Induction Machine

Consult your class or lab notes for the proper procedure to follow. If you were absent or inattentive that day then Google is your friend.

It's all in good ol' British Standard 7671 - again <sigh>.

Check the name plate for connection,afterwards apply rated voltage & measure no load starting & running currents by an analog clamp-on meter & let us know the readings. Normally LRC is about 6 to 8 times FLC. Confirm from manufacturers by giving them model,frame size,serial number etc

thanks for the response. I mailed the content to the manufacturers but they did not reply.

In the name plate, the connection is not mentioned.

name plate details: 5HP, 415 V, 7.5 A, 1425 rpm, Pf=0.83, Eff=0.85

The no load current (L-L) for delta connection is 3.8 A while same for the star connection is 0.9 A. As the no load current of induction machine is normally 30-50% of rated current, so it looks that machine is made to work in delta.

Comments!!

Then pick up the phone and use that instead. Time waits for no-one!

Can you measure no-load starting current too & inform us?.

At the instant of switch-on, the motor is not rotating, and has no idea what it is connected to mechanically. So the no-load starting current is the same as the full load starting current.

As a rule-of-thumb, one usually takes 5 times the full load current as stated on the rating plate attached to the motor.

When power is switched on to any motor the current taken has several modes like inrush,starting,no-load,even FLC,LRC currents like synchronous generator under fault condition taking subtransient,transient & syn reactances/current. I have asked OP to measure & give no-load starting current or can somebody give recorded/measured values or graphs by datalogger/poweranalyser when an induction motor is switched on at noload & with load.

Dear AAA65,for a 3hp 400V 5A delta motor noload starting current was 21A, give your reading

Sensible fellow, AAA665!

You went and measured it!

Note the maker did not give a rated voltage (415V) which could not be applied direct to a winding without exceeding ratings.

Your professor, by giving you a motor with two windings per phase, gives more options for measurement...

You can connect the two windings of a phase in series and connect these pairs in star to the 415V supply. This gives 120V per winding versus 415V rated. This gives a low flux density and maximum inductance - closest to a sinusoidal current on which the simple theoretical models are based. Obviously, locked rotor and pull-out torque will be minimal compared to 415V per winding.

Delta connection of series windings gives 207.5V per winding. Star connection of paralleled phase windings gives 240V per winding.

With 415V delta, you can try opening one winding in a phase and find what it does to current no-load and full-load in each phase.

Theoretically, with series windings, star, this motor could be 1610/930V supply - if the insulation would take it!

Someone needs to explain why the no-load current is of any particular interest.

It has the same importance as the no-load losses in a transformer, it gives the magnetizing branch impedance for the motor, plus the iron/no load losses for those people that like to keep their motors warm by running them with no mechanical load.

Then there are those charlatans who want to help you lower those losses using their magic boxes that somehow neutralize this current even though flipping the switch to "off" is a lot cheaper.