In　order　to　realize　the　wide　speed　range　of　the　sensorless　control　for　permanent　magnet　synchronous　motor　(PMSM),　a　composite　control　strategy　of　I/f　startup　control　combined　with　the　sliding　mode　observer　(SMO)　control　is　proposed.　In　low-speed　region,　I/f　control　with　speed　open　loop　and　current　closed　loop　is　adopted.　The　rotor　is　accelerated　from　standstill　to　a　certain　speed　by　following　gradually　increasing　frequency.　In　high-speed　region,　the　SMO　is　used　to　achieve　sensorless　control,　where　the　saturation　function　is　adopted　to　decrease　the　chatter.　When　the　rotor　speed　reaches　the　target　value,　the　smooth　transition　of　the　two　control　strategies　is　achieved　by　designing　a　proper　transition　status.　As　a　result,　the　experimental　results　verify　that　the　sensorless　control　system　can　work　reliably　and　stably　with　smooth　switching　transition　and　good　control　performance.　©　2020　IEEE.