Op-Amp circuit with input referenced to other ground.

[drillz]

Dear Readers,

I am designing phase loss detection circuitry for the input 3-phase supply for an electric drive.

This circuitry must simply do the following….
1. Detect when all three input phases are present
2. Detect when one input phase has been lost
3. Detect when two input phases have been lost.

-And be able to distinguish which of the above three events is occurring.

I have designed a simple circuit for this using two op-amps.
-However, I have the problem that the op-amp voltage supply that I must use is isolated from the three-phase power ground.

Here is the basic schematic for the electric drive….

(The phase loss circuitry will simply provide a pulsed output from IC2 (comparator) when one phase has been lost.)

I have put in the protection diodes as above to guard against the input from the three-phase, three-pulse rectifier going outside the op-amps’ supply rail.

I am wondering if the above circuit is a good idea. I would admit that the op-amps’ ground (“ground 1”) is floating with respect to the three-phase ground. However, my circuitry is basically a waveform shape detector, and will not depend too much on the absolute value of the inputs.

I would be very grateful for any ideas as to how I can mitigate the fact that the op-amp ground is different from that of the three-phase input.

 

[Noway2]

Why not use opto isolators to separate the line side from your electronics?

This way you could avoid the floating neutral problem and you would have a lot better protection against the 460Vac blasting your low voltage circuit.

 

[Madcow]

How does the curent flow in the R1 R2 divider
if the return path is to ground 1 and not DC bus
ground?

 

[ScottyUK]

Adding to the above, distinguish the loss of two phases from loss of all three unless you have a neutral, something which would be unusual for a drive. One phase on its own won't power anything.


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If we learn from our mistakes I'm getting a great education!

 

[DHambley]

Use a differential amp with very high input resistance so you can get by with small 1206 or 1210 resistors. Now you've got your input signal referenced back to GND1 which is what you want.