42
Unidrive Regen Installation Guide
www.controltechniques.com
Issue Number: 2
The rms value of the current can be estimated from the following formula:
Where:
k is 1 for simple rectifier-input systems,
√
2 for Regen systems
V
DC
is DC bus voltage
å
lf
s
is the sum of the products of motor cable lengths and switching frequencies of all drives in the system, including in the case of regenerative
systems the Regen drive with the total DC cable length
l is total cable length in metres
f
s
is switching frequency in kHz
If all drives operate at 3kHz, the expression can be simplified to:
I
RMS
= 4.85 x 10
-4
kV
DC
√
l
Example
A Regen system operating with a supply of 400Vac giving a DC bus voltage of 620V at 3kHz switching frequency and a cable length of 1km (motors
+ DC) has an I
RMS
of:
I
RMS
= 4.85 x 10
-4
kV
DC
√
l
I
RMS
= 4.85 x 10
-4
x
√
2 x 620 x
√
1,000
I
RMS
= 13.4A
The I
RMS
is the total current line to ground, therefore each capacitor will have to carry 4.5A.
Ground leakage current
The value of capacitance required means that the ground leakage current exceeds the usual safety limit of 3.5mA. The user should be
aware of the high leakage current. A permanent fixed ground connection must be provided to the system.
Discharge time
Resistors must be fitted in parallel with the capacitors to ensure that they discharge when the supply is removed. The resistor values should
be chosen so that the discharge time is no longer than for the drive itself. Typically values of about 5M
Ω
are suitable, and are high enough
not to cause the system to fail a simple insulation test.
I
RMS
2.8
10
4
–
kV
DC
Σ
lf
s
×
=
WARNING
WARNING
