Lost Amps

[Yorks Sparky]

Basically the circuit is of of an air heater battery consisting of : 3 x 2Kw heaters wired in star to a 3Ph & N supply , plus a capacitor start 1 Ph motor on L1 phase .
Total amps of heater = 8.5 amps / phase , motor draws 6.5 amps on L1 phase when run separately . But when both are run together the amps on L1 reads 11.5 amps !!!!! .
Any ideas people ?? . Theres something from the dim and distant past about this but dammed if I can remember . Any help or thoughts much appreciated .

 

[IQ Electrical]

The motor might draw 6.5 amps until the starter winding is disconnected at full load speed?

 

[Yorks Sparky]

Had clamp meters on supply to heaters & supply to motor & another one measuring total current of unit . 8.5 amps for heaters, 6.5 for motor , total current 11.5 all measured at the same time & when left running . Motor not running hot & heaters working ok . Brain now running hot !!

 

[Engineer54]

Had clamp meters ''on supply to heaters'' & '' supply to motor '''& ''another one measuring total current of unit'' . 8.5 amps for heaters, 6.5 for motor , total current 11.5 all measured at the same time & when left running . Motor not running hot & heaters working ok . Brain now running hot !!

How did you measure the total current with one amp meter?? You will not get a coherent reading placing a clamp amp meter around all three phase conductors... You need to check again, checking each phase conductor at a time, to find the actual current being drawn individually and the total current drawn by this equipment.


Only other way, is by use of a 3 phase amp meter, with 3 CT clamps, even then you would need to ensure you incorporate the L1 lead to the fan within the L1 CT clamp

 

[Knobhead]

Re-check the motor current when running. 1hp should only be about 3.5A. Also remember warm air is easier to move than cold, it makes quite a difference to fan loading.

 

[bomjac]

Think vectors and power factor. Highly resistive load of the heater + highly inductive load of the motor means that the total current on the phase is the vector sum of the two. (ie 8.5 amps @ 0 degrees + 6.5amps @ 60 degrees perhaps)

 

[Knobhead]

Just done a quick recalc. 1hp @230V .85pf & .85eff should equal 4.84A with the motor fully loaded. Have a look at http://en.wikipedia.org/wiki/AC_power if your interested in power factors.
But the fact that warm air is easier to move should be kept in mind.

 

[Yorks Sparky]

Thanks for the replies guys .
My quandary is over the total amps being drawnon L1 , IE motor is drawing 6.5 amps , heaters 8 amps both of which are OK but the total load on L1 is only reading 11.5 amps .All the reading were taken at the same time using CTs & remained constant . Still pondering over the fact that the total load should be 14.5 amps but the meter is reading 11.5 amps , ie 3 amps low .

 

[Des 56]

When all else defies logic,maybe the equipment that is giving those values should become the main suspect

 

[Yorks Sparky]

Thats what I thought , but all meters were checked & were found to be ok .
Should add we manufacture heating equipment but don't remember seeing this problem before .

 

[bomjac]

The total current in the leg supplying the motor / heater combo is the sum of the currents flowing through the leg.

What you have not taken into account is the fact that the current flowing through the heater is 8.5 amps at a phase angle of zero (due to pure resistive load) and the current through the motor is at a very high angle (almost purely inductive load). The only way you can add these currents is to add them as vectors, and the resulting vector is the proper current. By lag it means that the motor current is lagging behind the voltage (in this case maybe as much as 80 degrees) and the heater current is in phase (angle of zero degrees)

The (rough) sketch below shows this visually and should help you picture it better. The 80 degrees is a pure guess, it is way too late to even consider working out the proper angle so dont flame me if its wrong.

 

[Yorks Sparky]

The lagging was the bit the old grey cells had lost after 40 yrs . Even managed to find the old protractor , a few lines & one happy sparky . Many thanks .

 

[bomjac]

Ye sorry for being so abrupt on my original response, IIRC I was at work and pretending to work when i posted it. Surprised me at the level of reactive amps in a single phase. I knew they were pretty inneficent, but never realised they were as bad as that. From those values a 3ph Star connected motor would be much more appropriate, probably would drop your amps down to 1.5a per phase.

 

[Yorks Sparky]

No problem with orig response . Normally we do use 3 phase motors but these units are used by a petro - chem company and they have to be able to run them from a single phase supply if need be ( L1 L2 L3 strapped together) hence the 240V motor . Again many thanks .

 

[MarkieSparkie]

With an unbalanced three phase system the waveforms are likely to be a distorted sine-waves, particularly the heavy loaded phase, Unless you are measuring with a true RMS ammeter your measured magnitudes are likely to be erroneous when the motor is in circuit