3 phase Current Rating 3

[alekky]

Could someone tell me if the current rating on the name plate of a 3 phase 415v piece of equipment, refers to the line current or the phase current.

Thanks for any replies

Alekky

 

[motorsdirect]

The current ratings on a three phase motor normally refer to full load current per phase.
Jeff

http://www.motors-direct.co.uk

 

[tinfoil]

On a balanced 3-phase motor, why would the phase current and line current values be different?

 

[rbulsara]

The name plate rating is always "Line" current that you will measure in each line (conductor) connected to 3 motor terminals or to be more specific input to the motor controller.

The current in the actual windings depends on how they are connected. If only three conductors are connceted to the motor terminal, you need not worry about the motor is start or delta connceted. Most motor are delta connceted.

In case of a motor with a star-delta starter, the rated current in on the line side of the starter and you will have six leads conncected between the motor and the starter. In this case during normal run, (delta connection) the current is each of those six conductor will be 0.58 times the line current,as they are part of the windings (phase) conceted in delta.

 

[alekky]

Thanks for the replies

Tinfoil

The phase current and line current is equal with a star connection ( Ip = Il ), but with a delta connection, the phase current is equal to the line current divided by 1.73 ( Ip = Il / 1.73 , this applies to balanced loads.

Rbulsara

So if the nameplate says 25 amps, and the motor is connected in star, then the current in each of the 3 conductors / lines is 25 amps, and also the current through each winding is also 25 amps.

But if the motor is connected in delta, and the nameplate says 25 amps, does this refer to line current or winding current.

Sorry if i'm repeating myself, but this has got me confused.....Doh

 

[rbulsara]

Line current and NOT the winding current.

 

[rbulsara]

To be more specific Line current in its designed configuration.

Motor with six leads, the nameplate current is the rated current, at full load while connected in delta, unless indicated otherwise.

You can have a motor which is desinged to operate in star conncetion such as most medium voltage motors.

But the nameplate rating is always the "Line Current"

 

[busbar]

 
Problems with basic terminology—on an international basis in electrical subjects (within a discussion group like Eng-Tips.com} seem to stem from engineers on one continent trying to {sometimes mistakenly} envision practices and terminology on another continent.

For IEC regions, it seems like low-voltage motors are fairly uniformly designed with six-lead stators, with nameplates usually reflecting ratings with the external connections forming wye or delta arrangements—like ? (230V) and Y (400V)—having a logical root(3):1 relationship.

For ANSI areas, low-voltage motors may be found as nine-lead motors with nameplate voltages having a 2:1 ratio—230 and 460V, but generally not externally rewireable as ? or Y configurations. The voltage selection is made with stator coils externally configurable to parallel (230V) or series (460V) arrangements.

The IEC six-lead practice seems to default in ANSI medium-voltage motors—often having nameplate voltages of ? (2300V) and Y (4000V.)

One is not necessarily right or wrong—just differences in addressing the local/historical practice of “dual-voltage” motors.

 

[UKpete]

Alekky, I'm guessing you're from the UK as you mention 415V plus we have "lekkies". It is now officially 400V although if you measure it in the UK it is usually around 415V.

Motor nameplates refer to line currents not winding currents, as stated by rbulsara. It doesn't actually specify this in IEC 60034-1 but I think it is assumed because the nameplate information is intended for the user; only a motor designer would actually be interested in the value of winding current.

Confusion can arise because IEC nameplates often quote ratings for both the star and delta connection (as described by busbar). This is because the larger motors say 3kW and above typically have six wires i.e. both ends of each phase are available in the terminal box to allow the use of a star-delta starter. The direct-on-line connection should be delta. Lower power motors for which star-delta starting isn't necessary may be star connected.

So the dual ratings on the nameplate are not really necessary in my opinion and only serve to confuse (e.g. on a large motor we only want the delta connected information as we don't have a 3-phase system with 230V line voltage to enable the use of the star connection).

But whether you connect it star or delta on a supply of appropriate voltage, the rated currents always refer to LINE currents.

 

[motorsdirect]

YEH. Sorry for any confusion...terminology and waffle at its very best!!
The current raitng on the motor plate is Line current per phase, and is normally the full load rating.
hope you have it all clear now.
Jeff

http://www.motors-direct.co.uk

 

[alekky]

Thanks for all the replies lads, this is my first post on this site, and you've all helped me alot.

Ukpete, yes i'm in the UK, up in the North East.

My handle comes from when i was an apprentice, i had a job to do in the deepest darkest part of the petro chem plant, where the operators used to eat apprentice electricians for their breakfast, one of the operators said to me " you must be Alec" to which i replied no, my names Graeme, and they all started laughing, i was very naive in those days, and it wasn't untill i told my Electrician, that he explained that all Electricians are Alecs, hence the handle Alekky.

Regards
Alekky