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”.  







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Block Diagram



Ramp Circuit


Speed Amplifier


Current Amplifier


 Firing Circuit


Power Circuit


Types of DC drives 



 Checking Thyristor


Voltage feedback Vs Tacho feedback


Current Limit


Constant HP & constant Torque


Field Weakening


Using CRO 



Motor runs at full speed


Fuses Blow


Hunting in speed


Motor gives jerks


Belts vibrate


Speed control not satisfactory 



Speed drops on load


Motor overheats


Sparking on commutator


DC drives and Power Factor


Your comments and suggestions 


Drive stops all of a sudden 


Motor not able to drive the load


DC to DC isolation


 Motor or Drive?