Can I oversize RFI filters?

[Cow]

I have a customer that wants to plan for future motor upgrades by installing larger filters now instead of having to buy them twice. For instance, they have a 125HP well that needs a filter, but they want me to size a filter for a possible 200HP well upgrade. Does this affect the performance of the filter being that much oversize? We typically use MTE RFI filters if it matters.

I figured I'd ask you guys first before I call someone at MTE.

Thanks.

 

[Ingenieur]

Should not matter
it's tuned for a freq
just higher current/power capacity

http://www.mtecorp.com/pages_lang/wp-content/uploads/rfi_emi.pdf

looks like the same PN for 125 thru 200 HP

 

[GoldDigger]

Should not matter
it's tuned for a freq
just higher current/power capacity

http://www.mtecorp.com/pages_lang/wp-content/uploads/rfi_emi.pdf

looks like the same PN for 125 thru 200 HP

:thumbsup:

For the same space and cost you can get better filtering if you have a lower load current requirement. But as long as the unit chosen meets your customers filtering specs, the worst that can happen is that it costs more.

 

[Cow]

Good deal, thanks guys!

 

[Jraef]

Very SLIGHT additional losses in an oversized filter, but I wouldn't hesitate if there was a stated purpose.

 

[Sahib]

Very SLIGHT additional losses in an oversized filter, but I wouldn't hesitate if there was a stated purpose.

Bigger size, smaller overall loss. No?

 

[Jraef]

Bigger size, smaller overall loss. No?

The copper losses are based on current, so they stay pretty much the same. But the current carrying components that changes with size are the inductors (chokes) and bigger inductors means slightly more magnetizing current. So the losses as a percentage of load are technically greater if those inductors are bigger for no good reason.

 

[Sahib]

The copper losses are based on current, so they stay pretty much the same.

Not so, my dear jraef, as copper loss will be lower due to lower resistance of bigger size filter.

 

[Jraef]

Not so, my dear jraef, as copper loss will be lower due to lower resistance of bigger size filter.

Yes, but the "copper" losses are I²R, so the effect of I is much more than the effect of R, and it is mostly the load that determines I, so not much difference. The higher magnetic losses from hysteresis and eddy currents in the unnecessarily larger choke will be greater than the slightly lower I²R losses.

But really, this is splitting hairs. It's not going to be much net difference as I said in the start.

 

[GoldDigger]

I think that the question asked was whether, for a given fixed load current (until possibly changed down the line) the losses would be greater for an oversized filter.
The answer, IMHO, is that it would depend on whether the resistive (copper) losses, which would be lower, or the inductive (iron) losses, which would be greater, dominate. If the filter were designed similar to transformers, making the copper and iron losses roughly equal at full load, the net difference might be rather small.

 

[Sahib]

I think that the question asked was whether, for a given fixed load current (until possibly changed down the line) the losses would be greater for an oversized filter.
The answer, IMHO, is that it would depend on whether the resistive (copper) losses, which would be lower, or the inductive (iron) losses, which would be greater, dominate. If the filter were designed similar to transformers, making the copper and iron losses roughly equal at full load, the net difference might be rather small.

I based my statement on the fact that higher is the capacity of an equipment, greater is its efficiency. So for a given output, the total loss will be lower, higher the equipment capacity as its efficiency is also higher.True, efficiency is maximum when copper loss is equal to iron loss, but that maximum efficiency increases with increase in equipment capacity.

 

[GoldDigger]

I based my statement on the fact that higher is the capacity of an equipment, greater is its efficiency. So for a given output, the total loss will be lower, higher the equipment capacity as its efficiency is also higher.True, efficiency is maximum when copper loss is equal to iron loss, but that maximum efficiency increases with increase in equipment capacity.

So if I need 10kW to feed my load, I can choose a 75kW transformer to feed it and thereby get greater overall efficiency?
I don't think that is necessarily so.

 

[Sahib]

So if I need 10kW to feed my load, I can choose a 75kW transformer to feed it and thereby get greater overall efficiency?
I don't think that is necessarily so.

Check how efficiency changes with load for a given equipment capacity and make your choice.

 

[Ingenieur]

in this case it's the same filter PN for 125-200 kva so moot

but in general if we assume a different filter PN for 125 and 200 (basically heavier conductors for increased ampacity) would not the X (LC) be the same since they are tuned for the same freq band?
and if X is the same wouldn't losses be the same since losses = I^2 X?
actually a bit lower for the larger one since R would be smaller since the conductors would be larger to increase current capacity?