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Motor FLC, Rated current, running current and nominal current.

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Good evening,

1) Is the motor current on the nameplate always the full load current: the maximum current the motor can draw without there being damage to its winding insulation.

2) Often in motor books, I see the term nominal current? is this the current that the motor draws under normal operating conditions under load?

3) What the the motor rated current? is this the same as the FLC?





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Hello Ewan

Yes, confusing isn't it. Many terms that look the same and aren't, and many others that look different but are actually the same!!

When it comes to motor ratings, there is a maximum current rating that the motor should be operated at and this is referred to as the nominal current, or the rated Full Load Current. Operation at current levels above this will not cause an immediate failure, but will cause the operating temperature of the windings to be elevated and shorten the life of the insulation.

A good rule of thumb to apply to insulation products is that "Every Ten degree C rise, halves the life of the insulation" If the ambient temperature is very low, then you can run at a higher current as it will take a higher power dissipation to lift the temperature to the maximum temperature.

Another myth is al about temperature rise. The performance of the insulation is about end point temperature, or absolute temperature. Temperature rise calculations assume a prescribed ambient temperature to start with and I have never seen a fixed ambient temperature! You need to be very careful of the ambient temperature on which the ratings are based. - some times high at say 45C and sometimes low at say 25C.

In USA, it is also common to see a service factor applied. You can reduce the rating and allow a compensating overload so that the average power dissipated is equal to the full load dissipation.

Bottom line, talk to the manufacturer of the plant you are looking at. They are not all the same!! Comparisons must be under the same conditions.

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When the rotor is not turning, we refer to it being at 100% slip. (The difference between the rotor speed and the supply frequency is 100%)

At 100% slip, the induction motor will always draw Locked Rotor Current  (LRC) irrespective of the shaft load.

The LRC of an induction motor is a function primarily, of the rotor design and on older motors would typically be 550% to 650% of the Full Load Current of the motor. With modern high efficiency motors, this is more commonly in the range of 850% to 1150% FLC.

When we connect a "Delta" motor in star, the effective motor ratings drop to one third. The FLC drops to one third and the LRC will also drop by a similar ratio.

So if we look at an older motor, I would expect that the start current at zero shaft speed would be around six times the FLC of that motor if Delta connected, and around 6/3 time FLC for the motor connected in star. In reality, the Start current in Star will not be exactly as suggested above because of impedances reflected back to the supply by the difference wiring configuration.

If we consider a high efficiency motor with a LRC of 900%, then I would expect that the start current in star to be close to 300% FLC  (900% / 3).

Many text books and papers do not reflect the increased start current caused by the shallow bars in modern high efficiency motors.

Best regards,


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