Electricity: motive power systems – Induction motor systems – Primary circuit control
Reexamination Certificate
1998-10-06
2001-12-18
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Induction motor systems
Primary circuit control
C318S729000, C318S771000, C318S794000, C318S795000, C318S796000, C318S814000, C310S179000, C310S180000, C310S184000, C310S200000
Reexamination Certificate
active
06331760
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of electrical motors, and more specifically to high efficiency polyphase induction motors.
2. Description of Related Technology
Induction motors are well known in the electrical arts. The typical induction motor is comprised of 1) a series of electrically interconnected windings (one for each phase in a polyphase system), 2) a stator containing these windings and having a magnetically permeable core, and 3) a rotor having an electrically conductive structure (“cage”). The polyphase windings are most often arranged in the well known “delta” or “wye” configurations, as show,n in
FIGS. 1
a
and
1
b
, respectively. While single phase induction motors can be made, such motors develop no starting torque, and therefore require a separate motive force to initiate rotor movement. For these and other reasons, the great majority of all induction motors in use are polyphase (typically three-phase).
As its name implies, the induction motor works on the principle of inductive (versus electrical) power transfer from the stator to the rotor. In simple terms, power is transferred to the rotor by virtue of the electromotive force (emf) induced in the rotor conductors due to the relative motion of those conductors through a magnetic field. Since no torque on the rotor is generated when no relative motion exists, the rotor must necessarily turn slower or “slip” in relation to the stator field. This slip is typically on the order of 2-5%. Generally speaking, greater slip corresponds to greater rotor torque and greater current draw. Note also that a secondary magnetic field is generated by the rotor due to the current low within the rotor conductors. The electromotive force (emf) induced within the rotor “leads” the induced rotor magnetic field, since the rotor conductors have a non-zero reactance.
The polyphase induction motor provides several advantages over other types of motors, including most notably a comparatively simple construction which obviates the need for any brushes or slip rings, good efficiency, high starting torque, and good reliability.
Despite the aforementioned advantages of the induction motor, several limitations also exist. For example, while producing a high torque, the motor draws a very high current when starting, typically requiring, some sort of current limiting device to prevent excessive current from damaging the windings. This high inrush current can also place operational limitations on the motor (for example, by requiring the operator to start multiple motors in staggered fashion rather than simultaneously), and further adversely impact the motor current source by tripping distribution feeder protection (breakers) or even the source output breaker in the case of large motors if the operator is not cautious.
Additionally, when heavily loaded or overloaded, the typical induction motor draws a high current as the rotor is slowed by the counter torque of the increased load, thereby necessitating the use of a separate protective device or circuit. Accordingly, to compensate for these large currents and limit the possibility of winding damage during such hitch load conditions, the stator magnetic flux density must be maintained by design at comparatively low levels during normal operations. Hence, a larger motor than would otherwise be required to produce the same power output is needed, since some of the capacity of the motor is effectively unused.
In addition to the limitations described above induction motors experience inefficiencies or losses relating to their construction and the use of alternating current as the motive energy source. Alternating, current losses are generally related to 1) the AC resistance of the winding conductor(s); and 2) the properties of the magnetic stator core (“core losses”). Core losses are attributable to a variety of effects including most notably magnetic hysteresis and eddy (circulating) currents.
Magnetic hysteresis is the property by which the application of an external magnetic field (H) to a material results in the temporary alteration of the magnetic domains (dipoles) within the material such that a residual magnetic flux (B) is created. After initial magnetization, when the external field is applied, the domains realign in accordance with the field and produce an internal magnetic flux. At some point, however, increasing the applied external field will produce no further increase of the internal field; this condition is referred to as saturation. As the external magnetic field is reduced in intensity and reversed, the now-aligned dipoles within the material maintain the internal field density at a comparatively high level until coerced by the reversed external field to realign in an alternate orientation. This process is represented by the hysteresis curve of FIG.
2
. So-called “hard” materials (such as those typically used in permanent mag,nets) have a comparatively high coercivity, such that a significant external field is required to realign their dipoles. “Soft” magnetic materials (such as those in motor stator cores) are more easily aligned with little external field, although some minimum field strength is required. The area within the hysteresis curve of
FIG. 2
is related to the coercivity of the material; a hard material has a greater bounded area than a soft material under the same conditions, and therefore greater hysteresis losses for the same ac frequency.
It can be seen that energy must be utilized to realign the magnetic domains within a permeable material, thereby creating losses. This is further evidenced by the fact that alternating current systems using magnetic cores generate heat within the stator due strictly to hysteresis effects. At alternating frequencies on the order of 50-60 Hz, these losses can be quite significant.
Eddy currents are essentially localized circulating currents within a conductor or magnetic core. Eddy currents arise due a variety of factors including spatially and/or temporally non-uniform magnetic fields, and conductor material imperfections or inclusions. Eddy currents result in lower device efficiency and power loss since energy is ultimately dissipated from the device in the form of heat and secondary electric/magnetic fields generated by the circulating currents. Heat generation by a motor is a critical factor, since it can affect the overall power rating of the motor (for a given input voltage frequency, and physical configuration), and also may reduce the longevity of the motor.
The effects of eddy currents may be mitigated through the use of smaller diameter, layered, or segmented conductors or core elements thereby reducing the magnitude of the circulating current. Another method of reducing eddy current losses is to control the magnetic flux density (and uniformity thereof) in the region of the conductors and core.
Prior art designs have attempted to control stator core flux density and mitigate the effects of hysteresis and eddy current losses. Of particular relevance are U.S. Pat. Nos. 4,063,135, 4,132,932. 4,152,630, and 4,187,457 assigned to Wanlass, which generally disclose, inter alia, an alternative induction motor winding arrangement using one or more capacitors in series with the phase (stator) windings of the motor; see
FIG. 3
a
and
3
b
, which illustrate two exemplary prior art winding configurations. This series arrangement in theory is meant to maintain the stator flux density at a comparatively high level during normal operation through the storage of energy in the series capacitors, without high input voltages resulting in high input currents. Additionally, in theory, the series capacitor limits the energy transfer to the rotor, thereby maximizing rotor current for the given voltage and frequency input. U.S. Pat. No. 4,095,149 also assigned to Wanlass discloses a system by which the stator core is maintained in partial saturation as a function of motor load, thereby in theory reducing the energy storage capacity of the stator core and hence the losses associated the
Knobbe Martens Olson & Bear LLP
Nappi Robert E.
Smith Tyrone
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