Slip ring

Another concept is brushless excitation, in which an ac generator (exciter) is direc tfy coupled to or mounted on the motor shaft. The ac exciter has a stator field and an ac rotor armature which is directly connected to a static controllable rectifier on the motor rotor (or a shaft-mounted drum). Static control elements (to sense synchronizing speed, phase angle, etc.) are also rotor-mounted, as is the field discharge resistor. Changing the exciter field adjusts the motor field current without the necessity of brushes or slip rings. Brushless excitation is suitable for use in hazardous atmospheres, where conventional brush-type motors must have protective brush and slip-ring enclosures. 

Bowl drive shaft requires grounding other than through gear boj earings (e.g., brushes, slip rings)... 

This is a vital relationship, which reveals that during start-up and until such speed, the reactance of the motor windings / 2> the rotor current will also remain almost the same as the starting current and will fall only at near the rated speed. (Refer to the current curves in Figures 1.5(a) and (b)). The initial inrush current in a squirrel cage induction motor is very high. In a slip-ring motor, however, it can be controlled to a desired level. (Refer to Section 5.2.1.)... 

This is a very useful nomogram to determine the performance of a motor with the help of only no-load and short-circuit test results. In slip-ring motors, it also helps to determine the external resistance required in the rotor circuit to control the speed of the motor and achieve the desired operating performance. Slip-ring motors are discussed in Chapter 5. The concept behind this nomogram is that the locus of the rotor and the stator currents is a circle. Consider the equivalent circuit of an induction motor as shown in Figure 1.15, where... 

The maximum value of the output and torque of the motor can be obtained by dropping perpendiculars CC and CC3 on the output and torque lines respectively from the centre C.CjCi and C C4 indicate the magnitude of the maximum output and torque, respectively, that the motor can develop. This torque is the pull-out torque Tpp. In slip-ring motors it can be obtained at any speed on the normal speed-torque curve by inserting a suitable resistance into the rotor circuit to vary the slip. 

Because of heavy start-up inrush eurrents, the use of LT motors should be preferred up to a medium sized ratings, say, up to 160 kW, in squirrel eage motors and up to 750 kW in slip-ring motors. For still higher ratings, HT motors should be used. 

Figure 1.18(b) Screen protected drip proof slip ring motor (Cooling system ICOA1)... 

The electrolytes are non-corrosive and the electrodes do not corrode with time. This feature is of special significance when compared with an ordinary liquid resistance starter used commonly for slip-ring motors. Electrolytes do not deteriorate and therefore do not require replacement. The evaporated liquid can be replenished with drinking water when the level of the electrolyte falls as a result of evaporation. In Europe such starters have been used for over 15-20 years. Electrolyte switching is a costlier proposition compared to direct on-line or star/delta switching due to additional shorting contactor and timer, and the cost of electrolyte, its tank and thermostatic control etc. The cost may. 

Byatt. J.R., Selection of Primary Starters for Cage Motors and Secondary Starters for Slip-ring Motors, AOIP Electrical, UK. Kajiji, Y.H., Liquid Resistance Starting, AOYP Engineering Co. (P) Ltd, Mumbai, India. 

Starting and Control of Slip-ring Induction Motors... 

Automatic speed control of slip-ring motors 5/95... 

Since the performance of an induction motor can be varied by altering the rotor parameters, a slip-ring motor, through its rotor circuit, can be made to suit any specific torque and speed requirement. 

Restriction in starting current and a requirement for high starting torque to accelerate heavy rotating masses sometimes limit the use of a squirrel cage motor. For such applications a slip-ring motor provides a better alternative. 

Starting and control of slip-ring induction motors 5/85... 

The rotor voltage, refers to the standstill secondary induced e.m.f.. between the slip-rings. Whereas the rotor current, refers to the full load rotor current, when the slip-rings are shon-circuiled. 

Table 5.1 Recommended number of starling steps of a resistance unit to switch a slip-ring motor...

Table 5.3 Variation in torque and output with speed in slip-ring motors...

To achieve a better torque, the slip-ring rotors arc normally wound in star, in which case the rotor current is v3 time more than in delta for the same output. Also since the torque is proportional to the rotor current equation (1.1), the torque developed will be greater in this case. 

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