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Unidrive M702 User Guide 127
Issue Number: 3
8.2 Maximum motor rated current
The maximum motor rated current allowed by the drive is greater than
the Maximum Heavy Duty Current Rating (11.032). The ratio between
the Normal Duty rating and the Maximum Heavy Duty Current Rating
(11.032) varies between drive sizes. The values for the Normal and
Heavy Duty rating can be found in section 2.3 Ratings on page 11. If the
motor Rated Current (00.046) is set above the Maximum Heavy Duty
Current Rating (11.032), the current limits and the motor thermal
protection scheme are modified (see section 8.3 Current limits on
page 127 and section 8.4 Motor thermal protection on page 127 for
more information).
8.3 Current limits
The default setting for the current limit parameters is:
• 165 % x motor rated current for open loop mode
• 175 % x motor rated current for RFC-A and RFC-S modes
There are three parameters which control the current limits:
• Motoring current limit: power flowing from the drive to the motor
• Regen current limit: power flowing from the motor to the drive
• Symmetrical current limit: current limit for both motoring and regen
operation
The lowest of either the motoring and regen current limit, or the
symmetrical current limit applies.
The maximum setting of these parameters depends on the values of
motor rated current, drive rated current and the power factor.
Increasing the motor rated current (Pr 00.046/05.007) above the Heavy
Duty rating (default value), will automatically reduce the current limits in
Pr 04.005 to Pr 04.007. If the motor rated current is then set to or below
the Heavy Duty rating, the current limits will be left at their reduced
values.
The drive can be oversized to permit a higher current limit setting to
provide higher accelerating torque as required up to a maximum of
1000 %.
8.4 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 Rated Current (05.007) ≤ Maximum Heavy Duty Current (11.032)
Figure 8-1 Motor thermal protection (Heavy Duty)
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 8-2 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
1.05
Base speed/
frequency
50% of base
speed/frequency
K
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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