Control Techniques Digitax ST Advanced User's Manual page 76

Parameter
Menu 4
structure
Motor total
current (Pr 4.01)
as a percentage
of motor rated
2
I t protection operates in this region
current
105%
70%
Proportion of rated speed Pr 5.08.
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%. 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 = -(Pr 4.15) x ln(1 - (K x Pr 5.07 / Pr 4.01)
trip
Alternatively the thermal time constant can be calculated from the trip time with a given current from:
Pr 4.15 = -T / ln(1 - (K / Overload)
trip
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)
The thermal model temperature accumulator is reset to zero at power-up and accumulates the temperature of the motor while the drive remains
powered-up. Each time Pr 11.45 is changed to select a new motor, or the rated current defined by Pr 5.07 or Pr 21.07 (depending on the motor
selected) is altered, the accumulator is reset to zero. If Pr 4.15 is set to zero the motor thermal protection system is disabled and the accumulator
remains at zero.
4.17 Reactive current
Bit SP
Coding
Range ±DRIVE_CURRENT_MAX A
Update rate 4ms write
The drive reactive current is shown in this parameter. This parameter is normally 0 unless operating in the field weakening range.
4.18 Overriding current limit
Bit SP
Coding
Range 0 to TORQUE_PROD_CURRENT_MAX %
Update rate Background write
The current limit applied at any time depends on whether the drive is motoring or regenerating and also on the level of the symmetrical current limit.
Pr 4.18 gives the limit level that applies at any instant.
76
Keypad and
Parameter x.00
display
Motor speed as a
50% 100%
percentage of base speed
2
)
2
)
2
) = 89
FI DE Txt VM DP ND
1 1 2 1
FI DE Txt VM DP ND
1 1 1
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Parameter Advanced parameter
description format descriptions
Max. permissible
continuous
current
Pr 4.25 = 0
Pr 4.25 = 1
RA NC NV PT US RW BU
1 1
RA NC NV PT US RW BU
1 1
Serial comms Electronic
protocol nameplate
PS
PS
1
Digitax ST Advanced User Guide
Performance
Issue Number: 1