Patent Application
20060197398
The invention relates to a method to circumvent slot-fill limitations in the design of DC energy conversion machines such as motors and generators through the use of composite windings and to machines including composite windings.
The performance of a motor or generator is determined by its winding. Wire gauges and the number of turns are the variables that define a winding. The speed and strength of a motor are controlled through the selection of wire gauge and number of turns. In a generator, the voltage and current output are controlled through the selection of wire gauge and number of turns.
There are almost no limits for how thick a wire can be wound in such energy conversion machines. As the diameter of a wire wound on the armature increases, the first constraint becomes the manufacturing equipment. A winding machine can only handle wire in a certain size range. However, new fixtures can usually shift the range to larger dimensions. The next limit is the width of the slot opening. The specified wire may be so large that it does not fit between the poles. To get around this constraint, instead of winding one bobbin with thick wire, two or more bobbins may be made with thinner wire. For example, instead of winding an armature with one wire with 0.5 mm2 in cross-sectional area, one can use two wires with a 0.25 mm2 cross-sectional area and obtain the same performance. Because of the availability of fixture re-designs and the use of multiple windings, there are no practical limits to how large a wire may be used in these machines.
When it comes to the number of turns in a winding, there are no techniques to circumvent limitations. Every turn of wire takes up space in the armature slot that is at least equal to the cross-sectional area of the wire. As the number of turns increases, the slot becomes fuller. At some point, the area of the slot cannot accommodate the wire bundle. Specifically, the slot is full, and no more turns can be added. The only design option is to shift to a package of larger diameter and/or length.
The invention discloses a method and a energy conversion machine that circumvents these slot limitations. A first aspect of the invention is a machine comprising an annular stator and a rotatable rotor facing a surface of the stator, the rotor including a plurality of rotor slots. The machine also includes a first winding in at least one of the plurality of rotor slots, the first winding having a first cross-sectional area and a first number of turns, and a second winding in the at least one of the plurality of rotor slots, the second winding having a second cross-sectional area different from the first cross-sectional area and having a second number of turns.
A second aspect of the invention is a method of manufacturing a machine including a stator and a rotor wherein the rotor includes rotor slots. The method including the steps of installing a first winding in at least one of the rotor slots, the first winding having a first cross-sectional area and a first number of turns, and installing a second winding in the at least one of the rotor slots, the second winding having a second cross-sectional area different from the first cross-sectional area and having a second number of turns.
Additional aspects and features of the inventive machine and method are described hereinafter.
The various features, advantages, and other uses of the present invention will become more apparent by referring to the following detailed description and drawing in which:
In traditional energy conversion machine designs, the slot-fill limit in windings is encountered when trying to increase motor strength and/or reduce speed in the case of a motor or when trying to increase current and/or reduce voltage in the case of a generator. Larger diameter wire is necessary to increase the stall torque, making a motor stronger, or to increase the short circuit current rating of a generator.
This larger diameter wire, of course, takes up more area inside the slot. Similarly, to reduce motor no-speed or to reduce the open-circuit voltage rating of a generator, winding turns must be added to the armature. Every winding turn reduces the available slot area. An example best illustrates this problem. Assume the existence of a motor 10 wound with eight (8) turns of magnet wire of gauge A as partially illustrated in
The theoretical performance curve 16 of the motor 10 of
A composite winding can achieve the desired performance without an increase in package size, keeping a machine smaller and less expensive than the alternative. Instead of adjusting turns and changing the gauge of the wire, a composite winding controls the performance of the machine with at least two different windings in the armature. One winding is mainly used to control speed in a motor or voltage in a generator, and the other is manly used to control torque in a motor or current in a generator. Thus, while one bobbin is wound on top of the other, as is sometimes done in so-called double-windings, the bobbins have different turns and wire gauges.
For simplicity, the description that follows is described with reference to a motor. Thus, the winding of the composite winding that is used generally to control speed in a motor and voltage of a generator is called a speed winding, and the winding of the composite winding that is used generally to control torque in a motor and current of a generator is called a torque winding. Consequently, the performance curve of a motor, which reflects torque vs. speed, is described herein. However, the performance curve of a generator, which reflects current vs. voltage, is similar.
More specifically, the method for selecting the windings is based in part on manufacturing constraints as purely theoretical calculations will result in an impossibly small wire gauge for the speed, or voltage, winding and one turn for the torque, or current, winding. Depending on the available winding equipment, there is a limit to how thin a wire one can wind. Also, in a motor, the number of turns in the torque winding are used to control stall current.
Knowing these factors, one approach to composite windings is developed. First, according to standard methods known in the art, one calculates the winding that will produce the desired performance in the machine. If the winding exceeds slot fill-limits, a composite winding can be considered. To determine the size of the composite winding, one would first define the wire gauges. The speed winding wire gauge is selected as, for example, the smallest cross-sectional area wire that the manufacturer can wind. One small wire that could be used is 23 awg wire, which has a cross-sectional area of 0.258 mm2. The torque winding cross-sectional area At is then estimated as follows:
At=(Kt)(A)−As; where
The second step would be to define the number of turns. This is done by solving two simultaneous equations:
Ts+Tt=(Ks)(T); and
TsAs+TtAt=(Kw)(SA); where
In the example, Kw is 0.5, Ks is 1.71, T is 10 turns and SA is 11.71 mm2. Thus, solving for Ts yields:
Ts+Tt=(1.71)(10); and
Ts=17.1−Tt.
Substituting Ts into the second equation yields:
(17.1−Tt)(0.258)+Tt(0.518)=(0.5)(11.71); and]
Tt=5.55 turns or 6 turns.
Thus,
Ts=17.1−Tt=11.55 turns or 12 turns.
The formulas used, and the principles behind them; show that zero-current (no-load) speed is independent of wire gauge and that stall torque is unrelated to the number of turns. Test samples validate these conclusions. Specifically, three sets of three direct-drive motors were built with different sets of windings. One set had a speed winding comprising 12 turns of 23 awg. Another group had a torque winding comprising six (6) turns of 20 awg. The final units had a composite winding comprising 12 turns of 23 awg and six (6) turns of 20 awg calculated according to the method described above.
Composite windings are useful in at least three design instances. First, they can be used to relieve slot-fill limitations and to increase torque/current, reduce speed/voltage, or both. Second, such windings can reduce the size of the machine package in many designs but in particular in those with low slot fill. Finally, the composite windings can be used to eliminate gear boxes from machines with low reduction ratios (up to 5:1). These applications result in smaller, lower cost machines.
