Current Inrush of a Re Accelerated Motor 3

[Photocell]

Hi

Please tell me what will be the magnitude of current taken by a three phase induction motor when the motor is running at full speed and the voltage applied to it suddenly resduces to zero and then come back again (e.g when the contactor of the motor drops and makes again.) Will this be greater than motor's DOL starting starting current or less than that.

Thanks

Photocell

 

[digitrex]

I had tried this before:

We had a 200kW 415V motor for replacing bearings in our workshop. Then we wanted a solo run test but our motor test bay in our workshop is too small for this motor.

My technician did tried runningw(without my knowledge) with the undersize motor starter in workshop, and it ended up with all the lights in the workshop dimmed and all the 315A fuses were blown.

Then, I wound a rope on the motor shaft for many turns so that when I pulled the rope fast, the shaft will spins. While the shaft was spinning we started the motor again and there it runs on no load and it didn't blow the fuses.

Now you get an answer to your question?

 

[electricpete]

I think there are two separate cases to be considered.

If the motor has been deenergized long enough for the field to substantially decay (> 1.5* open circuit time constant as defined in NEMA MG-1, typically >10 cycles), then as is suggested above, the duration of the (re) starting current will be lower and possibly the magnitude will be lower than during start from standstill.

If the motor is deenergized only a short period of time such that field has not decayed significantly, there is a possibility that there will be out-of-phase condition between the re-closing power and residual motor voltage. In this case you may have much more than locked rotor current and you may have potentially damaging torques.

=====================================
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[jbartos]

Comment: It depends. The restarting current can be lower and it can also be higher. There are two aspects.
One is the instant at which the supply voltage is applied with respect to the motor internally generated electromotive force. If this instant is in the most adverse instant the restarting current will be higher than the motor normal starting current.
Other aspect is the motor deceleration due to load, e.g. the motor slip conditions affecting the rotor R/s. If the load does not decelerate the motor fast, and the first aspect condition is least adverse, the restarting current can be lower than its normal starting current.

 

[Photocell]

Digitrex: My problem is little bit different than yours. We have a 300 kW motor and had a loose connection in the control wiring of the motor starter that eventually led to the drop off of contactor. We beleive that the contactor had re applied to re accelerate the decelarating motor. The motor took heavy current and eventually resulted to operate the undervoltage protection on the feeding bus.

electricpete: Yes. I beleive that the time taken by the contactor was very short and motor internal emf could be out of phase thus causing heavy current flow.

jbartos: Problem is that it is not practically possible analyse the phase difference between line voltage and motors internal emf. But one thing is clear when the motor will start to decelrate then the frequency of the motors emf will be less than that the line voltage frequency (fr = s*f).

Is there any source where i can find a theoratical analyses of this issue ?

Thanks for your prompt reply

Photocell

 

[dpc]

You might try a Google seach for "fast bus transfer". This problem has been analyzed in some detail due to desire to rapidly transfer power plant auxiliary loads between transformers.

 

[jraef]

This phenomenon occurrs regularly in open transition reduced voltage starting, such as autotransformers and star-delta. In a multi-stage process with an open transition, the motor is breifly disconnected from the line. When reconnected, the regeneration can be enough out of phase with the line that it creates a very high voltage and current spike. In particular the star-delta issue has bee discussed in this forum many times. Do a keyword search for "transition spkike" and you will find many refernces to articles and papers on that subect. I know of a good one but can't put my hands (actually my mouse) on the link at the moment. I'll post back if I find it later.

"Venditori de oleum-vipera non vigere excordis populi"

 

[electricpete]

Can you describe the protection of the motor/circuit and what actually tripped.

If there was trip of an instantaneous relay or magnetic element of breaker, that would definitely point towards current exceeding assymetrical locked rotor current, and the scenario of out-of-phase reclosing.

On the other hand if it was an overload trip, you might consider the scenario that the circuit made/broke several times, resulting in heating equivalent to several starts in succession.

=====================================
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[jbartos]

Comment on Photocell (Electrical) Apr 1, 2004 marked ///\\
jbartos: Problem is that it is not practically possible analyse the phase difference between line voltage and motors internal emf. But one thing is clear when the motor will start to decelrate then the frequency of the motors emf will be less than that the line voltage frequency (fr = s*f).
///There are devices beginning to appear to provide a motor restart "on the fly," e.g.

http://www.automotioninc.com/pdfs/cc998.pdf

for: "DC servo"

http://www.schneiderelectric.com.tr/html/en/products/pcp/atv/atv38.htm

for: "catch on the fly"
etc. for more info\\

 

[Photocell]

jraef: I'll be waiting for ur mouse to move on. Couldnt understant the last line "Venditori de oleum-vipera non vigere excordis populi"

electricpete: yes it can be a case of several time reclosure because voltage dips were detected on the feeding bus solid state voltage protection relay's event records.

Thanks all of you once again for your interest and support


Photocell

 

[digitrex]

I just come across an article about effect of voltage dip on induction motors. May be this is of interest to you.

http://www.measurlogic.com/pdf/dpi/pq6.pdf

 

[jraef]

This is a reprint of the article I was thinking of, but it is all there.

http://www.mastercontrols.com/EngInfo/Articles/Nailen/TA_RNail.htm

Translation is roughly "Snake-oil sellers can't thrive without an ignorant populace"

"Venditori de oleum-vipera non vigere excordis populi"

 

[testtech]

A mangement company had a vane axial fan were the key kept getting ripped from the shaft. This was a 200 HP fan. Not jog duty rated. They got tired of paying to rebuild or replace the motor. I placed a power analyzer on load side of the contactor. The analyzer caught a disturbance every Saturday AM. Each Saturday AM, the Emergency generator exercised and when this happened, the fan motor contactor instantly opened and closed at the command of the building automation. When the contactor closed, the EMF produced by the motor was out of phase with the incoming current inrush. This produced a huge torque on the shaft, which eventually tore out the key. Not an answer to your question, but an illustration of the magnitudes involved in a short duration interruption. By the way, they installed a relay to delay reclosure of the contactor and never had a repeat of this problem.

 

[jbartos]

Comment: After reading the article in:

http://www.measurlogic.com/pdf/dpi/pq6.pdf

the first reference:
[1] Mulukutla S.S., ...
should be:
A critical survey of considerations in maintaining process continuity during voltage dips while protecting motors with reclosing and bus-transfer practices
Mulukutla, S.S. Gulachenski, E.M.
Dept. of Electr. & Comput. Eng., Northeastern Univ., Boston, MA, USA;
This paper appears in: Power Systems, IEEE Transactions on

Publication Date: Aug. 1992
On page(s): 1299 - 1305
Volume: 7 , Issue: 3
ISSN: 0885-8950
Reference Cited: 8
CODEN: ITPSEG
Inspec Accession Number: 4242037

-----------------------------------------------------
Abstract:
Plantwide shutdowns can be avoided where voltage dips of short duration on the power distribution system are a primary cause. A voltage dip where contactors drop open and reclose can be very serious because of the nature of motors and their driven loads during voltage transients. The general nature of the problem and criteria for safe reclosing or transfer of several motors on the same bus are presented. The mathematical development and modeling techniques for induction and synchronous motors are indicated, and simple easy-to-use expressions are given for determining the currents and torques due to out-of-phase reclosing. Various protective schemes against out-of-phase reclosing and transfer, as well as methods of riding through voltage dips, are discussed

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

 

[Photocell]

Thanks all of you. I think that the things are now well unsderstood. Specially the article refered by digitrex

http://www.measurlogic.com/pdf/dpi/pq6.pdf

was a great help.

Thanks again and i conclude this discussion. I will come back with some other questions that are still in my mind.

Photocell