**Subject :**Electrical Machines II (AC Machines)

**Unit :**Synchronous motor

## Synchronous-motor losses and efficiency

**Synchronous-motor losses and efficiency:**

*Fig: Power flow diagram for a synchronous motor*

The flow of power through a synchronous motor, from stator to rotor and then to shaft output, is shown in Fig. As indicated in the power-flow diagram, the total power loss for the motor is given by

Where:

*P _{scl}* = stator-copper loss

*P _{fcl}* = field-copper.loss

* P _{core}* = core loss

* P _{f,w}* = friction and windage loss

* P _{stray}* = stray load loss

Except for the transient conditions that occur when the field current is increased or decreased (magnetic energy stored or released), the total energy supplied to the field coils is constant and all of it is consumed as I2R losses in the field winding. Just as in the case of the synchronous generator, the overall efficiency of a synchronous motor is given by

Generally, the nameplates of synchronous motors and manufacturers’ specification sheets customarily provide the overall efficiency for rated load and few load conditions only. Hence, only the total losses at these loads can be determined. The separation of losses into the components listed in the equation of P_{loss} needs a very involved test procedure in the laboratory.

However, a closer approximation of the mechanical power developed can be calculated by subtracting the copper losses of the armature and field winding if these losses can be calculated. The shaft power can then be calculated subtracting the mechanical losses from the mechanical power developed.