Is the locked current rating of a motor the highest current I should expect when starting a motor? Is the locked rotor current rating the same as magnetization current? If not then how should the magnetization current be considered when selecting motor protection?

  • \$\begingroup\$ If the motor happens to be spinning in the opposite direction when you apply power then the current will exceed the LRA, possibly to the point of damage. \$\endgroup\$ – Spehro Pefhany Sep 20 '14 at 0:05
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    \$\begingroup\$ Thank you for that. I think thats what is happening in our case. \$\endgroup\$ – SivaDotRender Sep 21 '14 at 16:50

The magnetization current of an AC motor would be that current that flows into the stator should the rotor be removed. It's difficult to measure for that reason because any amount of movement/loading of/on the rotor is going to affect it.

Like a transformer, an AC induction motor has an equivalent circuit and it is sensible (for both motor and transformer) that the magnetization current is significantly lower than the load current due to the rotor transferring power to something (or the secondary winding transferring power to a load).

This diagram shows the equivalent circuit and note Im: -

enter image description here

Current Im flows thru inductor Xo and this current is largely unrelated to what happens on the rotor. It follows that the true magnetization current of the machine is with the rotor removed.

  • \$\begingroup\$ Thank you for the answer. By the looks of it the magnetization current should be insignificant when selecting motor protections. This booklet was also a great resource jimfranklin.co.uk/microchipdatasheets/00887a.pdf \$\endgroup\$ – SivaDotRender Sep 19 '14 at 15:19
  • \$\begingroup\$ The magnetising branch for a blocked rotor test is assumed to be absent, just as in a transformers short-circuit test \$\endgroup\$ – JonRB Sep 19 '14 at 22:32
  • \$\begingroup\$ @jonrb is the comment for me or the op? \$\endgroup\$ – Andy aka Sep 19 '14 at 23:19
  • \$\begingroup\$ OP. Blocked rotor is one of the two characterising tests, it is done at reduced volts to produce rated current. If he is after peak current at could be seen (apart from the 5x startup current) then a stalled rotor will be the other case & the mag branch can be ignored \$\endgroup\$ – JonRB Sep 20 '14 at 6:30

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