Series Capacitor 3p Motor Starting

Change the cable...

No. Replace the cable or consider adding a soft starter circuit to reduce the startup current and hence the voltage drop.

By changing the cable... I meant to replace it with one large enough to eliminate voltage drop as an issue. I should have explained that I guess.

Oh... you were saying no to his question. I thought you meant no to my statement about changing the cable...

Long day...

How did this cable come to be too small? Was the motor up-sized, long cable run, high-inertia load, ??? jack has recommended a soft starter, which could work. If the problem is high inertia load, some type of mechanical clutch might work. I was thinking y-delta starting, but then you would need to run 3 more conductors, so you might as well up-size the ones you have, plus avoid additional contactors. If the cable is just outright undersized, replace it.
In any case, forget the capacitor idea.

Replace the cable, otherwise the overcurrent protective device(s) will be ineffective in clearing a fault before the cable melts. Cable sizing protocols are given in British Standard 7671.

Use a shunt capacitor for starting/pfc.

Good idea, I used a shunt capacitor and a soft starter on a 11 kW motor with a input supply 3 x 25A 400V and it is working alreasy 13 years without troubles.

and the size of cable you are using is?

change the cable.. as already stated! Forget any type of capacitors, unless you want a fire!

Change the cable if you want to avoid a fire, damage to property, loss of production and loss of life. You also need to find out WHO sized the cable and shoot him!

Now someone will come up with " use a VSD!" NO! The cable must be sized to handle the in-rush current regardless of how you start it!

You might say the cable is buried, it will be hard to replace/change.. Rubbish! If you want to, there is ALWAYS a way! There are no options or quick fix solutions to your problem. Cost of replacing the cable.. who cares when you compare it to the cost of lost production, loss of and damage to building/property and what about the cost of a life?

You haven't given us the size of the motor or the size and length of the cable - all of this would affect the answer.

Your motor can take 8 times the full running current when it starts. You would need the fuse / mcb sized so as not to trip when you start the motor but also sized to protect your cable.

If its a small installation then likely the right answer is change the cable but get someone to look at your calculations first. If its a particularly long start up time then you can use a soft starter and limit the current.

If the volt drop is too great at start up chances are you have cattled the motor or certainly on the way.

I may have misunderstood the question. Careful with series capacitors, resonance might be a problem.

may result in high volages.

The concept you are proposing is a long established solution to this sort of problem, especially in large medium voltage motors. You can research it further by doing a web search on "capacitor assisted motor starting". But there are very significant risks in this, some of which are the risk of resonance and over voltage mentioned by member . But most importantly, it only addresses a SYMPTOM of the problem, not the problem itself.All of the suggestions of increasing the conductor size would be the real solution to the PROBLEM. However, that is not always possible.

If you must continue to pursue the cap-assisted starting angle, be aware that is requires very PRECISE timing of when you turn the caps OFF. For this to have any appreciable effect, the capacitors must be very GROSSLY over sized compared to Power factor Correction Capacitors. If you think about this, a PFCC system is used to add VARs to the circuit to correct a lagging PF of 0.8 to around 0.95, so by maybe 10-15%. To assist in starting a motor, these caps must correct a PF that is in the area of 0.1 to 0.2, to the caps must supply 80-90% of the motor kVA as kVARs!

If those caps are left connected once the motor PF drops to normal, at about 80% speed, then you will be SEVERELY over exciting the circuit. That will then cause a severe over voltage, motor heating and a very high probability of over exciting other connected loads and causing resonance with any other capacitors in use, as in EVERY form of Switched Mode Power Supply now found in nearly all electronics.

So to safely accomplish Cap-Assisted Starting, you must use a precise method of determining motor speed and, in conjunction with contactor interrupt time, make sure the capacitors are removed from the circuit at 80% of motor speed. I have seen people "guess" on this and be off by just a second or two, which causes a severe voltage spike in the line that has destroyed other connected equipment. Bottom line, it is NOT worth pursuing on most LV motors. The reason people use this on MV motors is because the other alternatives are far far more expensive in MV.

First of all kindly check your cable if can carry the running current of the motor. If the cable is ok. Then the cheapest solution is to take another power source of control supply. To avoid voltage drop of your control circuit during starting. If you have ligthing power source is more than enough to supply the motor contactor.

We expercience same problems here in Saudi Arabia.

in fact, I wonder if this in series capacitor has a positive boosting task in the circuit. I do not ask it practically (İn practice a resonance is always a risk). If we use an inductor in series with the motor this will have a drop effect on the start up. If we use capacitor then can we say this will have a reverse effect so that the motor will start more easily.

By the way, shunt capacitor seems reasonable,but do not we have a resonance risk here also? .Also since the effective impedance of these two guys during a high frequency start up condition, then does this not mean that start up current will tend to increase?