges and costly
demand charges from the resultant current
spike. Upon starting, the controller gradually
increases the voltage supplied to the motor until
full-line voltage has been reached. All settings,
such as ramp-up time (less than 0.5 sec) and
initial torque, are preset and do not require
modication.
Locked Rotor Amp value is the higher current as
measured on mechanically blocked compressor
tested under nominal voltage. The LRA value
can be used as rough estimation for the starting
current. However in most cases, the real starting
current will be lower. A soft starter can be applied
to reduce starting current.
LRA (Locked Rotor Amp)
Electrical data, connections and wiring