1.8 Startup Procedure

When the motor is in Stop condition, the position is usually unknown. The BEMF observer cannot work when the speed is lower than a minimum value which is determined by the motor and the application. There is a necessity of an open loop startup procedure, which allows the motor to reach the minimum speed at which the observer can work correctly. At this point the speed loop can be closed and the control assumes the form already described in the block scheme.

Since the effective position is not known, an arbitrary position (d, q) reference system will be used. Hence the quantities which are referred to as d-quantity or q-quantity in this process will not have any determined relationship with respect to the rotor position, until the speed loop is closed.

The important parameters to be chosen for the startup procedure are as follows:

• Start up speed: The speed reached during the startup phase. This is a parameter which can be modified by the user. It should be high enough to allow a good behavior of the phase estimation process.
• Start up time: The time required to reach the start up speed; it should be long enough to allow the estimation algorithm to stabilize (to recover from the initial condition errors).
• Start up current (i_start): The current level imposed during the startup; the value of this current (for a particular load) should be set to the minimum value of the current which can spin the motor in an open loop without stalling.