Theory Of Operation - Emerson AMPGARD RVSS Instruction Booklet

AMPGARD RVSS
Reduced Voltage Soft-Starter
User Manual
AMPGARD SL vacuum contactor, the Bypass contactor, is mounted
on a bracket attached to the top shelf of the lower compartment.
The Bypass contactor is used to bypass the SCRs in the RVSS truck
when the motor reaches full speed. Handles attached to the front
of the RVSS truck Power Poles are used during truck installation or
removal (see Figure 43).
1.4

Theory of operation

1.4.1 AC Motor theory
A review of AC motor starting theory and methods will help to
understand how the AMPGARD RVSS starts and protects a medium
voltage motor.
There are several methods for starting medium voltage three-phase
AC motors. The method chosen for any application is determined
by a combination of factors: the connected load starting and
running torque requirements, available AC power supply, process
requirements and cost. Some starting methods include features that
address the mechanical limitations of the load and/or the available
electric power. The Eaton Solid State Soft Starter addresses those
issues in a way that has significant advantages over the other
methods.
To understand AC motor starting methods, it is necessary to
understand the motor characteristics, explore starting methods, and
learn how a soft starter is different from an across-the-line starter. A
comparison of three different starting methods follows, and explains
how a soft starter provides benefits not found in the other methods.
In all of the following examples, AC power at constant frequency is
applied.
1.4.1.1 AC motor characteristics
The mechanical design and construction of AC motors make them
relatively inexpensive. Having their electromagnets mounted in the
motor frame makes the rotor design simpler and easier to balance.
Because AC motors use magnetic induction to transfer power, there
are few mechanical parts to wear out, so they can last a long time
with relatively little maintenance.
1.4.1.2 Motor Current and Torque
AC induction motors are so-called because their frame (stator)
electromagnets induce currents and magnetic fields in the rotor
conductors. Because AC motors produce torque from the interaction
of stator and rotor magnetic fields, and the rotor field strength
depends on the applied stator current, AC induction motors produce
torque that is proportional to the square of the applied current. That
is, with 50% of their rated applied voltage, they will deliver 25% of
their rated torque (0.5 x 0.5).
1.4.1.3 Motor Heating
AC motors produce considerable internal heating while starting.
The resistance of the stator winding conductors will generate heat
proportional to the applied current squared. – so if six times rated
current is applied, the motor will heat up at 36 times its normal rate.
AC motors are often self-cooled by rotor-mounted fans that circulate
air in the gap between stator and rotor. The faster the motor turns,
the more the motor is cooled. The rotor fan cooling efficiency is low
at low speeds, providing adequate cooling for full-load operation only
when the motor is running at rated speed. During a start, the motor
is heating rapidly due to high currents while lacking rated cooling. If
a start takes too long, or if the motor stalls and doesn't start at all,
motor insulation can suffer permanent heat damage, shortening the
motor's life.
1.4.1.4 AC motor torque and applied voltage
Under steady state conditions of voltage and frequency, AC motor
torque is proportional to the square of voltage. Under starting
conditions, the motor torque is proportional to the square of applied
voltage. This means that an AC motor's starting torque, when fed
from a variable voltage source, will have much less starting torque
available compared to across the line, full voltage starts.
Instruction Booklet IB020003EN
For example, an AC motor, at 50% voltage, will produce no more
than 25% torque, and so forth.
Whenever a soft starter is applied to a motor and load combination,
care must be taken to confirm that the load will not require more
torque to start than the motor can deliver while the supply voltage is
less than full voltage. If the motor cannot deliver sufficient starting
torque, it will stall.
1.4.2 Starting Performance
AC motor starting performance is determined by the motor design
and by the applied power. Different designs (such as NEMA A, B,
C or D) will produce different torque profiles as they accelerate
from stopped to full speed. Once at speed, they all deliver torques
proportional to their rated horsepower and speed.
A typical NEMA B frame motor can initially draw between six and
ten times rated current during a start. The current drawn during
acceleration depends upon the motor's electrical design and
eventually reduces to rated current at rated speed and load. Starting
torque for this type of motor are usually around 200% of rated full
load torque initially, reducing to less during acceleration, peaking
before full speed and then reducing to no more than rated torque
at rated nameplate speed. To accomplish a successful start, the
motor's delivered torque must be enough to break away the motor
and load and accelerate them to full speed. See Figure 1.
600
Percent
Current
Locked Rotor
200
or
Stall Torque
Percent
Torque
100 100
Rated Nameplate Torque, Speed, Current
0 10 20 30
Percent Speed
Figure 1. NEMA B motor typical speed - torque diagram.
In every case, AC motor starting performance depends on the
voltage and current applied to the motor. Voltage is furnished by the
plant supply system, and resultant current is determined by what
the motor requires and what the supply can deliver.
Since the motor draws higher than rated current during starting,
if the connected supply system is weak, its voltage at the starter
terminals may sag, causing brownouts or tripping protective devices.
1.4.3 Starting methods
The simplest and least expensive of the three starting methods is
an across-the-line starter. This approach works well when the power
supply system can tolerate high starting currents and the mechanical
load can tolerate high starting torques. When the electrical supply
system is weak, or when the connected mechanical load cannot
tolerate abrupt, jerking starts, across-the-line starters may not be an
acceptable starting method.
Effective January 2021
NEMA B Design Motor
Speed / Torque / Current
Curves
(Across the Line Start)
Pull Out
or
Breakdown Torque
Rated Operating Point
(at full load)
40 50 60 70 80 90 100
Synchronous
Operating
(Nameplate)
Speed
EATON www.eaton.com
Speed
13