Does an O/L exists for a load fed by a PWM signal? 1

[unclebob]

I have a consultant asking for this. He has an application (ozonation process in water treatment plant) that is fed by a PWM source.
He's using a Schneider type LR9F5569 class 20 O/L that trips the moment power is applied on the ozonation equipment.

Any suggestion?

 

[waross]

Use a thermal O/L.
An electronic O/L is apt to trip on instantaneous.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[JuanBC]

Hi,

When saying "PWM Source" are you meaning a "Variable Frecuency Drive"?

Usually drives are self-protected against Overloads (O/L). Externally, you only require protection against Short-circuits

Regards,

JBC
.......
"The more I read, the more I acquire, the more certain I am that I know nothing"

 

[itsmoked]

May be a problem with the ozonation equipment. It should be pretty resistive and not 'spikey'. There shouldn't be any in-rushes between PWM pulses. If it's tripping there may be a mechanical problem leaving the system shorted. Or, if it's a corona type design something arcing could be causing a short.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Most bi-metal OL relays will trip too early because of ohmic heating of the elements due to the effects of PWM pseudo AC and harmonic current passing through them, but it depends a lot on the carrier frequency (PWM pulse rate). In addition, they can be affected by the cable capacitive charging current on long motor runs, further exacerbating the nuisance tripping issues. There are a few out now, such as Rockwell's 140M-D8V motor protector, that have been specifically designed to withstand those effects up to about 4kHz CF, but not many other mfrs have followed suit yet. I see no evidence that Schneider has addressed this issue yet, but I'm not all that familiar with everything they offer. That specific SSOL shows that it is only rated to be used at 50/60Hz, that's usually the telltale sign that you can't use it behind a VFD..

Eutectic melting alloy type OL relays are completely insensitive to this, but are becoming anachronistic due to RoHS issues with the alloys and are already no longer allowed anywhere other than North America.

Some more recent advances in SS OL relay technology, i.e. those using Rogowski Coils as the current sensing elements, are now more immune to the effects of PWM outputs. Rogowski Coils are expensive though, so you usually don't see them used on low cost SSOLs like the Schneider LR9 series.

JuanBC,
If there is more than one motor down stream of a VFD, you must provide individual OL protection for the motors, the VFD looks at the entire circuit as one.


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