CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending U.S. patent application Serial No. 08/624,906, filed March 27, 1996. This prior-filed application is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

This invention relates to variable speed electromagnetic machines which can be adapted for use as a motor, a generator, or a dynamic brake. Many applications would be greatly improved by a motor that operates at variable speeds, without requiring adjustment of the frequency, wave form, or applied voltage of the power applied to the motor. Similarly, it would be highly advantageous to have a generator which produced an electrical current of a desired frequency, wave form, and voltage, despite changes in the rotational velocity of the input shaft to the generator. The present invention is intended to fill these needs in the art.

SUMMARY OF THE INVENTION

According to a first aspect of this invention, a variable speed electromagnetic machine is provided comprising a rotor and a stator, both of which are mounted for a rotation with respect to a reference element. An output element is coupled to one of the rotor and the stator for transmitting power between said one of the rotor and the stator at an external system. A variable load is coupled to the other of the rotor and the stator to apply a variable torque tending to retard rotation of the other of the rotor and the stator. This variable load reacts against the reference element, and the stator electromagnetically interacts with the rotor to produce at least one of torque and electrical power.

According to another aspect of this invention, a variable speed electromagnetic machine includes a rotor comprising a rotor gear, and a stator compπsing a stator gear. A gear assembly is coupled to the rotor gear and to the stator gear and is mounted for rotation relative to both the rotor

and the stator. A variable torque is coupled to the gear assembly to vary the rotational speed of the gear assembly by varying the torque applied to the gear assembly. The speed of the electromagnetic machine can be varied within a wide range by properly selecting the torque that is applied to the gear assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1 , 2, and 3 are schematic representations of electromagnetic motors that illustrate the operating principles of selected embodiments of this invention. Figures 4, 5, 6 and 6a are cross-sectional views of four preferred embodiments of the present invention.

Figure 7 is a cross-sectional view of a gear motor that incorporates another presently preferred embodiment of this invention.

Figure 8 is a cross-sectional view of a variation of the embodiment of Figure 7.

Figure 9 is a cross-sectional view of a gear motor that incorporates another embodiment of this invention.

Figure 10 is a partial cross-sectional view of a variation of the embodiment of Figure 9. Figure 11 is a partial cross-sectional view of another variation of the embodiment of Figure 9.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Turning now to the drawings, Figures 1 through 3 will be used to provide a general overview, and then preferred embodiments of the invention will be discussed in conjunction with Figures 4 through 11.

GENERAL OVERVIEW

The present invention can be adapted to the widest variety of electromagnetic machines, including motors, generators, and dynamic brakes. By way of example, Figures 1 through 3 show schematic views of

three motors that operate at variable speeds without requiring any change to the frequency, wave form, or voltage of the applied power

As shown in Figure 1 , the gear motor 10 includes a rotor 12 and a stator 14 The rotor 12 and the stator 14 can be any conventional electπcal rotor and stator for a motor or generator Simply by way of example, the stator 14 can include conventional three-phase stator windings, and the rotor 12 can be a conventional rotor suitable for use with such a stator Though not shown in Figure 1 , conventional devices as such as slip rings are used to provide electrical power to the stator 14 The rotor 12 interacts electromagnetically with the stator 14 in the conventional manner to produce torque or electrical power

As shown in Figure 1 , an output shaft 16 is connected to the rotor 12 to rotate therewith Bearings 18, 20 mount both the rotor 12 and the stator 14 for rotation with respect to a reference element R The reference element R is shown schematically in Figure 1 , but in many embodiments the reference element R will be a casing or housing for the motor 10

As shown in Figure 1 , a variable load 22 is coupled to the stator 14, and this variable load 22 reacts against the reference element R By properly controlling the variable load 22, the speed of rotation of the output shaft 16 can be controlled between zero rpm and the maximum rated speed of the motor 10 For example, in order to provide a gradual increase in the rotational speed of the output shaft 16, the variable load 22 can initially apply zero torque to the stator 14 When the stator 14 is energized, the rotor 12 and the output shaft 16 will remain stationary, and the stator 14 will rotate at the design speed in the reverse direction As the variable load 22 is gradually increased, the stator 14 will slow down, and the speed of the rotor 12 and the output shaft 16 will increase At the limit, when the variable load 22 is sufficient to hold the stator 14 in a fixed rotational position with respect to the reference element R, the rotor 12 and the output shaft 16 will rotate at the design speed of the motor 10

Many variations are possible to the motor 10 within the scope of the present invention For example, as shown in Figure 2, the output shaft 16' can be connected to the stator 14 and the variable load 22' can be connected

to the shaft of the rotor 12 Operation of the motor 10' of Figure 2 is substantially the same as that of the motor 10 Figure 1 , except that it is the stator 14 that provides power to the extemal system via the output shaft 16', and it is the rotor 12 that drives the variable load 22' In some applications it is important to increase the operating efficiency of the motor at intermediate speeds In these cases, a system such as that shown in Figure 3 may be preferable In this case, the variable load 22" takes the form of a variable hydraulic pump, which supplies pressurized hydraulic fluid to a hydraulic motor 24" The hydraulic pump 22" is coupled via gears 26", 28" to the stator 14 The pump 24" provides a variable load, similar to the vaπable load 22 of Figure 1 , but much of the power required to drive the variable pump 22" is recovered via the motor 24" The motor 24" is configured to rotate the output shaft 16" such that the power of the motor 24" adds to the power provided by the rotor 12 and the stator 14 Of course, the motor 10" Figure 3 can readily be adapted such that the output shaft is connected to the stator as shown in Figure 2 In this case, the rotor would drive the variable pump, and the motor would drive the stator Similarly, the variable pump can be replaced with an electrical generator, and the hydraulic motor 24" can be replaced with an electrical motor

SPECIFIC EMBODIMENTS

Turning to Figure 4, this drawing shows a cross-sectional view of a motor 30 which is similar to the motor 10 shown in Figure 1 In this case, the rotor 32 is connected to an output shaft 34, and the stator 36 is connected via spur gears 38, 40 to a brake 42 The brake 42 forms a variable load, and causes the motor 30 to operate as described above in conjunction with

Figure 1 The gears 38, 40 provide a speed multiplication to optimize performance of the brake 42

The motor 30 of Figure 4 allows speed changes from zero to full speed at very low cost and low energy efficiency At full speed, the brake 42 locks, and is removed from the energy equation The motor 30 therefore provides conventional motor efficiency when operating at full speed This arrangement works well in applications where an electπcal motor is used to start a high

torque load such as a ball mill. In this case, the brake 42 is used during start¬ up only, so total energy loss isjnsignificant. The motor 30 can be started at no load.

Figure 5 shows another motor 50 which operates according to the principles discussed above in conjunction with Figure 3. The motor 50 includes a rotor 52 that rotates in unison with an output shaft 54. A stator 56 is mounted for rotation with respect to the rotor 52 and the motor case 51. The stator 56 is connected via gears 58, 60 to a variable hydraulic pump 62, which supplies pressurized hydraulic fluid to a hydraulic motor 68. The hydraulic motor 68 is connected via gears 64, 66 to the output shaft 54.

The motor 50 of Figure 5 provides high control efficiency from zero to full torque throughout the speed range. If desired, the motor 68 and pump 62 can be switched and reversed to change rotational direction. The motor 50 of Figure 5 allows rotational change without switching where the load torque is always in one direction, as in hoisting applications. In this way, full control of the load on a hoist can be provided without using brakes to stop or to change the output rotational direction. The cost of stator construction to rotate at speed is proportional to the top rotational design speed. In an 1 ,800 rpm motor, the stator rotates between plus and minus 1 ,800 rpm, while the rotor rotates between 0 and 3,600 rpm.

The motor 70 of Figure 6 is in many ways similar to the motor 50 of Figure 5 discussed above, and comparable elements bear the same reference numeral. In this case, the hydraulic motor 68 rotates a gear carrier 72 via gears 74, 76. The gear carrier 72 forms part of a differential which applies torque to the output shaft 54 via the gears 78, 80, 82. It should be noted that the motor 70 is quite similar to the motor of Figure 10 in terms of the operation of the differential.

The motor 70 of Figure 6 allows constant horsepower throughout about 85% of the speed range, and is well suited for applications such as hoisting. The torque from 15% of maximum speed to zero speed is quite controllable. In this way the hoisting cycle can be made brakeless. With very high torque capability at zero speed the lowering cycle can be made regenerative or nonregenerative, as desired.

Figure 6a shows a modified version of the motor of Figure 6. In this modification, an additional hydraulic motor 69 is mounted to the gear case to drive the rotor 52 and the output shaft 54 directly. This motor 69 is powered by the pump 62, and the hydraulic unit 68' functions as a pump or as a motor, depending on the output speed. Typically, the motor 69 is larger capacity than the unit 68', which is larger capacity than the pump 62.

When the output shaft 54 of Figure 6a is at zero speed, the pump 62 will be out of service and the unit 68' will be at zero displacement and half output speed. The motor 69 will be at full displacement and zero speed. This arrangement can provide very high torque on the output shaft 54 at low speed. As the speed of the output shaft 54 increases from 0 to 50% of full speed, the unit 68' goes from half speed to zero speed and full displacement. The motor 69 will go to 50% speed at zero displacement and will be clutched out of service above 50% speed. The unit 68' will act as a motor above 50% output speed, while the pump 62 will act as a retarding load on the stator 56.

At 50% output speed, the only active hydraulic unit is the unit 68', and it is at zero speed. 50% of the output power is developed by the rotor 52, and 50% of the output power is developed by the stator 56 and transferred through the differential gear to the rotor 52 and the output shaft 54.

In summary, the large hydraulic motor 69 is operative at low speeds only. Hydraulic flow stops at 50% output speed, and the intermediate hydraulic unit 68' is operative as a motor at high output speeds. This arrangement can provide an efficient variable speed electric motor that can develop full horsepower over 90% of its speed range, yet is economical.

In general, the decision as to whether or not to use differential gearing will depend on the requirements of the particular application, particularly the efficiency and manufacturing cost requirements. In general, it will often be preferable to avoid differential gearing for small motors and generators, because small capacity hydraulic units are less expensive than differential gear units and high efficiency operation is often not a prime objective. Similarly, it will often be preferable to use differential gearing in large motors and generators, because (1 ) large hydraulic pumps and motors are often

speed restrictive, (2) differential gearing can substantially reduce the size and expense of such pumps and motors, and (3) differential gearing can increase efficiency

Hydraulic pumps and motors should be chosen as appropriate for the particular application In general, the present invention is suitable for use with the widest variety of pumps and motors Simply by way of example, and without intending any limitation to the scope of the present invention, hydraulic pumps and motors such as Model 7, available from RHL (U K), have been found suitable in one application Similarly, brakes such as Model TBA (7 m-lb), available from Dana Corp , have been found suitable for another application

The embodiment illustrated in Figure 7 is a gear motor that operates at variable speeds without requiring any change to the frequency, wave form, or voltage of the applied power The gear motor of Figure 7 is one example of an electromagnetic machine 110 Electrical generators and dynamic brakes are other examples

The electromagnetic machine 110 includes an output shaft 112 that is rigidly secured to a first flange 114 The first flange 114 is rigidly secured to a cylindrical shell 120, which is in turn secured to a second flange 116 The second flange 116 supports a cylindrical sleeve 118

The cylindrical shell 120 supports a stator winding 122, which in this embodiment is a conventional three-phase stator winding The entire assembly is mounted for rotation on bearings 124, 126 Power is supplied to the stator winding 122 via conventional brushes 128 The stator winding 122 and the elements that rotate with the stator winding 122 together make up a stator 130 As used herein, a stator is an electromagnetic structure that interacts with a rotor to produce torque or electrical power The term "stator" is intended broadly to cover rotating devices, as shown for example in Figure 7 The electromagnetic machine 110 also includes a rotor 132 which is mounted for rotation in unison with a rotor shaft 134 The rotor shaft 134 is supported by bearings 136, 138, 140

The bearings 126, 140 are mounted to a gear housing 142 which in this embodiment does not rotate. Within the gear housing 142 is mounted a stator gear 144, which is in this embodiment is a ring gear that is keyed for rotation with the cylindrical sleeve 118, and therefore with the stator 130. Also mounted within the gear housing 142 is a rotor gear 146, which in this embodiment is a spur gear keyed for rotation with the rotor shaft 134, and therefore with the rotor 132.

Also mounted within the gear housing 142 is a rotatable gear assembly 148. The gear assembly 148 includes a rotatable gear carrier 150, which is mounted for rotation on bearings 152, 154. The gear carrier 150 carries a shaft 156 which is mounted for rotation in bearings 158, 160. First and second gears 162, 164 are keyed to rotate in unison with the shaft 156. The first gear 162 is meshed with the rotor gear 146, and the second gear 164 is meshed with the stator gear 144. The gear carrier 150 supports a spur gear 166 such that the two elements 150, 166 rotate in unison. The gear 166 is meshed with another spur gear 168, which is in turn keyed to a shaft of a torque gate 170. In this embodiment, the torque gate 170 includes a variable volume hydraulic pump 172. Hydraulic fluid from the pump 172 is applied via a conduit 176 to a variable volume hydraulic motor 174. The variable volume hydraulic motor 174 is coupled to drive the rotor shaft 134.

The torque gate 170 is a torque control device selected as appropriate for the particular application. The torque gate 170 in Figure 7 retards rotation of the gear assembly 148 and allows the gear assembly 148 to transmit energy between the stator 130 and the rotor 132. The torque gate 170 can take many forms. In small machines such as hand drill motors, the torque gate 170 may take the form of a current-controlled mechanical friction brake 178 which retards rotation of a brake disc 180 that rotates with the rotor shaft 134, as shown in Figure 8. On a large machine such as an oil rig draw works motor, the torque gate can take the form shown in Figure 7. With this arrangement, the hydraulic motor 174 recovers torque energy from the pump 172 back into the electromagnetic machine 110, thereby providing particularly efficient operation.

Figure 9 shows another embodiment 110' of the electromagnetic machine of this invention In Figure 9 the same reference numerals are used as in Figure 7 for comparable parts The same reference numerals with an added prime are used for parts which, though somewhat different in configuration, perform similar or related functions as the corresponding unpπmed reference numeral elements of Figure 7 The following discussion will take up only the elements associated with the primed reference numerals in Figure 9

As shown in Figure 9, in the machine 110' a stator gear 144' takes the form of a bevel gear, as does the rotor gear 146' In this embodiment the gear assembly 148' includes a gear carrier 150' in the form of a differential gear cage The gear carrier 150' is mounted for rotation by bearings 152', 154', and the gear carrier 150' supports two collmear shafts 156' The shafts 156' support respective first and second gears 162', 164' The gears 162', 164' are bevel gears which are both in constant mesh both with the stator gear 144' and the rotor gear 146' This assembly forms a mechanical differential If desired the gear assembly 148' can include only a single gear 162', 164'

The gear carrier 150' supports on its exterior surface a spur gear 166' which is meshed with a spur gear 168' that supplies rotation to a torque gate 170' In the machine 110' the differential is closed In this example the torque gate 170' controls rotation of the gear assembly 148' to operate at a speed between zero and 900 rpm (for a 1 ,800 rpm motor) This arrangement transfers approximately one-half of the power from the stator 130 to the output shaft 112 at full speed, bypassing the gear unit The torque gate 170" controls output speed in a manner similar to that discussed above

Figure 10 shows a variation of the embodiment of Figure 9 In Figure 10 double-primed reference numerals have been used to show corresponding elements to those marked with single-primed reference numerals in Figure 9 In Figure 10 the rotor shaft 134 and the flange 116 are shown in fragmentary form, and the remaining elements of the stator and the rotor are not shown However, the unshown elements can be identical to those shown in Figure 9

Note that in the embodiment of Figure 10 only a single gear 164" couples the rotor gear 146" to the stator gear 144". Thus, in this case the gear assembly 148" includes only the gear carrier 150", the shaft 156", the gear 164", and the gear 166". In the embodiment of Figure 10 the torque gate 170" takes the form of a variable volume hydraulic pump 172" which is coupled via a conduit 176" to a variable volume hydraulic motor 174". The variable volume hydraulic motor 174" rotates a shaft 178" which is keyed to a gear 180". The gear 180" is in constant mesh with a gear 182" that is in turn keyed to the sleeve 118 and thereby to the stator (not shown). In this way the hydraulic motor 174" applies torque to the stator so that this torque is added to the torque in the output shaft 112 (Figure 9) developed as a result of the action of the pump 172".

Preferably, the hydraulic pump 172" and the hydraulic motor 174" are both variable volume units. When the output shaft is at full speed, the hydraulic pump 172" will be at zero speed, with the volume at full. The hydraulic motor 174" will be at full speed with the volume at zero. The hydraulic pump 172" will develop full torque load and zero hydraulic flow. When the output shaft is at zero speed, the hydraulic motor 174" will be at zero speed and full volume. The pump 172" will be at full speed with the volume at zero. The hydraulic motor 174" will develop full torque and zero hydraulic flow.

As the volume is modulated between the hydraulic pump 172" and the hydraulic motor 174", the output shaft can be modulated both in torque and speed to meet the needed load requirements. The control system shown in

Figure 10 will be nearly 100% energy efficient at zero and full speed, and will exhibit a somewhat reduced efficiency at intermediate speeds.

Figure 11 shows a further variation of the embodiment of Figure 9 As before, the rotor and stator, though not shown in Figure 11 , can be identical to the structures shown in Figure 9. Corresponding elements in Figures 9 and 11 are provided with the same reference numerals.

Figure 11 shows a variation in which the hydraulic motor 174" is coupled by a journalled shaft 184" and a bevel gear 186" to a second bevel

gear 188". The second bevel gear 188" is mounted on a rotatable shaft 190" that is in turn mounted to a gear carrier 165". The gear carrier 165" is keyed to both the gear carrier 150" and to the gear 166". The gear 188" meshes with a bevel gear 192" that is keyed to the shaft 134. In the embodiment of Figure 11 the power developed in the hydraulic pump 172" is used by the motor 174" to slow the shaft 134, thereby increasing the speed of the output shaft 112 (Figure 9).

Operation of the Embodiments of Figure 7-11

For the purposes of the following description it will be assumed that the rotor 132 of Figure 7 is rotating the rotor shaft 134, which is rotating the rotor gear 146 in the clockwise direction (as viewed from the gear housing end). The rotor gear 146 will cause the gear 162 to rotate in a counterclockwise direction. In the absence of extemal torque on the gear carrier 150, the gear 164 will turn freely in the ring gear 144, and the gear 164 will cause the gear carrier 150 to rotate clockwise as the gear 164 turns in the ring gear 144. At this stage the torque gate 170 is applying no torque to the gear carrier 150 via the gears 166, 168, and the stator 130 and the output shaft 112 are stationary.

The function of the torque gate 170 is to select or adjust the torque and thereby the power that the rotor 132 can deliver through the gears 146,

162, 164 to the ring gear 144. When the torque gate 170 is set at zero torque, no torque passes through the gears 162, 164, and the gear carrier 150 will be turning clockwise at the full design speed of the output shaft 112. When the torque gate 170 is set to adjust the torque on the gear carrier 150 to an equivalent torque greater than the load torque on the output shaft 112, the gear carrier 150 will slow down, and the stator 130 and the output shaft 112 will begin to rotate in a counterclockwise direction. As long as the torque gate 170 supplies more torque than the torque of the load, the output shaft 112 will accelerate in rotational velocity until it reaches the full design speed (counterclockwise), and the gear assembly 150 is at a standstill. The desired speed under the currently prevailing load for the output shaft 112 is easily obtained by adjusting the torque gate 170 to allow

sufficient power to pass from the rotor 132 through the gear train 146, 162, 164, 144 to the stator 130 and thereby to the output shaft 112. The torque gate should be sized to handle the torque of the shaft 112.

Since the rotor 132 and the stator 130 operate with opposite directions of rotation, the torque developed energy in the stator winding 122 is added to the shaft 112 directly and does not go through the gear unit.

When a prime mover (not shown) is coupled to the shaft 112 to supply power to the shaft 112 at a given speed, the electrical power generated by the electromagnetic machine 110 increases as the torque gate 170 is set to higher torques and decreases as the torque gate 170 is set to lower torques.

In order to provide a specific example of gearing that can be used in the electromagnetic machine 110, the following details of construction are provided. These details are of course intended only by way of illustration

Table 1 - Gear Tooth Count Table

Gear Tooth Count

144 60

146 30

162 30

164 12

Table 2 - Gear Speed Table

Gear Gear Speed (RPM)

Shaft 112 at 0 Shaft 112 at 300 RPM RPM

144 0 300

146 1800 1500

162 1800 1500

164 1800 1500

166 300 0

Tables 1 and 2 provide the tooth count for selected gears, and the gear speed for selected gears in two modes of operation The center column

of Table 2 is for the output shaft 112 at a standstill, and the right hand column of Table 2 is for the output shaft 112 at full design speed of 300 rpm. Note that when the output shaft 112 is at 0 rpm the gear 166 and therefore the gear assembly 150 rotate at 300 rpm. The following general considerations apply to the embodiments described above. In general, in order for the electrical rotating magnetic flux energy imparted to the stator to be added directly to the output shaft, the stator and the rotor are rotated in opposite directions. When a braking force is applied to the rotating assembly, this braking force will tend to slow the assembly down, thereby causing the stator to rotate. If a driving force using the energy developed by the braking force is added directly to the rotor, this driving force will not aid the torque gate or braking force in developing torque at the output shaft. If a driving force using the energy developed by the braking force is added to the rotor through a mechanical differential to slow the rotor down, this driving force will aid the torque gate or braking force in developing torque at the output shaft. If the developed torque in the output shaft is greater than the load torque, the output shaft will accelerate, and can accelerate to full design speed. If the developed torque in the output shaft is less than the load torque, the output shaft can decelerate. If necessary, the output shaft can decelerate through zero speed and begin to rotate in a reverse direction.

CONCLUSION

The electromagnetic machines described above can operate as a variable speed motor, as a variable speed generator, or as a variable speed dynamic brake with equal ease.

The electromagnetic machines 30, 50, 70, 110, 110' provide a number of important advantages. First, the speed of rotation of the output shaft 112 can be made to vary without altering the frequency, waveform, or voltage of the applied power to the stator windings. In general, if a type of motor can rotate on a given electrical power, that motor can be made into a variable speed gear motor using the techniques discussed above.

Furthermore, the machines 30, 50, 70, 110, 110' provide an inherent torque limit to limit shock loads jnto the machine. Shock loads greater than this applied torque cause a speed reduction rather than increased torque in the driver. The average conversion efficiency of the machines 30, 50, 70, 110,

110' can be very nearly the same as that of a standard gear motor.

The control energy required to operate the torque gate 170, 170', 170" is zero when the output shaft 112 is operating at full speed, and this control energy is comparatively small at speeds lower than full speed. The rotor runs at design full speed relative to the stator at all output speeds of the shaft. For this reason cooling of the stator winding and the rotor is always sufficient. This means that the machines 30, 50, 70, 110, 110' can operate at any speed, including a stalled condition, with full output torque on the shaft without damaging the stator winding or the rotor. The machines 30, 50, 70, 110, 110' when operating as a motor can always start at no load with zero torque at 42, 62, 170, 170', 170". This will allow use of efficient, low resistance rotors without excessive damage to the machine 30, 50, 70, 110, 110' or the power system in starting. The stator develops horsepower in the output shaft in the ratio of the speed of the shaft to the relative speed of the rotor to the stator.

With respect to the electromagnetic machines of Figures 1 through 11 , since both the rotor and the stator rotate, power can be supplied to the electromagnetic machine or power can be taken from the electromagnetic machine via either the rotor, the stator, or both simultaneously. The speed and output or input torque of the machine can be controlled by applying torque to the rotor or the stator or both, or through gearing between the rotor and the stator. The variable load that applies this torque can be controlled to provide a constant input or output torque over the entire speed range or a constant horsepower output over a speed which ranges from a low speed to full speed.

Of course, it should be recognized that a wide range of changes and modifications can be made to the preferred embodiments described above. The widest variety of stators and rotors can be used, including stators and

rotors for both synchronous and asynchronous machines. Similarly, a wide variety of devices can be used.for the variable load 22, 22', 22" and the torque gate 170, 170', 170", all of which provide torque as a control input. Other types of loads, such as hysteresis brakes, eddy current brakes and DC or AC generators, can be used for the variable load or the torque gate.

Furthermore, the gear train can be modified substantially from the illustrated gear train. For example, a spur gear may be substituted for the gear 144, and a reversing idler gear may be used between this spur gear and the gear 164. In effect, the gear train operates as a mechanical differential. This allows speed control for the shaft 112 as a result of torque applied by the torque gate 170. The differential gearing permits control of speed at relatively low torque and low speed, providing a relatively low power loss of control energy while the machine 110 operates at a high power level.

It is therefore intended that the foregoing detailed description be regarded as an illustration and not as a limitation of the scope of this invention. It is the following claims, including all equivalents, which are intended to define the invention.