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EATON
www.eaton.com
Instruction Booklet
IB020003EN
Effective January 2021
AMPGARD RVSS
Reduced Voltage Soft-Starter
User Manual
1.4.3.1 Across-the-line Starters
An across-the-line starter applies full connected AC voltage directly
to the motor terminals, and only the motor’s electrical characteristics
and the available supply determine how much current the motor
draws during the start. Typically, an across-the-line starter will
employ a three-pole contactor to connect power to the motor and
will include protection and measuring devices to protect the motor
and starter. There are no features in an across-the-line starter to
address supply line or mechanical load limitations, or to effect how
much heating the motor insulation endures during the start.
Across-the-line starts at full voltage demand very high starting
currents, produce higher than rated torques, and higher than normal
heating. The torque jerk that the motor produces from an across-the-
line start can stress couplings and drive shafts and may damage the
mechanical load.
1.4.3.2
Reduced Voltage Starters
Reduced voltage starters are applied when conditions external to
the starter require a different starting method. If the connected
mechanical load cannot tolerate an across-the-line motor’s starting
torque pulse, or if the shop power system cannot deliver the
six- to ten-times FLA required by an across the line start, then an
alternative starting method must be employed.
Reduced voltage starting is a method that temporarily applies lower-
than-line voltage to initiate the start, then applies full line voltage
after the motor and load have accelerated to near rated speed.
This approach reduces the motor starting current but delivers lower
starting torques. The driven load must be such that it can be started
and accelerated by the lesser motor torques, or else the motor and
load will not start at all.
There are two types of reduced voltage starters: those using
switching contactors (two-step), and those using power
semiconductors, or SCRs (soft starters).
Two-Step Starters
These starters first connect the motor to a chosen lower-than-line
voltage, then connect it to full line voltage once the motor is near
full speed. Since all starters include at least one contactor for AC
power switching, two-step starters include additional contactors
to switch in the lower voltage, then apply full line voltage at an
appropriate moment. The power devices producing reduced voltage,
whether reactors or autotransformers, are used only during the start,
and are bypassed once the motor is at speed.
Reduced voltage starting softens the mechanical jerk associated
with across-the-line starting, by reducing the motor torque produced.
If the connected load does not require higher-than-rated starting
torques to accelerate, reduced voltage starters may be applied,
since they will produce lower starting torques. They also have the
benefit of reduced motor heating, reduced starting currents and
lower mechanical stress on the drive train.
Reduced voltage starts take longer to accelerate the load than full
voltage starts, because motor acceleration depends upon motor
starting torque. More torque means higher acceleration, and shorter
times to speed. Less torque lowers acceleration and extends the
time spent getting to rated speed.
To apply reduced voltage starting, the starter must be designed
based upon detailed knowledge of what the connected mechanical
load requires, acceptable starting times for the process, and the
electrical supply system’s available starting current.
Soft Starters
The AMPGARD RVSS is unlike two-step starters using contactors
and reactors or autotransformers. It uses power electronics and
closed-loop control to produce a smooth application of increasing
voltage. Because solid-state semiconductor output can be precisely
controlled and varied without steps, with the benefit of precise
current control and built-in starter protection features.
Like the reduced voltage starter using contactors, the soft starter
provides variable voltage to the motor. Unlike the two-step
starters, the soft starter uses power semiconductors to start and
accelerate the motor and load. Once the motor is at full speed, a
bypass contactor shorts around the SCR power-poles, and passes
line voltage directly to the motor. The SCR power-poles are then
switched off until either another start is needed, or a controlled
(motoring) stop is required. Because the SCRs are active only during
a start cycle, they are not rated for continuous duty, and cannot be
operated continuously during normal motor running.
A soft starter is not an adjustable frequency drive, and does not
provide variable speed control of an AC induction motor. Soft
Starters provide variable voltage but constant frequency power to
start the motor while lowering power supply system demand and
reducing shock to the mechanical components.
Adjustable frequency drives, using more complex, sophisticated
and expensive power design and control, create adjustable voltage,
adjustable frequency power for the motor. Because it generates
its own output voltage and frequency, this type of drive can start
and run a motor at any speed while providing full rated torque, all
without exceeding the motor’s rated full load current. Motor torque
is directly proportional to motor current, with no excessive starting
current demand.
1.4.3.3
Soft start considerations
When starting a fan-cooled induction motor, starting current and
times are important parts of the starting conditions. AC induction
motors experience their most demanding duty during a start, when
higher than rated currents are delivered to the motor while it is
operating at less than rated speed. The higher currents generate
I
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R heating at much higher rates, while the motor, if fan cooled, is
producing lower than rated cooling.
For these reasons, it is important to get the motor started and
accelerating as soon as possible after start initiation. Ampgard RVSS
provides current limit control that enables less than across the line
inrush currents, and includes protection for motor stall (detection
of no motor rotation) and failure to reach transition speed within a
specified time. Of course, the trade-off of lower starting currents
is longer starting times, so a compromise must be struck between
short start times and less motor heating.
1.4.4 SCRs for variable voltage motor control
1.4.4.1
SCRs when used in soft starters
Figure 2.
Gate
Cathode
Anode
Silicon controlled rectifier symbol.
Silicon Controlled Rectifiers are solid state devices that have the
directional conduction characteristics of a diode but will only be
switched into conduction by a control gating signal. Like diodes, they
can only be triggered when forward (or positively) biased from anode
to cathode. They continue to conduct until the current goes to zero.
Like diodes, SCRs can be used in rectifier circuits, to produce DC
(with ripple) from AC. They can also be used to create variable-
voltage AC from constant AC.
In a soft-starter, each power-pole has two or more SCRs arranged in
parallel circuits with one device oriented in each direction.
Figure 3. SCRs connected for soft start operation.