Frequency Inverter is Over Current

Be aware that when frequency converters start up, they ramp up from zero speed to maximum speed (or up to the limit you specify). While ramping up, their current can be quite high. In your case, high enough to trip the over-current alarm.

Try this:

Measure the starting current as the frequency converter starts up. If the converter can take it, set the over-current parameter to just above the starting current. It will go down as the frequency ramps up. Remember, however, that if you increase the over-current setting, you may be removing the motor's protection.

Alternatively, if your drive has a trip delay parameter, set it so that the current has time to go down before it starts monitoring the current.

regards,

Vulcan

thank you for your help,

i did all that already ,even i made the start ramp time higher .

am sure now that the problem was in the inverter , but will reply you when i change it .

...even made the start ramp time higher.

Ooooh! All you did was increase the time that the motor is running at overload condition.

Remember what I said, at low frequencies, the motor will draw a higher current. When you increase the ramp-up time, the motor stays at high current longer. It's worse when you have a load on the hoist when you start the drive.

I'm not saying that your drive is not defective. You'll need to eliminate the easiest reasons first.

You could try running the hoist motor without load (better decouple it from the hoist mechanism) to see if it still trips the drive. Then again, your motor might be defective but you probably have checked that already. Your hoist mechanism might also have jammed.

Might you have mis-entered some parameters like the motor voltage? supply voltage?

There must be a half-dozen or more different possibilities here. Try to eliminate them one by one.

regards,

Vulcan

thank you Mr Vulcan ,

sure i already eliminated all the possibilities were available , also i run the motor with full load using contactors and it's working fine , except that it jumps on start up .

that's why we needed to install the inverter .

but am thinkin now that i didn't raise the frequncy more than 50Hz , maybe that can be it .

Not my field - but a series NTC thermistor might be sufficient - if so (and suitably chosen) it's a much cheaper and more robust way of reducing the start-up jump.

change the module inside the inverter or replace the C.P.U thats all the fault will eliminated thanks me after getting the 100% result ok

hi Amer,

why don't you bypass the inverter and check the other equipments are working ok ,ie ;the motor and the winch, if ok than check there start current and same u feed in to the inverter . your Inverter should take less start current than the normal start of the motor and the ramping speed of the motor more slower than the normal running of the motor . Also connection of the motor ,it should be wye connection for a winch operation .If all found to be ok than it is the Inverter problem .

Regards

Adil

Another idea:

• If the inverter has a "reduced motor voltage at start-up" capability, it might be worth doing that to reduce the motor start-up current.

Try that.

What is the brand of the drive you installed?

Pls give more information on this. Is this a new installation or is this an existing winch (operating DOL before) and you want to add the inverter for the speed control feature?

Depending on what type of drive you have, the 20amp setting is for the protection of the motor, once the start up cycle completed. The drive will normally calculate (by thermal image etc.) the protection for the start up cycle from this setting.

Overcurrent trips on inverters normally indicates either an overload on the motor (hence my first question) or a faulty motor. Did you test the insulation on the motor (both phase to phase and phase to earth)?

Hello

It might be a mechanical problem -bearing might have ceased -

try to rotate rotar by hand -whether you feel free movement

try to look at boot settings-if you provide some volatge boost-it may help to start without taking more starting current

things are so simple let me solve your problem with 1 call i have done specialization in the feild of variable frequency drive n dc drive n servo as well so plz contact me 0092-21-3462155739

Try to adjust the current limiter of the converter if it has any. If the start up freq is much different then your mains freq it can (and most likely will) cause a greater current consumption. Unless your motor is faulty!?

Was this without a load on the motor (uncoupled)?

Could be something as simple as a parameter out of whack on the inverter. Have you tried to jog the motor?

What is the make and model of the inverter?

What Mode is the drive running? Pwm inverter drive usually have three mode of operation, volts/hz, vector and open loop vector. The motor will perform very differently depending on how the drive is set up.

I worked at the ship yard where we had the very same issue on the CNC Routers

It would trip irregularly, and got progressively worse.

I was not quite sure of the problem until I dug into it.

The manuals was translated from Japanese, which was not very accurate, the only thing that was accurate on it was the logic.

The error signal from Frequency Invert was displayed with a sequence of flashing lights, the flashing lights indicated from the manual it was a speed pot problem. ? :/

After farther investigation, only then I determined it was a Frequency Inverter issue

The problem I did have at the time was that there were basically (2) types of an inverters

1.) Domestic which was about $15,000.00 and readily available

2.) And a European which was $30,000.00-$40,000.00 plus 8 weeks del.

It required a European. I had better been right, it was and I was.

I was able to basically limp along ok by adjusting the ramp time for start-up, only a temporary fix, that decayed as time went on, until it was not worth starting it, which the replacement came the following day fortunately

phoenix911

in that case you had a problem in the capacitor bank of the inverter , you only needed to replace the capacitors .

amerlathqani:

I am only relaying the information at hand from my memory....

This occurred back in 1992. Procurement investigated and relayed information back to me. With the information that you can only purchase the unit itself.

If I had more experience at the time, I would have challenged them,

An basically what you are saying can be applied to the OP

Thank you,

phoenix911

I think I must be missing something that every-one else replying to this post already knows:

My understanding of most types of motor is that they consume more current while they are starting up. Often much more current.
So why does everyone here address only the inverter?

Its the motor start current, at start up it will be between 6 to 8 times the running current, which works out to be between 84 and 112 amps.....generally around 6 times.....

You need either a bigger inverter or one that handles such momentary currents better........

Best of luck!!!!

As you say 'GA from me' coz you damn right!

I wasn't sure of that earlier. I figured that if he has chosen the fuses right and the inverter can also handle the same current or even more than it is alright, but it is not. The speed of the inverter's Current Sensing circuit is lot faster than the blow speed of the fuses. I guess the fuses are slow-blow, anyway, as there's no need for fast-blow ones. And yeah, by the time his fuses would blow, the onboard micro with its current sensor circuit will do the analysis and decides to shut down. Meaning that he never should have bother using higher current rating fuses than before either.

If you meant me with a GA, I never got it!! Did you forget?

In deed I did forget. It was late at night I suppose but I've just made up for it though. Can I give you two instead?

One is correct and enough......thanks.

Overcurrent trip is a peak current trip. The current limiter has no effect normally at this level as I assume the inverter is 7.5kW rated (16-18A) and will have an overcurrent trip level of 200-350% of this rating.

The winch application in itself throws up several questions:

How is the brake controlled? Is it possible that the inverter 'fights' the brake too long? Is the brake malfunctioning mechanically? Does the brake maybe come back on for an instant when starting through logic or wiring or electrical problem (chatter)? - Maybe you can lower the winch to the floor, make sure brake is released manually (pushbutton to apply again instantly on release) then try to raise a short distance. If there is no problem look into brake methodology to sequence better with inverter operation.

Another possibility is loose connections etc, not always as obvious as it may sound - double check everything thoroughly, this is often a culprit causing OC error.

The start of rotation / torque could also mean motor or wiring short circuit developing, but if the motor has always been good this is less likely.

OC error can also be caused by calculation errors caused by vector control, if in Vector try v/f just to see if this cures the problem. If it does, investigate how to use vector control better.

Most likely is brake malfunction or a wiring problem - you need to eliminate one possibility after another to arrive at the reason for the problem.

It would be helpful to give more information than offered as this is pretty minimal, is it new, have you just fitted the inverter, what rating is it, what control are you using, what checks have you made already etc???

Maybe its easier to arrange local advice from an experienced engineer with relevant inverter knowledge to be more sure of a good outcome. Good luck.

I tend to disagree with most of what you have written, its simply not true.

I have researched on the web for a document that will show you the errors of thinking that you have made (as many do) and have found many references that show just why AC motors take such a large current on start up, unless some form of "Reduced current" starter is installed between the supply and the motor.

Here I am quoting from a .pdf document that you can read in full at the following link:-

http://ww1.microchip.com/downloads/en/AppNotes/00887a.pdf

------------------------------------------------------------------

STARTING CHARACTERISTIC

Induction motors, at rest, appear just like a short circuited transformer and if connected to the full supply voltage, draw a very high current known as the "Locked

Rotor Current." They also produce torque which is known as the "Locked Rotor Torque". The Locked Rotor Torque (LRT) and the Locked Rotor Current (LRC) are a function of the terminal voltage of the motor and the motor design. As the motor accelerates, both the torque and the current will tend to alter with rotorspeed if the voltage is maintained constant.

The starting current of a motor with a fixed voltage will drop very slowly as the motor accelerates and will only begin to fall significantly when the motor has reached at least 80% of the full speed. The actual curves for the induction motors can vary considerably between designs but the general trend is for a high current until the motor has almost reached full speed. The LRC of a motor can range from 500% of Full-Load Current (FLC) to as high as 1400% of FLC. Typically, good motors fall in the range of 550% to 750% of FLC.

-------------------------------------------------------------------

I feel that this explains not only the "Why" but also the "amount" in an easy to understand manner.....

Hope you find this helpful in the future.

Dear Andy,

the motor is under inverter control and is not the same as your research into 'starting' a motor on a mains supply.

In fact, motor characteristics in inverter control are similar to the final slope of the slip curves your text describes e.g. when at no load at full speed, 14A motor should be about 6-7A. Full load is 14A. 150% load would be about 20A and this then applies across the frequency (speed) spectrum. up to rated speed. Locked rotor current and so on is irrelevant.

In training, I often describe that with inverters you do not 'start' a motor, its always at full speed (for that frequency) and you adjust this speed, you then see the current associated with the load demand required by the machine.

I assume you have no experience of inverters since you query my comments, but I have worked with VFD for over 30 years and supply lift and hoist applications etc. as a specialist.

I must say that in trying to assist in this case, I am disappointed that readers cannot even recognise help when it is offered through a lack of basic understanding of the topic in question. Hopefully, we live and learn but your confidence is disagreeing with my comments is very ill founded, I think. Speak to someone you know who understands these applications and then commenting would be a more constructive input, in my opinion. I hope further research and effort will lead you to a better understanding of my comments.

All indications are of an "over current", no matter what the in between reasons are......

Obviously current needs to be reduced or the output increased to handle the currents...

I will not believe that a 14 amp running current motor will run correctly on a 20 amp inverter!!

Lets wait and see what happens.....

'I will not believe that a 14 amp running current motor will run correctly on a 20 amp inverter!!'

Try asking anyone who deals with inverter applications and open your mind to a little more knowledge and information rather than making even more and more strange statements.

Probably over half of all existing applications of VFD are selected for an output the same or nearly the same as the motor flc rating with no more current required even at starting. High starting load applications may need 150 - 200% flc, most VFDs offer 150% of continuous rating for 60s or more, oversizing is not needed for most applications.

If you are on the right track (very questionable at this time I feel personally, looking at results), then you must be the man to "FIX" this problem......but you are seemingly unable to do that either!!!!

Lots of telling me that "I have no idea what I am talking about" (which could be right, I have no problems with that either way....), but no fix or problem identification from you for the OP.......!!

You seem keener to put me down verbally, than doing anything positive for the original poster!!!

Remember one thing, this has NEVER worked.......we are not fixing a problem that has arisen later........think about that point MOST carefully!!!

Maybe you (and anyone else who is "cleverer" than simple old me....) should all aim your combined talents better in helping the OP to get running, rather than just talking.....

My thoughts on a possible area for the OP to try out (on which I am not alone):-

Maybe it is a brake problem that is causing the overload at start (that has already been mentioned, but no reaction from the OP on that point as far as I remember, why?), that must be the simplest thing to test (with no load of course) by wiring it separately and "disengaging" it before trying to move the motor......or mechanically disabling it completely for the test duration......

That's a comment from me, that at least could possibly help the OP further, instead of bickering over nothing.....(or nothing proved either way....!)

Change your aiming point, do something useful......crowing comes later when or if the problem is found and fixed.

Have a great day anyway.......

PS Vulcan in post #14, seems to have some good ideas as well......

I think what I previously failed to realise (I said this was not my field) is that a properly matched inverter-motor system starts up with reduced Voltage as well as reduced frequency. That means that you don't need the back EMF at start-up to avoid initial over-current problems.

That would point towards one of two possible problems - a mismatch between inverter characteristics and the intended mechanical load*, or (as suggested by others) a brake that is not being released (i.e. the load is not as intended). Assuming proper control systems, I imagine that the problem is probably not overcurrent as such - more that the controller is sensing that the motor has not accelerated; the crude report will be "overcurrent" because that was the historical symptom, so many users expect this.

It's perhaps a shame that the experts chose to ignore my query, and similarly not to clarify the actual position when responding to your objections.

*A load with inertia that exceeds the calibration range of the inverter could cause such problems.

What method inverters use on start up to keep their current limited to their inductive load, like a motor ?

The laws of physics does allow for high current draw on start up for any inductive load w.r.t. nominal current at operating speed. It's just normal, is it not?

So, in order to keep the start up current limit under control there must be something but - what is it?

The OP claims he has installed over 200 of these things before, and providing he's an engineer, then he surely should have some problem solving skills!!!

He should have an other inverter unit at hand to test that particular motor with or know enough about the principal operation of the inverter/s he installs on regular basis. If he doesn't, then I no wonder he cannot solve the problem because he is technically not up for it!

He probably only knows how to hook things together, if he does at all, and that is not good enough. Sometimes people tend to over look basic things (like checking the fuse not in his case though) and I have a feeling he does in this case, if he has installed so many of these things already.

Thanks for the explanation Malcolm.

In any case the OP is still ignoring something, if he has done so many installations and providing nothing is faulty!

I never new how the current control is done in VFDs but now i can see what you mean and you have cleared me up on current vs. XL and how freq. vs. speed ofcourse.

I have a few questions though:-

-What is the 'manual' starting voltage as opposed to Vector (automatic)?

-What values the software monitor to do the 'vector' adjustment?

-How many values a smart type of VDF can or has to monitor on average to get an efficient motor operation besides speed control?

-What about torque control is it adjustable or depends on the power supply (like where battery vs. mains)?

Hello Isti,

Although Amerlathkani says he has experience with inverters in setting up etc, he didn't say that this was in the hoist application so I assume this may be new and different? Please refer back to my original post for a few things I thought could be checked.

In brief reply:

Manual means setting a fixed v/f relationship, so the VFD outputs a frequency and that then means the voltage for that Hz. From these, the motor demands current based on this and its load.

Automatic (& Vector) are similar in that the inverter mathematical model estimates the load condition and then adjusts voltage to achieve desired current for the calculated load.

Not sure exactly what you mean but important user inputs are normally motor rated V, A, kW, nr poles, FLC (Ir) and NLC (Io) and Stator resistance and Leakage Reactance, the last two or 3 are often estimated by an inverter tuning procedure of the motor if you want to improve on default settings already existing. The VFD 'knows' what voltage its outputting, and measures current (therefore knowing actual current and angular displacement to voltage) to run its mathematical model.

Torque control is not as good as speed since most applications are the latter (more development done) but can be handled by VFD, in my experience, much better using closed loop vector as the information is more secure and allows much faster and more accurate calcs, particularly at low speed and low torques.

Battery or mains has little effect, both supply the inverter stage with adequate DC if in specification.

Back to original query, I have seen many reasons for OC tripping in hoist applications, many to do with brake but also loose connections (crimp catching on insulation is not so obvious, feels tight), brake relay on / off due to logic problems, vector calcs errors, starting voltage too high, output contactor opening briefly etc. Its rare to find as VFD maloperation but I do not know make, age, or other information to assess.

Kind regards

Hello Malcolm,

Again you gave a lot of info and I believe some of the things you have said about the possible causes could hide his answers to the problem/s.

I noticed on this blog a lot of you talking about the motor breaking could cause the problem due to a malfunction. Here's a posting Charging Coils in a Three-Phase AC Induction Motor System where after while, going in circles for not understanding his question, the guy explained the VFD they build, copied from another one, never switch off its power to the motor when the speed control is brought to a stop but rather keeps the motor under power at zero phase shift.

And when I finally understood his Question all he wanted to know why is that instead of cutting off power to the motor full stop?

My conclusion to him was to keep the motor under constantly break when the it is not running. In my opinion is what the VDF was designed to do. Run the motor maybe under heavy load and if it's necessary break it instantly as it stops and un-break it as it starts rotating in the desired direction. Thus no need for relay and seperate dc break power, am I right?

Hi Isti,

Its not my area of experience as I have worked largely using VFDs in applications, more my forte, I guess. Neither is my background in electronics / electrical but I will offer an opinion from what I know.

There's no torque without current, there's no magnetisation without current also. On upper devices in PWM would seem to only pass minimal current whilst charging capacitance, I can't think of any good reason from a motor or inverter viewpoint why you would do this (although it does no harm, other than the motor is ON when you think it is OFF (stopped).

So, all upper devices ON does nothing that I can think of. If you want DC to slow or to hold at zero speed, you can do this by VFD output where PWM controls the DC current giving a low torque which can operate with the VFD rated current available. For faster braking you would need much more DC (larger inverter) and you would not do this as it is much more effective to use motor negative slip to effect braking by ramping down the VFD output..

Zero speed closed loop is well known using Forward and Reverse PWM as required to keep in 'position' at zero speed. It can also be achieved open loop with some inverters but the control is then, of course, slower and less precise.

DC and Zero Speed hold are inverter features available without extra external devices, you can also Start at Zero Speed (useful on lift and hoist but closed loop as fast response needed when mechanical brake lifts.

Further, you can use Pre-magnetisation to avoid motor flux delay times on rapid response mechanical handling applications, thus saving the time you normally have to wait whilst the motor fuxes up to produce torque.

Finally, you can alos use low voltage and trickle currents as an anti-condensation method.

Hope this is something like you were meaning?

Cheers

Hi Isti ,

my only reason to mention that i installed 200 inverters already was to give the engineers here an idea about my background , if i read that in a post i will not be replying to the poster " check the fuse " . also in my third post #20 i mentioned that i eliminated all the possible external causes and run the winch with full load , which means that i don't have a problem with the brake or the mechanism or the motor itself .

you are assuming that i " only know how to hook things together, if i do at all " .

regarding my problem solving skills, i have an experience of ten years working on maintainance and development in all kinds of industrial control systems from measuring instruments to production lines .

BTW, 2 of the 200 inverters i installed were tower crain hoist drives , and yes i faced a lot of problems that i had to work out on a very friendly environment 200 meters above the ground . also i worked on various applications ; pump, fan, vibrator, mixer and a lot more .

thanx to Mr Malcolm , his informations are very useful and valuable .

No worries mate, but you didn't even bother to ask Malcolm questions and it was Andy who started pushing the issue more forward on a very logical ground since you did not elaborate on things.

And after Malcolm kicked in to debate it with Andy and since initially Malcolm's explanations did not make much sense I felt jump in as he also stated that he used to get involved, or still does I think, in this field.

After he cleared up a few things it became clearer what he was on about however, if you do not give enough info then it is hard to know what is going on.

Indeed, when I used to work as a service personel sometimes I had to phone up the customer to make sense about his problems due to lack of enough info.

Even here we can only help one another if you either give a better explanation for starters. Now lucky you that Mal could point out a few things to me yesterday (as I gather now you would have been observing quietly) which not only made sense but most likely had solved your problem.

The fact that I said 'you might be over looking some issue/s' to me appeared very possible as that can happen, even to the best of us, on a job when you have a lot of things on mind at times (regardless how many you might have installed previously).

Andy,

On this topic, I agree with MalcolmK. I did many pump station conversions to VFD inverters. The previous controls varied from DOL, to wound-rotor motors with secondary electrodes in a salt solution tank (varying level gave smooth speed control but lots of maintenance issues), to first-generation 6-step thyristor drives with insanely huge load-side reactors. The oversizing we did was to allow the motors to be run at their service factor current with the inverter in a very high-temperature ambient. I have also done a number of inverters on conveyor belts---a constant torque application similar to a hoist.

I strongly suspect the problem lies in how the hoist handles its brake. If this is an electrically released brake, with the release done by the voltage applied to the motor windings (and that is a common method of doing a motor brake), then the application needs to have the brake coil independently energized at line voltage with conductors powered independently of the motor windings. I would suggest a small contactor that is closed for the duration of time the inverter is running, with the brake coil wired through this contactor.

The inverter's parameters would include a minimim frequency high enough to keep the winch from freewheeling backwards at the instant of starting (perhaps 5-10Hz) and enough torque boost to handle the constant-torque load of a winch (compared to the variable torque load of a pump). Probably there should be a deceleration time of zero, so the brake can be energized the moment the run command is turned off. If this is not a line-regenerative inverter, the application may need a dynamic braking resistor to keep the DC bus voltage from going too high during lowering of a heavy load.

Be safe--John M.

...electrically released brake

Egads, you're right! I suddenly remembered one application where we used a brake-motor (that's what we call them). We decided to change it to an inverter driven unit and wondered why the motor kept tripping on start up. It took several minutes before somebody remembered it.

It was powered via the motor's power supply. When the motor was fed voltage, the brake disengaged and allowed the motor to start. When we put in the inverter, the reduced voltage starting meant that the brake remained engaged during the ramp up causing the inverter to trip off.

We laid out new cable for the brake along with separate controls for it. It was made to disengage about a second before the inverter was started.

Yup, certainly a possibility. GA from me.

regards,

Vulcan

Where do you get the supply for the brake? Is the brake fitted on the back of the motor or between the motor and the winch? I know common practice (if the brake is piggy back on the motor) is to tap the supply for the brake, from the motor terminals (saves cabling and a feeder).

This is not a problem if the motor is started DOL, but the brake will not release if the motor is supplied via a vsd, because the output voltage and frequency from the drive continually changes to give full flex density in the motor and hence full torque (MalcolmK has covered the theoretical part of this statement).

The brake coil is rated for a specific voltage and frequency (in our case 400v @ 50 cycles, i don't think you gave your normal supply voltage rating?), the brake coil will therefore only operated at this voltage and frequency (with a +- 5% tolerance). The lower frequency and voltage output from the drive during start up, will therefore not operate the brake coil and the brake will only release once the motor get to 100% speed (100% output voltage and frequency from VSD).

The only way to solve this problem (if your problem) is to supply the brake coil from a different source (100% voltage and frequency)

Regards

Hello dear,

Firstly there is no need to use a high rated inverter to such small motor,

U must first clear that what the type of the mechanical coupling used to drive that motor ;such it is a very important point to startup the frequency converter,

then I think u should search in that driver parameters for the parameters that responsible for the time of starting current, such u can define to the drive that your starting current will an X time , but watchout u must sure that it will not affect your overcurrent protection ...

I hope that help

OOOHHHH!!! You guys all good, 'You Have the Technology' for real. I guess that the motor are trying to catch-up with the load by sucking more juice as demand call for. The motor are asking for more juice in order to make it happen like a 'horse without head' straigthforward an triggering the limits in the attempt off the pit. Lot of torque there in that 'Puppy' unless as pointed out before, the motor is Shot. Can Happen too as mentioned. Everything can happen. Is a 'Detective Job' kind-off.

So far all explanations make good sense here to me so we migth start troubleshooting this 'Puppy' out with good clues, I'll Bet! You Got It................

Winch Juice,

MC