DYNAMIC BRAKING CIRCUIT FOR A HYBRID LOCOMOTIVE

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the benefits, under 35 U S C §119(e), of U S Provisional Application 60/745,153 entitled "Dynamic Braking for a Hybrid

Locomotive" to Donnelly et al filed April 19, 2006, which is incorporated herein by this reference

FIELD The present invention relates generally to a method for configuring dynamic braking circuits for a locomotive which are operable down to very low speeds

BACKGROUND

Railroad locomotives typically use a pneumatic braking system controlled by an independent brake (that is separate from the train brakes) The locomotive may include a dynamic braking system whereby the traction motors are reversed so that they generate braking power which is commonly dissipated in a large resistive grid on the locomotive

U S 6,027, 181 discloses a system for a locomotive which includes a blended braking system combining a pneumatic braking system for the train with a dynamic braking system on the locomotive

The present inventors have disclosed a system for controlling a dynamic and regenerative braking system for a hybrid locomotive which employs a control strategy for orchestrating the flow of power amongst the prime mover, the energy storage system and the regenerative braking system in a U S Patent Application 1 1/200,879 filed August 9, 2005 entitled "Regenerative Braking Methods for a Hybrid Locomotive" which is also incoiporated herein by reference

As presented in U S Patent Application 1 1/200,879 and in U S Provisional Application 60/745, 153 entitled "Dynamic Braking for a Hybrid Locomotive", the concept was to recover energy from the traction motors to either dissipate this power in resistive grids (dynamic braking) and/or feed this power into a DC bus if the DC bus is equipped with any means of energy storage, such as for example, a battery pack, a capacitor bank and/or a flywheel system As shown in Figure 23 of U S Patent

Application 11/200,879, the proposed method consists in reversing the power flow of the armature and field windings in series to switch from the motoring to braking mode In this configuration with both windings in series, it may be difficult to control the power drawn from the traction motors in braking mode In U S Patent Application 1 1/200,881 filed August 9, 2005 entitled

"Locomotive Power Train Architecture", Donnelly et al have further disclosed a general electrical architecture for locomotives based on a plurality of power sources, fuel and drive train combinations The power sources may be any combination of engines, energy storage and regenerative braking This application is also incorporated herein by reference

In rail yard switching operations, for example, a locomotive may be operated primarily at low speed (speeds less than about 15 mph) with multiple stop and starts In these situations, the braking system is worked hard and is a high maintenance system on the locomotive Further, if the brake system locks up, it can cause wheel skid which can result in flat spots developing on the skidding wheels Flat spots are a further costly high maintenance operation usually requiring wheel replacement

There thus remains a need for a locomotive braking system that can be used in conjunction with or instead of a mechanical or pneumatic locomotive braking system, that is particularly suited for operations at low speed These and other needs are addressed by the various embodiments and configurations of the present invention which are directed generally to utilizing the locomotive's traction motors to return energy from braking to a least one of the locomotive's diesel engines, DC electrical bus, energy storage system or dynamic braking system in a way that minimizes wheel skid and in a way that provides seamless braking action at or near 0 mph

SUMMARY

These and other advantages will be apparent from the disclosure of the invention(s) contained herein The various embodiments and configurations of the present invention are directed generally to a dynamic braking method for a locomotive which minimizes the

tendency for wheel skid and can be used preferentially instead of the locomotive's pneumatic or mechanical braking systems The invention disclosed herein may be used on a conventional diesel-electric locomotive, a multi-engine diesel-electric locomotive, or a hybrid locomotive comprised of one or more engines and one or more energy storage systems The energy produced during braking can be utilized or discarded If utilized, it can be stored in an energy storage system such as for example a battery pack or a capacitor bank or it can be used to power the electrical braking control and auxiliary power systems on the locomotive If discarded, it can be routed to a dissipative resistive grid or can be dissipated by routing it through an alternator, for example, an induction or synchronous alternator, to do work against the locomotive's engine (engine braking)

In a first embodiment, traction motors are connected in parallel for high-speed motoring and high-speed braking When connected in parallel, the traction motor circuit includes current path to a common DC bus which alternates with a low resistance free-wheeling current path The free-wheeling time is controlled by selecting the duty cycle for a pair of transistors, typically IGBTs

Pairs of traction motors are switched to a series connection configuration for low-speed motoring and low-speed braking When connected in series, the traction motor circuit includes current path across a common DC bus which alternates with a high resistance free-wheeling current path also through the DC bus The freewheeling time is controlled by selecting the duty cycle for a pair of IGBTs and the free-wheeling current is prevented from a runaway buildup by the voltage on the DC bus It is possible, for example, to include only the low-speed circuit for a yard switching locomotive, where operation is commonly less than about 15 mph During low speed dynamic braking , the circuit provides current through the motor field every cycle During low speed motoring, the same IGBTs are controlled and the direction of current through the armature remains unchanged Only the reverser position is changed to switch between low speed motoring and low speed dynamic braking This allows for motoring through zero speed with the same configuration as dynamic braking through zero speed By doing this, there are no contactors to set up when going from dynamic braking to motoring With no

contactors setup, the independent locomotive pneumatic brake can be blended, when necessary, with the dynamic braking without any loss of response

In low speed braking mode, there is always current flowing through the traction motor armatuie and field coil so that, even at zero speed, theie is a braking force that will resist further motion

The circuit of the first embodiment can become unstable m certain circumstances since the field and armature coils are m series This can be overcome, foi example, by using well-known analogue contiol methods or very fast-acting digital control methods In a second embodiment, a zeio oi low resistance bypass can be added acioss the field coils of the ciicuits of the fiist embodiment to control potential instabilities aiising fiom the field and armatuie windings connected electπcally in seπes such that they create a positive feedback condition and can, in some circumstances, cause a runaway current build-up if not propeily controlled An IGBT is used to contiol the shunt iesistor connected across each field coil when two traction motois are opeiated m a seπes configuration This allows independent control of traction motoi torque foi low speed motoiing or dynamic braking and is useful for eliminating non-synchionous wheel slip duimg motoring or dynamic biakmg (i e when only one of the wheel sets is slipping) By utilizing the dynamic braking circuit configui ations described above during low-speed braking, the possibility of wheel skid such as can occui when pneumatic biakes lock-up can be effectively eliminated This, m turn, pi events flat spots fiom developing on the locomotive wheels Thus, the two embodiments of the piesent invention have the advantage of substantially i educing locomotive downtime and maintenance which aie significant pioblems, for example, in yard switching opei ations For example, multiple locomotives have been used m yard switching operations involving long tiains to minimize wheel skid occurrences and pneumatic brake maintenance when only the locomotives' independent braking systems are used This is a wasteful practice since the multiple locomotives can geneiate fai moie power, pioduce more emissions and consume far more fuel than lequned When the dynamic biaking methods of the present invention are used, the independent pneumatic brakes

of the locomotive need only be used in heavy braking or emergency braking situations This practice will substantially eliminate occurrences of wheel skid most typically associated with pneumatic brake systems Thus locomotive brake maintenance problems can be minimized while using only one locomotive with concomitant savings in fuel costs and reduction of emissions

As can be appreciated, the methods of dynamic braking disclosed herein can be blended with the locomotive's independent brake system for example in switch yard work where speeds often are low and there are frequent starts and stops The method of dynamic braking can also be blended with the train's automatic brake system for example in road switchers and/or passenger trains where speeds are often high

The above-described embodiments and configurations are neither complete nor exhaustive As will be appreciated, other embodiments of the invention are possible utilizing, alone or in combination, one or more of the features set forth above or described in detail below The following definitions are used herein

A locomotive is generally a self-propelled railroad prime mover which is powered either by a steam engine, diesel engine or externally such as from an overhead electrical catenary or an electrical third rail

A traction motor is a motor used primarily for propulsion such as commonly used in a locomotive Examples are an AC or DC induction motor, a permanent magnet motor and a switched reluctance motor

An engine refers to any device that uses energy to develop mechanical power, such as motion in some other machine Examples are diesel engines, gas turbine engines, microturbines, Stirling engines and spark ignition engines A prime power source refers to any device that uses energy to develop mechanical or electrical power, such as motion in some other machine Examples are diesel engines, gas turbine engines, microturbines, Stirling engines, spark ignition engines or fuel cells

An energy storage system refers to any apparatus that acquires, stores and distributes mechanical or electrical energy which is produced from another energy source such as a prime energy source, a regenerative braking system, a third rail and a

catenary and any external source of electrical energy Examples are a battery pack, a bank of capacitors, a compressed air storage system and a bank of flywheels

Dynamic braking is implemented when the electric propulsion motors are switched to generator mode during braking to augment the braking force The electrical energy generated is typically dissipated in a resistance grid system

Regenerative braking is the same as dynamic braking except the electrical energy generated is recaptured and stored in an energy storage system for future use

The independent brake is typically the pneumatic brake system on a locomotive The automatic brake is typically the pneumatic brake system for a train and usually includes the locomotive's pneumatic brake system

A blended brake system is a combination of brake systems such as the combination of the dynamic and independent brakes on a locomotive or the dynamic and automatic brake systems on a train An electrical energy converter refers to an apparatus that converts mechanical energy to electrical energy Examples include various types of alternators, alternator- rectifier combinations and generators

A power control apparatus refers to an electrical apparatus that regulates, modulates or modifies AC or DC electrical power Examples are an inverter, a chopper circuit, a boost circuit, a buck circuit or a buck/boost circuit

An IGBT is Insulated Gate Bipolar Transistor which is a power switching device capable of sequentially chopping a voltage waveform at a very fast rate

Locomotive speed is the speed of the locomotive along the tracks and is typically expressed in miles per hour or kilometers per hour Engine speed is the rotary speed of the engine output drive shaft and is typically expressed in rpms

Alternator speed is the rotary speed of the alternator rotor and is typically expressed in rpms The alternator speed is commonly the same as engine speed since they are usually directly connected with no intermediate gearing Traction mode is the same as motoring mode where the vehicle is accelerating or maintaining speed

Braking mode is where the vehicle is decelerating under application of at least one braking system

As used herein, "at least one", "one or more", and "and/or" are open-ended expiessions that are both conjunctive and disjunctive in opeiation For example, each of the expressions "at least one of A, B and C", "at least one of A, B, or C", "one or moie of A, B, and C", "one or more of A, B, oi C" and "A, B, and/or C" means A alone, B alone, C alone, A and B together, A and C together, B and C togethei , oi A, B and C together

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a circuit diagiam illustrating the ciicuit of the piesent invention in high speed motoiing mode

Figuie 2 is a circuit diagram illustiating the ciicuit of the present invention in low speed motoπng mode Figure 3 is a circuit diagram illustrating the circuit of the present invention in high speed braking mode

Figure 4 is a circuit diagram illustrating the circuit of the piesent invention in low speed braking mode

Figure 5 is a circuit diagram illustrating the circuit of the present invention in low speed motoring mode with independent control of the field coil shunt resistor

Figure 6 is a circuit diagram illustrating the circuit of the piesent invention in low speed braking mode with independent contiol of the field coil by a shunt iesistor

DETAILED DESCRIPTION High and Low Speed Dynamic Braking Circuit Approach

The method of locomotive biakmg disclosed heiein provides a seamless dynamic braking action at low speeds down to 0 mph and even at 0 mph provides a resistive torque that can prevent rollback Typically on a tram, a brake pipe is blended with the locomotive's independent brake using the automatic biake contiol The biake pipe is the line that goes to all of the cars in the tram and is used for long, ovei the ioad operation In the switch yard, the biake pipe is often not connected to the tiam

from the locomotive as it takes time to charge the train's air system Often, only the locomotive's independent brake is the only braking system used in switch yard operations

Blended braking is an existing concept for a locomotive which combines the locomotive's independent air brake with some dynamic braking The method of locomotive braking disclosed herein is more properly called preferential braking since, for most operations, only the dynamic braking system is used at low speeds, with the locomotive's pneumatic braking system used only during very heavy braking or in emergency braking situations The principal advantages of the locomotive braking method disclosed herein are D the braking system works effectively at low speeds Conventional dynamic braking does not work effectively at low speeds for either AC or DC locomotives, although a boost circuit can be used to raise the voltage from the motor terminal volts to a higher voltage D the braking system substantially reduces the likelihood of wheel skid and the flat spots that can develop during skid D the braking system can be blended with the locomotive's independent brake in a way that is transparent to the operator

D the dynamic braking system disclosed herein uses a minimum of power switching components

D this method of dynamic braking allows the engineer to apply brakes more aggressively since the likelihood of wheel skid is substantially reduced

Figure 1 is a circuit diagram illustrating the circuit of the present invention in high speed motoring mode High speed is considered in the range of about 15 mph and higher The circuit is comprised of an electrically positive terminal 108 and an electrically negative terminal 109 which are connections to a DC bus (not shown) A capacitor 105 is also shown The capacitor 105 provides filtering action and can also store significant energy during switching between the various modes For example, the capacitor 105 should be able to store an amount of energy comparable to that stored inductively m the motor windings and therefore can limit the voltage developed across the IGBTs as well as absorb switching voltage transients As can be appreciated,

electrical energy can flow to or from the DC bus, depending on the voltage across the DC bus and the voltage across the traction motors In high speed motoring configuration, switches 111 and 113 are closed and conducting while switch 112 is open and non-conducting In high speed motoring, electrical energy flows from the positive terminal 108 and through the two traction motors connected in parallel In Fig Ia, IGBTs 102 and 103 are conducting while IGBTs 101 and 104 are nonconducting Current flows through armature coils 122 and 124 and their respective field coils 121 and 123 as shown by current flow arrows and propulsive power is developed by the traction motors Field coils 121 and 123 are connected by reverser contacts which control the direction of current flow through the traction motor field coils In forward motoring, by convention used herein, current flow arrows through the field coils are in the same direction as the armature current flow arrows In Fig Ib, IGBTs 102 and 103 are switched off (non-conducting) while IGBTs 101 and 104 remain non-conducting In this configuration the currents in the traction motors are free wheeling, with the by-pass diodes of IGBTs 101 and 104 serving as the freewheeling diodes During the free wheeling portion of the duty cycle, current flows through the traction motors but no propulsive power is developed The duty cycle of IGBTs 102 and 103 (which are the same) therefore controls the average propulsive power flowing from the DC bus to the traction motors Figure 2 is a circuit diagram illustrating the circuit of the present invention in low speed motoring mode (about 15 mph or less) The circuit is comprised of an electrically positive terminal 208 and an electrically negative terminal 209 which are connections to a DC bus (not shown) In low speed motoring configuration, switches 21 land 213 are open and non-conducting while switch 212 is closed and conducting In low speed motoring, electrical energy flows from the positive terminal 208 and through the two traction motors now connected in series In Fig 2a, IGBTs 202 and 203 are conducting while IGBTs 201 and 204 are non-conducting Current flows through armature coils 222 and 224 and their respective field coils 221 and 223 as shown by current flow arrows and propulsive power is developed by the traction motors Field coils 221 and 223 are connected by reverser contacts which control the direction of current flow through the field coils In Fig 2b, IGBT 202 and 203 are

switched off (non-conductmg), IGBTs 201 and 204 remain non-conducting In this configuration, the motors are free wheeling, with the by-pass diodes of IGBTs 201 and 204 serving as a fi ee- wheeling diodes The free wheeling path includes circulation thi ough the DC bus In low speed free-wheeling the combined back emf of the two- series connected armatures is sufficiently high to oveicome the DC bus voltage so that the free-wheeling current continues to flow The current path through the DC bus requires a path that may be completed by an auxiliary load, a dynamic breaking gπd and/oi an energy storage system During the free wheeling portion of the duty cycle, cuπent flows through the traction motois but no propulsive powei is developed The duty cycle of IGBTs 202 and 203 (which are the same) therefoie contiol the aveiage propulsive powei fiom the DC bus to the ti action motois IGBTs 201 and 204 aie switched off when IGBTs 202 and 203 are on, and IGBTs 201 and 204 are switched on when IGBTs 202 and 203 aie off By using the DC bus as part of the freewheeling path duiing the energy build-up phase of low speed motoiing, the tendency of the cuπent to build-up too iapidly oi too high is eliminated

Figure 3 is a ciicuit diagram illustrating the circuit of the present invention in high speed biaking mode and is suitable for dynamic braking or legenerative braking methods The circuit is comprised of an electrically positive terminal 308 and an electrically negative terminal 309 which are connections to a DC bus (not shown) In high speed braking configuration, switches 31 1 and 313 are closed and conducting while switch 312 is open and non-conducting The two ti action motors are shown connected in parallel acioss the DC bus terminals 308 and 309 In high speed biaking, electrical energy flows intermittently fiom the negative terminal 309 to the positive terminal 308, thus ietummg eneigy to the DC bus (not shown) In Fig 3a, IGBTs 301 and 304 are conducting while IGBTs 302 and 303 are non-conducting Cuπent flows through armature coils 322 and 324 and then lespective field coils 321 and 323 as shown by cuπent flow arrows Field coils 321 and 323 are connected by leveisei contacts which control the direction of cuπent flow through the ti action motor field coils In forwaid biaking, by convention used herein, current flow aπows through the field coils are m the opposite diiection as the armature current flow arrows as the leveiser contacts have been switched from motoring The current inci eases and

magnetic energy is built up and stored in the armature windings In Fig 3b, IGBTs 301 and 304 are switched off (non-conducting) while IGBTs 302 and 303 remain off The current flow is through the by-pass diodes of IGBTs 302 and 303 In this configuration the energy stored in the traction motors reaches a voltage level higher than that on the DC bus and is released Current flows back to the positive terminal 308 of the DC bus and energy is returned to the DC bus By modulating the current, the duty cycles of IGBTs 301 and 304 (which are the same) control the maximum voltage level and power flow from the traction motors to the DC bus as well as controlling the braking force applied by the traction motors Figure 4 is a circuit diagram illustrating the circuit of the present invention in low speed braking mode and is suitable for dynamic braking or regenerative braking methods The circuit is comprised of an electrically positive terminal 408 and an electrically negative terminal 409 which are connections to a DC bus (not shown) In low speed braking configuration, switches 411 and 413 are open and non-conductmg while switch 412 is closed and conducting The two traction motors are now shown connected in series In low speed braking, electrical energy flows intermittently back and forth between the negative terminal 409 and the positive terminal 408, but returns a net energy to the DC bus (not shown) In Fig 4a, IGBTs 402 and 403 are conducting while IGBTs 401 and 404 are non-conducting Current flows through armature coils 422 and 424 and their respective field coils 421 and 423 as shown by current flow arrows Field coils 421 and 423 are connected by reverser contacts which control the direction of current flow through the traction motor field coils In forward braking, by convention used herein, current flow arrows through the field coils are in the opposite direction as the armature current flow arrows as the reverser contacts have been switched from motoring The free wheeling path includes circulation through the DC bus In low speed free-wheeling, the combined back emf of the two- series connected armatures is augmented by the DC bus voltage so that the freewheeling current rises rapidly and magnetic energy is built up and stored in the armature windings The current path through the DC bus requires a path that may be completed by an auxiliary load, a dynamic braking grid and/or an energy storage system In Fig 4b, IGBTs 402 and 403 are switched off (non-conducting) while

IGBTs 401 and 404 remain off Current flows through the by-pass diodes of IGBTs 401 and 404 In this configuration the energy stored in the traction motors reaches a voltage level higher than that on the DC bus and is released and flows back to the positive terminal 408 of the DC bus By modulating the current, the duty cycle of IGBTs 402 and 403 (which are the same) control the voltage level and power flow from the traction motors to the DC bus as well as controlling the braking force applied to the traction motors during low speed braking By using the DC bus as part of the free-wheeling path during the energy build-up phase of braking, the tendency of the current to build-up too rapidly or too high is eliminated For low speed dynamic braking, the two diagonal IGBTs (as shown in Figure

4) are turned on at once for the buildup portion of the duty cycle and off for the free wheeling portion of the duty cycle In the freewheeling mode, the diodes on the opposite comers of the IGBTs conduct By turning on the diagonal IGBTs, a current flows through the motor field coils every cycle For low speed motoring, the same IGBTs are controlled and the direction of current through the armature remains unchanged Only the reverser position is changed This allows for motoring through zero speed with the same configuration as dynamic braking through zero speed By doing this, there are no contactors to setup when going from dynamic braking to motoring With no contactors setup, the locomotive's pneumatic brake can be blended, when required, without any loss of response

In a switch yard, the reverser can have its direction changed while the locomotive is in idle notch and the locomotive can then be notched up at low speed with out fear of plugging (that is, without fear of unwanted current build-up which can result in damage or burn out of the IGBT's free-wheeling diodes) An additional benefit of operating in such a mode for motoring is that if there is a direction change, the locomotive can travel through zero speed and go the other way without coordinating the contactor positions with direction of travel This reduces the chance of 'plugging' through a diode and losing control and burning out the diode as could happen if the high speed motoring configuration is used with a reverser not set to match the direction of rotation

As can be appreciated, the method of dynamic braking disclosed herein can be blended with the locomotive's independent brake system for example in switch yard work where speeds often are low and there are frequent starts and stops The method of dynamic braking can also be blended with the train's automatic brake system for example in road switchers and/or passenger trains where speeds are often high High and Low Speed Dynamic Braking Circuit Approach with Independent Field Control

When in low speed motoring or braking mode, two traction motors are connected in series It therefore becomes possible for wheel slip to occur on one of the wheel sets but not on the other controlled by the two traction motors Such non- synchronous wheel slip can be detected as follows The speeds of the two traction motors (or axles which the traction motors typically drive via a fixed gear ratio linkage) can be measured Any substantial difference in wheel speeds between the two wheel sets is a direct indication of wheel slip Examples of rotary speed sensors include tachometers such as axle alternators or reluctance pickups on the bull gear

Alternately the speeds of the two traction motors can be estimated by measuring the motor volts across each of the series-connected motors and evaluating the voltage difference between the two measured motor volts Typically the motor volts are within about 5% of each other during normal operation When the motor volts across one of the traction motors increases rapidly during motoring, it is an indication of wheel slippage by the wheels controlled by that traction motor Both direct measurement and estimating methods are well-known locomotive techniques for detecting the occurrence of wheel slip

In an embodiment of the present invention, a switch such as an IGBT is connected in series with the field coil shunt resistor of each traction motor This IGBT allows some of the motor current to by-pass the field coil when a slip condition is detected, so as to reduce the torque applied to the wheels associated with that traction motor That is, if one set of wheels is determined to be slipping, then the speed of the traction motor for which wheel slip is occurring can be controlled by a desired adjustment of the field coil of that traction motor by controlling the duty cycle of the IGBT

Figure 5 is a circuit diagram illustrating the circuit of the present invention in low speed motoring mode with independent control of the field coil shunt resistor This figure is similar to that of Figure 3 but shows a shunt resistor and IGBT in parallel with the field coil of each traction motor Typically, a shunt resistor and switch in parallel with the field coil of each traction motor are used at higher locomotive speeds to reduce the back emf of the field coil so as to provide more current to the armature when the back emf approaches the level of the total motor volts

IGBT s 504 and 514 can be operated with duty cycles ranging from 0% to 100% and so can be used to control the current through the field coils as is well known for higher locomotive speeds As used in the present invention, these IGBTs can also be used to control non-synchronous wheel slip at low speeds by prescribing a selected duty cycle between 0% and 100%

The arrows depicting current flow are the same as those shown in Figure 3 In Figure 5, the wheels associated with traction motor 51 1 have been detected to be slipping IGBT 514 is operated with a selected duty cycle to allow some current through the shunt resistor 513 and thereby reduce the current through field coil 512 At low speeds (about 15 mph or lower), this will reduce the torque applied by traction motor 51 1 Fig 5a illustrates the power-on mode of the circuit and Fig 5b illustrates the free-wheeling mode of the circuit as was described previously in Figure 2 In Figure 5, the wheels associated with traction motor 501 have not been detected to be slipping IGBT 504 remains off and no current is allowed through shunt resistor 503 therefore has no effect on the current in field coil 502

Figure 6 is a circuit diagram illustrating the circuit of the present invention in low speed braking mode with independent control of the field coil shunt resistor This figure is similar to that of Figure 4 but shows a shunt resistor and IGBT in parallel with the field coil of each traction motor

IGBT s 604 and 614 can be operated with duty cycles ranging from 0% to 100% and so can be used to control the current through the field coils as is well known for higher locomotive speeds As used in the present invention, these IGBTs can also be used to control non-synchronous wheel slip at low speeds by prescribing a selected duty cycle between 0% and 100% Wheel slip at low speeds can occur during

dynamic braking if the braking torque applied by one of the two series traction motors is substantially different Typically, wheel skid will not occur during dynamic braking The arrows depicting current flow are the same as those shown in Figure 5 In Figure 6, the wheels associated with traction motor 611 have been detected to be slipping IGBT 614 is operated with a selected duty cycle to allow some current through the shunt resistor 613 and thereby reduce the current through field coil 612 At low speeds (about 15 mph or lower), this will reduce the torque applied by traction motor 611 Fig 6a illustrates the power-on mode of the circuit and Fig 6b illustrates the free-wheeling mode of the circuit as was described previously in Figure 4 In Figure 6, the wheels associated with traction motor 601 have not been detected to be slipping IGBT 604 remains off and no current is allowed through shunt resistor 603 therefore has no effect on the current in field coil 602

A number of variations and modifications of the invention can be used As will be appreciated, it would be possible to provide for some features of the invention without providing others For example, in one alternative embodiment, a low speed high-power yard switcher locomotive might use only the low speed circuit of Figures 3 and 4 and eliminate switches 311, 312 and 313 in Fig 3 and switches 41 1, 412 and 413 in Fig 4

The present invention, in various embodiments, includes components, methods, processes, systems and/or apparatus substantially as depicted and described herein, including various embodiments, sub-combinations, and subsets thereof Those of skill in the art will understand how to make and use the present invention after understanding the present disclosure The present invention, in various embodiments, includes providing devices and processes in the absence of items not depicted and/or described herein or in various embodiments hereof, including in the absence of such items as may have been used in previous devices or processes, for example for improving performance, achieving ease and\or reducing cost of implementation

The foregoing discussion of the invention has been presented for purposes of illustration and description The foregoing is not intended to limit the invention to the form or forms disclosed herein In the foregoing Detailed Description for example, various features of the invention are grouped together in one or more embodiments for

the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment Thus, the following claims are hereby incorporated into this Detailed Description, with each claim standing on its own as a separate preferred embodiment of the invention

Moreover though the description of the invention has included description of one or more embodiments and certain variations and modifications, other variations and modifications are within the scope of the invention, e g , as may be within the skill and knowledge of those in the art, after understanding the present disclosure It is intended to obtain rights which include alternative embodiments to the extent permitted, including alternate, interchangeable and/or equivalent structures, functions, ranges or steps to those claimed, whether or not such alternate, interchangeable and/or equivalent structures, functions, ranges or steps are disclosed herein, and without intending to publicly dedicate any patentable subject matter