Tuesday, February 26, 2013

Scherbius Drive of Electrical Machines

      Scherbius Drive is an early method for adjustable speed drive of wound rotor induction machines. It can handle speed regulation using  Scherbius Regulating Machine and an Autotransformer that includes tap changer. User can adjust the required speed using the tap changer of the autotransformer. The main motor speed is independent of the load conditions. 

      Scherbius drive is more efficient than rotor resistance speed control method and speed regulation is independent of the load condition. However, the overall system is very complex, bulky and costly. This kind of motor drive technique is now obsolete due to the rapid development of semi conductor business. The new kind of motor drive techniques are totally contains the semi conductor based inverters. The overall efficiency of the drive is very high, size and cost are significantly lower than Scherbius Drive technique.

Sherbius Drive for Wound Rotor Induction Machine
Characteristics of Scherbius Machine;
Scherbius Machine is actually a polyphase commutator machine with 3 phase stator exciting windings and a commutator set in armature. The rotor is wounded single phase but designed to give 3 phase voltage ;therefore, three sets of commutator carbon brushes placed 120 degrees apart electrically to for 3 phase operation.

The stator excitor winding is energized with the rotor current of the main induction machine. The excitor induced emf magnitude can be calculated with the following well known formula;

e = 4.44 * ɸ * Ns * f

Note : The actual signal is a sine wave with frequency f, this formula gives the magnitude.
Similarly induced emf in rotor commutators will be 

E = 4.44 * ɸ * Nr * n * p/2

Ns is effective number of turns in stator windings
Nr is effective number of turns in rotor windings
             ɸ   is flux per pole
             f   is the frequency of stator exciting voltage

Note : Please see the topology for rotor and stator connections. Rotor and stator terminals are connected each other via an autotransformer with changing turns ratio say Kt.

E / e = Kt

Kt = (Nr * n * p) / (Ns * f * 2)

f = (Nr * n * p) / (Ns * Kt * 2)

Please notice that the frequency is totally depending on Kt , since pole number and number of turns values are constant. Moreover, since the Scherbius machine rotor is coupled with an auxiliary induction machine the n value will be nearly constant ( only very small slip changes). Thus, the frequency is determined by the transformer tap postion.

Note : If the Scherbius Machine was a slip ring machine, the rotor frequency will be f –fr. However, commutators changes the reference frame to synchronously rotating reference frame with speed fr, which will in turn gives the output frequency f (same as the one for stator side).

Note : The recovered energy from main motor is converted to mechanical torque at the rotor of Scherbius Machine, which returns the energy to the mains with the help of aux induction machine.

Note : The topology provides a closed loop control of rotor current such that the required speed is always constant with adjusted autotransformer tap position.

REFERENCE: ADKINS, B. , GIBBS, W. J. Polyphase Commutator Machines. 1951

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