Thursday, January 17, 2013

Motor Action

Motor Action

Owing to the identity of generator and motor structure, a generator may be made to operate as a motor . this is especially simple to do in the case of a shunt generator operating into a d.c. line . in this case, it will suffice to bring down the field current so that the armature emf becomes smaller than the line voltage . As a result , the greater line voltage will cause a reversal of the armature current Iarm which will now be produced by the difference between the line voltage and the armature emf

Iarm = (VEarm)/rarm                               (13.6)

By interacting with the magnetic field of the machine, this current will produce a driving rather than a braking torque. Owing to the driving torque, the armature will no longer need a prime mover for Its rotation, and it can be disengaged. In this way, an electric ma­chine can be switched from generator action into motor action.

Of course, proceeding in reverse, we can switch a shunt-wound machine from motor action into generator action, provided there is a prime mover available to drive it. It should be stressed, however, that such a transition from generator action to motor action without making any changes in internal connections applies only to shunt­ wound machines. In the case of a series-wound machine, a transi­tion from motor to generator action necessitates the reversal of cur­rent in the armature or field winding.

Figure 36 shows how power is distributed in a motor. The pow­er PI taken from a supply line is divided between the armature circuit, Parm (which is the greater portion of the total), and the field circuit, Pt = VIf (which amounts to a few percent of the total). A small fraction of the armature winding. and the remainder is converter to mechanical power Pmech . By subtracting from it the core (or iron ) losses Pc (due to hysteresis and eddy current ) , and the friction and windage losses Pf,w (in the bearings, at the commutator, etc.), we obtain the useful power available at the motor shaft, P 2 .

Motor Action

Special mention should be made of the part played in a motor by the armature emt, Earm . Since its direction is opposite to that of the current , it is called the counter-emf . on the basis of Eq . (13.6) ,

the voltage across the armature terminals is

V = rarmIarm + Earm

This equation can be converted into a power equation if we mul­tiply its left- and right-hand sides by I arm

VIarm = Parm + Earm

This equation can be converted into a power equation if we multiply its left and right-hand sides by Iarm

VIarm = Parm = rarmI2arm + EarmIarm = rarmI2arm + Pmech

because

VIarm - rarmI2arm = Pmech = EarmIarm

is the mechanical power developed by the machine.

As follows from the power equation, the power in the armature circuit is the sum of the copper loss. or the power lost as heat, rarmI2arm end mechanical power. The latter is directly proportional to the counter-emf Earm . The presence of counter-emf is typical of the conversion of electric into mechanical energy by an electromag­netic device. Hence we may conclude that the higher the counter­ emf Earm , the higher the efficiency of a motor. For this reason, the armature voltage rarmIarm during operation of medium and large motors is a few percent (2-5%) of the rated voltage.

We have examined how a machine can be brought out of gener­ator into motor action. In most cases, however, an electric machine intended to operate as a motor is caused to do so by suitably star­ting it, for which purpose it is connected to a d.c. line. In the circum­stances, no emf is induced and V = rarmI arm so long as the armatu­re remains stationary (n = 0). The armature winding resistance rarm is relatively constant and small, so, unless measures to the contrary are taken, the armature current I arm at starting would be 25-40 times the operating current in the steady state . such a starting current would be disastrous for the commutator , armature winding, and line. It is avoided by connecting a field rheostat, rr, in series with the armature winding in all d.c. motors except those rated at not over 0.25 kW.

In the steady state, the speed of a d.c. motor, Eq. (13.1), is inver­sely proportional to the main magnetic field Φ and directly propor­tional to the terminal voltage VEarm. Therefore it can he ad­justed by varying either the magnetic flux or the voltage across the armature.

A d.c. motor can be reversed by reversing the current in the field circuit or the armature circuit of the machine. If the current is re­versed in the two circuits at the same time, the machine will keep rotating as before.

The performance of a d.c. motor depends on how its main mag­netic field is excited.