Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2001-06-29
2004-09-07
Mullins, Burton (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S052000, C310S062000, C310S180000, C310S214000
Reexamination Certificate
active
06787948
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to rotating machines such as electric motors and generators and, more particularly, to cooling the field windings of the stator of electric motors, alternators and generators.
2. Prior Art
There have been very many proposals intended to improve the operation of transducers for electrical power/mechanical power conversion (motors, generators, or alternators). However, there are still areas where the use of electric motors remains impractical, for example for use as the main drive of a vehicle such as an automobile. Present electric motors are generally too large, heavy, and produce too little power (especially at high speed) for commercial use in a vehicle such as an automobile.
One problem associated with electrical machines, such as electric motors, is that it is necessary to cool them because they generate heat which reduces their efficiency. Motors that are driven by inverters have a high frequency component of current in their windings due to the high frequency pulse width modulation of the inverter. This high frequency component adds to the losses in the stator winding by both the increased RMS current it represents and by the skin and proximity effects in the wire. Conductor losses in a motor represent a large part of the total losses in a well designed motor.
At present, such machines may be cooled by blowing air through or over them. For heavy duty applications it is known to spray oil onto the rotor and stator assemblies and into the gap between them using a high pressure pump. A scavenger pump may also be provided to collect the sprayed oil for re-cycling. The need for optimization in cooling is even more important as some applications of electric motors demand high efficiency in compact packages.
A common configuration for such motors is to have an inner rotor mounted on a straight shaft supported by bearings on the ends. The bearings are mounted in end covers that support and locate the rotor in the center of a current-carrying stator. The rotor contains multiple current-carrying bars which run length wise parallel to the shaft and are located near the outer circumference of the rotor. Heat is produced in the rotor and stator when the current in the stator excites the bars. Heat dissipation limits the design of the stator.
In a typical electric rotating machine where heat dissipation is required, it is customary to circulate a cooling medium in the outer structural jacket of the machine if the cooling medium is a liquid. Whereas, if the medium is air, the flow is then routed through the center and outside shell of the device.
A transverse cross-section of the stator field windings of a prior-art motor is illustrated in FIG.
1
. In this design, the electromagnetic conductors
100
are housed in a slot
102
of a stator
104
. The conductors
100
are cooled by the longitudinal flow of a synthetic oil between the interstices of the wires large external radii and the region formed by an interior wall of a slot liner
106
. Maintaining this cross-sectional area open along the longitudinal axis of the stator
104
has proven to be very problematic during the manufacturing process of the motor and this creates a high fluid flow pressure drop. This problem ultimately manifests itself when the fixed power output of the oil pump delivery system cannot deliver the designed mass flow of the oil that is needed to sufficiently cool the conductors
100
where enormous heat is generated due to internal electrical resistance.
This particular electric motor designed for an electric vehicular application produces 450 lb-ft of torque and 250 HP at normal steady state conditions but coolant oil leaking from the stator slots into the air gap between the stator and rotor bodies causes hydrodynamic drag which leads to significant power transmission inefficiencies. Furthermore, the incessant impact of the rotor into this entrapped oil causes hardware damage due to cavitation and heat build up. Thus, the cooling oil meant to absorb the heat from the conductors
100
typically leaks from the interstices, causing an obstruction to the mechanical function of the motor.
The process to form these interstices in each slot
102
is tedious and fraught with manufacturing risk.
FIGS. 2
a
to
2
d
illustrate this.
FIG. 2
a
illustrates Nomex/Kapton/Nomex (NKN) slot liners
106
which are placed in each stator slot
102
followed by a pair of pre-formed solid magnetic conductors
100
. A center stick
108
made of a modified fiberglass cloth saturated with a high temperature phenolic resin (PCGP-HT) material placed between them separates the upper and lower conductors
100
.
As illustrated in
FIG. 2
b
, four 0.030 inch diameter steel wires
110
are then inserted into the interstitial space between the conductors
100
and slot liners
106
. As illustrated in
FIG. 2
c
, a PCGP-HT top stick
112
is then carefully inserted into place. After making the welded connections on the wires and buss ring terminals, the entire stator sub-assembly is then dipped into a bath of Doryl B-109-9 electrical insulating varnish under vacuum. After the excess varnish is drained, the stator sub-assembly is placed in a curing oven.
Referring now to
FIG. 2
d
, half way through the curing process, taking great care so as not to tear the slot liner
106
, the four steel wires
110
are then gently pulled out and removed to expose the interstitial oil cooling flow passages
114
. The curing process is continued so that the remaining varnish coating will form a secondary dielectric insulation
116
on the magnet wires as well as fuse the slot liner end papers and steel laminates, thus forming a means of primary containment for the cooling oil along with maintaining oil flow passages.
There are major problems associated with this cooling design and methodology. Keeping the interstitial area open along the longitudinal axis of the stator has proven to be very problematic since manufacturing variation in the cross-section tends to create relatively high fluid flow pressure drops. Another issue is that the varnish, being a thin liquid, does not completely fill the gaps between the slot liner end papers, top sticks and the steel laminates, thus compromising the integrity of the oil's primary containment. However, the biggest issue with this sealing technique is that incurred by the large temperature differential of 140 degrees Centigrade between the inner diameter of the stator and the outside diameter of its aluminum alloy casting. The different thermal coefficients of expansion of the elements, mainly in the stator slots, create a complex system of 3-dimensional expansion and contraction due to this temperature differential of the operating motor. This movement creates such stresses in these sealing joints that the glass-like crystalline structure of the varnish tends to fracture. Thus, these differential thermal expansions and contractions lead to the destruction of any sealing provided by the varnish.
All these problems can be compounded to such a magnitude that the pressurized oil coolant will breach any flaw in its containment and flow radially towards the center of the stator, filling the air gap between the stator and rotor. At the relatively low speeds of 5,000 RPM or less, the cooling medium is churned and ground by the spinning rotor causing a drag force and hence mechanical losses to the output of the motor. At the maximum operating speed of 15,000 RPM, the cooling medium is subjected to heavy churning which not only results in major propulsion inefficiency, but causes high, localized temperatures that breaks down the oil's viscosity and hence, cooling capacity.
SUMMARY OF THE INVENTION
Therefore it is an object of the present invention to provide a rotating machine with cooled stator field windings having internal passages which provide an increased amount of cooling than is provided by prior art methods for cooling rotating machines.
In order to increase heat conduction away from the field windings of a stat
Lyons Arthur Paull
Peterson William Anders
BAE SYSTEMS Controls Inc.
Esatto, Jr. Paul J.
Krauss Geoffrey H.
Mullins Burton
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