Lra (Locked Rotor Amp); Mcc (Maximum Continuous Current); Max Oper. A (Maximum Operating Amp); Winding Resistance - Danfoss MLM Series Application Manuallines

A G
PPLICATION UIDELINES

LRA (Locked Rotor Amp)

MCC (Maximum Continuous
Current)
Max Oper. A (Maximum
Operating Amp)

Winding resistance

Electrical connections

PSC wiring
PTCSCR wiring
18
E ,
LECTRICAL DATA CONNECTIONS AND WIRING
LRA is the higher average current as measured on
a mechanically blocked compressor tested under
nominal voltage. LRA is printed on the nameplate.
The MCC is the current at which the internal mo-
tor protection trips under maximum load and low
voltage conditions.
The Max Oper. A is the current when the compres-
sor operates at maximum load conditions and
10% below nominal voltage.
This value which is the max rated load current for
the compressor is new on the nameplate.
Winding resistance is the resistance between
indicated terminal pins at 25°C (resistance value
+/- 7%).
Winding resistance is generally low and it re-
quires 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
diff erent value than 25°C, the measured resist-
ance must be corrected with following formula:
MLZ / MLM single phase scroll compressors are
designed to operate without any assistance. If
PSC wiring with a run capacitor only is the default
wiring solution for single phase MLZ and MLM
compressors.
The start winding (C-S) of the motor remains in
circuit through a permanent (run) capacitor. This
permanent (run) capacitor is connected between
the start winding (S) and the run winding (R).
If the starting torque of the PSC wiring is not suf-
fi cient due to pressures not fully equalized during
the off -cycle or some voltage drop during start-
ing, the PTCSCR wiring might be an option. PTC-
SRC wiring provides more motor torque than PSC
wiring but less than CSR wiring. The PTC is wired
in parallel to the run capacitor.
FRCC.PC.015.A4.02
The LRA value can be used as a rough estimation
for the starting current. However in most cases,
the real starting current will be lower. Many coun-
tries have defi ned limits for the starting current
in domestic use. A soft starter can be applied to
reduce starting current.
This MCC value is the maximum at which the com-
pressor can be operated in transient conditions
and out of the application envelope. Above this
value the overload will switch off to protect the
motor.
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.
a + t
amb
R = R
tamb 25°C
a + t
25°C
t : reference temperature = 25°C
25°C
t : temperature during measurement (°C)
amb
R : winding resistance at 25°C
25°C
R : winding resistance at t
amb
coeffi cient a= 234.5
starting within the defi ned voltage range, PSC
wiring is suffi cient.
N L₁
Thermostat
Run capacitor
When starting the compressor, the PTC, which
is at low resistance, provides additional starting
current to the motor's start winding. The current
passing through the PTC causes it to heat up and,
at a certain temperature, change to a very high re-
sistance. At this time the motor is up to nominal
speed and the run capacitor determines the cur-
rent through the start winding. The PTC remains
amb
C
IOL
S R