Very high starting current of induction motors 3

[farooquekhann]

The starting current for some of our 6.6 kV motors is more than 12 times of rated current. What could be the possible reasons for such a high starting current?

 

[waross]

How are you measuring the starting current?

http://electrical-engineering-portal.com/motor-starting-problem-and-high-motor-inrush-currents[/URL]]Why is Inrush Current So Much Higher Than LRC?
The basic answer is… LRC is not the only component of inrush current. This raises the question: “What else is there?”

LRC is a steady state current. That is, it remains constant so long as the rotor is not moving. Motors, however, are highly inductive loads. Like all inductive loads they generate an initial transient (short lived) response which causes the load to draw more current.

The steady state LRC is symmetrical when voltage is near zero. The initial transient response raises the LRC curve so that it is no longer symmetrical – thus giving it the name “asymmetrical offset”. This asymmetrical offset usually lasts only a few cycles as the current settles to a normal steady state LRC, which dies off as the motor begins to rotate (refer to Figure 1).
Figure 1: Current waveform showing an asymmetrical inrush
Figure 1: Current waveform showing an asymmetrical inrush

The asymmetrical offset is dependent mainly upon at which point on the voltage wave the circuit is energized. If the circuit is energized at a voltage maximum, there is no asymmetrical offset and the inrush current is essentially the LRC for that current phase.

However, if the circuit is energized when the voltage is zero the initial inrush current is made completely asymmetrical, that is, shifted from the nominal current axis (refer to Figure 2).

This makes the inrush current greater than the LRC for that current phase.

Also, in a three phase system, the odds of one of the phases being at or near voltage zero when starting a motor is very high. This explains the source of nuisance tripping. Considering the actual asymmetrical inrush current could be, according to NEMA manufacturers, as much as two times the LRC. A HMCP circuit breaker (that is set based on the LRC) and is used with a high efficiency motor will experience nuisance tripping during energizing.

Thus, the inrush could be 18 times the FLC – much higher than the 13 times FLC that the HMCP circuit breaker may be set to by the NEC.

Bill
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"Why not the best?"
Jimmy Carter

 

[7anoter4]

Thanks,waross.

 

[LionelHutz]

It is a rather vague question without details such as how this current was determined or the motor data. It could simply be the motor design has a high LRC combined with a power system capable of providing that current without an excessive voltage drop which would instead further limit it.

 

[crshears]

Would have been a better fit in the Electric Motors & Generators Engineering forum...just sayin'

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]