Control Techniques Digitax ST - Page 75

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Digitax ST Advanced User Guide 75
Issue Number: 1 www.controltechniques.com
The motor is modelled thermally in a way that is equivalent to the electrical circuit shown as follows.
The temperature of the motor as a percentage of maximum temperature, with a constant current magnitude of I, constant value of K and constant
value of motor rated current (set by Pr 5.07 or Pr 21.07) after time t is given by
Temp = [I
2
/ (K x Motor rated current)
2
] (1 - e
-t/τ
) x 100%
This assumes that the maximum allowed motor temperature is produced by K x Motor rated current and that τ is the thermal time constant of the point
in the motor that reaches its maximum allowed temperature first. τ is defined by Pr 4.15. The estimated motor temperature is given by Pr 4.19 as a
percentage of maximum temperature. If Pr 4.15 has a value between 0.0 and 1.0 the thermal time constant is taken as 1.0.
Pr 4.25 can be used to select 2 alternative protection characteristics (see diagram below). If Pr 4.25 is 0 the characteristic is for a motor which can
operate at rated current over the whole speed range. If Pr 4.25 is 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 is 1.05, so that above the knee of the characteristics the
motor can operate continuously up to 105% current.
Switching freq Proportional gain Integral gain
3kHz Pr 4.13 Pr 4.14
4kHz Pr 4.13 x 1.5 Pr 4.14
6kHz Pr 4.13 x 2 Pr 4.14
8kHz Pr 4.13 x 2 Pr 4.14 x 1.3
12kHz Pr 4.13 x 2.6 Pr 4.14 x 1.3
4.15 Thermal time constant
Coding
Bit SP FI DE Txt VM DP ND RA NC NV PT US RW BU PS
111
Range 0.0 to 3000.0
Default 20.0
Second motor
parameter
Pr 21.16
Update rate Background read
4.16 Thermal protection mode
Coding
Bit SP FI DE Txt VM DP ND RA NC NV PT US RW BU PS
1 111
Range 0 to 1
Default 0
Update rate Background read
I /(K*Motor Rated Current)
22
Temp
C
R