Danfoss MCD 201-075-T6-xxx [21/24] N soft start application guide
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Danfoss MCD 201-075-T6-xxx [21/24] N soft start application guide

Danfoss MCD 201-075-T6-xxx [21/24] N soft start application guide
MCD 200 Design Guide
Soft Start Application
Guide
■ Soft Start Application Guide
■ Application Guide
This section provides data useful in the selection
and application of soft starters.
■ Reduced Voltage Starting
When started under full voltage conditions AC induction
motors initially draw locked rotor current (LRC) and
produce locked rotor torque (LRT). As the motor
accelerates the current falls and the torque increases
to bre akdown torque before falling to full speed levels.
Both the magnitude and shape of the current and
torque curves are dependent on motor design.
7 x FLC
6 x FLC
5 x FLC
3 x FLC
1 x FLC
4 x FLC
2 x FLC
10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
CURRENT (% Motor Full Load Current)
ROTOR SPEED (% Full Speed)
2 x FLT
1 x FLT
TORQUE (% Motor Full Load Torque)
Full Voltage Stator Current
Full Voltage Start Torque
Sample Load Torque Curve
177HA267.10
Motors with almost identical full speed charac teristics
often vary significantly in their starting capabiliti es.
Locked rotor currents range from as low as 500%,
to in excess of 900% of motor FLC. Locked rotor
torques range from a s low as 70%, to highs o f
around 230% motor full load torque (FLT).
The motor’s full voltage current and torque
characteristics set the limits for what can be achieved
with a reduced voltage starter. For installations in which
either minimis ing start current or maximising start
torque is critical, it is important to ensure that a motor
with low LRC and high LRT characteristics is used.
When a reduced voltage starter is used, moto r start
torque is reduced according to the following formula.
T
ST
=St
art torque
I
ST
=Start current
LRC =Motor Locked Rotor Current
LRT =Mot
or Locked Rotor Torque
Start current can be reduced only to the point where the
resulting start torque still exceeds the torque required
by the load. Below this point motor acceleration will
cease and the motor/load will not reach full speed.
The m ost common reduced voltage starters are:
• Star/Delta starters
• Auto-transformer starters
• Primary resistance starters
• Soft starters
Star/Delta starting is the cheapest form of reduced
voltage starting, however performance is limited.
The two most significant limitations are:
1. There is no control over the level of current
and torque reduction; these are fixed at one
third of the full v o ltage levels.
2. There are normally large current and torque
transients as the starter changes from star to delta.
This causes mechanical and electrica l stress often
resulting in dama ge. The transients oc cur because
as the motor is spinning a n d then disconnected
from the supply it acts as a generator with output
voltage which may be a t the same amplitude
as the supply. This voltage is still present when
the motor is reconnected in delta configuration,
and can be exactly out of phase.
The result is a current of up to twice locked rotor
current and four times locked rotor torque.
Auto-transformer starting offers more control
than the star/delta method, however voltage
is still applied in steps.
Limitations of auto-transformer starting include:
1. Torque transients caused by switching
between voltages.
2. Limited number of output v oltage taps restricts the
ability to closely select the ideal starting current.
3. High price for models s u itab le for frequent or
extended starting conditions.
4. Cannot provide an effective reduced voltage start
for loads with varying start requirements. For
instance, a material conveyor may start loaded
or unloaded. The auto-transformer starter can
only be optimised for one condition.
Primary resistance starters also provide greater
starting control than star/delta starters. However,
they do have a number of characteristics that
reduce their effectiveness.
These include:
1. Difficult to optimise start performance when
commissioning be cause the resistance value must
MG.17.C2.02 - VLT is a registered Danfoss trademark
21

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