METHOD OF DETERMINING A MAXIMUM OPERATING SPEED OF AN ELECTRIC MOTOR

Abstract

A method that involves determining the maximum operating speed of an electric motor under varying torque conditions, specifically at speeds surpassing its rated speed. Initially, the motor is operated at its highest speed across different torque values, establishing a data set of real measurement points. These points illustrate the relationship between the maximum motor speed and the applied torque. A coefficient (COF), ranging from 0 to 1 and dependent on motor characteristics, is selected. Two curves are then established: the first for lower maximum speeds, represented by

[00001]${}_{ref}=\frac{{\mathrm{COFP}}_{nom}}{T_{meas}},$

and the second for torques below a defined limit torque T.sub.lim expressed as

[00002]${}_{ref}=\frac{{\mathrm{COFP}}_{nom}}{f\left(T_{meas}\right)},$

where is a polynomial function derived from the measurement points. The appropriate curve is then used to ascertain the maximum motor speed for a given torque, based on whether the torque is greater or lesser than T.sub.lim.

Claims

1. A method of determining a maximum operating speed of an electric motor subjected to a defined torque, at a speed higher than the rated speed of the motor, comprising: operating the motor at its maximum speed, for different torque values, and determining, for each torque value, the corresponding maximum motor speed, to obtain a set of real measurement points representing the evolution of the maximum motor speed as a function of the torque applied to the motor, selecting a coefficient COF between 0 and 1, for example depending on the motor characteristics, determining a first curve representing the evolution of the maximum speed .sub.ref as a function of the measured torque T.sub.meas corresponding to the actual measurement points, for low maximum speeds, said first curve being defined by ${}_{\mathrm{ref}}=\frac{\mathrm{COF}*P_{\mathrm{nom}}}{T_{\mathrm{meas}}},$ where .sub.ref is the maximum speed, P.sub.nom is the rated power and T.sub.meas is the measured torque of the motor, defining a limit torque T.sub.lim corresponding to the torque at the intersection of the first curve with the set of measurement points, determining a second curve representing the evolution of the maximum speed .sub.ref as a function of the measured torque T.sub.meas, corresponding to the actual measurement points, for measured torques T.sub.meas lower than T.sub.lim said first curve being defined by ${}_{\mathrm{ref}}=\frac{\mathrm{COF}*P_{\mathrm{nom}}}{f\left(T_{\mathrm{meas}}\right)}$ where f is a polynomial function, said function being determined from the measurement points, and determining, from a defined torque T, the corresponding maximum motor speed, from the equation of the first curve if TT.sub.lim and from the equation of the second curve if TT.sub.lim.

2. The method according to claim 1, wherein said function is a polynomial function.

3. The method according to claim 2, wherein said function is a function defining a Bzier curve.

4. The method according to claim 1, wherein said method further comprises: determining an initial torque T.sub.0 from which the maximum speed .sub.ref of the motor decreases, said maximum speed being constant when the torque applied to the motor is less than said initial torque, and defining the maximum motor speed .sub.ref subjected to said defined torque being equal to a constant maximum value .sub.max if said defined torque TT.sub.0.

5. (canceled)

6. A non-transitory computer-readable recording medium on which is recorded a computer program comprising instructions for implementing the method according to claim 1 when the computer program is executed by a processor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Further features, details and advantages will become apparent from the detailed description below, and from an analysis of the appended drawings, in which:

[0039] FIG. 1 schematically illustrates a lifting device comprising an electric motor subjected to a load,

[0040] FIG. 2 schematically illustrates the various stages of the process described herein,

[0041] FIG. 3 is a diagram showing the evolution of the maximum speed of an electric motor as a function of the torque to which the motor is subjected.

DESCRIPTION OF EMBODIMENTS

[0042] FIG. 1 shows a lifting device for a crane, for example, comprising a load-bearing electric motor. The motor comprises a stator and a rotor coupled to a shaft, the rotation of the shaft moving the load. The speed at which the load is moved is linked to the speed of rotation of the motor shaft, also known as the motor speed. Thus, during operation, the motor is subjected to a torque whose value depends on the load to be moved.

[0043] FIG. 2 schematically illustrates a method of determining a maximum operating speed of an electric motor subjected to a defined torque, at a speed higher than the rated speed of the motor.

[0044] FIG. 3 illustrate the evolution of the maximum speed of a motor as a function of the torque applied to the motor. More particularly, the evolution is represented by a series of measurement points gathered from experiments conducted on a lifting device or a test bench, both incorporating a motor to which a load has been applied. This figure provides a detailed view of how the maximum speed of the motor varies in response to different levels of torque.

[0045] This variation in maximum speed reveals three distinct zones, a first zone (T.sub.measT.sub.0) where maximum speed is essentially constant and equal to .sub.max, a second zone (T.sub.0T.sub.measT.sub.lim) where speed decreases with torque, and a third zone (T.sub.measT.sub.lim) where speed also decreases with torque, but more rapidly than in the second zone.

[0046] In the proposed method, the motor is operated at its maximum speed (S1 in FIG. 3), for different torque values (i.e., with different loads), and, for each torque value, the corresponding maximum motor speed is determined, to obtain a set of real measurement points representing the evolution of the maximum motor speed as a function of the torque applied to the motor.

[0047] To that aim, the load may be progressively increased or decreased, per increment for example. The torque applied to the motor and the maximum speed may be measured or determined directly or indirectly, for example by calculation from another measurement.

[0048] Then a coefficient COF between 0 and 1 selected, for example depending on the motor characteristics (S2).

[0049] A first curve C.sub.1 representing the evolution of the maximum speed .sub.ref as a function of the measured torque T.sub.meas corresponding to the actual measurement points, may then be determined for low maximum speeds (S3). Said first curve may be defined by

[00011]${}_{\mathrm{ref}}=\frac{\mathrm{COF}*P_{\mathrm{nom}}}{T_{\mathrm{meas}}},$

where .sub.ref is the maximum speed, P.sub.nom is the rated power and T.sub.meas is the measured torque of the motor.

[0050] A limit torque T.sub.lim corresponding to the torque at the intersection of the first curve with the set of measurement points may then be determined (S4).

[0051] Then, a second curve C.sub.2 may be determined (S5), said second curve representing the evolution of the maximum speed .sub.ref as a function of the measured torque T.sub.meas, corresponding to the actual measurement points, for measured torques T.sub.meas comprised between T.sub.0 than T.sub.lim said second curve being defined by

[00012]${}_{\mathrm{ref}}=\frac{\mathrm{COF}*P_{\mathrm{nom}}}{f\left(T_{\mathrm{meas}}\right)}$

where is a polynomial function, said function being determined from the measurement points.

[0052] Said function is a polynomial function, for example a first order polynomial function.

[0053] In such case, the equation

[00013]${}_{\mathrm{ref}}=\frac{\mathrm{COF}*P_{\mathrm{nom}}}{f\left(T_{\mathrm{meas}}\right)}$

can be read

[00014]${}_{\mathrm{ref}}=\frac{\mathrm{COF}*P_{\mathrm{nom}}}{\left(a*T_{\mathrm{meas}}+b\right)},$

where a and b are scalars. In this case, in order to determine the second curve, it is first necessary to determine the values of the scalars a and b.

[0054] For this purpose, one option may be to solve a system of two equations with two unknowns by choosing two measurement points defined by the coordinates (.sub.ref1; T.sub.meas1) and (.sub.ref2; T.sub.meas2), the equations can then be defined as follows:

[00015]$\begin{array}{cc}{}_{\mathrm{ref}1}=\frac{\mathrm{COF}*P_{\mathrm{nom}}}{\left(a*T_{\mathrm{meas}1}+b\right)}& \left(\mathrm{Equation}1\right)\end{array}$$\begin{array}{cc}{}_{\mathrm{ref}2}=\frac{\mathrm{COF}*P_{\mathrm{nom}}}{\left(a*T_{\mathrm{meas}2}+b\right)}& \left(\mathrm{Equation}2\right)\end{array}$

[0055] The values .sub.ref1, T.sub.meas1, .sub.ref2, T.sub.meas2, COF et P.sub.nom being known, a and b can be determined.

[0056] Alternatively, a and b can be determined by the following equations:

[00016]$\{\begin{array}{c}=\min \left(\max \left(\frac{\mathrm{COF}*\mathrm{FRS}}{{}_{\max }*\left(T_{\lim }-T_0\right)}-\frac{T_0}{T_{\lim }-T_0},0\right),1\right)\\ a=\left(1-\right)\\ b=*T_{\lim }\end{array}$

[0057] where FRS is the rated frequency of the motor. The rated frequency of a motor refers to the specific frequency at which the motor is designed to operate most efficiently and effectively. This frequency is typically specified by the manufacturer (for example on the motor nameplate) and is closely tied to the electrical power supply standards of the region where the motor is intended to be used.

[0058] Then, the corresponding maximum motor speed may be determined (S6), from a defined torque T, from the equation of the first curve if TT.sub.lim and from the equation of the second curve if T.sub.0TT.sub.lim.

[0059] In other words, for a defined torque T, the corresponding maximum speed can be determined by the following equations:

[00017]$=\frac{\mathrm{COF}*P_{\mathrm{nom}}}{T}\mathrm{if}T{T}_{\lim }$

[00018]$=\frac{\mathrm{COF}*P_{\mathrm{nom}}}{a*T+b}\mathrm{if}{T}_{0}T{T}_{\lim }$

with the above-mentioned function (first order polynomial function),

[00019]$={}_{\max }\mathrm{if}T{T}_0.$