Understanding active inrush current limiter

Thread Starter

coolro18

Joined Jul 10, 2020
17
I came across this device and was interested in understanding its functionality,
https://www.mouser.com/ProductDetail/MEAN-WELL/ICL-16R?qs=vLWxofP3U2wgD7jPA%2Bx9JQ==

Could something like this work with an AC motor that has extremely high inrush current? I wasn't sure what the datasheet meant by 23A inrush limiting current, whether that was the max this device could handle or the cap on output given that the running load is <16A.
 

MaxHeadRoom

Joined Jul 18, 2013
20,894
The specs mention a bypass relay, and a set current limit, I would imagine there is some form of active circuit used to limit the current to the 23a, which is then bypassed by the relay mentioned after a certain time or when the inrush has subsided.
IOW the maximum inrush experienced is limited to 23A
Max,
 

Thread Starter

coolro18

Joined Jul 10, 2020
17
The specs mention a bypass relay, and a set current limit, I would imagine there is some form of active circuit used to limit the current to the 23a, which is then bypassed by the relay mentioned after a certain time or when the inrush has subsided.
IOW the maximum inrush experienced is limited to 23A
Max,
So, a spike of 100A during the inrush cycles would be compatible with its circuitry?
 

andrewmm

Joined Feb 25, 2011
389
This device is basicaly a NTC , with a delay turn on solid state relay across the NTC.
At start up, the NTC is high resistance, and slows the surge, as the NTC warms up, the curren tcan rise. After a period , the NTC is bypassed.

Its VERY expensive for what it is .
 

shortbus

Joined Sep 30, 2009
7,760
Could something like this work with an AC motor that has extremely high inrush current?
So, a spike of 100A during the inrush cycles would be compatible with its circuitry?
The "inrush" of any electric motor is what the motor takes to start turning from a dead stop. Where do you assume this "100A spike" is going to come from? Not much can stop that from coming in from the mains, but it has nothing to do with the "inrush current" when you start a motor. The only thing that will stop the overload of the motor staring is a fuse or breaker , and if that is what is happening you need a circuit with both a heavier wire gauge and then a larger amp fuse or breaker, NEVER just add a higher amperage device to an existing circuit without making sure the wire is heavy enough.

Have a read of -
Quote, " When an electric motor, AC or DC, is first energized, the rotor is not moving, and a current equivalent to the stalled current will flow, reducing as the motor picks up speed and develops a back EMF to oppose the supply. AC induction motors behave as transformers with a shorted secondary until the rotor begins to move, while brushed motors present essentially the winding resistance. The duration of the starting transient is less if the mechanical load on the motor is relieved until it has picked up speed." from - https://en.wikipedia.org/wiki/Inrush_current#Motors

Or - https://www.jadelearning.com/blog/understanding-motor-starting-inrush-currents-nec-article-430-52/
 

MrAl

Joined Jun 17, 2014
7,666
I came across this device and was interested in understanding its functionality,
https://www.mouser.com/ProductDetail/MEAN-WELL/ICL-16R?qs=vLWxofP3U2wgD7jPA%2Bx9JQ==

Could something like this work with an AC motor that has extremely high inrush current? I wasn't sure what the datasheet meant by 23A inrush limiting current, whether that was the max this device could handle or the cap on output given that the running load is <16A.
Hello,

It is a bit of a trick to select an NTC inrush current limiter.
First you have the 25 deg C resistance R25, then the max current temperature resistance rating Rmin which is the minimum resistance, then you have the max power rating, then you have the time to change resistance from R25 to Rmin.
That's kind of a lot to think about because if the device is underrated it will heat up too fast and thus it could reach Rmin before the motor even turns one complete revolution.
Also, after it reaches Rmin the max power rating kicks in and if it is too much power for the device it will burn up. Even without a problem it could reach a temperature as high as 250 deg C during normal operation so it has to be located away from other heat sensitive components as well as whatever connects to the two leads. If you use a bypass relay with it, you'll have to get the timing right too or else it either wont limit the current or it will leave the NTC in the circuit for too long and thus burn it up.
So selecting an NTC surge current limiter (with or without a relay) is not that easy. But these things are clear:
1. Has to take the right time to heat up to get to Rmin.
2. Has to have Rmin such that the normal operating current does not burn up the device.
3. Has to have R25 such that it actually limits current to an acceptable value.
4. If a relay is used, it has to be timed correctly.

What this really means then is it is best to get a ready made solution that has the specifications that you need and be done with it.

I dont know what your normal operating current is, but let's say it is 20 amps, and your max inrush is 100 amps. That means you need a device that limits 100 amps with a normal current rating of 20 amps. You might do ok to get close to those specs though. It may however require some trial and error unfortunately unless you want to build up something yourself from scratch where you can set all the ratings and limits yourself. It might not be too easy but you can experiment a little that way too and get what you need using somewhat standard components. You will of course have to do some careful testing.
 

shortbus

Joined Sep 30, 2009
7,760
I dont know what your normal operating current is, but let's say it is 20 amps, and your max inrush is 100 amps
How on a single phase AC or even a DC motor are you going to change the inrush current by a separate device? I read his question differently than the others in the thread. In rush current is a given part of a motor design, if the current isn't met the motor won't run. I take it he is worried about hurting the motor by using a higher current line or source to the motor. And as you know any working electrical device will only take from the supply what is needed. A short circuit in the device withstanding.
 

MrAl

Joined Jun 17, 2014
7,666
Hello,

As anyone who has ever used a speed control will testify to as you turn the speed down the motor starts it just starts more slowly and the presumption is that the full load is not being applied.

The idea is that you use a device to lower the initial current and get the motor turning a little so that the full voltage, when applied, is now working on a motor that is turning already even if not at the max speed and thus the required current is lower.

The other idea i think is that the expectation is that the motor is not under full load at the time of starting. Thus without a limiter of any kind the starting current is still high but doesnt last as long.

Now picture this. We have the regular larger motor connected to a smaller motor that turns the shaft of the larger motor. The smaller motor inrush current is 10 amps while the larger is 30 amps.
We start the smaller motor, that starts the larger motor shaft turning. Then we apply the full voltage to the larger motor and turn off the smaller motor voltage. The inrush current to the larger motor is not as great now because it is already turning a little. The smaller motor itself can not start with the full load applied however.

With the inrush current limiter, the hope is that the larger motor will start to turn with a smaller current, even though it wont get up to full speed. Once started turning, the inrush requirement goes down.

Now if this did not work with any motor anywhere, there would be no such thing as inrush current limiter devices used with motors. Even more so though is that a motor would not be able to start up using a speed control with the control set low. An example of this is the well known Dremel tool used for cutting and grinding and drilling. It has no problem starting up with the speed control set very low. However, if you tried to start it up into a heavy load that would normally take the full power to get it to turn it probably would not start at all just sit there and hum, until maybe you turned the speed control up higher. The typical Dremel drill stalls pretty easy with load when the speed control is set lower, so it would have to be started with little or no load.

So the second requirement is probably that the load has to be light when the motor is starting up. An air conditioner for example has much less load when it first starts up, as long as it was off for a while. If it was running normally then switched off, then after only 1 second turned back on again, it would probably not start up but trip the thermal cutout if it was not already electronically controlled. The older models do that because all they have is a thermal cutout device which turns the unit off if it is started immediately after turning it off. It heats up and breaks the current, then after it cools it tries again. Once the pressure in the lines dies down a bit it will be able to start up gain because the motor will not be under full load.

But maybe you could look up some examples on the web see what you think after reading one or two.
Here is one example:
A 120vac air conditioner draws 30 amps at startup but the circuit breaker is 20 amps while the normal run current is 16 amps. Motor inductance is about 0.0018 Henries.
The solution is to use an Ametherm part number: MS35 10018 Inrush Current Limiter.
Does it work? I dont have that air conditioner and that inrush limiter part so i cant test it. I also did not go over the design to try to verify it.
 
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