Must i isolate?
Must i isolate?
(OP)
I want to design a dc motor controller.(300W, 24V dc motor) I plan to use H-Bridge. What i wonder is, must i isolate control circuit from the power circuit completely? I don't want it to be cost much. If i isolate everything then the circuit will be more complex and will cost much. What do you suggest me? Can i obtain a reliable drive without isolation? If i isolate everything, what can i use for voltage and current measurements which doesn't cost much.





RE: Must i isolate?
It is unlikely that the insulation within a 24v motor will safely withstand full mains voltage between windings and frame for any length of time. That could be very dangerous.
RE: Must i isolate?
I'm lost on Warp's response "full mains"?
RE: Must i isolate?
Sorry, my misunderstanding.
RE: Must i isolate?
RE: Must i isolate?
Probably the main thing to avoid is using a common return wire for both dc power to the motor, and as a signal return wire. There should be a "power circuit" and a "control circuit" that have quite independent supply and return systems. These two grounds can be connected together but only at a single point.
The rest is all about good wiring practice, bypassing, and even cable screening if the cable runs are very long.
RE: Must i isolate?
RE: Must i isolate?
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RE: Must i isolate?
1-If your product is medical or in a wet environment, you might have to isolate anyway to prevent exposing operators or patient to high frequency current when touching the controls.
2-If you use a micro processor, the capacitive coupling with the "grounded" environment could cause problems. Make sure that all the lines are terminated properly.
3-If you use potentiometers and switches, make sure that they can handle the high frequency voltage spikes that will be present between their circuit and operator. Membrane switches could be a problem.
Otherwise it may work well.
RE: Must i isolate?
abfer you have not stated whether this is a product or a one-off.
The point I want to make tho is if this is a system YOU will be installing, and can control the wiring, you can get away with out isolation, barring failure saftey protection as mentioned above. But if this is something installed by others, say this is an off-the-shelf product you will sell to others, ISOLATE IT! Otherwise some systems will work, others won't, and worse some will sorta work! You will find yourself on the phone for the rest of your life talking to the unhappy ;(
RE: Must i isolate?
"2-If you use a micro processor, the capacitive coupling with the "grounded" environment could cause problems. Make sure that all the lines are terminated properly"
Can you please explain more detail this or suggest me a source to help me learn this subject.
Actually i made a drive simpler than this circuit without islation with one switch. It doesn't contain sensitive measurements. Measurements affected from the high frequency noise however. As they weren't sensitive i solved them by software. But i didn't faced serious problems or failures in control circuit. It has 3 different grounds interconnecting at one point. I think if i care more to reduce noise i can success.
RE: Must i isolate?
One very effective way is to fit opto isolators close to the processor, and run individual twisted pair two wire circuits back to the motor. Proportional control or measurement can be via pulse width modulation or voltage to frequency conversion so that the opto isolators operate in digital mode. That is always the best and safest way.
The whole thing need not be isolated totally, but isolate all the long wiring runs at the processor end, that are noise sensitive.