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Instruction Booklet IB020003EN
Effective January 2021
Code found on a motor nameplate, Use a code range to calculate
the motor locked rotor amps based upon the motor full load amps.
For low inertia loads, use 65% to 80% of the calculated locked rotor
amps as an RVSS initial current limit setting, if there are no power
system or mechanical system constraints to overrule that.
For example, a 500 HP , 4160V, three phase motor with an efficiency
of 90% and a power factor of 0.85 (since no motor nameplate is
available in this example to give FLA directly) would have an FLA of
approximately
FLA = (746 x HP) / (1.732 x efficiency x power factor x Voltage)
= (746x500) / (1.732 x 0.9 x 0.85 x 4160)
= 373,000 / 5512
67.7 amps
If that motor has a low inertia load, and a Locked Rotor Code
of C (corresponding to a Locked Rotor kVA/HP of 3.55 - 4.0), a
recommended current limit setting would be 65% of the calculated
locked rotor amps times the Locked Rotor kVA/HP value, or
0.65 x 67 .7 x 4.0 = 176 amps.
The actual current limit would be determined by trial starts. If the
calculated value works, use it. If not, gradually increase the current
limit over successive trials until a suitable current limit that meets
the application requirements is obtained.
Table 40. NEMA locked rotor codes
NEMA
code
letter
A
B
C
D
E
F
G
H
J
K
L
M
N
P
R
S
T
U
V
For low inertia loads the Ramp Time should be in the range of 4 to
8 Seconds with initial voltage of 30% and current limit ranging from
65% to 80% of the locked-rotor current based upon the code on the
motor nameplate.
Incomplete sequence time
The incomplete sequence time can be determined by experiment
during commissioning. After several successful starts, the
Incomplete Sequence Time can be set from the measured time
needed to start the motor and load.
It can also be calculated if the motor thermal behavior is known.
Often, vendor supplied motor data includes a Thermal Limit Curve.
This curve shows how the motor will heat up when subjected to
108
EATON www.eaton.com
Locked rotor
kVA/HP
0 - 3.15
3.15 - 3.55
3.55 - 4.0
4.0 - 4.5
4.5 - 5.0
5.0 - 5.6
5.6 - 6.3
6.3 - 7.1
7.1 - 8.0
8.0 - 9.0
9.0 - 10.0
10.0 - 11.2
11.2 - 12.5
12.5 - 14.0
14.0 - 16.0
16.0 - 18.0
18.0 - 20.0
20.0 - 22.4
22.4 and up
various levels of current, usually multiples of FLA. If the motor
Thermal Limit Curve is available, its response to overcurrents can be
used to calculate a suitable Incomplete Sequence Time.
Figure 98 shows an example Thermal Limit Curve for a 300 HP 4160
V motor.
The following formula shows how to calculate this time, using
the anticipated successful start current limit setting (as a multiple of FLA),
•
the motor Locked Rotor Code (see Table 40),
•
and the motor Locked Rotor Time (also known as the Hot Stall Time) from the
•
Thermal Limit Curve.
For example, if a motor can be started using 400% FLA (a multiplier
of 4), and it has a Locked Rotor Code of F (Locked Rotor kVA/HP of
5.0 to 5.6), and its Locked Rotor Time (LRT) from the vendor curve is
12 seconds, then
Incomplete sequence time
= LRT x (LR Code / Current Limit multiple)
= 12 x (5.6 / 4)
= 12 x (1.4)
= 12 x 1.96 = 23.52 sec.
2
2
So the RVSS Incomplete Sequence Time parameter could be set to
23 seconds and provide adequate motor thermal protection.
Any changes in the current limit setting will require a recalculation
and resetting of the Incomplete Sequence Time parameter.
Figure 121. Example motor thermal limit curve
General Notes - field trials and adjustments
For high inertia loads where the geometry-mass of the rotor and
load are large, such as a large blower, the ramp can be extended as
the accelerating torque will be required for a much longer time. The
ramp can be adjusted out to 12 to 15 seconds and the current may
be as high as 80% of locked-rotor code.
For conveyors and material handling applications where a stiction
at breakaway, it may be necessary to use Kick-Start, set as high as
80% of full voltage, with a pulse width of one second.
It may also be useful on applications with longer cable lengths to
also use a Kick-Start set at 30% of full voltage for up to 1 second to
condition the cables before ramping.
AMPGARD RVSS
Reduced Voltage Soft-Starter
User Manual
2
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