Inrush Current Mitigation

Thread Starter

powerfade

Joined Jan 19, 2022
34
I was hoping I could get some insight as to how I mitigate an inrush current situation that I'm experiencing. My current set up is 208v 3phase feeding a 75KVA transformer to boost the voltage to 480v to run a CNC waterjet. The current service in the shop is 100amp. I've done some research and sort of understand that when I shut the breaker off, one of the coils typically becomes energized, because it's not at the 0 level leading to the inrush current the next time I turn on the breaker which blows one of the 100amp fuses at the main. I know that's probably a very bad description, but I'm by no means an EE. I've read about Control Switching Devices, Phase Syncing Devices, soft starters. I'm trying to wrap my head around what I would need to do so that I don't keep blowing fuses everytime I switch the breaker on. Appreciate any input.

Thanks
 

seanstevens

Joined Sep 22, 2009
105
Welcome to AAC!
I have used soft starters using a MOSFET to slowly pass the current through to the rest of the system, also have used low-value thermistors doing the same kind of thing on low power devices but those were all DC circuits. For AC and especially high power devices such as yours and 100A, I would imagine active soft starters would be more difficult than using a relay and resistor/thermistor type of inrush controller such as the one in figure 6 of this link:
https://sound-au.com/articles/inrush.htm

But there are a lot of clever folks here and I am sure someone will help.
 

Marley

Joined Apr 4, 2016
470
I'm not sure what you mean by:
when I shut the breaker off, one of the coils typically becomes energized, because it's not at the 0 level leading to the inrush current the next time I turn on the breaker which blows one of the 100amp fuses at the main
Hopefully you have a 3-phase MCB (circuit breaker) in the feed to this transformer. This should be of a rating (A) and type (current - time curve) more sensitive than the main fuses so this always trips first.

There are many ways to reduce the inrush current. One way is to connect the transformer via 3 series resistors (1 in each phase) that limits the current to a value that will not trip the circuit breaker (at least not straight away). A short time later, these resistors are shorted out using a contactor (a power relay). This time would be a second or so.

The resistors would be high power types, probably around 1Ω or so. Able to carry 200A for a short time. You would probably have to arrange some kind of overheat cut-out in the event of the shorting contactor not operating. The contactor would be operated by a delay-on timer.
 
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MisterBill2

Joined Jan 23, 2018
11,612
I am not sure how a soft starter would even work feeding a transformer. They are intended for use with motors, at least the ones that I have dealt with.
And I find this statement rather puzzling: " when I shut the breaker off, one of the coils typically becomes energized, because it's not at the 0 level leading to the inrush current the next time I turn on the breaker which blows one of the 100amp fuses at the main " There may be a bit of residual magnetism, but nothing else left in a transformer after the power is switched off.

I have seen some poor designs where only two of the three phases are disconnected, and I regard that as a totally stupid trick to save a bit of cost. And no apology for offending those who see no problem with it!

So I am wondering if the input to the CNC machine has alternate input voltage possibilities by means of transformer connection changes. Or does the CNC machine have 480 volt motors? If not, than a change of the transformer could be the solution
 

LowQCab

Joined Nov 6, 2012
1,930
A 100-Amp, 208/120-Volt-Panel is downright cheezy for a production facility.
I didn't even know they made them that small.
3-Phase-Panels usually start-out at 200-Amps.

The only other viable alternative to the Resistors is to up-grade your Electrical Service.
Probably the most cost-effective way to do that is to have a second Meter and Panel installed.
You may even be able to get a 480-Volt Service brought into the Building,
which would eliminate the need for the Step-Up-Transformer altogether.
This would probably save You around 10% on your Electric Bill,
and free-up a good sized chunk of floor-space as well.

You may also have other equipment that could benefit from
a conversion to 480/277-Volt 3-Phase Power.

Talk to a COMMERCIAL-ONLY Electrical-Contractor, ( no Residential-Companies ).

It may also be possible to rewire the Motor(s) and Electronics in
your Water-Jet for 208-Volt operation
.
.
.
 

Tonyr1084

Joined Sep 24, 2015
6,866
For what limited experience I have I'll say this: When working in the aircraft industry, one function test I was assigned was to test the radio rack cooling fans. Each fan was three phase and each single phase had its own breaker. Aircraft breakers are not like the breakers in the home or shop panels, you have to energize all three at once otherwise the first one energized will pop back out before you can get the other two energized. When I first did the test I thought there was a problem because the breakers kept popping. I troubleshot the whole system more than once and couldn't find any faults. So I concluded the fan was bad. When I requested a replacement part I was asked if I was energizing all three breakers at the same time. "No." I said. "Try it" was the response. It worked. I spent days working on that system all for nothing. But I did learn something. It looks like you may be in the same learning curve.

Marley said:
Hopefully you have a 3-phase MCB (circuit breaker) in the feed to this transformer.
Yes - absolutely. If you have individual breakers and try to switch each one independently you WILL blow the fuses. Do you have a single "Three Phase" breaker? Or are they individual breakers? Individual breakers should produce the effect you're seeing. I say "Should" but I don't know that for sure. On three phase - I am limited in experience. Very limited.

And MisterBill2 said:
I am not sure how a soft starter would even work feeding a transformer. They are intended for use with motors, at least the ones that I have dealt with.
Again, yes - absolutely. You don't soft start a transformer. There's no reason to. A transformer is simply a coil of wire with a ferrous core and another coil of wire. Depending on the number of coil-turns and the size of the wire you get a transformer that either transforms the voltage up or down to the desired voltage and current. 10 amps at a 120 volt to 12 volt transformer would ideally drop the voltage to 12 volts and 100 amps (in a perfect world). That's all a transformer does. There's no moving parts, hence no need for a soft start. A motor is a different animal. When it starts - at first it appears as an extreme electrical load because the motor has barely begun to spin. As the motor spins up the load on the electrical supply drops because the motor is spinning. In such a case you'd want a soft start - for a motor. But not a transformer.
 

Thread Starter

powerfade

Joined Jan 19, 2022
34
I'm not sure what you mean by:


Hopefully you have a 3-phase MCB (circuit breaker) in the feed to this transformer. This should be of a rating (A) and type (current - time curve) more sensitive than the main fuses so this always trips first.

There are many ways to reduce the inrush current. One way is to connect the transformer via 3 series resistors (1 in each phase) that limits the current to a value that will not trip the circuit breaker (at least not straight away). A short time later, these resistors are shorted out using a contactor (a power relay). This time would be a second or so.

The resistors would be high power types, probably around 1Ω or so. Able to carry 200A for a short time. You would probably have to arrange some kind of overheat cut-out in the event of the shorting contactor not operating. The contactor would be operated by a delay-on timer.
I'm not sure what you mean by:


Hopefully you have a 3-phase MCB (circuit breaker) in the feed to this transformer. This should be of a rating (A) and type (current - time curve) more sensitive than the main fuses so this always trips first.

There are many ways to reduce the inrush current. One way is to connect the transformer via 3 series resistors (1 in each phase) that limits the current to a value that will not trip the circuit breaker (at least not straight away). A short time later, these resistors are shorted out using a contactor (a power relay). This time would be a second or so.

The resistors would be high power types, probably around 1Ω or so. Able to carry 200A for a short time. You would probably have to arrange some kind of overheat cut-out in the event of the shorting contactor not operating. The contactor would be operated by a delay-on timer.
I'm not sure what you mean by:


Hopefully you have a 3-phase MCB (circuit breaker) in the feed to this transformer. This should be of a rating (A) and type (current - time curve) more sensitive than the main fuses so this always trips first.

There are many ways to reduce the inrush current. One way is to connect the transformer via 3 series resistors (1 in each phase) that limits the current to a value that will not trip the circuit breaker (at least not straight away). A short time later, these resistors are shorted out using a contactor (a power relay). This time would be a second or so.

The resistors would be high power types, probably around 1Ω or so. Able to carry 200A for a short time. You would probably have to arrange some kind of overheat cut-out in the event of the shorting contactor not operating. The contactor would be operated by a delay-on timer.
So the breaker is not 3 Phase MCB breaker, but rather a Siemenes BL 3 Phase Type BL 100amp breaker. Sounds like I would need to change that. I had a friend who is a residential electrician wire it up for me.

I believe the motor is a 480v 30 HP motor. Thank you all for your input. I am learning so much!!!
 
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Thread Starter

powerfade

Joined Jan 19, 2022
34
For what it's worth, when I moved into the shop, the panel stated that there was 200amp service in my unit, but l didn't realize until later that it was only 100amp service.

Also will a MCB breaker fit in a standard panel that uses Siemens Bolt on Circuit Breakers?

Thanks again everyone!
 

Tonyr1084

Joined Sep 24, 2015
6,866
View attachment 258188This is the current breaker in the panel.
Since it is a simultaneous trip breaker it should be fine. If you're seeing voltages AFTER you shut it off then there's possibly something seriously wired wrong. Either that or the breaker is malfunctioning. I've never heard of a breaker malfunction - but anything is possible. But shutting off the breaker should not be blowing main fuses.
 

Thread Starter

powerfade

Joined Jan 19, 2022
34
Since it is a simultaneous trip breaker it should be fine. If you're seeing voltages AFTER you shut it off then there's possibly something seriously wired wrong. Either that or the breaker is malfunctioning. I've never heard of a breaker malfunction - but anything is possible. But shutting off the breaker should not be blowing main fuses.
The exact way thing that happens is:

Turned on circuit breaker and main 100amp fuse blew on leg one. Put a new fuse in and I am able to turn everything on and all works fine. Week later turn on the circuit breaker again and fuse blew out. Put new fuse in and all works again. 3rd time, went to turn on circuit breaker fuse blew again, but this time on leg 2 of the 3 phase. Put a new fuse in and all works fine. Haven't turned on again because I assume I'm going to blow another fuse.

Hope that makes sense.
Thanks so much
 

Tonyr1084

Joined Sep 24, 2015
6,866
Sorry for asking dumb questions but what is the amperage rating of the fuses? I ask because it doesn't make sense unless something is incorrect or wired incorrectly - or the wrong fuses are in place. I don't know if they should be (or if there is such things as) fast blow or slow blow fuses in that size. But fuses are blowing for some reason. It's best to get to the bottom of it before you end up hurting something. Or burning something down. Or killing someone. You're talking about severely dangerous voltages. I've been hit with 110 a number of times but have never had the pleasure of experiencing 220. Or anything higher. Except the snap of an ignition coil on an automobile.
 

DC_Kid

Joined Feb 25, 2008
924
My guess, wrong sized breaker or type/size of fuse.
Can you amp-clamp a line to see what the amps on an inrush event?
Next best advice is, get an electrician who has experience in 208/480 wye and delta to look at it.
 
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Thread Starter

powerfade

Joined Jan 19, 2022
34
Sorry for asking dumb questions but what is the amperage rating of the fuses? I ask because it doesn't make sense unless something is incorrect or wired incorrectly - or the wrong fuses are in place. I don't know if they should be (or if there is such things as) fast blow or slow blow fuses in that size. But fuses are blowing for some reason. It's best to get to the bottom of it before you end up hurting something. Or burning something down. Or killing someone. You're talking about severely dangerous voltages. I've been hit with 110 a number of times but have never had the pleasure of experiencing 220. Or anything higher. Except the snap of an ignition coil on an automobile.
The amperage on each of the fuses is 100amp. I've looked for slow blow fuses, but unable to find them. Here is a reference pic of the fuse. I was thinking somehow the inrush current from the transformer was causing the failure, but not sure now.jlln100-1.jpg
 

ThePanMan

Joined Mar 13, 2020
357
If the MAIN'S are 100 amp and you're switching a 100 amp load - any other circuitry that might be drawing current will easily put the fuse over limit and blow it. Breakers tolerate higher startup current to a degree. A sustained 100 amp draw shouldn't blow a breaker but it might blow a fuse.
 
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