Types of Motors - 2


Vertically Mounted Motors
  
Vertically Mounted Motors are used for applications such as pumps and blowers. These motors are similar to their horizontal counterparts, except that they have few differences in the design. The most obvious example is in their ability to withstand axial loads.vertically mounted motors

Horizontally mounted motors do not have to withstand axial loads. Hence, roller bearings or ball bearings can be used. In vertically mounted motors, angular ball bearings are usually used to help the motor withstand axial thrust.

In case of higher thrust loads, special thrust bearings will be used.


Double Squirrel Cage Motors

One of the main drawbacks of the squirrel cage induction motor is that it has very low starting torque. Hence, for applications requiring high starting torques, the slip ring motor is the only choice.

However, by providing a double squirrel cage rotor to the induction motor, the starting torque of the motor can be increased.

The double squirrel cage induction motor consists of a rotor which has two cages. The outer cage consists of rotor bars with high reactance and low resistance. The inner cage, on the other hand, consists of rotor bars with low reactance and high resistance.

When the motor is started, the slip is high. As a result, the frequency of the currents induced in the rotor is high. This causes the rotor currents to flow in the outer cage. Thus the torque of the motor increases due to the high initial resistance in the outer cage.

When the motor reaches the rated speed, the slip between the stator and the rotor decreases and the frequency of the rotor currents drop. This causes the reactance of the inner cage to drop. Thus the current flows through the inner cage.

During normal operation, current flows through both the inner and the outer cages as the reactance is very low.

Thus, the double cage induction motor provides excellent speed torque characteristics during start-up.

The double cage induction motor is used in applications such as mixers and crushers in the industry where high initial torque is necessary. It is cheaper than the slip ring motor and does not require the complicated and costly starter circuits like the slip ring motor.


Applications of Squirrel Cage Motors

Squirrel Cage induction motors find wide applications in industries and in homes. Their rugged constructions and low maintenance makes them the motor of choice for many requirements.

Applications

Lathes and turning equipment
Pumps
Industrial Drives
Fans and Blowers
However, the downside of squirrel cage motors is that they draw a very heavy current when switched on due to the absence of back-emf. Hence, they require special starters.

They also have low starting torque. Hence, they are seldom used in applications such as lifts and cranes.

Another disadvantage of the squirrel cage motor is the poor speed control. However, with the advent of Variable Frequency drives, this disadvantage has been overcome.