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Powerdrive F300 User Guide 159
Issue Number: 2
8.2 Motor thermal protection
A dual time constant thermal model is provided to estimate the motor temperature as a percentage of its maximum allowed temperature.
The motor thermal protection is modelled using losses in the motor. The losses in the motor are calculated as a percentage value, so that under these
conditions the Motor Protection Accumulator (04.019) would eventually reach 100 %.
Percentage losses = 100 % x [Load related losses + Iron losses]
Where:
Load related losses = (1 - K
fe
) x (I / (K
1
x I
Rated
))
2
Iron losses = K
fe
x (w / w
Rated
)
1.6
Where:
I = Current Magnitude (04.001)
I
Rated
= Rated Current (05.007)
K
fe
= Rated Iron Losses As Percentage Of Losses (04.039) / 100 %
The Motor Protection Accumulator (04.019) is given by:
Pr 04.019 = Percentage Losses x [(1 - K
2
) (1 - e-
t/τ1
) + K
2
(1 - e-
t/τ2
)]
Where:
T = Motor Protection Accumulator (04.019)
K
2
= Motor Thermal Time Constant 2 Scaling (04.038) / 100 %
τ1
= Motor Thermal Time Constant 1 (04.015)
τ2
= Motor Thermal Time Constant 2 (04.037)
K
1
= Varies, see below
If Pr 04.025 is 0 the characteristic is for a motor which can operate at rated current over the whole speed range. Induction motors with this type of
characteristic normally have forced cooling. If Pr 04.025 is 1 the characteristic is intended for motors where the cooling effect of motor fan reduces
with reduced motor speed below 50 % of base speed/ frequency. The maximum value for K1 is 1.05, so that above the knee of the characteristics the
motor can operate continuously up to 105 % current.
Figure 8-1 Motor thermal protection (Normal Duty)
Both settings of Pr 04.025 are intended for motors where the cooling effect of the motor fan reduces with reduced motor speed, but with different
speeds below which the cooling effect is reduced. If Pr 04.025 is 0 the characteristic is intended for motors where the cooling effect reduces with
motor speed below 15 % of base speed/frequency. If Pr 04.025 is 1 the characteristic is intended for motors where the cooling effect reduces with
motor speed below 50 % of base speed/frequency. The maximum value for K1 is 1.01, so that above the knee of the characteristics the motor can
operate continuously up to 101 % current.
When the estimated temperature in Pr 04.019 reaches 100 % the drive takes some action depending on the setting of Pr 04.016. If Pr 04.016 is 0, the
drive trips when Pr 04.019 reaches 100 %. If Pr 04.016 is 1, the current limit is reduced to (K - 0.05) x 100 % when Pr 04.019 reaches 100 %.
The current limit is set back to the user defined level when Pr 04.019 falls below 95 %. The thermal model temperature accumulator is reset to zero at
power-up and accumulates the temperature of the motor while them drive remains powered-up. If the rated current defined by Pr 05.007 is altered,
the accumulator is reset to zero.
The default setting of the thermal time constant (Pr 04.015) is 89 s which is equivalent to an overload of 150 % for 60 s from cold.
0.00
0.70
1.00
Pr = 0
Pr = 1
04.025
04.025
1.00
1.01
Base speed/
frequency
50% of
base speed/
frequency
15% of
base speed/
frequency
K
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