A method of and an apparatus for starting a piston type compressor (C) driven by a permanent magnet synchronous motor (M) controlled by a frequency converter (FC), the method comprising at least following steps:
a) indicating the position of the piston highest pressure point and aligning the rotor of the motor (M) at the optimal starting angle prior to the actual compression operation by
providing a pre-pulse to rotate the rotor of the motor (M) at least one full rotation at a certain frequency (f 2 ),
during rotation measuring the load torque by means of the frequency converter as well as indicating the maximum load torque point and
at the end aligning the rotor of the motor (M) at the optimal starting angle of about 180° from the maximum pressure angle, and
b) at the beginning of the actual compression operation
setting the rotor position opposite to the maximum torque angle by feeding direct current to the stator winding and
starting the motor by supplying maximum motor current to the motor to give the maximum torque and acceleration.