Capacitors discharge

[electrolitic]

High voltage capacitors connected in parallel with induction motor needs a dedicated discharge circuit before next motor starting? Thanks.

 

[Skogsgurra]

No, the motor windings discharge the capacitors within seconds. And, even if it didn't, there is not much of a problem to start an induction motor with charge left on the capacitors. But, as said, there shouldn't be any charge.
Are you sure that it isn't capacitors in a VFD DC link you are thinking about?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[RRaghunath]

Generally, the capacitors come with internal discharge resistors which are capable discharging the capacitors fully in less than 10-minutes time.
The capacitor specifications I have seen included this stipulation.
This means you need to have minimum of 10-minutes gap from motor stop to next start.
I guess this is not much and is common (to provide 10-minute delay) with all HV motors.

Rompicherla Raghunath

 

[crshears]

As Gunnar pointed out, upon motor shutdown/disconnection from the line, all AC effects/impedance will disappear, and the vastly lower resistance of the solidly-connected-in-parallel motor windings will collapse any capacitor charge within seconds; all the capacitor internal resistors will ever do is waste some energy keeping the cans warm whenever the motor is in service.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

You may generally restart an induction motor with capacitors in a few seconds.
It is NOT SAFE to restart an induction motor with capacitors in a FEW CYCLES.
Severe mechanical damage including bent or broken shafts may occur.
Restarting an induction motor within a few cycles is dangerous.
Restarting an induction motor with capacitors within a few cycles is more dangerous.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Yes, Bill. The capacitors keep flux alive for an extended period and the restart is just as bad as a Wye/Delta start.
BTW, what does the OP mean when he says high voltage capacitors? A few hundred volts or the range above MV - i.e. HV?

BTW BTW: Are the capacitors PFC? There are other ways to skin that cat. Automatic phase compensation, for instance. That doesn't kill the contactor at switch-on. Which a motor connected set of capacitors may do.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[RRaghunath]

I agree in this case where capacitors are directly connected to motor terminals, Discharge can takes place through the motor windings. This is the best way.
Internal discharge resistors with capacitor bank are required only if the capacitor bank is connected with a separate circuit breaker, like PFC capacitors connected to the switchboard busbars.

Rompicherla Raghunath

 

[electrolitic]

Thanks all of you. The concern is related to a 4,16kV system, induction motor 4,0kV,low inertia load, capacitors bank connected to the motor terminals, DC control power from batteries and AC undervoltage relay not fast enough to detect a deep grid peak. It makes sense this concern related to re-start? If real, how to minimize it?

 

[crshears]

Hey there electro,

For most of us, 4160V motors barely qualify as MV, let alone HV...

"Deep grid peak" seems a contradiction in terms to me...do you mean some sort of drop-out of <1 second duration? If the AC 27 relay doesn't even "see" it, is the main motor contactor even opening? You may not actually have a problem, and if you do, please describe it in greater detail; "more information leads to better answers"...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[electrolitic]

After all these years, sometimes my wish is to leave this forum. I suppose that is absolutely clear when a thread is written by someone that is not fluent in English. Most of answers are educated and respectly sent. Others don't. I had received answers explaining the risk of re-start after few cycles. When I tried to explain "dc control voltage fed by batteries and Grid Undervoltage relay ANSI 27 not fast enough to detect a short and deep grid drop out" I supposed that feeder circuit breaker would be kept closed and motor would be "re-started" just after few cycles, that involves risks, as explained by some of you. I never would reply with comments like "few it is not a technical unit. How to define it". So, if my english is enough to be understood: A very deep and fast Grid drop-out not detected by 27 can be a risk for circuis where capacitors are connected in parallel to motors? If yes, how to deal with it?Thanks.
I suppose that is not easy to write correctly in Swedish, Japanese, French or languages spoken in others high tech countries.

 

[LionelHutz]

Is the grid actually being turned on and off during this drop-out or is it a droop/sag/brownout/etc type of event?

 

[waross]

I never would reply with comments like "few it is not a technical unit.

This is a much more technical statement;
"A very deep and fast Grid drop-out"

You may generally restart an induction motor with capacitors in a few seconds.

This should read "Without capacitors"
Few; The exact time varies depending on the rate at which the motor drops speed. More than one cycle but generally less than ten cycles = A few cycles. It depends.

A few hundred volts or the range above MV - i.e. HV?

What is the applied voltage? 230V, 380V. 400V, 480V, 600V, 690V?
Generally above about 50% of normal voltage = a few hundred Volts. It depends.
A motor with capacitors: When the supply voltage is interrupted, the motor starts to slow down and the back EMF starts to decay.
As the speed drops, the back EMF will be going in and out of phase with the grid voltage.
At close to normal speed the capacitors tend to hold up the back EMF and the back EMF tends to hold up the capacitor voltage.
It depends.
Bottom line; If a spinning motor with significant back EMF is connected to the grid at an instant when the back EMF is out of phase with the grid, the motor may violently pull into step with the grid frequency. This has been known to break or bend shafts and may damage driven equipment. How long for the back EMF to drop to a safe value? It depends. How long for the back EMF phase angle to reach a dangerous angle? It depends.
You continually speak of a motor restart.
A grid dip where the motor remains connected to the grid and, importantly, the other loads remain connected to the grid would be considered by some to be a "Ride through" event.
Finally, if you want exact information concerning a specific motor under real world conditions, then you may have to do some real world testing of the motor under real world conditions.
Until then, a "few" is as accurate technical description as you may get. The phrase A few cycles may be replaced with the phrase "It depends." While that may be more accurate, a "few" may be more informative.
It depends.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

By the way:
Eng-tips has a money back warranty.
Double your money back if you are unsatisfied with the FREE information.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

What Lionel said; I was seeking clarification. I meant no offense, and if I inadvertently gave such, mille pardons / gomen nasai...

If my understanding is correct, I would not see where what you are describing would be a danger to the motor/capacitor combination, the crucial difference being as to whether there is an electrical separation / opening of physical contacts between the motor/capacitor combination and the grid supplying it.

PS: I read Bill's post only after I had made mine, and was therefore a bit out of sequence with the thread; sorry for any confusion.

That said, I must agree that terminology makes or breaks human interaction, and if in fact what you are describing is indeed a "ride-through," which is how I understood it, I don't think I misled you.

 

[itsmoked]

electrolitic; Part of the problem is that it depends on the load. If the load has a lot of inertia, say a loaded conveyor belt, then a motor with connected capacitors could keep generating for a considerable amount of time and getting re-closed on would have problems. Hence "few" and it could be actually a lot more than a few.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[RRaghunath]

If I understood the problem correctly, you have DC powered Vacuum contactor to control the motor. There is U/V relay that is time delayed.
Now, if the power supply returns within the U/V time delay the motor gets powered back risking the motor health.
The solution is to make the U/V protection instantaneous by making the time delay zero. This way, the motor Contactor would open as soon as the power to the motor is lost.