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Diagnostics Optimization CT MODBUS RTU Technical Data
210 E300 Design Guide
Issue Number: 1
Time for Motor Protection Accumulator to reach 100 %
Assuming a single time constant model is being used (i.e. Motor Thermal Time Constant 2 Scaling (B23), the time for the Motor Protection
Accumulator (J26) to change from its initial value to 100 % is given by the following equation:
Time to reach 100.0 % = -τ1 x ln[(1 − C
1
) / (C
0
− C
1
)]
C
0
represents the conditions that have persisted for long enough for the Motor Protection Accumulator (J26) to reach a steady state value. If the
motor current and speed are I
0
and w
0
then,
C
0
= [(1 - K
fe
) x (I
0
/ (K
1
x I
Rated
))
2
] + [K
fe
x (w
0
/ w
Rated
)
1.6
]
C
1
represents the conditions that begin at the start of the time being calculated. If the motor current and speed are by I
1
and w
1
then,
C
1
= [(1 - K
fe
) x (I
1
/ (K
1
x I
Rated
))
2
] + [K
fe
x (w
1
/ w
Rated
)
1.6
]
Example 1:
The effect of iron losses are not modelled (K
fe
= 0), Motor Thermal Time Constant 1 (B20) = 89 s, the initial current is zero, Motor Rated Current (B02)
≤ Maximum Heavy Duty Rating (J05) and the new level of current is 1.5 x Motor Rated Current (B02).
C
0
= 0
C
1
= [1.5 / (1.05 x 1.0)]
2
= 2.041
Time to reach 100.0 % = -89 x ln(1 − 1/C
1
) = -89 x ln(1 − 1/2.041) = 60 s
This is the default setting for Open-loop and RFC-A modes allowing an induction motor to run at 150 % rated current for 60 s from cold.
Example 2:
The effect of iron losses are not modelled (K
fe
= 0), Motor Thermal Time Constant 1 (B20) = 89 s, the initial current is Motor Rated Current (B02),
Motor Rated Current (B02) ≤ Maximum Heavy Duty Rating (J05) and the new level of current is 1.5 x Motor Rated Current (B02).
C0 = [1.0 / (1.05 x 1.0)]
2
= 0.907
C1 = [1.5 / (1.05 x 1.0)]
2
= 2.041
Time to reach 100.0 % = -89 x ln((1 − C1) / (C0 − C1)) = -89 x ln[(1 − 2.041) / (0.907 − 2.041)] = 7.6 s
This is the default setting for Open-loop and RFC-A modes allowing an induction motor to run at 150 % rated current for 7.6 s after running under
rated conditions for a significant period of time.
Motor Protection Accumulator Reset
The initial value in the Motor Protection Accumulator (J26) at power-up is defined by Motor Protection Accumulator Power-up Value (B21) as given in
the table below.
The Motor Protection Accumulator (J26) is reset when Motor Thermal Time Constant 1 (B20) is set to 0.0. Note that this is not possible in the
standard product as the minimum parameter value is 1.0.
Motor Protection Accumulator Warning
If Percentage Losses > 100 % then eventually the Motor Protection Accumulator (J26) will reach 100 % causing the drive to trip or the current limits to
be reduced. If this is the case and Motor Protection Accumulator (J26) > 75.0 % then [Motor Overload] alarm indication is given and Motor Overload
Alarm (L21) is set to one.
Motor Protection
Accumulator Power-up
Value (B21)
Motor Protection Accumulator (J26) at power-up
Power Down The value is saved at power-down and is used as the initial value at power-up.
Zero The value is set to zero
Real Time
If a real-time clock is present and if Date/Time Selector (H32) is set up to select the real-time clock then the value
saved at power-down is modified to include the effect of the motor thermal protection time constants over the time
between power-down and power-up. This modified value is then used as the initial value at power-up.
If no real time clock is present then and this option is selected then the value saved at power-down is used as the
initial value.
B21 Motor Protection Accumulator Power-up Value
Mode Open-loop: RFC-A, RFC-S
Minimum 0 Maximum 2
Default 0 Units
Type 8 Bit User Save Update Rate Background write
Display Format Standard Decimal Places 0
Coding RW, TE
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