Jim Bryan
EASA Technical Support Specialist

The squirrel cage induction motor (SCIM) functions by applying a voltage to the stator winding and inducing a voltage across the air gap in the rotor circuit. The squirrel cage rotor consists of a lamination stack with slots to ac­commodate some number of rotor bars and shorting (end) rings that tie all the bars together. The squirrel cage consists of bars and end rings that are typi­cally made from alloys of aluminum or copper. This squirrel cage is illustrated in Figure 1 with the lami­nation stack removed.

Magnetic fields
When three-phase sine wave power is applied to the stator, the peak of the sine wave can be said to travel around the winding. As this peak travels over a set of rotor bars one pole pitch apart, it induces a voltage in those bars similar to the effect of a transformer primary on the secondary. The end rings provide a path for current flow produced by this in­duced voltage (see Figure 2).

Whenever a current flows through a conductor, a magnetic field is formed. The rotating peak of the sine wave voltage creates a rotating magnetic field. (See Figure 3.) The current induced in the rotor bars creates a matching magnetic field with an opposite polarity. These opposite po­larities are attracted to each other so the induced rotor field tries to follow the sta­tor field, thus producing torque to turn the shaft. The rotor field does not keep up with the stator field. This difference in speed is termed slip and produces current that results in torque. The more the difference in speed, the greater the torque produced. As the motor load is increased and more torque is required, the motor slows relative to synchronous speed; this is called increased slip. The more the applied load the greater the slip and the slower the motor turns.

Rotor bar material
The materials and profile of the rotor bars and end rings have an impact on this torque profile and the slip. The torque produced follows a curve based on these parameters as shown in Figure 4.

The National Electrical Manufac­turers Association (NEMA) and the International Electrotechnical Commission (IEC) separate these curves into classes that are then used to determine the suitability of a motor design for an application. Each design letter has minimum torque values es­tablished for significant points on the curve as shown in Table 1.

The rotor bar material affects this curve by its conductivity (the inverse of resistivity). Generally, the lower the conductivity, the higher the starting torque produced. Table 2 lists the conductivity of several common materials that might be used in a rotor. The designer uses these properties in choosing the material for the rotor. For instance, to meet the required torque values for an application, he chooses pure aluminum (M2) with a conductivity of 50-55%. If we decide to replace the bars and end rings with pure copper (M50), the conductiv­ity will increase to 100% and the starting torque will be diminished. If we want to use copper, an alloy such as M42 bronze with 55% conductivity will more closely match the original torque curve.

Rotor bar profile
The bar profile utilizes the skin effect of the conductor. That is, the higher the frequency, the closer to the outer surface of the conductor the cur­rent flows. The material in the center may not be used at all. Figure 5 shows a sample of the original power trans­mission cable coming from the power station at Hoover Dam in the U.S. This utilized the skin effect by leaving the conductor hollow with only a 0.125 inch (3mm) thick ring of conducting material. This resulted in a much lighter line and the ability to circulate coolant through the cable. 

The frequency of the current in the rotor circuit is equal to the slip frequency. At start up, the slip is 100%, so the rotor frequency equals line fre­quency, which would be 60 hertz (Hz). At this time, the current is concentrated in the portion of the bar closest to the air gap due to the skin effect as shown in Figure 6. As the motor accelerates, the slip is decreased and the rotor frequency goes down. At full load, the frequency will be very low. As an example, if a motor has a synchronous speed of 1800 rpm and a full load speed of 1740 rpm, the slip is 1800 – 1740 = 60 rpm. Dividing 60 rpm by 60 seconds/minute gives 1 cycle per second or 1 Hz. When this occurs, the current is evenly distributed throughout the bar cross section as shown in Figure 7.

A few of the possible bar shapes are shown in Figure 8. Many of these shapes take advantage of the skin effect to increase the motor starting torque. The top of the bar nearest the air gap has a smaller cross section thus decreasing the conductivity of that portion. This is called a dual cage bar because originally it was done with two bars of different size and material. This reduced conductivity increases the torque produced as the motor ac­celerates until at full load speed the entire bar is used to carry the current flow without overheating the bar.

Stator slot-rotor bar combination
Another important factor in ro­tor design is the relationship of the number of rotor bars compared to the number of stator slots. If enough rotor bars line up with stator slots, the torque can be affected. It is the misalignment of the bars and slots that produces torque; as more bars line up, the torque is reduced. Problems associated with this are known as cogging and torque cusps. A third problem can occur if the combination produces resonance in the motor components. The electrical noise produced by the resonating parts can be quite high. For our discussion, we will concentrate on the torque issues. Table 3 shows slot bar combinations that are known to produce these prob­lems. Subtract the number of rotor bars from the number of stator slots. If the result matches a number on the table, the indicated problem is likely. These combinations should be avoided. More information can be found on this sub­ject and on rotor bar skew to address the problems in the Currents article, “The effect of rotor skew, cusp and cogging on motor starting,” published in May 2015.

Conclusion
Although the rotor appears to be a black box, several considerations must be addressed to properly design and maintain the rotor’s contribution to the application of the motor. Changes in these parameters will result in changes in the performance of the motor and may be detrimental to the application of the motor. Understanding the in­teraction between the rotor and stator helps to achieve successful and reliable utilization of the equipment.

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Categories: AC motors, Rotors, Design/theory/application, Testing, Motor testing

Tags: Rotor design

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