Drag Cup Induction Motors

Guys, this is something i could know about drag cup type induction motors. If you know any additional thing, please share -
Whenever we talk of induction motors, invariably, we talk of two types i.e. squirrel cage induction motors and slip ring or phase wound induction motors. But only few of us might know about the kind of induction motor named as drag-cup induction motor. The name of this motor is on account of its peculiar rotor construction. This type of rotor is chiefly employed in AC servo motors in control system applications.
Construction: The key feature of this motor is its extremely low rotor inertia. Special arrangements are made in the design so as to achieve great reduction in the rotor inertia of the motor. For this, the mechanical linking of the current carrying conductors of the rotors with the magnetic circuit is removed. The rotor is made up in the shape of a drag cup. The rotor in this motor is directly coupled to the motor shaft. For an induction motor the value of slip at which motor gives maximum torque is given by Smt = R2/X2 where R2 = rotor resistance and X2 = rotor reactance. In drag cup type motor the value of rotor resistance is so adjusted to give the value of Smt = 1. Thus the motor gives maximum torque at the starting. The rated flux density is kept at a lower value so as to avoid non-linearity due to saturation effects.
Torque – speed characteristics: the rotor resistance adjustment (as discussed above) gives us torque-speed characteristic which is a fairly straight line. A straight line translates uniform and finite damping co-efficient as well as a properly defined transfer function for the motor.
The proper working of the motor greatly depends upon the fabrication of the motor. This is so because for this motor, a very thin walled cup has to be manufactured. This cup rotates in two thin annular air gaps. So any irregularity in the cup may produce unbalanced magnetic drag and may disturb proper functioning of the motor. Moreover, the length of airgap can not be increased beyond a limit as it would cause increase in the magnetizing currents and hence losses.
Applications: the motor finds applications in fields where instant rotation of motor is required. That is what this motor is meant for. Low inertia enables the motor to respond to even very small input signals. The motor is extensively used in feedback control systems as a derivative feedback giving instrument. It finds application is tachogenerators, odometers and rheometers. Rheometers are nothing but devices used to measure a liquid or slurry flows on application of forces. Rotational cylinder type rheometers employ drag cup type motors. However apart from this the motor is not as such used. That is the reason why most of us are unknown of this type of motor, but now you know!!
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