When a motor is started there is what people commonly refer to as "insush current" This inrush current consists of the magnetising inrush current to create the magnetic field in the windings and the motor starting current which is a function of the motors Locked Rotor Current. The magnetising current lasts briefly only for maybe a 1/10th of a second before it is diminished to the value of the starting current. The magnetising inrush current is higher than the starting current, however my question is by what factor? I have heard that a typical multiplier for this inrush current is 1.6 times the Locked rotor current of the motor? Does anyone know if there is a multiplier for determining this current?
Also I believe the LRC is a function of motor slip with the greatest LRC occuring when slip=1 or when the rotor is at rest. However if the rotor is already spinning (draft on a fan) when power is applied to the motor would the slip at start be something less than 1 and therefore make the LRC less than the LRC that the motor would have at rest? If this is the case then would the magnetising inrush current for a motor be less on a motor that had a rotor already spinning prior to applying power? Would this diminished magnetising inrush current value be a multiplier (as mentioned above) times the new LRC of the motor that is already spinning?
Also I believe the LRC is a function of motor slip with the greatest LRC occuring when slip=1 or when the rotor is at rest. However if the rotor is already spinning (draft on a fan) when power is applied to the motor would the slip at start be something less than 1 and therefore make the LRC less than the LRC that the motor would have at rest? If this is the case then would the magnetising inrush current for a motor be less on a motor that had a rotor already spinning prior to applying power? Would this diminished magnetising inrush current value be a multiplier (as mentioned above) times the new LRC of the motor that is already spinning?