165Diagnostics and TroubleshootingProblem Cause/Solution/TroubleshootEncoder slippage/mounting(PM motors)The position of the rotor must be known for synchronous (PM) motorsfor the drive to properly commutate the stator magnetic field andgenerate torque. Performing a encoder/rotor position learn (LF.03 =SPI or PLrn Encoder Pole Position Learn) determines a correspondingencoder position offset value for a given rotor position. The encoder isa mechanical extension of the rotor and therefore acts as an electricalcommutator.If the mechanical relationship (eg. mounting) between the motorand encoder changes (eg. slippage), the position information from theencoder does not accurately reflect the actual rotor position resulting inthe actual commutation angle being incorrect.When the commutation angle is not correct, more current is required toproduce a given amount of torque. Large enough changes will resultin very high current draw and low torque production. This leads tothe motor being unable to move (stalling) or unable to hold the load(movement in direction of load, eg. empty car counterwieights pull carup). In this case, the current is often reaching the corresponding 0.LF36Maximum Torque limit or the peak current rating of the drive,If the encoder/rotor position is re-learned and determined tobe different than the previous value of the LF.77 Encoder PolePosition by more than 4,000 counts, then this is a clear indicationthat the mechanical relationship between the motor and encoderhas changed.In most cases, encoder slippage has occurred or there is anencoder mounting issue. The accumulation of slippage may occurover distance (between a few inches of movement or the entirehoistway), over time (sometimes after several years of operation), orfrom a change in direction (sometimes due to loose encoder mounting).The suggested course of action would be to fi rst inspect the encodermounting (in many cases, the encoder may actually be mounted tight),remove the encoder and inspect again, and re-install the encoder thenrelearn the encoder position (it will be different than before the encoderwas removed). If issues persist, re-learn the encoder/rotor position.If large difference between learns persist (it is important to move carbetween learns to accumulate slippage, if this is the issue), continue toinspect the motor and encoder for mounting issues.
