Inrush Considerations

I am building a small machine that is equipped with stepper motors. The power supply for the stepper drivers is 48v at 12.5a but the thing has a 20ms inrush of 50a when turned on. I haven't designed the electrics around this inrush value. Will this affect anything greatly?

I have installed various transformers in the past and have used time delay fuses on the primary just to deal with things like this.

Stuart

If its a problem, find a dead 300 watt or larger computer power supply, pull the NTC resistor out of it and install it in series with the ac line for your power supply. You can use a terminal block to hold it, they get hot so keep pvc insulated wires away from touching it.

Afaik din rail mount ntc inrush prevention devices are available but i have not gone looking for them.

What you can do for that is install a series resistor in the power line. Then set up a relay to bypass the resistor.

The relay should not close until the voltage on the 48V bus is up to perhaps 30 to 36 volts. That way the inrush will be significantly cut, although 50 A is actually not that bad. I'd bet the 50A is all that would register on your meter, but the actual peak is quite a bit more.

The new inrush current will be limited by the combination of the series resistor and the winding resistance of the transformer, vs the mains voltage. The resistor has to be a low enough value that the output voltage will actually reach the relay closure trip point.

Instead of a resistor, I like to use the NTC inrush protectors, because they are rated for pretty high peak power, without being physically large.

atomarc said:

I am building a small machine that is equipped with stepper motors. The power supply for the stepper drivers is 48v at 12.5a but the thing has a 20ms inrush of 50a when turned on. I haven't designed the electrics around this inrush value. Will this affect anything greatly?

I have installed various transformers in the past and have used time delay fuses on the primary just to deal with things like this.

Stuart

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Depends on what you plan as a PSU.

At only 20 ms and a mere 4:1 ratio, you may not need to do anything at all. Most any OTHER load on-planet has some form of "inrush" and PSU just have to deal with that anyway. PSU with uber-sophisticated regulation and protection a possible exception. IOW - "dumber" can be better for this sort of situation.

How expensive in time and cash would it be to dummy-up workalike parts, try that and see if it is even a problem atall?

You didn't specify what type of circuit your using to feed the power supply, but I don't think you have much to worry about. There are tripping curves available for breakers and fuses, that show tripping points for instantaneous magnetic tripping levels and longer time thermal tripping values.

SqD breakers are one of the most responsive breakers in the industry. So looking at the trip Curves for their QO series would be a good example to use for comparison. You could also look up Bussman fuses if you plan on using standard fuse protection for their curves.

In this document, on page 20 is the curve for the standard QO breaker.
static.schneider-electric.us/docs/Circuit%20Protection/Miniature%20Circuit%20Breakers/QO-QOB%20Circuit%20Breakers/0730CT9801R108.pdf

It shows that at 3 seconds, the thermaltrip level is 3 times the breaker rating minimum and abut 6.5X maximum.

So if your using a 20A 120V circuit, you can draw up to 60A for 3 seconds, before you get into the thermal trip region, and much higher for the instantaneous magnetic trip region. Way longer than 20 milliseconds. Other protection devices will have their own curves, so you may want to look at a few others to ensure that you are covered, if you plan on marketing the machine.

SAF Ω

Soft start power supply circuit if you know how to work with electronic devices. Better than brute force methods.

In rush can be a significant issue, but I don't think it is an issue in your case. I built a 10KW isolation transformer for a sailboat several years ago and I solved the issue with a simple light bulb in series with the in feed line and a switch that that shorts across the light bulb as a start/run set up. It works perfectly.

atomarc said:

I am building a small machine that is equipped with stepper motors. The power supply for the stepper drivers is 48v at 12.5a but the thing has a 20ms inrush of 50a when turned on. I haven't designed the electrics around this inrush value. Will this affect anything greatly?

I have installed various transformers in the past and have used time delay fuses on the primary just to deal with things like this.

Stuart

Click to expand...

Get some ptc inrush limiters for a couple bucks at digikey and put in series with one side of ur ac power input. U can parallel them for higher current.

Sent from my SM-G900V using Tapatalk

The circuit is protected with a double pole 30 amp 'B' curve breaker which I believe will handle that inrush just fine. My real concern was a enclosed power relay in that same circuit that's rated for 30 amps which doesn't indicate whether it will tolerate that inrush without ill affects.

This isn't a commercial product..my use only.

This particular switching power supply is (input) 120v/240v selectable. While inrush at 240v is 50a, inrush at 120v is only 25a. Seems like I could merely wire it for 120v and get the same output but half the inrush.

Stuart

The breaker is NOT the only, or even the most important consideration regarding inrush current.

The inrush for a given voltage is dependent on the resistance vs voltage. For a switching power supply, there is no particular limit aside from the ESR of the capacitors. You will NOT get the lower inrush by setting to 120V if you have it operating at 240. You WILL get other problems, however.

Putting a regular NTC inrush protector in series, if it is properly rated, will definitely help with the inrush current. It MAY cause operational problems, however. Maybe less so with a switching supply than with a transformer based supply.

The problem is that the inrush protector has a relatively high resistance when cold, to limit the current, and goes down in resistance when it heats up. A properly rated one will go to a fairly low resistance with the "normal current" in the circuit.

But, if the circuit has intermittent high currents, as a stepper supply may have, then the protector (the "NTC") may be allowed to cool off and go to a high resistance. At that point, a high current drain may lead to a drop in voltage, which can be enough to bother the circuit. If you have a switching supply, that is rated for any voltage from 90 to 264, as some are, that may be no issue at 240V (might be a problem at 120V).

But if the supply is switchable, either to 120 OR 240, then it has little reserve, and the voltage drop may be an issue, the supply may not be able to maintain output voltage without sags. With transformer based power supplies, the voltage drop is even more of a problem, and can lead to problems in operation of the steppers.

Inrush protectors also have ratings which must be taken into account. They are often rated for the connected capacitance and voltage. The capacitance tells the energy (current * time) that the thing has to handle at the operating voltage. If you have a transformer in series, that can be difficult to figure, but with a switching supply you know tha capacitance. If you do not use a properly rated part, it will fail, probably failing "open".

As for the real problem.... The rectifiers and capacitors in the power supply take a big hit when the inrush occurs. That shortens life, and can lead to a failure. The capacitors "do not like" huge current surges. Rectifiers have limits also, although they often can stand more.

So there are reasons for using some form of inrush limiter even if the breaker is OK. And there are reasons for using a relay to switch the protector out when the power supply voltage has come up.

VFDs and other drives can have this issue too. Huge inrush to charge up the bus capacitors. The larger drives will have a pre-charge circuit to slowly (in electrical terms) charge the caps before turning things over to the full load capability.

I currently have three drives in the shop that are powered all the time. I have always felt this was the better option than constantly cycling power on and off. I just may do that with this power supply as there is also a small drive in the machine.

Leave a protected circuit ON that has the drive and power supply, and 'control' the other circuit via a panel pushbutton through a power relay for the other circuit.

The master disconnect will kill power to the entire enclosure when the machine is down for extended periods of time or access to the panel is needed.

This would alleviate the inrush to both the power supply and the VFD every time the operator hits the 'control power' button on the panel.

Right now..this sounds like it would work OK!

Stuart

Don't know about your supply, but many switchers will not start correctly, and can even fail, if you try to limit the inrush current. I would never put a soft start device on a switcher without consulting the manufacturer.

Conrad Hoffman said:

Don't know about your supply, but many switchers will not start correctly, and can even fail, if you try to limit the inrush current. I would never put a soft start device on a switcher without consulting the manufacturer.

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If a packaged SMPS needs inrush protection, it will likely already have it. If you then add a second level of it, you may cause the original protection, which may be a resistor and relay, to fail, due to the voltage getting high enough for the device to work, but not high enough for the resistor to be cut out by the relay, so that it tries to pull full current through the resistor.

Most any decent SMPS should not have that problem, since the relay is operated by the low voltage supply and controls, so if the unit can operate, the relay will have closed. If the voltage is too low, the controls know that and will not run the unit,

If the unit does NOT have any built-in protection, it may not need it, but is not so likely to fail. It may not work with the external protector in place, depending.

Transformer based supplies can have the same issues with excessive inrush, especially at 48V and above.