Induction Motor Breakdown Slip Prediction for Propulsion Traction Applications

A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyrights whatsoever.

This invention relates generally to a control system for an AC traction motor and, more particularly, to a system using information measured and supplied by a vehicle propulsion system controller to predict traction induction motor slip speed at maximum torque (breakdown) conditions in near real time and control the operation of the inverter connected to the motor by adjusting a slip limit command.

A system of thermal protection for AC traction motors using temperature estimations to calculate torque limits and blower speed requirements was disclosed in commonly assigned Vanek et al., U.S. Pat. No. 5,446,362, issued Aug. 29, 1995. In Vanek et al., the thermal protection apparatus for a motor having a stator, a rotor, a blower fan, and an inverter includes measuring apparatus for measuring ambient air temperature of the motor and atmospheric air pressure of the motor. A computer is provided for using the measured ambient air temperature and atmospheric air pressure values for estimating a plurality of temperatures in the AC traction motor and for calculating a plurality of motor torque limits imposed by the plurality of estimated temperatures. A control system is provided for controlling the inverter by providing a signal comprising the lowest of the calculated plurality of motor torque limits to an inverter controller. The control system can also be used to calculate a plurality of blower speed requirements imposed by the plurality of estimated temperatures and set the blower speed of the motor to the highest of the calculated plurality of blower speed requirements.

The system of Vanek et al. is useful for calculating torque limits and blower speed requirements. However, conventionally obtained design margins can limit propulsion system torque capability at maximum vehicle speeds.

It would be desirable to reduce such system design margins to allow for increased propulsion system torque capability at maximum vehicle speeds. In the present invention, accurate predictions of traction induction motor slip speed at maximum torque conditions (breakdown conditions) are used to reduce the system design margin. The predictions of traction induction motor slip speed at maximum torque conditions can be made using inductance parameters and a rotor cage resistance estimation.

Briefly, in accordance with an embodiment of the present invention, a method of controlling an inverter to provide protection for an AC traction motor includes starting the inverter, estimating a rotor cage resistance, calculating a slip limit command from the estimated rotor cage resistance, and using the slip command to control the inverter. Calculating the slip limit command can include determining a first slip limit using the rotor cage resistance, a stator leakage inductance, a rotor leakage inductance, a magnetizing leakage inductance, a number of motor poles, and a fundamental frequency of a voltage applied to the motor; determining a second slip limit using a predetermined maximum slip limit, a predetermined minimum slip limit, and a switch dependent on initial flux build up conditions; and calculating the slip limit command as the lower of the first and second slip limits.

In one embodiment, determining the second slip limit includes subtracting a previous value of the second slip limit from the maximum slip limit to obtain a differential slip limit; limiting the magnitude of the differential slip limit to a predetermined value; adding the limited differential slip limit to the previous value of the second slip limit to obtain an adjusted slip limit; determining a maximum of the adjusted slip limit and the minimum slip limit; if the initial flux build up is not complete, determining the second slip limit as the minimum slip limit; and, if the initial flux build up is complete, determining the second slip limit as the maximum of the adjusted slip limit and the minimum slip limit.

Although a preferred embodiment includes comparing the first and second slip limits, in another embodiment, the second slip limit is obtained and used as the slip limit command. The second slip limit is useful because it provides a technique for providing a minimum required slip during flux build up and a slow ramp up to a maximum permitted slip during operation.

The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, where like numerals represent like components, in which:

FIG. 1A is a block diagram of a vehicle AC propulsion system.

FIG. 1B is a block diagram of an inverter controller of the present invention.

FIGS. 2A-2I include computer source code listings and dictionaries of parameters and output variables which can be used to by a computer to provide data for use in accordance with the present protection method.

FIG. 1A is a block diagram of a vehicle 10 AC propulsion system. A vehicle power source 22 may comprise any desired power source including, for example, a diesel engine/alternator, battery, or wayside power source such as a third rail or high voltage catenary. Electrical power is conditioned by solid state inverter drives shown as inverters 12 (only one of which is shown in FIG. 1A) which have respective inverter controllers 16 to regulate the voltage and frequency applied to respective AC traction motors 14 in a manner consistent with the needs of the vehicle application. Inverters 12 are coordinated by a single propulsion system controller (PSC) 18 which controls each respective inverter controller and inverter for each axle (axles not shown) of the vehicle. A vehicle level control system 20 can be used to send commands to PSC 18. Ambient temperature sensor 26, ambient pressure sensor 24, and vehicle speed sensor 25 supply data to the propulsion system controller. AC traction motor 14 includes a stator 48 and a rotor 50 having an inlet end 52 and a discharge end 54.

FIG. 1B is a block diagram of an inverter controller of the present invention. A sub-processor 28 processes motor data, motor parameters, and ambient conditions sensed by temperature sensor 26 and pressure sensor 24 (shown in FIG. 1A) to obtain, as disclosed in aforementioned Vanek et al., U.S. Pat. No. 5,446,362, a torque limit, a blower speed command, and a rotor cage resistance estimation. The rotor cage resistance estimation is used in the present invention at slip limit calculation device 30.

FIGS. 2A-2I include computer source code listings, a dictionary of parameters, and a dictionary of output variables which can be used to by a computer to provide data for use in accordance with the present protection method. The equations shown in the algorithm of FIGS. 2A-2I are all for purposes of example only. There are a number of techniques for modeling the losses, temperatures, and rotor cage resistance, and the invention is not limited to the specific techniques shown in FIGS. 2A-2I.

In one embodiment, the flowchart software resides within the propulsion system controller (PSC) 18 and is executed by an on-board microprocessor. When PSC 18 is active, the protection system is executed continuously at regular intervals for each vehicle traction motor using the input signals defined above. At the conclusion of each interval, the output signals are passed to vehicle system controller 20.

Briefly, as disclosed in aforementioned Vanek et al., U.S. Pat. No. 5,446,362, after the inverter is started, the ventilating air ambient temperature and the atmospheric pressure of ventilating air are measured in the local area ambient in which the motor is operated. The measured ambient temperature and atmospheric pressure, as well as additional vehicle system input signals (motor data) are supplied to the algorithm. Initial temperature variables are set and the phase current RMS value is calculated.

Then, if necessary, the protection system is initialized by setting each of stator and rotor node temperature to an initial temperature value TINT which is calculated for each node using the equation:

wherein T.sub.p is the previous temperature stored immediately prior to inverter shut down; Tamb is the present ambient temperature; Tamb.sub.p is the previous ambient temperature; t is the elapsed time in minutes since PSC 18 has been without power; and const is the machine thermal time constant in minutes assuming a rotor at a standstill with no ventilation. The time constant, which varies with motor size, is 34.6 minutes in one embodiment.

The temperature at the inlet end of the motor is calculated using the formula on line 6586 of FIG. 2D. To determine the temperature at the outlet end of the motor, the input value of BSPEED (blower speed) is evaluated. If BSPEED is less than 0.5, then the outlet air end air temperature is set to equal the inlet end air temperature because the blower is off. If BSPEED is not less than 0.5, then the air temperature at the outlet end of the motor is calculated with the total fundamental and harmonic losses (lines 5926-5976 of FIG. 2C), the total air rise conductance (lines 5926-5976 of FIG. 2C), and the ambient temperature, as shown in the equation of line 6590 of FIG. 2D.

In one embodiment, the stator and rotor nodal temperatures are calculated with numerical integration of system differential equations. For example, numerical integration methods such as Runge-Kutta methods or, as shown in lines 6595-6700 of FIG. 2D, Euler's method, can be used. The rotor and stator average temperatures and the rotor endring and rotor core differential temperatures are calculated using the equations in lines 6710-6714 of FIG. 2D. The average rotor cage resistance per phase (ohm) R2AVE and the average stator resistance per phase (ohm) R1AVE are calculated using motor specific constants and the calculated average rotor and stator temperatures using, for example, the equations in lines 6716-6718 of FIG. 2D.

Referring again to FIG. 1B, sub-processor 28 supplies the average rotor cage resistance per phase R2AVE, which is hereinafter referred to as rotor cage resistance R.sub.r, to the slip limit calculator 30.

The slip limit calculator additionally receives the following fixed parameters which are characteristic for each particular motor's electrical design:

Ls: unsaturated stator leakage inductance/phase (H)

Lr: unsaturated rotor leakage inductance/phase (H)

Lm: unsaturated magnetizing leakage inductance/phase (H)

f: fundamental frequency of voltage applied to motor (Hz)

P: number of poles.

To obtain an estimated maximum slip speed limit sliplimit.sub.est1, the slip limit calculator then performs the following equation: ##EQU1## Sliplimit.sub.est1 is then compared to a sliplimit.sub.est2, which is determined as discussed below, with the smatter of the two signals being used (as determined by minimum calculator 44) as a slip limit command (sliplimit.sub.COM) to adjust the inverter.

Maximum slip limit (sliplimit.sub.max) and minimum slip limit (sliplimit.sub.min) are predetermined values of slip which are set according to specific traction motor requirements and capabilities. In one embodiment, the minimum slip limit is set to 30 (RPM) and the maximum slip limit is set to 50 or 60 (RPM).

At switch 40, if it is determined that initial flux has not yet been built up to a point where the flux is approximately equal to that which would be expected based on DC (direct current) voltage value and speed (condition T), then the sliplimit.sub.min value will be used as the sliplimit.sub.est2 for comparison with the sliplimit.sub.est1. After the initial flux build up is complete (condition F), switch 40 will send a sliplimit.sub.est2 signal based on the maximum (as determined by maximum calculator 38) of the sliplimit.sub.min and an adjusted maximum slip limit (sliplimit.sub.adj).

A limiter 34 is used to slowly ramp up the sliplimit.sub.adj. First the previous value of the sliplimit.sub.est2 (which is held by element 42) is subtracted from the sliplimit.sub.max with the resulting differential slip limit (sliplimit.sub.diff) then being scaled, if necessary, by limiter 34 to provide a limited differential slip limit (sliplimit.sub.difflim). For example, in one embodiment, if the sliplimit.sub.diff is greater than 1 RPM, the sliplimit.sub.difflim can be clipped to 1 RPM, and if the sliplimit.sub.diff is less than or equal to 1 RPM, the sliplimit.sub.difflim is unchanged from the sliplimit.sub.diff. Sliplimit.sub.difflim is then added to the sliplimit.sub.est2 to provide the sliplimit.sub.adj. Thus, for every execution of the loop, sliplimit.sub.adj increases at a rate controlled by the number of executions per second and the maximum limit of limiter 34 until sliplimit.sub.adj becomes equal to sliplimit.sub.max. After the flux has built up, because sliplimit.sub.adj is always greater than sliplimit.sub.min, sliplimit.sub.adj will be passed on to sliplimit.sub.est2. So sliplimit.sub.est2 will ramp from sliplimit.sub.min to sliplimit.sub.max after the initial flux build up is false (no longer occurring). Therefore, sliplimit.sub.COM will ramp from sliplimit.sub.min to either sliplimit.sub.max or sliplimit.sub.est1. Typically sliplimit.sub.est1 will be less than sliplimit.sub.max because the level of sliplimit.sub.max is normally selected to be above the slip required for the hottest motor. Once sliplimit.sub.est1 is in control, then sliplimit.sub.COM will follow the slip limit dictated by the motor temperature up to the sliplimit.sub.max. Thus this embodiment provides for the limiting of current initially because the slip is limited to a minimum value, and, after the flux has been built up, the slip limit is moved to the required operating limit under a controlled ramp. The initial minimum limit followed by a controlled ramp also ensures that the torque does not overshoot when the flux builds up.

After the sliplimit.sub.COM is determined, the value can be used to limit the slip in a motor by a technique such as disclosed in commonly assigned Kumar, U.S. Pat. No. 5,480,220, for example, wherein a slip limit (MSPM, as shown in FIG. 3 of Kumar) is compared with another slip limit value which is determined using a maximum speed value, an actual speed value, and a torque limit command, and the minimum of the two slip limit values is used to set a torque regulator slip positive limit (TRSPM).

The protection system thereby can use the slip limit command to adjust the torque limit command and control operation of the inverter. This "adjustment" will result in a change of torque limit due to slip limit analysis only if the latest slip limit command is different than the previous slip limit command.

The present invention can be used to reduce the propulsion system design margin because a predetermined value of maximum permissible motor torque can be set to a higher value in reliance upon the use of slip speed analysis for predicting breakdown conditions. Thus, the maximum permissible motor torque for a given motor can be increased using the present invention. Alternatively, if the maximum permissible motor torque is sufficiently large for a particular application, cost savings can be achieved using the present invention to decrease the motor size while maintaining the same value for the maximum permissible motor torque.

Unlike the present invention wherein the slip limit is based on the present operating condition of the motor, the slip limit of a conventional system is fixed. In one conventional embodiment, the slip limit is based on the coldest operating conditions of the motor, and, when the motor heats up, such slip limit will limit the amount of torque the motor can produce. Thus, the motor/axle rating will be the hot motor's capability with the cold motor slip limit. These prior control implementations could limit motor capability at levels in some cases, depending on the motor construction and temperature range, by about 30%.

Another conventional technique is to base the slip limit on hottest motor operating conditions. In this case, when a motor is running normally it will be colder compared to the maximum temperature. Hence, the slip limit is not effective because the slip limit enforced due to the hottest motor is beyond the breakdown torque capability of the motor.

Although the preferred embodiment of the present invention includes determining and comparing sliplimit.sub.est1 and sliplimit.sub.est2, in another embodiment of the present invention, sliplimit.sub.est2 is obtained and used as the slip limit command (sliplimit.sub.COM) without comparison to a sliplimit.sub.est1.

Generally it is preferable to operate an induction motor well within the breakdown torque limit because, as the operation becomes closer to the breakdown torque region, the amount of current required to produce torque increases. At low speeds with nominal voltages, it is generally not required to operate near the breakdown torque limit. The only time operation near the breakdown torque limit is needed is at very high speeds or at high speeds with low input voltages where the flux on the machine is low.

Elements 32, 34, 36, 38, 40, and 42 of FIG. 1B, which are used for obtaining sliplimit.sub.est2, limit the slip of the machine to low levels until the flux builds up and then limit the increase in the slip at a controlled rate until the slip limit reaches a desired level. Under normal operation at low speeds, sliplimit.sub.min is not generally reached. At starting, the flux in the machine is low, and to produce torque, at least a minimum level of slip is needed. The embodiment of FIG. 1B provides the minimum level (sliplimit.sub.min) and thus controls the slip and hence the current until the flux builds up. In a conventional locomotive or transit application, the torque is applied at a ramped rate. Limiting the slip to a minimum value, as in the embodiment of FIG. 1B, also helps prevent overshoot in torque as the flux builds up.

While only certain preferred features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.