Which of following statements is true for a slip ring induction motor? 

The correct answer is option 1):(Efficiency is less.)

Concept:

Slip-ring induction motor:

• A slip ring induction motor is referred to as an asynchronous motor as the speed at which it operates is not equal to the synchronous speed of a rotor
• The rotor of this type of motor is wound type. It comprises of a cylindrical laminated steel core and a semi-closed groove at the outer boundary to accommodate a 3-phase insulated winding circuit.
  
• The three terminals of a rotor and three start terminals connecting through slip rings are connected to a shaft. The aim of the shaft is to transmit mechanical power.
• Construction is complicated because of slip rings and brushes.
• Frequent maintenance is needed for slipping induction motors due to brushes and external  resistance
• High starting torque compared to squirrel cage induction motors about 2.5 times full load torque. Pull-out torque can be achieved at even zero speed. Squirrel cage induction motors take 6 to 7 times the full load current but slip-ring induction motors take a low starting current of approx 2.5 to 3.5 times the full load current.
• The starting torque of a slip ring induction motor can be increased by adding external resistance to the rotor ​
• Slip-ring induction motors have lower efficiency than squirrel cage induction motors. Hence option 1 is correct
• The power factor of slip-ring induction motors is low as compared to squirrel cage motors. The operation of the slip ring induction motor is more sensitive to the fluctuations in supply voltage as compared to the squirrel cage motor