|
DC to DC isolation in DC drives.....
|
|
|
...Read further to know more about the reasons |
|
|
There are
different levels of voltages present in a DC drive. These levels differ
widely from voltages as low as millivolts to 440 volts. Refering the block
diagram of a typical Drive, one will notice that the components which have
both voltages, very high and very low, present around them are Pulse
Transformers. These are connected to gate and cathode of thyristors on one
side and pulses of 15 to 24 volts on the other side. In addition to
above, if voltage feedback arrangement is used for speed control of the
motor, high voltage of armatuere is present on components around speed
amplifier. This armature voltage is only divided to a lower value by
reisistor network but not Isolated galvanically. That means the components
like ICs, resistors, capacitors, transistors are all at high level of voltage
unless isolated. Lack of isolation has disadvantages like.. a) unsafe surrounding for trouble shooting. b) faults reflect on PCBs in a very damaging
manner, destroying PCBs and components making these irrepairable. DC to DC
isolators are available commercially as ready made boxes, and these are used
to isolate 440 volts of armature to typcial 0-10 volts level before connecting
to feedback circuit. Typical DC to DC isolators have two power supplies
inside. One is used for modulator on input side, and the second one is used
for the De-modulator on output side, with a transformer in between isolating
two circuits galvanically.
|
|
|
Typical
specifications of a DC to DC isolator include whether the input / output is
unipolar or bipolar, whether the input can accept 0-10 volts as well as
0/4-20 mA, whether output is 0-10 volts or 0/4-20 mA, what is the operating
supply voltage etc. There are some isolators for isolating “Armature Voltage
Feedback” which do not require external power supply! The other area requiring DC to DC
isolation is a case where two Control panels of different make having
different polarity constraints need to be interconnected. It is essential to
isolate signal and use any one terminal of output as “ground”.
|
|
|
|
Download PDF
version of these pages.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Speed control not satisfactory
|
|
|
|
|
|
|
|
|
|
|
|
|