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Soft-Starting Solenoid Coils

08/08/2009 7:18 PM

What is best practical method to soft start a AC or DC solenoid coils drawing heavy inrush currents while switching on ?

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#1

Re: soft starting solenoid coils

08/08/2009 10:36 PM

There are soft start circuits for motors already packaged up by a host of companies. (I recommend using the GlobalSpec search in the upper right corner of your window to look for one.) But solenoids are inductors. The current lags the applied voltage so the initial current is zero. In a capacitor the current leads the voltage. Connecting a capacitor to a voltage source will produce a huge current surge limited only by wiring and voltage source resistance. Most likely your problems though come from turning OFF the solenoid. The voltage spike to maintain current in the inductor can and will damage circuits that don't account for this. To guard against this you need a snubber network in parallel with the inductor.

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#4
In reply to #1

Re: soft starting solenoid coils

08/09/2009 1:53 PM

Let me disagree with the original poster. Qoute

....inductors. The current lags the applied voltage so the initial current is zero. In a capacitor the current leads the voltage. Connecting a capacitor to a voltage source will produce a huge current...

On the contrary, at the very first switching on an inductor appears as a short circuit and a capacitor as a discontinuity in the circuit.

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#6
In reply to #4

Re: soft starting solenoid coils

08/09/2009 4:39 PM

While I did misunderstand what presumably the original questioner wanted with soft-starting a solenoid, I did get the current vs. voltage of an inductor correct. Look at the current vs. voltage of an Inductor transient response. You will see that the current rises after the voltage (aka lags).

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#10
In reply to #4

Re: soft starting solenoid coils

08/09/2009 11:04 PM

Yes, let there be no question, at first connect, the current in an inductor is zero and ramps up according to I=1/LR. The poster's comment is incognizant of this and he may benefit from knowing that a DC solenoid does not have a high current inrush. An AC solenoid however does, since initially it has a much lower inductance which rises as the core enters the body of the coil thereby reducing current flow tremendously once fully engaged.

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#16
In reply to #4

Re: soft starting solenoid coils

08/10/2009 8:59 PM

Yuri,

In either case there is an inrush current for a capacitor connected across a Voltage source and an inrush current for an inductor connected across a Voltage source you and Redfred are focusing on timing due to lead and lag issues that have nothing to due with the Op's question about inrush limiting.

Jon

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#2

Re: soft starting solenoid coils

08/08/2009 10:52 PM

Soft starting of solenoid coils s a lot different that soft starting motors. The trouble with AC coils is that, as a magnetic device, there is a "knee point" in the magnetic field strength that has you either in a state where it is too weak to pull in, or over the edge and fully operating. There is no "soft" about it, even if you ramp the voltage; it's all or nothing.

But it can be done with DC coils and a little PWM circuit. Westinghouse years ago perfected a "soft closing" DC coil system for their Advantage Series motor starters and contactors (now Eaton / Cutler Hammer). The reason was not the inrush issue, but rather as a way to avoid contact bounce. Indirectly though, it softened the coil inrush as well. So what they did for AC control circuits was to build-in the PWM coil and miroprocessor controller; it was there, and the coil was DC, but the user never saw that. Many other contactor manufacturers use that technique now as well.

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#3
In reply to #2

Re: soft starting solenoid coils

08/09/2009 11:28 AM

Ah, so the "soft" part of this process applies to the motion of the slug, not the current in rush. Learn something new every day.

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#5

Re: Soft-Starting Solenoid Coils

08/09/2009 4:39 PM

Self heating thermistors in series with the solenoid coil have been used for many years for oil burner delayed opening fuel oil valves, likely if you have an oil burner to heat your home, it is fitted with such a valve.

When voltage is first applied the thermistor exhibits high resistance, as the thermistor heats the resistance decreases until sufficient current flows through the solenoid coil to open the valve, a simple,method of "Soft starting" a solenoid, relay, or contactor coil, works just as well on AC, or DC.

Regards CEKM

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#7
In reply to #5

Re: Soft-Starting Solenoid Coils

08/09/2009 4:50 PM

Possibly the inductance of some solenoids are significantly lower with the slug out of the ON position than with the slug in the OFF position. This lower inductance could draw more AC current, requiring the soft start approach you suggest. But more current just might be required to "grab" the slug sitting partially outside of the solenoid. But this should only be an effect with AC solenoids.

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#8
In reply to #7

Re: Soft-Starting Solenoid Coils

08/09/2009 5:03 PM

Redfred,

Your point is well taken, an an anecdote to your comment, I learned many years ago while doing service work when I wanted to defeat the operation of an AC powered solenoid by removing the coil to close the valve the coil would overheat. I learned to take with me a steel bolt, or other ferrous object about the size of the solenoid stem, and insert in to the coil to prevent burnout of the coil. I have passed this "Trick", on to many tech's that I have supervised, sometimes too late!.

Regards CEKM

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#11
In reply to #7

Re: Soft-Starting Solenoid Coils

08/10/2009 6:22 AM

Hi Redfred,

the inductance change is quite noticeable, to such an extent that dirt between the pole faces causes a sufficient increase of current to overheat the coil.

I have never had a problem with solenoids with regard to the inrush but I do know that the primary of a transformer does exhibit a low resistance to the source until the magnetic field has built up, so why not a simple coil?

Chas

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#12
In reply to #11

Re: Soft-Starting Solenoid Coils

08/10/2009 9:30 AM

The high inrush current of a transformer in a power supply is typically because of the initial charging of the capacitors in subsequent stages. I believe you'll find the inrush current of only the transformer to be negligible.

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#14
In reply to #12

Re: Soft-Starting Solenoid Coils

08/10/2009 5:26 PM

Ummm, tell that to transformer manufacturers!

There is an INITIAL inrush of current into any inductor, because until the inductance is established by the interaction of the magnetic fields, the coil is nothing less than a dead short circuit, limited only by the series resistance of the windings themselves! This inrush typically only lasts about 100ms at the most, but can easily be 12X the FLA rating of the coil for that instant. This by the way is the accepted industry standard for use in selecting transformer primary circuit protection, explained in detail in IEEE Std. 242

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#15
In reply to #14

Re: Soft-Starting Solenoid Coils

08/10/2009 8:48 PM

JRaef,

Sooo...initially the inrush current is limited by the resistance of the coil and as the voltage increases the current and flux increase rapidly then the current decreases due to entry of the plunger.

Transformer? Loaded or unloaded secondary? Does this look right? Does the current jump up like that as the flux goes into saturation. Is that much different than a solenoid inrush current waveform?

Jon

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#17
In reply to #5

Re: Soft-Starting Solenoid Coils

08/10/2009 9:08 PM

CEKM,

If the solenoid has just been operated

and you want, again to operate it

and the thermistor is still hot

it will not.

- Dr Suess

Jon

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#18
In reply to #17

Re: Soft-Starting Solenoid Coils

08/11/2009 1:30 AM

kudukweller9,

You are correct, I was offering a KISS (Keep It Simple Stupid) minimum parts count, inexpensive suggestion that works well for applications when a slight delay is not a significant factor, and quick repetitive operation is not necessary. My suggestion is not an elegant fits all solution.

CEKM

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#19
In reply to #18

Re: Soft-Starting Solenoid Coils

08/11/2009 3:12 AM

CEKM,

I wonder if this would work for repetitive inrush limiting for dc.

The Resister value determines the current limit. 1.25/R

It can output 10mA to 1.5A continously. It is internally limited at typically 2.2 Amps.

Inputing 3Volts higher than output can provide 1.2 to 37 volts output. There are LM150 3 amp and LM138 5 amp versions too. There is a negative regulator LM137.

AC can be rectified and filtered to use this to drive the solenoid.

For line voltages something else would have to be used possibly based on a triac circuit.

Jon

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#20
In reply to #19

Re: Soft-Starting Solenoid Coils

08/13/2009 5:33 AM

kudukdweller,

Yes, that circuit works well, I use a version of that circuit with a potentiometer for providing adjustable current limiting.

But why would it be needed on a DC coil since the inrush current is "softened" by the inductance of the coil, and the current actually opposed by the back EMF of the movement of the solenoid slug, the current reaching maximum after the coil and slug are magnetically saturated, then limited purely by resistance of the coil wire.

Not so with an AC coil where the inductive reactance changes drastically from low to high when the coil slug is pulled up into the coil. As I explained in an earlier reply, I learned this when defeating a valve operation by removing the coil from the stem and having the coil overheat and burn out. I now insert a steel bolt or other ferrous object in the coil when removed from the valve.

Regards CEKM

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#21
In reply to #20

Re: Soft-Starting Solenoid Coils

08/13/2009 4:39 PM

CEKM,

why would it be needed on a DC coil since the inrush current is "softened" by the inductance of the coil,

It was an answer for the fast repetitive thermistor inrush compensation. "If" there was an inrush problem with a dc driven coil the circuit would provide for rapid repitition where a heated thermistor would not.

Fast repetitive driving with ac would cause some heating too due to the period that the plunger is out. A solenoid that forces a plunger to the center instead of from the outside takes care of the ac inrush problem.

It seems that people are trying to work with what they have instead of having what works.

Jon

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#9

Re: Soft-Starting Solenoid Coils

08/09/2009 8:47 PM

It is normal for a AC or DC solenoid coils drawing heavy inrush currents while switching on. The inrush current needed to increase the magnetic flux of the coil to full magnetism in a split second so that plunger or the actuator can work as designed according to the manufacture design of these solenoid valves. Soft start of these solenoid coil or valve may cause unnecessary plant operation problem to the system due to the delay of the valve energizing.

All these solenoid coils inrush current had been calculated and design as according to the specification of the manufacture. So don't worry about these inrush current.

What is more importance is that you have to make sure that your power requirement is enough for these solenoid coils to operate.

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#13

Re: Soft-Starting Solenoid Coils

08/10/2009 1:58 PM

Sprajah,

We tend to think of inrush as having to to do with the first moment of the power it takes to stabilize the inner workings of electronic assemblies that have circuit elements that require Capacitive charging or like old radios that have low resistance filaments until they are hot. Or the power draw that momentarily dims my lights when I turn on my big Plasma TV

For the solenoid, inrush is the power applied until the plunger is pulled in. When the plunger is pulled in it draws much less power.

If the plunger doesn't move in the coil gets hot.

As a solenoid closes, the flow of current decreases. The peak INRUSH CURRENT in the coil when the solenoid is open is several times greater than the "solenoid closed" HOLDING CURRENT due to A.C. IMPEDANCE which increases as the solenoid closes.

An Oscillocope with a current probe wrapped onto the feedwire to the solenoid and a voltage probe connected to the wire we could see graphically the relationship of which you speak as the power is first applied and the subsequent inductive changes effected by the movement of the plunger into the solenoid coil.

The thermistor approach to inrush limiting may do the trick if a slight delay is not an issue.

Jon

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#22
In reply to #13

Re: Soft-Starting Solenoid Coils

08/13/2009 7:33 PM

Hi Jon,

ask, and it shall be delivered unto you!

the oscillagraphs show the same solenoid operation, the first at 20ms/ div and the second at 500µs. Upper traces are the voltage and the lower trace the current measured with a high freq current clamp. The voltage from a regulated PSU at 26V, the coil resistance was 20Ω.

The notch in the curve I suppose to be the slug bottoming out. (there is no economiser coil/contact) Note there is no inrush, it behaves exactly as per college course 40+ years ago. This wasn't too surprising as the current in a dc coil is limited by the resistance of the winding.

I started to set up a similar demo for a transformer winding but got interrupted, I will try tomorrow to post the results of that.

regards

Chas

Jon,

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#23
In reply to #22

Re: Soft-Starting Solenoid Coils

08/13/2009 8:27 PM

Nice Job Cap, you get a GA from me. I liked seeing the effect of the what must be the solenoid slug being pulled in and stopping (the notch I expect). Nothing beats a speculative argument like real data. When you test the transformer, don't forget to note the secondary circuit; i.e. open circuit, short circuit, diode bridge charging a capacitor, fixed resistor.

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#24
In reply to #22

Re: Soft-Starting Solenoid Coils

08/13/2009 8:27 PM

Capblanc,

Great!

Thank you so much.

That is what I would expect for the dc operated solenoid.

I am thinking the OP's inrush issue is for the ac operated solenoid.

He asked: "What is best practical method to soft start a AC or DC solenoid coils drawing heavy inrush currents while switching on ?"

So we need to address each one separately and it looks like the dc one is done.

Now the ac one that does have a significant inrush issue is in need of a little massaging.

Your ac driven solenoid and contactor graphs should look like this.

I would like to see the real loaded and unloaded transformer graphs. This article doesn't mention any output load effects.

When a transformer is initially connected to a source of AC voltage, there may be a substantial surge of current through the primary winding called inrush current. This is analogous to the inrush current exhibited by an electric motor that is started up by sudden connection to a power source, although transformer inrush is caused by a different phenomenon.

We know that the rate of change of instantaneous flux in a transformer core is proportional to the instantaneous voltage drop across the primary winding. Or, as stated before, the voltage waveform is the derivative of the flux waveform, and the flux waveform is the integral of the voltage waveform. In a continuously-operating transformer, these two waveforms are phase-shifted by 90o. Since flux (Φ) is proportional to the magnetomotive force (mmf) in the core, and the mmf is proportional to winding current, the current waveform will be in-phase with the flux waveform, and both will be lagging the voltage waveform by 90o:

Let us suppose that the primary winding of a transformer is suddenly connected to an AC voltage source at the exact moment in time when the instantaneous voltage is at its positive peak value. In order for the transformer to create an opposing voltage drop to balance against this applied source voltage, a magnetic flux of rapidly increasing value must be generated. The result is that winding current increases rapidly, but actually no more rapidly than under normal conditions:

Both core flux and coil current start from zero and build up to the same peak values experienced during continuous operation. Thus, there is no "surge" or "inrush" or current in this scenario.

Alternatively, let us consider what happens if the transformer's connection to the AC voltage source occurs at the exact moment in time when the instantaneous voltage is at zero. During continuous operation (when the transformer has been powered for quite some time), this is the point in time where both flux and winding current are at their negative peaks, experiencing zero rate-of-change (dΦ/dt = 0 and di/dt = 0). As the voltage builds to its positive peak, the flux and current waveforms build to their maximum positive rates-of-change, and on upward to their positive peaks as the voltage descends to a level of zero:

A significant difference exists, however, between continuous-mode operation and the sudden starting condition assumed in this scenario: during continuous operation, the flux and current levels were at their negative peaks when voltage was at its zero point; in a transformer that has been sitting idle, however, both magnetic flux and winding current should start at zero. When the magnetic flux increases in response to a rising voltage, it will increase from zero upwards, not from a previously negative (magnetized) condition as we would normally have in a transformer that's been powered for awhile. Thus, in a transformer that's just "starting," the flux will reach approximately twice its normal peak magnitude as it "integrates" the area under the voltage waveform's first half-cycle:

In an ideal transformer, the magnetizing current would rise to approximately twice its normal peak value as well, generating the necessary mmf to create this higher-than-normal flux. However, most transformers aren't designed with enough of a margin between normal flux peaks and the saturation limits to avoid saturating in a condition like this, and so the core will almost certainly saturate during this first half-cycle of voltage. During saturation, disproportionate amounts of mmf are needed to generate magnetic flux. This means that winding current, which creates the mmf to cause flux in the core, will disproportionately rise to a value easily exceeding twice its normal peak:

This is the mechanism causing inrush current in a transformer's primary winding when connected to an AC voltage source. As you can see, the magnitude of the inrush current strongly depends on the exact time that electrical connection to the source is made. If the transformer happens to have some residual magnetism in its core at the moment of connection to the source, the inrush could be even more severe. Because of this, transformer overcurrent protection devices are usually of the "slow-acting" variety, so as to tolerate current surges such as this without opening the circuit.

Jon

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#25
In reply to #24

Re: Soft-Starting Solenoid Coils

08/19/2009 12:35 AM

Capblanc

Thanks for the detailed information, I learned a lot about what I had suspected, was a good explanation.

CEKM

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#26
In reply to #25

Re: Soft-Starting Solenoid Coils

08/19/2009 12:41 AM

CEKM,

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