Danfoss MCD 201-075-T6-xxx [22/24] N types of soft start control
Макгруп McGrp.ru
Задайте вопрос
Зарегистрироваться

Danfoss MCD 201-075-T6-xxx [22/24] N types of soft start control

Danfoss MCD 201-075-T6-xxx [22/24] N types of soft start control
MCD 200 Design Guide
be calculated when the starter is manufactured
and is not easily changed later.
2. Poor performance in frequent starting situations
because the resistance value changes as heat is
generated in the resistors during a start. A long
cool down period is required between starts.
3. Poor performance for heavy duty or extended
starts because heat build up in the resistors
changes the resistance value.
4. Cannot provide an effective reduced voltage start
for l oads with varying start requirements.
Soft starters are the most advanced of the reduced
voltage starters. They offer superior control over
current and torque as well as incorporating advanced
motor protection and interface features.
The main starting advantages soft starters offer are:
1. Simple and flexible control over starting
current and torque.
2. Smooth control of voltag e and current free
from steps or transitions.
3. Capable of frequent starting.
4. Capable of handling changing start conditions.
5. Soft stop control to extend motor deceleration times.
6. Braking control to reduce motor deceleration times .
■ Types of Soft Start Control
The term ’soft start’ is applied to a range of
technologies. These technologies all relate to motor
starting but there are significant differences in the
methods used a nd the benefits available.
Some of the key differences are described below.
Control philosophy: Soft starters can generally
be divided into two groups.
• Timed Voltage Ramp (TVR) systems
• Current controlled systems
TVR starters control voltage applied to the motor
in a preset manner and receive no feedback on
motor starting current. Control of start performance
is provided to the users through settings such as
Initial Voltage and R amp up time. Soft Stop is
also commonly av ailable and provides the ability
to extend motor stopping times.
Current controlled soft starters monitor motor current
and use this feedback to adjust voltage so that user
specified starting current is maintained. Soft Stop is also
provided as are range of motor protection functions.
Power assemblies: Soft starters can provide control
of one, two or all three phases.
Single-phase controllers remove the torque shock
associated with motor starting but provide no significant
current reduction. They must be us ed with a line
contactor and motor overload. They are suitable for
very small motors and should only be applied to light
applications with low to me dium start frequency.
Two-phase controllers control two phases while the
third phase is uncontrolled. These controllers provide
soft start and current reduction. Care should be taken
to ensure that the control algorithms of two-phase
controllers balance the output waveform in order to
provide a s ymmetrical waveform. B asic two-phase
controllers subject the motor to an asymmetrical
output waveform which creates a DC field in the
motor. This stationary DC field increases the required
start current and increases motor heating. Such
unbalanced controllers should not be applie d to
high inertia loads or in situatio ns with high start
frequencies. Three-phase controllers control all phases
and are best suited for very l arge motors.
External or internal bypass connection: The SCRs
in a soft starter can be bypassed once the motor
is up to speed. This reduces heat generation and
prevents damage to the SCR from overcurrent or
overvoltage events that occ ur while the motor is
running. Some soft starters include built-in bypass
contactors while other provide terminals for connection
of an external bypass contactor.
■ Understanding Soft Starter Ratings
The maximum rating of a soft starter is calculated
so the junction temperature of the power m odule s
(SCRs) does not exceed 125 °C. Five operating
parameters effect the SCR junction temperature:
Motor Current, Start Current, Start Du ration, Number
of Starts Per Hour, Off Tim e. 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 AC 53 utilisation categories
for describing a soft starter’sratings.
TherearetwoAC53codes:
1. AC53a: for soft starters used without
bypass contactors.
For example, the foll owing 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).
256 A: AC-53a 4.5-30 : 70-10
Starter Current Rating
Start Current
(multiple of FLC)
Start Time (seconds)
On-load Duty Cycle
Starts Per Hour
177HA280.10
MG.17.C2.02 - VLT is a registered Danfoss trademark
22

Содержание

Похожие устройства

Скачать