Current Inrush of a Re Accelerated Motor
Current Inrush of a Re Accelerated Motor
(OP)
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
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





RE: Current Inrush of a Re Accelerated Motor
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?
RE: Current Inrush of a Re Accelerated Motor
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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RE: Current Inrush of a Re Accelerated Motor
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.
RE: Current Inrush of a Re Accelerated Motor
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
RE: Current Inrush of a Re Accelerated Motor
RE: Current Inrush of a Re Accelerated Motor
"Venditori de oleum-vipera non vigere excordis populi"
RE: Current Inrush of a Re Accelerated Motor
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.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Current Inrush of a Re Accelerated Motor
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\\\
RE: Current Inrush of a Re Accelerated Motor
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
RE: Current Inrush of a Re Accelerated Motor
http://www.measurlogic.com/pdf/dpi/pq6.pdf
RE: Current Inrush of a Re Accelerated Motor
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"
RE: Current Inrush of a Re Accelerated Motor
RE: Current Inrush of a Re Accelerated Motor
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
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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
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RE: Current Inrush of a Re Accelerated Motor
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