Product Features; Motor Overload Protection - Danfoss vlt soft starter mcd 500 Operating Manual

Product Features

5 Product Features

5.1 Motor Overload Protection

The thermal model used for motor overload in the soft
starter has 2 components:
•
Motor windings: The motor windings have a low
thermal capacity and affect the short-term
thermal behavior of the motor. The motor
windings are where the current generates heat.
•
Motor body: The motor body has a large thermal
capacity and affects the long-term behavior of
the motor. The thermal model includes consider-
ations for the following:
- Motor current.
- Iron losses.
-
Winding resistance losses.
- Motor body and winding thermal
capacities.
-
Cooling during run and cooling at
standstill.
- The percentage of the rated capacity of
the motor. This sets the shown value for
the winding model and is affected by
the motor FLC setting among others.
NOTICE
Set parameter 1-1 Motor FLC to the rated motor FLC. Do
not add the overload rating as the soft starter calculates
this rating.
The thermal overload protection used in the soft starter
has several advantages over the thermal relays.
•
The effect of fan cooling is accounted for when
the motor is running.
•
The actual full load current and locked rotor time
can be used to tune the model more accurately.
The thermal characteristics of the windings are
treated separately from the rest of the motor
(that is the model recognizes that the windings
have low thermal mass and high thermal
resistance).
•
The winding portion of the thermal model
responds rapidly compared with the body
portion. Thus, the motor can be run closer to its
safe maximum operating temperature while still
being protected from thermal damage.
•
The percentage of motor thermal capacity used
during each start is stored in memory. The soft
starter can be configured to determine automat-
ically whether the motor has sufficient thermal
MG17K602
Operating Guide
•
The overload protection function provided by this model is
compliant with a NEMA 10 curve, but provides superior
protection at low levels of overload due to the separation
of the winding thermal model.
10000
1000
100
10
1
100
Illustration 5.1 Protection Degree Compared to Overload
1.
2.
3.
1) MSTC is the motor start time constant. It is defined as the
locked rotor time (in parameter 1-2 Locked Rotor Time) when
the locked rotor current is 600% of FLC.
Danfoss A/S © 05/2016 All rights reserved.
capacity remaining to complete another start
successfully.
The memory function of the model means that
the motor is fully protected in warm-start
situations. The model uses data from the real-
time clock to account for elapsed cooling time,
even if control power has been removed.
3
2
1
400 500 600 700
200 300
Current (%motor full load current)
1)
MSTC =5
1)
MSTC =10
1)
MSTC =20
5 5
800 900 1000
35