Tom Bishop, P.E.
EASA Senior Technical Support Specialist
Although unbalanced magnetic pull can affect other rotating electric machines such as DC motors and generators and single-phase motors and generators, our focus in this article will be on three-phase squirrel cage induction motors. The two main topics are unbalanced magnetic pull (UMP) and rotor pullover. For clarity we will begin by defining and briefly explaining these terms.
Unbalanced Magnetic Pull
Unbalanced magnetic pull (UMP) occurs when the rotor is not perfectly centered in the air gap between it and the stator, or the magnetic field is not perfectly uniform throughout the air gap. The result is that the magnetic attraction between the rotor and stator does not result in the exact same force all the way around the air gap. Simply put, UMP occurs when the air gap and or magnetic field between stator and rotor is not perfectly uniform. Thus, in any real (non-ideal) motor there will always be some level of UMP.
Twice Line Frequency Vibration
A power supply produces an electromagnetic attracting force between the stator and rotor that results in a frequency of vibration equal to two times the frequency of the power source (twice line frequency vibration). This vibration is extremely sensitive to motor foot flatness, frame and base stiffness, and how uniform the air gap is between the stator and rotor around the stator bore. It is also influenced by eccentricity of the rotor. Note: Reducing UMP also usually reduces the level of electrical noise.
A common misconception is that twice line frequency vibration varies with load. This comes from the belief that twice line frequency vibration excitation is due to a magnetic field generated by current in the stator that varies with load and creates a magnetic force that varies with load current squared. However, the ampere-turn strength of the stator and rotor tend to balance one another, except for the magnetizing ampere-turns that are created by the motor no load current. Thus, the main component of twice line frequency vibration is created by an unbalanced magnetic pull due to air gap dissymmetry and tends not to change with load.
On 2-pole motors, the twice line frequency vibration level may appear to modulate over time due to its close relationship with two times rotational frequency vibration. Problems in a motor such as a rub, loose parts, a bent shaft extension or elliptical bearing journals can also cause vibration at two times rotational frequency.
Nonsymmetrical Air Gap
Twice line frequency vibration levels can significantly increase when the air gap is not symmetrical between the stator and rotor, as shown in Figure 1.
This condition results in the force being greater in the direction of the smaller air gap. That is, an unbalanced magnetic pull will exist in the direction of the minimum air gap.
Force ≈ B2/d
Where:
B = Flux density
d = Distance across air gap
Not only is the stator pulled in one direction, but the rotor is also pulled in the opposite direction, to the side with the minimum air gap. This causes higher shaf t vibration, which is detrimental to bearing life. Note that in Figure 1 the rotor outside diameter (OD) is concentric with the axis of rotation, thereby causing the force to remain a maximum in the direction of minimum air gap. When there is stator-rotor contact, UMP typically results in a rub in a small area of the stator and around the entire outside diameter of the rotor.
Winding Changes
On motors where rotor pullover is a potential problem, single-circuit connections should be avoided. The influence of UMP can be decreased by increasing the number of parallel circuits or equalizing parallel circuits, particularly when the winding has two circuits. Another possibility, if the number of circuits is half the number of poles, is to use extra-long jumpers that interconnect groups on opposite sides of the stator. Examples include 1-10 jumpers for a 3-circuit 6-pole winding, and 1-13 jumpers for a 4-circuit 8-pole winding. Caution: Unequal grouping will limit the number of parallel circuits, often to no more than two circuits.
A redesign to a winding that results in increased magnetic flux densities makes the motor more susceptible to magnetic instability associated with UMP. Major factors for magnetic instability are the length of the shaft between bearings and the shaft diameter where the rotor core is mounted. The longer the length between bearings, or the smaller the shaft diameter under the rotor core (or both), the greater the potential for rotor eccentricity and possible stator-rotor contact. Since the air gap becomes smaller as the number of poles (e.g., 8 or more poles) increase, lower speed motors are more susceptible to magnetic instability than higher speed (e.g., 2 or 4 pole) motors.
Rotor Pullover
Rotor pullover means that the rotor core is being pulled (bent) toward the stator core, with a worst-case scenario occurring when the rotor comes into physical contact with the stator. UMP is a potential problem that can cause the rotor to distort and strike the stator winding. The magnetic pull varies as the square of the difference in the air gap (see Figure 2). Such things as eccentricity, rotor weight, bearing wear and machine alignment all affect the air gap geometry (see Figure 3).
The magnetic forces acting on the rotor are resisted only by the stiffness of the shaft. The more the shaft is deflected, the greater its resistance to being bent further. In a good design, shaft stiffness is more than adequate to resist the bending forces of an imperfect air gap. The UMP associated with rotor pullover varies as the square of the difference in the air gap (see Figure 2). The result may or may not be accompanied by physical contact with the stator. If contact does occur, the first evidence may be noise, vibration or catastrophic winding failure. If contact does not occur, evidence may be limited to noise or vibration.
Motor designers control UMP by limiting the acceptable amount of air gap eccentricity–usually to 5% of the average air gap for 2-pole motors and 10% of the average for motors with 4 or more poles. They also limit rotor runout to a maximum of 5% of the average air gap and select shaft size based on its ability to resist (shaft stiffness) these bending forces.
The potential for rotor pullover is a function of the air gap, concentricity, core length, air gap flux density and stator winding circuitry. The probability of rotor pullover into the stator is usually greatest during the starting cycle when the current is the greatest. If the rotor strikes the stator, it can usually be heard. Depending on the amount of contact, it may or may not
damage the rotor and/or stator parts. An inspection of the parts is the best way to confirm that this condition exists and how serious it is. The most common way to correct rotor pullover is to improve the air gap geometry by centering the rotor within the stator bore.
Broken Rotor Bar
If there is a broken rotor bar, no current will flow in that bar and no magnetic field will exist around that bar (see Figure 4). Therefore the force applied to that side of the rotor will differ from that on the other side of the rotor, resulting in an unbalanced magnetic force that rotates at one times rotational speed and modulates at a frequency equal to slip frequency times the number of poles.
This is one of the few conditions that cannot be detected at no load. However, if the rotor can be heated by means other than loading, such as reduced voltage locked rotor or multiple starts in quick succession at reduced voltage, another test can be performed. The rotor can then be checked for a thermal bow caused by the variation in rotor heating due to the broken bar(s). The resulting rotor unbalance and increase in UMP due to rotor eccentricity can create a high one times and a low level twice line frequency vibration. Rotor rubs due to rotor eccentricity typically show heavy smearing in a small area of the rotor’s outside diameter and around the entire stator bore.