Transformer Inrush

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mull982

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I have a control cabinet which is using a 3kVA 480V to 240V single phase transformer to provide power to several control relays in order to power field solenoid devices. The relays are set up such that a PLC fires two relays, a "solenoid relay" and "master relay" at the same time in order to provide power to the field soleniod when needed. After the master relay there is a 240V bridge rectifier in order to convert the AC power to DC to power the solenoids. The master relay is used to provide power from the transformer to the rectifier and then through the appropriate solenoid relay in order to provied power to the solenoid.

The 3kVA transformer has a 10A circuit breaker as well as 10A instantaneous fuses on its primary. We keep blowing the primary fuses for some reason during operation of the relays. The secondary side of the transformer does not supply power unless two of the relays mentioned above are pulled in, and this only lasts for about 500ms or so.

My questions is, when the two relays get pulled in, does this present an inrush to the secondary and primary of the transformer? When trying to measure current on the primary it is hard to get an accurate reading because everything is happening so fast, but using a fluke meter I was seeing 12 and 13A on the primary. With a 3kVA transformer I would expect to see 6.2A on the primary. Why was I seeing 12 and 13A on the primary, was this due to an inrush on the primary of the transformer? What size primary fuse should be used on a 3kVA transformer?

I know this is confusing but I would appreciate any help.
 
Yes it is possible you are getting large inrush currents that last for less than the 500ms time. Often inrush currents will only last a few cycles.
Your meter may not be fast enough to record the actual peaks. I would use a scope with a current probe to measure this.


Next you should check the data sheet (time curves) of the 10 amp fast blow fuses and see how long they will last with various inrush currents.

Is the primary turned on and off very often? You also get inrush to the transformer primary each time it is energized and may be weakening the fuse.

I have never found a fast blow fuse to be a good combination with transformer primaries (depending upon the fuse type and time curve of course).
 
Control transformers (depending upon manufacturer) has 25 to 50 X the FLA.
What kind & Class of fuse are you using? Class CC you can use at 300% and if it doesn't hold you can go up to 400% per the NEC
Just my $.02
 
Transformer inrush current only occurs when the transfromer primary is first energized. Depending upon the style of your transfromer the inrush will vary from maybe 6x FLA all the way up to 12X. For a standard power transformer I would use primary time delay fuses sized at 1.75 x FLA. For a machine tool transformer I would check with a fuse manufacturer.

But, if your secondary load also has an inrush associated with it the transformer primary will have to supply that current, again maybe 14x full load for a solenoid, whenever that load is applied. So, in your case it looks like your the inrush of your solenoids is causing an increase in your primary current (not really an inrush). The solution may be as simple as providing time delay fuses on the primary.
 
Could you give a bit more info please.

I assume there is one Master Relay and several Solenoid Relays. The Master always fires, but only one Solenoid Relay fires simultaneously.

What is the nameplate DC Volts and Watts on the Solenoids?

Are you saying the whole event only lasts for 500ms?

Are there filter capacitors after the Bridge Rectifier?

Is this a new installation or has it been working for a while?
 
You are likely blowing the fuses on inrush. The solenoids draw more current when the are moving the contacts than when they are holding them in place, and one can easily blow fuses if they are of an instantaneous type.

For a 3KVA 480V transformer I would expect a 10A fuse there using a primary only setup, or a 15A if using a primary+secondary setup. If you're blowing fuses, I have a couple comments.

1. Change the transformer protection fuses you are using to use a fuse with a long time delay such as a Bussman FRN/FRS series.

2. If you must protect the diode bridge, then you will need to use supplementary high-speed semiconductor fuses, sized properly to protect the diode according to it's ratings.

I've used this approach successfully on a similar applications, where I had DC contactors being powered through a transformer and bridge, and it was necessary to fuse the bridge per customer requirement.
 
jim dungar said:
Transformer inrush current only occurs when the transfromer primary is first energized. Depending upon the style of your transfromer the inrush will vary from maybe 6x FLA all the way up to 12X. For a standard power transformer I would use primary time delay fuses sized at 1.75 x FLA. For a machine tool transformer I would check with a fuse manufacturer.

But, if your secondary load also has an inrush associated with it the transformer primary will have to supply that current, again maybe 14x full load for a solenoid, whenever that load is applied. So, in your case it looks like your the inrush of your solenoids is causing an increase in your primary current (not really an inrush). The solution may be as simple as providing time delay fuses on the primary.
Click to expand...

Jim

It sounds like what your are saying, is that the primary of a transformer only has an inrush on the primary when the transformer is first energized, ie. power is supplieed to the transformer? This inrush can be 6-12 x FLA only lasting for a couple of cycles. From what youve stated it sounds like this inrush is only exibited when the transformer is first powered, and not during any other time during normal operation of the transformer?

What you also mentioned was that an inrush on the solenoid ( solenoid magnetizing inrush) on the secondary side may lead to an increase in current on the primary side, but this is not a primary inrush current but rather just an increase of current on the primary. Should this current on the primary raise above the 6.2A for which the 3kVA transformer is rated for? What is the maximum safe current that the primary should draw as a result of an action on the secondary?

As I mentiond in my first post I took some readings on the primary of the transformer. When none of the relays on the secondary are pulled in thus no solenoids are powered I read about 0.4A on the primary. This 0.4A is probably the result of electronics (PLC, HMI) on the secondary of the transformer. When the solenoids are firing, I am measuring up to 12 and 13A on the primary of the transformer? You are saying that this 12-13A reading is coming as a reslult of the secondary current inrush to the solenoids, and is a current increase on the primary but not an inrush? The current draw on the solenoids is about 20-25A on the secondary. Could it be possible that I am not even catching the full current increase on the primary becuse of everything happening so fast?

Just looking at the primary inrush alone on the transformer, shouldn't this have blown the instantaneous fuse during inrush if you are saying that inrush is 6-12X FLA?
 
stickelec said:
Could you give a bit more info please.

What is the nameplate DC Volts and Watts on the Solenoids?

Are you saying the whole event only lasts for 500ms?

Are there filter capacitors after the Bridge Rectifier?

Is this a new installation or has it been working for a while?
Click to expand...

Nameplate DC volts is 240V DC. Not sure of the watts but average current draw is 20-25A.

When I put a scope on the output and trigger the whole output waveform, it only appearst to last for about 500ms (.5s)

No there are not filter caps after the bridge rectifier.

This is a new installation.
 
mull said:
Jim

It sounds like what your are saying, is that the primary of a transformer only has an inrush on the primary when the transformer is first energized, IE. power is supplied to the transformer? This inrush can be 6-12 x FLA only lasting for a couple of cycles. From what youve stated it sounds like this inrush is only exibited when the transformer is first powered, and not during any other time during normal operation of the transformer?

What you also mentioned was that an inrush on the solenoid ( solenoid magnetizing inrush) on the secondary side may lead to an increase in current on the primary side, but this is not a primary inrush current but rather just an increase of current on the primary. Should this current on the primary raise above the 6.2A for which the 3kVA transformer is rated for? What is the maximum safe current that the primary should draw as a result of an action on the secondary?

As I mentiond in my first post I took some readings on the primary of the transformer. When none of the relays on the secondary are pulled in thus no solenoids are powered I read about 0.4A on the primary. This 0.4A is probably the result of electronics (PLC, HMI) on the secondary of the transformer. When the solenoids are firing, I am measuring up to 12 and 13A on the primary of the transformer? You are saying that this 12-13A reading is coming as a reslult of the secondary current inrush to the solenoids, and is a current increase on the primary but not an inrush? The current draw on the solenoids is about 20-25A on the secondary. Could it be possible that I am not even catching the full current increase on the primary becuse of everything happening so fast?

Just looking at the primary inrush alone on the transformer, shouldn't this have blown the instantaneous fuse during inrush if you are saying that inrush is 6-12X FLA?
Click to expand...

The transformer primary current will be whatever level is needed to supply the current the secondary load wants (up to the point that the transformer melts). So yes, a 20-25A secondary current would cause your primary to be 12-13 amps.

The magnetizing inrush current of a power transformer does not last very long so even with a 6-12x FLA inrush most protective devices can handle it. However, for control power transformers the 14-20x inrush is more difficult to accommodate without time delay device.
 
mull982 said:
Nameplate DC volts is 240V DC. Not sure of the watts but average current draw is 20-25A.

When I put a scope on the output and trigger the whole output waveform, it only appearst to last for about 500ms (.5s)

No there are not filter caps after the bridge rectifier.

This is a new installation.
Click to expand...

We probably need Jim or one of the other Engineers to comment, but I think due to the DC Ripple the 240vac to the bridge will be about 214vdc.

If my guess is correct, the current required for the DC Solenoid could be as high as 28 amps.
 
jim dungar said:
The transformer primary current will be whatever level is needed to supply the current the secondary load wants (up to the point that the transformer melts). So yes, a 20-25A secondary current would cause your primary to be 12-13 amps.
.
Click to expand...

Jim

What is typically the "safe" operating level of a transformer, or what you refer to as "the point that the transformer melts" Is this a particular current value based on transformer kVA, or does this have to do with both time and current such as what would be seen on a transformer thermal damage curve? What would you expect this level to be on the 3kVA transformer that I am referencing?

When a transformer is energized on the primary but has no secondary load on it, what magnitude of current is being drawn by the primary of the transformer? You have mentioned the inrush current or "magnetizing current" lasting a for a very short time, but after that inrush what kind of current is the primary drawing when sitting idle with no load on the secondary? Is there a magnetizizing current still on the primary of the transformer when sitting idle? Is this what is refered to as kVAR? Can this be measured?
 
mull982 said:
Jim

What is typically the "safe" operating level of a transformer, or what you refer to as "the point that the transformer melts" Is this a particular current value based on transformer kVA, or does this have to do with both time and current such as what would be seen on a transformer thermal damage curve?
Click to expand...
Yes, it depends on the thermal damage curve.

When a transformer is energized on the primary but has no secondary load on it, what magnitude of current is being drawn by the primary of the transformer? You have mentioned the inrush current or "magnetizing current" lasting a for a very short time, but after that inrush what kind of current is the primary drawing when sitting idle with no load on the secondary? Is there a magnetizizing current still on the primary of the transformer when sitting idle?
Click to expand...
Yes, there will always be some amount of primary current, even though it is very minor, used to maintain the magnetic field. The result of maintaining this field is the "core loss" of the transformer.

KVAR is a different animal and is not part of this discussion.
 
Magnetizing current:

Magnetizing current:

Transformers require a steady-state magnetizing current for their operation.

The equivalent circuit of the transformer comprises a shunt inductor, Lp, and a shunt resistor, Rc. The inductor current provides the magnetic field and uses a small amount of reactive power (KVARs). The resistor uses a bit of real power (core loss measured in KW).
 
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