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# Subtransient reactance of induction motor

## Subtransient reactance of induction motor

(OP)
How can I know the subtransient reactance for an induction motor if it is not mentioned in the motor data sheet ?

### RE: Subtransient reactance of induction motor

1/(Motor stating current multiples) gives Motor sub transient reactance in Per Unit terms.

### RE: Subtransient reactance of induction motor

#### Quote:

(Motor stating current multiples)
I generally see staring current in Per Unit Terms, typically 500% or 600%.
The first cycle, fully offset, sub transient reactance current may approach a peak of 2.8 times the RMS starting current.
The starting current is related to the motor inductive reactance which in turn is related to the rotor frequency or slip frequency.
RMS of a sine wave times root 2 equals the peak. 1.414
For a fully offset condition, the peak will be 2 x 1.414 or 2.828.
The actual value of the fist 1/4 cycle peak varies from start to start and is dependant on, the strength and direction of residual magnetism in the motor core, and the point on wave at which power is applied.
In many years in the field, I have only had a couple of issues with sub transient currents.
The most common issue was a common breaker feeding a group of motors, started as a group.
When the stars aligned and other conditions were met, the sub transient current would occasionally trip the instantaneous element of the feeder breaker.
Example: FLA = 10 Amps
Starting current @ 600% - 60 Amps
First 1/4 cycle sub transient current may approach 170 Amps.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

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