Understanding Soft Starter Ratings; Model Selection - Danfoss MCD3000 Series Manual

controllers employing constant current algorithms to
provide the best in soft start control.

Understanding soft starter ratings

The maximum rating of a soft starter is calculated so
the junction temperature of the power modules (SCRs)
o
does not exceed 125 C (257
parameters effect the SCR junction temperature;
Motor Current, Start Current, Start Duration, Number
of Starts Per Hour, Off Time. The full rating of a
particular soft start model must account for all these
parameters. A current rating on its own is not sufficient
to describe the capability of a soft starter.
IEC 60947-4-2 details the AC53 utilisation categories
for describing a soft starter's ratings. There are two
AC53 codes;
1. AC53a: for soft starters used without bypass
contactors.
For example, the following AC53a code describes
a soft starter capable of supplying a 256 A run
current and a start current of 4.5 x FLC for 30
seconds 10 times per hour where the motor runs
for 70% of each operating cycle. (Operating cycle
= 60 minutes / starts per hour)
•
Starter Current Rating: Maximum FLC rating
of the motor to be connected to the soft
starter given the operating parameters
specified by the remaining items in the AC53a
code.
•
Start Current: The maximum start current that
will be drawn during start.
•
Start Time: The time taken for the motor to
accelerate.
•
On-load Duty Cycle: The percentage of each
operating cycle that the soft starter will run.
•
Starts Per Hour: The number of operating
cycles per hour.
2. AC53b: for soft starters used with bypass
contactors
For example, the following AC53b code describes
a soft starter which, when bypassed, is capable of
36
o
F). Five operating
MCD3000 Series
supplying 145 A run current and a start current of
4.5 x FLC for 30 seconds with a minimum of 570
seconds between the end of one start and the
commencement of the next.
In summary, a soft starter has many current ratings.
These current ratings are dependent on the start
current and operational performance required by the
application.
To compare the current rating of different soft starters
it is important to ensure that operating parameters are
identical.

Model selection

ATTENTION
To fully understand the model selection
procedures it is important to have a good
knowledge of the fundamental principles of soft starter
ratings. Please read the previous section of this
manual, Understanding soft starter ratings.
Model selection can be made in two ways. The most
appropriate procedure is dependent on the individual
circumstances of the application. Technical support is
also available from your local supplier.
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This method is appropriate for typical industrial
applications that operate within the standard
MCD3000 ratings of 10 starts per hour, 50% duty
o o
cycle, 40 C (104 F ), <1000 metres.
1. Use the table below to determine the typical start
current required for the driven load.
2. Refer to the Current Ratings tables in the
Specifications section of this manual, and use the
typical start current identified above to select a
MCD3000 model with a FLC rating greater or
equal to the motor nameplate FLC.
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