mivey
Senior Member
When it starts.
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Is there a such thing as Asymetric Current?
- Thread starter Eduardo Maun
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mivey
Senior Member
So you are saying unbalanced voltage supplying a motor results in fault currents in the motor? Come on now.
Add: Asymmetrical fault current does not include all asymmetrical current in existence. See gar's post #5.
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mivey
Senior Member
Asymmetrical across the zero voltage reference plane. I know you know better than that. What are you driving at?
weressl
Esteemed Member
The initiation of the fault occurs at various voltage points between two phases. The phases are 120* to each other so for example when one crosses ) voltage the other phase is at a 2/3 peak value.
weressl
Esteemed Member
First, I did not say anything I just said listen to the kid. He did not say such a thing.
Unballanced load is not consdiered an asymetrical current.
Hameedulla-Ekhlas
Senior Member
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ok thanks for information. what I said was it right or wrong and please advise to know the mistake.
mivey
Senior Member
I know. He has only talked about fault current. I have no issue with fault current other than to say that might not have been the current they were asking about.
Please explain the purpose of asymmetric current controls on a motor then.
mivey
Senior Member
What you originally said was right. But it was not the only possible answer.
Hameedulla-Ekhlas
Senior Member
- Location
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I found some information in IEEE Std C37.96-2000-IEEE guide for AC motor protection
Please read it.
Motor circuits complying with the NEC are required to have one overcurrent unit (series tripping device,
protective relay, or fuse) in each phase conductor, or other approved means. When fuses are used, the consequences
of unbalanced operation and backfeeding of faults following a blown fuse must be considered. IOC
relays (50) are used to detect motor supply cable faults as well as severe stator faults. They may be connected
to trip directly, or through a short time delay to coordinate with the asymmetrical starting current
when set just above the locked-rotor current (see Figure 37 and Figure 38). When the motor kilovolt ampere
(kVA) rating is less than half that of the transformer (as a rule of thumb), instantaneous relays can be used
for phase protection. Where the starting current value approaches the fault current (motor kVA) greater than
half of the transformer rating), differential relays should be used.
Because of this, it will operate undesirably on the motor contribution to unbalanced faults on the supply system
and, therefore, must trip through a timer or be directionally supervised.
The application of phase-balance and negative-sequence overcurrent relays [and neutral overcurrent relays
(51N)] should consider CT characteristics under high-current conditions. Excessive CT burdens result in
current transformers saturating during fault conditions. Heavy motor-starting currents can also cause CT saturation,
especially when the starting current has a large asymmetrical dc component. This dc resulting from
motor inrush may last for a significant period of time, compared to an asymmetrical fault-current condition.
This is because of the much greater inductance/resistance L/R of the total circuit when starting a motor.
When the phase fault current at the terminals of a motor is considerably larger than the starting current or the
motor contribution to a fault, a high-set instantaneous trip unit can be set at 165% to 250% of locked-rotor
current to trip directly (see Figure 37). The general requirement for using this instantaneous unit or separate
instantaneous relay is that the setting be as low as possible, yet never operate during the starting period.
Because this type of relay may be susceptible to operation on dc offset, the inrush current value is multiplied
by a factor to account for the asymmetrical current value that may be obtained. The degree of offset is determined
by the angle of the sine wave at the time of motor energization. The duration of the offset is
determined by the X/R ratio. The factor?s value varies up to a maximum of 1.73. Higher asymmetrical currents
are usually associated with higher voltage equipment (4.16?13.8 kV) and stronger fault-current
IEEE
AC MOTOR PROTECTION Std C37.96-2000
sources. If the protective device effectively removes the dc offset from the current signal into it, the IOC can
be set more sensitively. If the inrush current is not accurately known, an additional 10% to 25% is usually
added as a safety factor when settings are calculated. The relay tolerance (usually 10%) is then added to
arrive at the final setting.
In instances where the bus may experience a fast transfer or reclosing, voltage and angle differences may
cause inrush currents exceeding those of normal motor starting. The instantaneous unit setting should then
consist of the calculated maximum asymmetrical current, at the maximum transfer voltage differential, plus
the relay tolerance and margin. Settings of 250% have been applied by some users.
Please read it.
Motor circuits complying with the NEC are required to have one overcurrent unit (series tripping device,
protective relay, or fuse) in each phase conductor, or other approved means. When fuses are used, the consequences
of unbalanced operation and backfeeding of faults following a blown fuse must be considered. IOC
relays (50) are used to detect motor supply cable faults as well as severe stator faults. They may be connected
to trip directly, or through a short time delay to coordinate with the asymmetrical starting current
when set just above the locked-rotor current (see Figure 37 and Figure 38). When the motor kilovolt ampere
(kVA) rating is less than half that of the transformer (as a rule of thumb), instantaneous relays can be used
for phase protection. Where the starting current value approaches the fault current (motor kVA) greater than
half of the transformer rating), differential relays should be used.
Because of this, it will operate undesirably on the motor contribution to unbalanced faults on the supply system
and, therefore, must trip through a timer or be directionally supervised.
The application of phase-balance and negative-sequence overcurrent relays [and neutral overcurrent relays
(51N)] should consider CT characteristics under high-current conditions. Excessive CT burdens result in
current transformers saturating during fault conditions. Heavy motor-starting currents can also cause CT saturation,
especially when the starting current has a large asymmetrical dc component. This dc resulting from
motor inrush may last for a significant period of time, compared to an asymmetrical fault-current condition.
This is because of the much greater inductance/resistance L/R of the total circuit when starting a motor.
When the phase fault current at the terminals of a motor is considerably larger than the starting current or the
motor contribution to a fault, a high-set instantaneous trip unit can be set at 165% to 250% of locked-rotor
current to trip directly (see Figure 37). The general requirement for using this instantaneous unit or separate
instantaneous relay is that the setting be as low as possible, yet never operate during the starting period.
Because this type of relay may be susceptible to operation on dc offset, the inrush current value is multiplied
by a factor to account for the asymmetrical current value that may be obtained. The degree of offset is determined
by the angle of the sine wave at the time of motor energization. The duration of the offset is
determined by the X/R ratio. The factor?s value varies up to a maximum of 1.73. Higher asymmetrical currents
are usually associated with higher voltage equipment (4.16?13.8 kV) and stronger fault-current
IEEE
AC MOTOR PROTECTION Std C37.96-2000
sources. If the protective device effectively removes the dc offset from the current signal into it, the IOC can
be set more sensitively. If the inrush current is not accurately known, an additional 10% to 25% is usually
added as a safety factor when settings are calculated. The relay tolerance (usually 10%) is then added to
arrive at the final setting.
In instances where the bus may experience a fast transfer or reclosing, voltage and angle differences may
cause inrush currents exceeding those of normal motor starting. The instantaneous unit setting should then
consist of the calculated maximum asymmetrical current, at the maximum transfer voltage differential, plus
the relay tolerance and margin. Settings of 250% have been applied by some users.
weressl
Esteemed Member
Never heard of it and it is not an IEEE defintion that is used in motor protection or control. Current or voltage unbalance relaying, yes. Asymetry control, no.
mivey
Senior Member
And do you think eveything you quoted is talking about the following?
weressl
Esteemed Member
In other words, all asymetrical fault currents are unbalanced, but only unbalanced fault currents are called asymetrical currents. Unbalanced currents due to the steady state or even transient load variant condition's resistance, impedance and voltage conditions are called simply unballanced but not asymetrical.
Hameedulla-Ekhlas
Senior Member
- Location
- AFG
yes, I was just telling regarding to asymmetrical current
mivey
Senior Member
That's just your opinion. That does not mean it matches the opinion of the test-giver. While it may not be common usage, it would not be un-heard of.
From a Siemens soft-start manual:
mivey
Senior Member
Asymmetrical Fault Current to be more precise.
mivey
Senior Member
Hameedulla-Ekhlas
Senior Member
- Location
- AFG
For finding the asymmetrical currents
IEEE Std C37.96-2000-IEEE guide for AC motor protection explains the below method:
the inrush current value is multiplied
by a factor to account for the asymmetrical current value that may be obtained. The degree of offset is determined
by the angle of the sine wave at the time of motor energization. The duration of the offset is
determined by the X/R ratio. The factor?s value varies up to a maximum of 1.73. Higher asymmetrical currents
are usually associated with higher voltage equipment (4.16?13.8 kV) and stronger fault-current
IEEE Std C37.96-2000-IEEE guide for AC motor protection explains the below method:
the inrush current value is multiplied
by a factor to account for the asymmetrical current value that may be obtained. The degree of offset is determined
by the angle of the sine wave at the time of motor energization. The duration of the offset is
determined by the X/R ratio. The factor?s value varies up to a maximum of 1.73. Higher asymmetrical currents
are usually associated with higher voltage equipment (4.16?13.8 kV) and stronger fault-current
weressl
Esteemed Member
That is Germlish, a translation from German to English by a German translator. The term is NOT used in the USA.
I am still waiting for your reference to the "asymetric current controls on a motor"!?
Hameedulla-Ekhlas
Senior Member
- Location
- AFG
no it is not translated please check the first page and read the red lines what is saying.
Sponsor
Power System Relaying Committee
of the
IEEE Power Engineering Society
Approved 30 March 2000
IEEE-SA Standards Board
Abstract:
Generally accepted methods of protection for ac motors are provided. This guide identifies
and summarizes the functions necessary for adequate protection of motors based on type, size,
and application. This guide does not purport to detail the protective requirements of all motors in
every situation.
Keywords:
ac motor protection, adjustable-speed drive motor protection, induction motor protection,
microprocessor-based motor protection, motor, rotating machinery protection, synchronous
motor protection, wound rotor motor protection
The Institute of Electrical and Electronics Engineers, Inc.
3 Park Avenue, New York, NY 10016-5997, USA
Copyright ? 2000 by the Institute of Electrical and Electronics Engineers, Inc.
All rights reserved. Published 12 September 2000. Printed in the United States of America.
Print:
ISBN 0-7381-2496-6 SH94857
PDF:
ISBN 0-7381-2497-4 SS94857
No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without the prior
written permission of the publisher.
Sponsor
Power System Relaying Committee
of the
IEEE Power Engineering Society
Approved 30 March 2000
IEEE-SA Standards Board
Abstract:
Generally accepted methods of protection for ac motors are provided. This guide identifies
and summarizes the functions necessary for adequate protection of motors based on type, size,
and application. This guide does not purport to detail the protective requirements of all motors in
every situation.
Keywords:
ac motor protection, adjustable-speed drive motor protection, induction motor protection,
microprocessor-based motor protection, motor, rotating machinery protection, synchronous
motor protection, wound rotor motor protection
The Institute of Electrical and Electronics Engineers, Inc.
3 Park Avenue, New York, NY 10016-5997, USA
Copyright ? 2000 by the Institute of Electrical and Electronics Engineers, Inc.
All rights reserved. Published 12 September 2000. Printed in the United States of America.
Print:
ISBN 0-7381-2496-6 SH94857
PDF:
ISBN 0-7381-2497-4 SS94857
No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without the prior
written permission of the publisher.
weressl
Esteemed Member
Stupid question. The world asymetric is only used in context of fault.
Voltage and winding impedance unballance will cause unbalance of current between the phases.
The answer would depend on what level of exam this is being asked. Is it an journeyman electrician's exam? A PE, or a graduate exam. The subject of a PhD thesis? The answers would cover a large range of responses, filling 10's of sheets of papers and lengthy calculations.
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