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98 Unidrive SP Advanced User Guide
www.controltechniques.com Issue Number: 7
If the rated current is above the maximum Heavy Duty rating then Pr 4.25 can also be used to select 2 alternative protection characteristics. Both
characteristics 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. The maximum value for K is 1.01, so that above the knee of the characteristics the motor can operate
continuously up to 1.01% current. (In Regen mode K = 1.01 over the whole operating frequency range.)
When the estimated temperature reaches 100% the drive takes some action depending on the setting of Pr 4.16. If Pr 4.16 is 0, the drive trips when
the threshold is reached. If Pr 4.16 is 1, the current limit is reduced to (K - 0.05) x 100% when the temperature is 100%. The current limit is set back
to the user defined level when the temperature falls below 95%. In servo and regen modes the current magnitude and the active current controlled by
the current limits should be similar, and so this system should ensure that the motor operates just below its thermal limit.
The time for some action to be taken by the drive from cold with constant motor current is given by:
T
trip
= -(Pr 4.15) x ln(1 - (K x Pr 5.07 / Pr 4.01)
2
)
Alternatively the thermal time constant can be calculated from the trip time with a given current from:
Pr 4.15 = -T
trip
/ ln(1 - (K / Overload)
2
)
For example, if the drive should trip after supplying 150% overload for 60 seconds with K = 1.05 then
Pr 4.15 = -60 / ln(1 - (1.05 / 1.50)
2
) = 89
Motor total
current (Pr 4.01)
as a percentage
of motor rated
current
Motor speed as a
percenta
e of base speed
100%
Max. permissible
continuous
current
100%
I t protection operates in this region
2
70%
50%
Pr = 0
Pr = 1
4.25
4.25
Open loop: Proportion of rated frequency Pr 5.06.
Closed loop: Proportion of rated speed Pr 5.08.
Rated current (Pr 5.07 or Pr 21.07) ≤ maximum Heavy Duty rating.
Motor total
current (Pr 4.01)
as a percentage
of motor rated
current
Motor speed as a
percenta
e of base speed
100%
Max. permissible
continuous
current
100%
I t protection operates in this region
2
70%
50%15%
Pr = 0
Pr = 1
4.25
4.25
Open loop: Proportion of rated frequency Pr 5.06.
Closed loop: Proportion of rated speed Pr 5.08.
Rated current (Pr 5.07 or Pr 21.07) > maximum Heavy Duty rating.
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