Rotor bar

The movement of rotor teeth around the stator produces a clogging effect, resulting into vibrations and noise. To reduce this effect, the common practice is not to provide the rotor slots parallel to the shaft axis but at an angle. This practice is known as rotor bar skew . A proper skewing can also improve the starting torque and reduce the starting cunent, in addition to the effects of space... 

To ascertain whether the stator or the rotor would fail first during a stalled condition, the thermal withstand time of the rotor should also be determined separately for the rotor bars and the end rings. The lowest values for the stator or the rotor will be the safe stall time for the entire motor. The limiting temperatures in rotor components may be considered as follows ... 

Excessive vibrations according to international codes can cause mechtinieal failure in the insulation by loosening wedges, overhangs, blocks and other supports that hold the stator and the rotor windings or rotor bars in their slots. Vibrations also tend to harden and embrittle copper windings and may eventually break them when they become loose (see also Sections 1 1.4.6 and I 1.4.7). 

Rotor stampings are loose or rotor bars are damaged... 

Misalignment causes vibrations, which may eventually lead to failure. The vibrations may also cause cracks between the rotor bars and the end rings. 

The unbalanced voltage will produce an additional rotor current at nearly twice the supply frequency. For example, for a 2% slip, i.e. a slip of 1 Hz, the negative sequence stator current, due to an unbalanced supply voltage, will induce a rotor current at a frequency of (2/- 1) = 99 Hz for a 50 Hz system. These high-frequency currents will produce significant skin effects in the rotor bars and cause high eddy current and hysteresis losses (Section 1.6.2(A-iv)). Total rotor heat may be represented by... 

Loose rotor bars are a common failure mode of electric motors. Two methods can be used to identify them. 

The first method uses high-frequency vibration components that result from oscillating rotor bars. Typically, these frequencies are well above the normal maximum frequency used to establish the broadband signature. If this is the case, a high-pass filter such as high-frequency domain can be used to monitor the condition of the rotor bars. 

The second method uses the slip frequency to monitor for loose rotor bars. The passing frequency created by this failure mode energizes modulations associated with slip. This method is preferred since these frequency components are within the normal bandwidth used for vibration analysis. 

The Fitzpatrick Co. roller compactor (Fig. 20) features two feed screws in series one horizontal and the second vertical. This system is designed to transfer powder bulk into a set of cantilevered rolls positioned in the horizontal mode one is fixed, the other floats. Operator interface allows for on-line monitoring and controlling feed screw speeds, roll pressure, and roll gap. Features such as on-line help and diagnostic functions, maintenance, and calibration screens are displayed. All functions are interfaced and adjustable by PLC for process control and report generating. The machines have vacuum deaeration and roll cooling capabilities. The sizing unit is separate from the compactor and is either a rotor bar or a hammer mill. 

However, up to 30% of the power consumption of a motor can be attributed to no-load losses because of windage (by cooling fan and air drag), friction in the bearings, and core losses that comprise hysteresis and eddy current losses in the motor magnetic circuit. Load losses include stator and rotor losses (resistance of materials used in the stator, rotor bars, magnetic steel circuit) and stray load losses such as current losses in the windings.f ... 

Look for cracks near the end rings or broken rotor bars. A new rotor may be required as repairs are usually temporary. 

To increase efficiency, it is necessary to minimize these losses. The rotors of energy-efficient motors are built with additional aluminum to reduce losses resulting from current flowing in the aluminum rotor bars. Additional copper is used in the stators to reduce losses in the motor. Most motors have 100 percent copper stator windings few are aluminum-wire-wound. More steel, together with special processing, is also used in the motors to reduce the... 

Since induction motors rely on rotor bars or windings to cut the flux of the rotating field to turn the rotor, they will operate at a speed slightly less than synchronous speed. 

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