WO2004022944A2

WO2004022944A2 - Method and device for delivering high torque at low amperage

Info

Publication number: WO2004022944A2
Application number: PCT/US2002/018788
Authority: WO
Inventors: Lester B. Manz
Original assignee: General Electric Company
Priority date: 2000-11-15
Filing date: 2001-11-15
Publication date: 2004-03-18
Also published as: WO2004022944A3

Abstract

A method for delivering high torque at low amperage includes reconnecting the windings such that there are more turns. Turns are increased by reconnecting the windings from a delta connection to a wye connection, reconnecting the windings from poles in parallel to poles in series, or connecting normally unused windings in series with normally used windings. An induction motor including at least one sensor to monitor application parameters and a switch configured to reconnect the windings such that the turns are increased.

Description

METHOD FOR DELIVERING HIGH TORQUE AT LOW AMPERAGE

BACKGROUND OF THE INVENTION

This invention relates generally to electric motors and, more particularly, to induction motors on adjustable speed drives.

Induction motors for electric vehicles require a high top speed and a substantial accelerating torque at low RPMs (0-600) for climbing steep grades. Typically, induction motors sacrifice accelerating torque to achieve high top vehicle speed, or sacrifice top vehicle speed to obtain substantial accelerating torque. The trade off between speed and torque is found in the design of a stator' s windings. Energizing the windings with alternating current produces a changing magnetic field or flux within a rotor core providing torque to a rotor shaft. When an adjustable speed drive is utilized, motor low speed peak torque is limited by the maximum amperes the drive can deliver. If the drive, at a given frequency, produced a higher voltage, the drive would deliver the same amount of torque at a lower ampere draw. But, because of the saturating nature of the iron in a stator core of the motor, to produce a higher voltage requires an increase in turns in the motor's stator windings. While producing a higher voltage is possible at frequencies where the rated voltage of the drive has not been achieved, at all frequencies above the frequency where rated voltage has been achieved, no more increase in voltage is possible. Accordingly, an addition of turns at a low frequency allows current to be reduced, but at higher frequencies, the higher number of turns reduces the available torque.

Typically, high amperage motor controls, one of which would be an adjustable speed drive, are used to partially overcome the high top speed versus substantial torque trade off. A motor control's manufacturing cost is related to the type of semiconductors employed. Since high amperage semiconductors cost more than low amperage semiconductors, high amperage motor controls are more expensive than low amperage motor controls.

Accordingly, there is a need for providing both high top vehicle speed and substantial torque with a low amperage motor control. BRIEF SUMMARY OF THE INVENTION

A method for delivering high torque at low amperage includes reconnecting the windings to increase the number of turns at low speeds while retaining the lower number of turns at high speeds. Turns are increased by reconnecting the windings from a delta connection to a wye connection, by reconnecting the windings from poles connected in parallel to poles connected in series, or by reconnecting normal windings in series with extra (normally unused) windings.

Reconnecting the windings from a delta connection to a wye connection increases the turns by a ratio of the square root of 3 (approximately 1.73). Reconnecting the poles from being in parallel to being in series increases the turns by a ratio of two. Reconnecting normal windings in series with extra windings increases the turns by the ratio of the sum of the number of turns in the normal windings and the number of turns in the extra windings to the number of turns,in the normal windings. Accordingly, multiple ratios can be selected through the use of extra windings.

Additionally, combinations of the above methods can be utilized. For example, to increase the turns by a ratio of 3.46 (2 * 1.73), delta connected poles in parallel are reconnected to wye connected poles in series.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a cross-sectional view of an induction motor;

Figure 2 is a schematic view of windings forming six poles for one of three phases in an induction motor;

Figure 3 is a schematic view of the windings shown in Figure 2 with the poles connected in parallel;

Figure 4 is a schematic view of the windings shown in Figure 2 with the poles connected in series;

Figure 5 is a schematic view of an embodiment of a switch operationally coupled to the windings shown in Figure 2;

Figure 6 is a schematic view of the embodiment shown in Figure 5 configured to provide poles connected in parallel; and Figure 7 is a schematic view of the embodiment shown in Figure 5 configured to provide poles connected in series.

DETAILED DESCRIPTION OF THE INVENTION

Typically, an induction motor with an adjustable speed drive as the motor's power supply is started at a low voltage and frequency. The motor is accelerated in a constant or nearly constant flux by increasing the voltage and frequency until the rated voltage of the drive is reached. The drive can be battery powered or receive power from a utility company. At a fixed frequency, flux is a function of the ratio of volts to turns. Therefore, given a fixed frequency to maintain a constant flux, increasing the number of winding turns requires an increase in the voltage resulting in lower amperage to deliver the same flux.

High torque is utilized to climb steep grades. Alternatively, high torque at a low speed may be required to get out of a pot hole, to carry loads in excess of vehicle capacity, or to push loads across a road surface in a "bulldozing" fashion. Accordingly, electric vehicle manufacturers sometimes specify a motor capable of climbing a specific grade such as a 20% grade (20 meters in the vertical direction for every 100 meters in the horizontal direction). High speed is typically not a requirement in climbing steep grades. Thus, the motor operates in a region where a drive delivers a voltage substantially less than the power supply's capacity. Reconnecting the windings to increase the number of winding turns provides the constant flux at a higher voltage and lower amperage, resulting in more torque per ampere at the low speed condition for climbing grades. Accordingly, high torque at low amperage is achieved, which allows low amperage motor drives to be used in electric vehicle applications.

The present invention allows a motor to deliver the same accelerating torque and top speed as prior art designs but with a lower amperage rated motor control, and at a lower cost. Alternatively, this invention allows a motor to deliver a higher top speed than prior art designs with the same amperage rated motor control and same accelerating torque. An additional alternative use allows a motor to provide a higher accelerating torque than prior art designs with the same amperage rated motor control and same top speed.

In an exemplary embodiment, an electric vehicle includes a switch to reconnect the windings such that the number of winding turns are increased from a normal number of winding turns. The switch, when returned to a normal position, decreases the number of windings from the increased number to the normal number. In another exemplary embodiment, power electronics and software sense vehicle parameters and automatically reconnect the windings such that the number of winding turns are increased when more torque is desired and, subsequently, the number of winding turns are decreased when higher speed is desired. Typical vehicle parameters include vehicle grade, motor speed, motor torque, motor amperage, drive capacity, and vehicle speed. In an alternative embodiment, one or more sensors are coupled to the switch and monitor and compare vehicle parameters. For example, current motor torque is compared against a preset value, and if the torque required to drive the vehicle exceeds the preset value then the windings are reconnected to provide the higher torque. Although, an exemplary embodiment is described in the context of an induction motor for an electric vehicle, it is contemplated that the benefits of the invention accrue to a wide variety of motors for various applications. In an application not involving an electrical vehicle, motor parameters including motor torque, motor speed, motor amperage, and power supply capacity are sensed.

Figure 1 is a cross-sectional view of an induction motor 10 coupled to a switch 12. Induction motor 10 includes a housing 14, a stator 16 including windings having a plurality of turns and a plurality of poles. Induction motor 10 also includes a rotor assembly 18 including a rotor core 20 mounted on a rotor shaft 22. Switch 12 is coupled to a sensor 24. In an exemplary embodiment, switch 12 is in a drive (not shown).

During operation, sensor 24 is configured to monitor parameters, as explained further below, and when a low speed/high torque condition is sensed, as explained below, switch 12 reconnects the windings such that a reduced current is achieved.

Figure 2 is a schematic view of windings 26 forming six poles 28, 30, 32, 34, 36, and 38 that form one phase of a three phase motor. In an exemplary connection, poles 28, 32, and 36 are connected is series forming a first pole group 40. Additionally poles 30, 34, and 38 are connected in series forming a second pole group 42. First pole group 40 is positioned between ending wires 44 and 46, and second pole group 42 is positioned between ending wires 48 and 50. In alternative embodiments, other known connections of poles 28, 30, 32, 34, 36, and 38 are utilized in series and parallel combinations. Figure 3 is a schematic view of windings 26 wired in parallel into a phase 52 of a three phase wye connection 54 including a plurality of other phases 56. The ending wires 44 and 48 are connected together to a motor line lead 58 while the ending wires 46 and 50 are connected to wye connection 54 forming a parallel connection 60. Other phases 56 are connected similarly and this configuration is utilized when motor 10 is operated under a normal operating condition, such as, for example, when motor 10 is operated at or near a top speed.

Figure 4 is a schematic view of windings 26 rewired in series to three phase wye connection 54. Ending wire 44 is connected to motor line lead 58 while ending wire 50 is connected to wye connection 58. Further, ending wires 46 and 48 are connected together forming a series connection 62 of windings 26 resulting in twice as many turns as parallel connection 60. Other phases 56 are connected similarly and this configuration is utilized when motor 10 is operated under a low speed / high torque condition with an increased voltage and, accordingly, a reduced current compared to parallel connection 60.

Figure 5 is a schematic view of an exemplary embodiment of switch 12 operationally coupled to windings 26 such that the number of winding turns can be increased and decreased when desired. For illustrative purposes, a connection to only a single phase is shown. Switch 12 is operationally connected to a first parallel-series switch 52 and a second parallel-series switch 54. For poles connected in parallel, switch 12 positions first parallel-series switch 52 in electrical contact with second pole group 42 and second parallel-series switch 54 in electrical contact with first pole group 40. Alternatively, for poles connected in series, switch 12 positions first parallel-series switch 52 in electrical contact with a series link 56 and second parallel- series switch 54 in electrical contact with series link 56. It is contemplated that other embodiments are possible, such as, for example, using a relay instead of parallel- series switches 52 and 54. In still other alternative embodiments, switch 12 can be a mechanical (hardware) switch or a software controlled smart switch.

Figure 6 is a schematic view of switch 12 (shown in Figure 5) configured to provide poles connected in parallel. Switch 12 is operationally coupled to parallel-series switches 52 and 54 that connect pole groups 40 and 42 in parallel. Series link 56 is unused in the parallel configuration shown in Figure 6.

Figure 7 is a schematic view of switch 12 (shown in Figure 5) configured to provide poles connected in series. Reconnecting pole groups- 40 and 42 from poles connected in parallel to poles connected in series increases the number of winding turns of windings 26 by a ratio of two. In addition to reconnecting poles connected in parallel to poles connected in series, other reconnections increase the turns of windings 26. Reconnecting windings 26 from wye connected to delta connected increases the number of turns, as does the addition of extra windings.

A method for increasing torque for an electric vehicle with an induction motor includes operating the vehicle, monitoring vehicle parameters, and connecting the windings to increase the number of winding turns when vehicle parameters reach preset values. For example, reconnect the windings when the vehicle is at a 15% or higher grade and vehicle speed is less than 3 miles per hour.

While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.

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Claims

WHAT IS CLAIMED IS:

1. A method for delivering high torque at low amperage in an electrical vehicle including an adjustable speed drive and an induction motor including connected windings, said method comprising the steps of:

energizing the induction motor;

monitoring vehicle parameters; and

reconnecting the windings to change the number of winding turns when vehicle parameters reach preset values.

2. A method in accordance with Claim 1 wherein said step of reconnecting the windings further comprises the step of reconnecting the windings from a delta connection to a wye connection.

3. A method in accordance with Claim 1 wherein said step of reconnecting the windings further comprises the step of reconnecting the windings from poles in parallel to poles in series.

4. A method in accordance with Claim 1 wherein said step of reconnecting the windings further comprises the step of connecting first windings in series with second windings.

5. A method in accordance with Claim 1 wherein said step of monitoring vehicle parameters further comprises the step of monitoring at least one of vehicle grade, vehicle speed, motor speed, motor torque, motor amperage, and drive capacity.

6. A method in accordance with Claim 5 wherein said step of monitoring at least one of vehicle grade, vehicle speed, motor speed, motor torque, motor amperage, and power supply capacity further comprises the step of comparing at least one of vehicle grade, vehicle speed, motor speed, motor torque, motor amperage, and drive capacity to preset values.

7. An induction motor comprising:

a motor housing; a stator mounted in said housing and comprising a bore therethrough, said stator including a plurality of windings having a plurality of a number of turns and a plurality of poles;

a rotor assembly mounted in said housing and extending through said stator bore, said rotor assembly including a rotor core and a rotor shaft;

a switch operationally coupled to said windings and configured to connect said windings such that said number of turns are increased; and

at least one sensor operationally coupled to said switch and configured to monitor motor parameters.

8. An induction motor according to Claim 7 wherein said switch further configured to reconnect said windings from a delta connection to a wye connection.

9. An induction motor according to Claim 7 wherein said switch further configured to reconnect said windings from poles connected in parallel to poles connected in series.

10. An induction motor according to Claim 7 wherein said windings include first windings and second windings, said switch configured to connect said windings is further configured to connect said second windings in series with said first windings.

11. An induction motor according to Claim 7 wherein said sensor further configured to monitor at least one of motor speed, motor torque, motor amperage, and power supply capacity.

12. An induction motor according to Claim 7 wherein said sensor further configured to compare at least one of motor speed, motor torque, motor amperage, and power supply capacity to preset values.

13. An induction motor according to Claim 7 wherein said switch further configured to reconnect said windings such that said number of turns are decreased to a normal number of turns.

14. An electric vehicle comprising: an induction motor comprising a motor housing, a stator mounted in said housing and comprising a bore therethrough, said stator including a plurality of windings having a plurality of a number of turns and forming a plurality of poles, and a rotor assembly mounted in said housing and extending through said stator bore, said rotor assembly including a rotor core and rotor shaft;

a switch operationally coupled to said induction motor configured to connect said windings such that said number of turns are increased; and

at least one sensor operationally coupled to said switch and configured to monitor vehicle motor parameters.

15. An electric vehicle according to Claim 14 wherein said switch further configured to reconnect said windings from a delta connection to a wye connection.

16. An electric vehicle according to Claim 14 wherein said switch further configured to reconnect said windings from poles connected in parallel to poles connected in series.

17. An electric vehicle according to Claim 14 wherein said windings include first windings and second windings, said switch configured to connect said windings is further configured to connect said second windings in series with said first windings.

18. An electric vehicle according to Claim 14 wherein said sensor further configured to monitor at least one of vehicle grade, vehicle speed, motor speed, motor torque, motor amperage, and drive capacity.

19. An electric vehicle according to Claim 14 wherein said sensor further configured to compare at least one of vehicle grade, vehicle speed, motor speed, motor torque, motor amperage, and drive capacity to preset values.

20. An electric vehicle according to Claim 14 wherein said switch further configured to reconnect said windings from an increased number of turns to a normal number of turns.