The salient pole synchronous machine without any field or extiting winding runs at synchronous speed due to the reluctance torque and is termed Reluctance Motor. These motors find application in automatic control systems, telemetering devices and systems, signalisation circuits, sound film systems etc. requiring synchronised or simultaneous movements. Simplicity of construction is the main attraction of such motors. They are generally required in fractional kW sizes having ratings from a few watts to several hundreds of watts.
Reluctance motors are started automatically by the induction motor torque developed by the currents induced in the solid rotor of the motor. Pull into synchronisation takes place due to the reluctance torque which is developed due to the tendency of the rotating magnetic field to retain the running rotor in minimum reluctance position to field flux. The condition is met when the rotor runs in step with the field and when the pole-axis coincides with the magnetic flux axis. As soon as the rotor is pulled into synchronisation, the relative speeed between the rotating field and the rotor core vanishes and the 'Induction Motor' operation ceases. When the motor is loaded, the axis of the poles shift away from the stator poles (a rotationg field axis) towards lagging as shown in figure b.
In a machine with cylindrical rotor, as shown in figure c, the reluctance torque is not created because of the lack of any difference in position of the rotor relative to the stator field.
The magnetic field of a reluctance motor is produced due to the armature reaction magnetic flux and therefore the motor draws the reactive (lagging) current required for producing the magnetic field from the power circuit and consequently operates with low power factor.
The analysis of the motor can be done following the method of analysis of salient pole synchronous motor with zero excitation. Phasor diagram of both the machines are given below:
From the above phasor diagram, we see that—
Ia = Id + Iq
Ut = Ud + Uq
Ut = Uf + jIdXd + jIqXq + IaRa (for synchronous motor)
Ut = jIdXd + jIqXq + IaRa (for reluctance motor)
and with the positions of phasors as in figure above, we have—
Id = Id + j0
Iq = 0 + jIq
Ud = −Ud + j0
Uq = 0 + jUq
So we get—
Ut cos δ = Uq = Uf + IdXd + IqRa (for synchronous motor)
Ut cos δ = Uq = IdXd + IqRa (for reluctance motor)
Ut sin δ = Ud = IqXq − IdRa
Now to get power equations, we have—
Complex power S = P + jQ = UtIa* = (Ud + Uq) (Id + Iq)*
= (– Ud + jUq) (Id + jIq)* = (– Ud + jUq) (Id − jIq)
= (– UdId + UqIq) + j(UqId + UdIq)
So, P = – UdId + UqIq and Q = UqId + UdIq
Using above equations and negecting armarure resistance (which is usually small, we get for synchronous motor—
and for reluctance motor—
So the maximum power of reluctance motor occurs at δ = 45° and maximum power is given by—
Power angle curves of both the motors are given below—
The maximum power of a reluctance motor increases with the increase in Xd/Xq. Measures are taken to increase the ratio as large as possible and for this purpose, rotor may be assembled at steel plants with non-magnetic strips (like Aluminium) inserted between them as given below—
Single phase reluctance motors are also used in practice and these are like capacitor motors.
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