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Diagnostics Optimization CT MODBUS RTU Technical Data
208 E300 Design Guide
Issue Number: 1
A dual time constant thermal model is provided that can be used to estimate the motor temperature as a percentage of its maximum allowed
temperature. The input to the model is the Current Magnitude, Total Output Current (J22). Throughout the following discussion Motor Rated Current
(B02) is used in the model. It should be noted that if the parameters that have been added in addition to those in Unidrive SP are left at their default
values the model is a simple single time constant model as provided in Unidrive SP.
Percentage Losses
The losses in the motor are calculated as a percentage value, so that under these conditions the Motor Protection Accumulator (J26) 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, Total Output Current (J22)
I
Rated
= Motor Rated Current (B02)
K
fe
= Rated Iron Losses As Percentage Of Losses (B24) / 100 %
The iron losses are relatively low in motors that have a rated frequency of 60 Hz or less, and so the motor could be modelled based on load related
losses alone. This can be done by setting K
fe
to zero. In motors where iron losses are significant, K
fe
defines the proportion of losses that are iron
losses under rated conditions (i.e. rated current and rated frequency). For example if the iron losses are 30 % of losses and other losses are 70 % of
losses under rated conditions Rated Iron Losses As Percentage Of Losses (B24) should be set to 30 %.
The value of K
1
defines the continuous allowable motor overload as a proportion of the Motor Rated Current (B02) before the Motor Protection
Accumulator (J26) reaches 100 %. The value of K
1
can be used to model reduced cooling at low speeds and to allow the motor to operate under
rated conditions with a small margin to prevent spurious trips. K
1
is defined in more detail later.
Motor Protection Accumulator
So far the steady state motor losses have been defined, but the motor model must estimate the temperature within the motor under dynamically
changing conditions, and so the Motor Protection Accumulator (J26) is given by the following equation.
T = Percentage Losses x [(1 − K
2
) (1 − e
-t/τ1
) + K
2
(1 − e
-t/τ2
)]where:
T = Motor Protection Accumulator (J26)
K
2
= Motor Thermal Time Constant 2 Scaling (B23) / 100 %
τ1 = Motor Thermal Time Constant 1 (B20)
τ2 = Motor Thermal Time Constant 2 (B22)
[(1 − K
2
) (1 − e
-t/τ1
) + K
2
(1 − e
-t/τ2
)] gives the effects of the thermal time constants in the motor. K
2
defines the ratio of the contribution to the Motor
Protection Accumulator (J26) value from each of the time constants. If K
2
is set to its default value of 0 then only Motor Thermal Time Constant 1
(B20) is included and the model will give the temperature of the main mass of the motor body. To give better protection to the motor, the model can be
used to model a particular point in the motor, for example the stator windings. This can be done by including an additional shorter time constant
representing the thermal impedance between the windings and the main mass of the motor body which can be modelled with Motor Thermal Time
Constant 2 (B22).
Reduced cooling with lower speed
If Motor Rated Current (B02) ≤ Maximum Heavy Duty Rating (J05) then K
1
is defined as shown below. If Low Speed Thermal Protection Mode (B44)
= 0 the characteristic is intended 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 Low Speed Thermal Protection Mode (B44) = 1 the characteristic is intended for motors where the
cooling effect of motor fan reduces with reduced motor speed below half of rated speed. The maximum value for K
1
is 1.05, so that above the knee of
the characteristics the motor can operate continuously up to 105 % of rated current.
B20 Motor Thermal Time Constant 1
Mode RFC-S
Minimum 1.0 Maximum 3000.0
Default 89.0 Units s
Type 16 Bit User Save Update Rate Background read
Display Format Standard Decimal Places 1
Coding RW
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