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78 Unidrive SP Advanced User Guide
www.controltechniques.com Issue Number: 7
The example shown is for a very rapid load change where the torque reference of the motor drive has been changed instantly from one value to
another. The proportional gain of the voltage controller defines the voltage transient because the integral term is too slow to have an effect. (In
applications where the motor drive is operating under speed control, the speed controller may only require a limited rate of change of torque demand,
and so the transient voltage may be less than covered in the discussion below.) If the set point voltage (Pr 3.05) plus the transient rise exceed the
over-voltage trip level the regen unit will trip.
When a 400V motor is operated above base speed from a drive in vector mode, fed from the regen unit with the same rating supplying a DC voltage
of 700V, and an instantaneous change of torque is demanded (i.e. -100% to +100%) the peak of the voltage transient (∆V) is approximately 80V if the
current controllers are set up correctly and the voltage controller uses the default gain. (Operating with maximum voltage on the motor, i.e. above
base speed, gives the biggest transient of power and hence the biggest value of ∆V.)
If the load change, drive voltage rating, motor voltage or DC bus set-point are different then ∆V is calculated from:
∆V = 80V x K
L
X K
RAT
x K
MV
X K
SP
Where:
K
L
= load change / 200%
K
RAT
= Drive voltage rating / 400
K
MV
= Motor voltage / 400
K
SP
= 700 / DC bus voltage set point
In some applications, particularly with a high DC bus voltage set point and low switching frequency it may be necessary to limit the rate of change of
power flow to prevent over voltage trips. A first order filter on the torque reference of the motor drive (i.e. using Pr 4.12) is the most effective method
to reduce the transient further. (A fixed limit of the rate of change of torque demand is less effective.) The following table gives an approximate
indication of the reduction in ∆V for different time constants. (As already mentioned the value of ∆V given if for an instantaneous change of torque
representing the worst case. In applications where a speed controller is used in the motor drive the transient will already include an inherent filter.)
The transient produced is approximately proportional to the voltage controller gain. The default voltage controller gain is set to give a value that is
suitable for most applications. The gain may need to be increased if the DC bus capacitance is high compared to two drives of similar rating coupled
together. However, care must be taken to ensure that the gain is not too high as this can cause excessive ripple in the DC bus voltage.
When the regen unit has powered-up through the soft-start resistor and the DC bus voltage has stabilised this bit changes from 0 to 1. When regen
mode is selected this bit is routed to the relay on terminals 41 (T41) and 42 (T42) as default. This output, or an alternative output, should be used to
control the soft-start contactor.
Time constant Change in ∆V
20ms x 0.75
40ms x 0.5
3.07
Close soft-start contactor
Drive modes Regen
Coding Bit SP FI DE Txt VM DP ND RA NC NV PT US RW BU PS
111
Update rate 4ms write
Regen unit
DC bus voltage
Active regen
unit current
ch3: dT= 194ms dV=2.24 V
50ms/div
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