Motor current confusion

Discussion in 'General Electronics Chat' started by GARYN, May 20, 2010.

  1. GARYN

    Thread Starter Member

    Sep 4, 2009
    If someone could clear up some slight confusion it would be much appreciated, i have read some contridicting answers on the net.
    I know the motor current formula is A= POWER/VOLTS*1.73*EFF.*P.F

    And i know the following for Star, I line = I phase, V line= 1.73*V phase

    Delta, I line= 1.73*I phase, V line= V phase

    Delta line current is 3 times more than if it was connected in star.

    Now the confusion starts, from what i have read you apply the top formula for calculating motor current weather its in Star or Delta.

    I know the nameplates tell you the info but sometimes it is missing or some of the data is worn away as i have had before.
    IF you know only the KW and Volts and want to know the current by applying the formula is this giving you Delta or Line current?

    If its the Star current and you want to connect it in Delta do you times this value by 3 to find delta value.
    If its the Delta value and you want to connect in Star, do you divide this answer by 3 to get the current.

    Or are there different formual for finding out the current of a motor if you connect it in star and delta (star /delta- starting).

    Am i over complicating this or missing something simple?

    Thanks in advance.
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    Both the line and motor winding currents will differ depending upon the connection.

    Star-Delta starting is commonly used (particularly with larger motors) to provide a soft start up of a motor rather than direct on line starting. You start in star to give a lower line (& winding) current then switch to delta mode to achieve rated motor torque / power.

    Consider a 3-phase supply with 240V phase-neutral voltage & 415V line-line voltage.

    For simplicity assume for a particular motor in star configuration that 240V across the motor windings gives a winding current of 10A. So the line current is also 10A.

    If the motor is switched to delta mode the motor windings are then connected to 415V. The winding current would then be 10*√3=17.32A. The line currents would then be 17.32*√3=30A.

    This line of reasoning really only applies at start up. With the motor running under load in either star or delta format would require a more detailed analysis based on a thorough knowledge of the motor parameters.
  3. GARYN

    Thread Starter Member

    Sep 4, 2009
    Thanks for the info, i understand the relationships you are describing but what i am getting at is you need to know one of the currents i.e STAR to use your formulas to find the current if connected in DELTA or vice versa. So if you only know the motor KW and Volts what current does the formula CURRENT=POWER/VOLTS*1.73*EFF*PF give you (star or delta) to give you a starting point (referance current to manipulate the formulas you gave).
  4. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    Your original formula starts with the motor Power.
    That will be very different if the motor is in star or delta and on the same voltage.

    A dual-voltage motor will have different sets of ratings for each mode.

    eg. a motor may be rated for 220V in delta and 415V in star so it can be used in different countries. The power rating will be similar in both modes but of course the current higher to achieve that when on the lower voltage supply.

    A motor instended for Star-Delta start will probably only have the power specified for Delta mode. The power in Star mode will be far smaller and currents also far lower - but then it's only running at a fraction of it's true power rating to allow a gentle start-up.
  5. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    It's been a long while since I looked inside a 3-phase motor terminal box. My recollection is that a motor configurable in either star or delta mode has a (minimum) 6 stud terminal arrangement. There are links used to connect the windings in star or delta - links in parallel from "top" 3 studs to "bottom" 3 studs would indicate the delta setup. Each phase is tied to a link. In star mode either the top or bottom row of studs are shorted and the phases are tied to the unlinked studs. There are other connections with more studs - such as you see in this link

    In the former connection (delta) arrangement the expectation is that the windings are at their rated voltage condition - since each winding is from line to line.

    Say you open the terminal box and note that you have an apparently delta connected 75HP rated motor [DOL start] with 415V 3-phase mains supply. If the efficiency is 94% at rated power and the power factor is 0.85 then the rated line current would be about 174 Amps. The windings would only have to be rated for ~100 Amps.

    If you open the terminal box and note the 75HP motor is apparently hard wired (linked) as star connected at 415V, you would have to surmise that the motor windings are not rated for 415V but rather for something like 240V. Since the motor is presumably still capable of delivering 75HP in this case then the line current is still 174 Amps. But the motor windings would also be capable of carrying 174 Amps.

    Presumably this is your dilemma - particularly where the nameplate is illegible and you basically only know the nominal rating. In the end you have to make an educated guess - perhaps as I suggest, based upon the physical connection of the windings as you find them.
  6. retched

    AAC Fanatic!

    Dec 5, 2009
    There is a way from measuring resistance between L1 and L2 lugs and L1 and 'N', but for the life of me, I cant remember. It was something along the lines of..If the resistance between L1 and L2 is more than twice L1 to 'N' it is delta...I cant remember.

    It may not even be true.. I remember being told this YEARS ago when I was an HVAC apprentice, just cant remember the method.

    Hopefully this will trigger someones memory and a response.
  7. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    Two ways you can do it;

    go to your manufacturers data, or go to the NEC and look it up in the table. The code inspector will look to see if you've provided the the apporpriate rated circuit including disconnect, wire size, overload, and overcurrent. They want nameplate, manufacturers data, then code book, in that order.
  8. GARYN

    Thread Starter Member

    Sep 4, 2009
    Thanks for the info, its been very helpful.
  9. russ_hensel

    Distinguished Member

    Jan 11, 2009
    You cannot compute the power using voltage times current in AC circuits unless you include the phase relationships. Sometimes this is done using complex numbers. I am sure there are lots of sources for info on this on the web.