BACKSPINNING MOTOR CONTROL

Abstract

Examples include a method of control implemented in a variable speed drive for controlling an electric motor during backspin, wherein the method comprises includes: determining, by the variable speed drive, a mechanical power value occurring at a backspin speed and an estimated load torque; determining, by the variable speed drive, a specific electrical losses profile occurring at a motor flux level, wherein the specific electrical losses profile coincides with the mechanical power value; determining, by the variable speed drive, a flux reference and a speed reference to be applied to the motor to coincide with the specific electrical losses profile ; and controlling, by the variable speed drive, the backspin speed of the motor to maintain the coincidence with the specific electrical losses profile.

Claims

1. A method of control implemented in a variable speed drive for controlling an electric motor during backspin, wherein the method comprises: determining, by the variable speed drive, a mechanical power value occurring at a backspin speed and an estimated load torque; determining, by the variable speed drive, a specific electrical losses profile occurring at a motor flux level, wherein the specific electrical losses profile coincides with the mechanical power value; determining, by the variable speed drive, a flux reference and a speed reference to be applied to the motor to coincide with the specific electrical losses profile; and controlling, by the variable speed drive, the backspin speed of the motor to maintain the coincidence with the specific electrical losses profile.

2. The method according to claim 1, wherein controlling the backspin speed of the motor comprises redetermining the specific electrical losses profile to update the flux reference.

3. The method according to claim 1, wherein controlling the backspin speed of the motor comprises extracting, from the specific electrical losses profile, an electrical losses value occurring at the estimated load torque, and updating the speed reference to the backspin speed corresponding to the electrical losses value.

4. The method according to claim 1, comprising determining a maximum electrical losses profile, and verifying that an electrical losses value occurring at the backspin speed and the estimated load torque are below the maximum electrical losses profile.

5. The method according to claim 1, wherein controlling the backspin speed of the motor is carried out until a predefined threshold is reached.

6. The method according to claim 5, wherein the predefined threshold is at least one from a group comprising: a minimum load torque, a minimum mechanical power value, a maximum backspin speed.

7. The method according to claim 1, wherein the backspin speed is a predefined backspin speed.

8. The method according to claim 7, wherein the predefined backspin speed is chosen from a group comprising a slower, a medium and a faster backspin speed.

9. The method according to claim 1, wherein determining the specific electrical losses profile comprises selecting the specific electrical losses profile from a plurality of electrical losses profiles occurring at a plurality of motor flux levels.

10. The method according to claim 9, wherein the plurality of motor flux levels range from 10% of a nominal flux to 150% of the nominal flux.

11. The method according to claim 1, further comprising selecting whether controlling the backspin speed of the motor includes changing the flux reference and/or the speed reference.

12. The method according to claim 1, further comprising detecting, by the variable speed drive, the motor backspinning.

13. The method according to claim 1, wherein the method comprises applying the method following a power outage.

14. A variable speed drive of an electric motor comprising a processor and a memory, the processor being configured to operate according to claim 1.

15. A non-transitory computer-readable storage medium comprising instructions which, when executed by a processor, cause the processor to carry out the method of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] FIG. 1 schematically illustrates an example of a variable speed drive connected to an electrical network and a motor.

[0028] FIG. 2 illustrates schematically an example of a variable speed drive controlling backspin speed of a motor following an electrical power outage.

[0029] FIG. 3 illustrates an example method.

[0030] FIG. 4 illustrates another example method.

[0031] FIG. 5 illustrates yet another example method.

[0032] FIG. 6 illustrates yet another example method.

[0033] FIG. 7 illustrates an example of a control system for the implementation of the methods of the present disclosure.

[0034] FIG. 8 illustrates an example of a step illustrated in FIG. 7.

[0035] FIG. 9 illustrates an example of another step shown in FIG. 7.

[0036] FIG. 10 illustrates an example of another step shown in FIG. 7.

[0037] FIG. 11 illustrates another example of a step of FIG. 7.

DETAILED DESCRIPTION

[0038] This disclosure applies to the controlling of an electric motor by a variable speed drive. A variable speed drive should be understood in this disclosure as an electronic, virtual or software implemented control unit for an electric motor.

[0039] As in the example illustrated in FIG. 1, the variable speed drive 10 may be connected, on the one hand, to an electrical network N and, on the other hand, to the electric motor M. The variable speed 10 drive may comprise an inverter module 12, a DC (Direct Current) power bus 14 and a converter module 16.

[0040] The inverter module 12 may comprise a diode bridge configured to convert a 3-phase AC (Alternating Current) voltage provided by the electrical network N to a DC voltage. The DC voltage outputted by the inverter module 12 may be applied to the DC power bus 14.

[0041] The DC power bus 14 can comprise two power lines connected together by a bus capacitor C.sub.bus configured to stabilize the voltage of the bus 14. The output of the DC power bus 14 may be connected to the converter module 16.

[0042] The converter module 16 can comprise several switching arms each comprising power transistors, for example of the IGBT (Insulated Gate Bipolar Transistor) type. The converter module 16 may be intended to cut off the voltage supplied by the DC power bus 14, to achieve a variable output voltage applied to the electric motor M.

[0043] The variable speed drive 10 may comprise a processor PROC, the processor PROC being configured to operate according to any of the methods hereby described. Processor PROC may comprise electronic circuits for computation managed by an operating system.

[0044] The variable speed drive 10 may comprise a non-transitory machine-readable or computer readable storage medium, such as, for example, memory or storage unit MEM, whereby the non-transitory machine-readable storage medium is encoded with instructions executable by a processor such as processor PROC, the machine-readable storage medium comprising instructions to operate processor PROC to perform as per any of the example methods hereby described. A computer readable storage according to this disclosure may be any electronic, magnetic, optical or other physical storage device that stores executable instructions. The computer readable storage may be, for example, Random Access Memory (RAM), an Electrically Erasable Programmable Read Only Memory (EEPROM), a storage drive, and optical disk, and the like. As described hereby, the computer readable storage may be encoded with executable instructions according to the methods hereby described. Storage or memory may include any electronic, magnetic, optical or other physical storage device that stores executable instructions as described hereby.

[0045] The variable speed drive 10 according to this disclosure is connected to a driven load. By driven load, it should be understood that the motor M may drive the load, or reciprocally, the load may drive the motor M. When the motor drives the load, electrical power is consumed by the motor M to produce mechanical power. Reciprocally, when the load drives the motor M, the load applies a load torque on the motor to spin the motor M, to produce electrical power. Electrical power may be understood as a combination of voltage and current. Mechanical power may be understood as a combination of speed and torque.

[0046] FIG. 2 illustrates the behavior of the electrical motor M under an electrical power outage. Electrical power outages occur when the voltage provided by the electrical network N to the variable speed drive 10 is cut off. A power outage may occur prior to the applying the methods described hereby. A power outage may be detected and trigger the methods described hereby.

[0047] Following an electrical power outage, occurring at (0) on FIG. 2, the torque produced by the electric motor M can drop. As illustrated at (1) on FIG. 2, the electric motor M may decelerate under the action of the load torque. The motor M may generate electrical power, which may be stored by the DC power bus capacitor C.sub.bus, resulting in a rising DC bus voltage.

[0048] The motor M may reach an area close to a zero speed, at (2) on FIG. 2, wherein the electric motor M can no longer generate electrical power. The bus voltage may drop due to electrical losses occurring in the variable speed drive 10 and the electric motor M. To avoid a switching off of the variable speed drive 10, occurring when the bus voltage decrease below a critical level, the electrical power stored by the DC power bus capacitor C.sub.bus may be used to establish backspin. Backspin is to be understood as the reverse spinning of the motor M. Backspin may be established by the motor M producing mechanical power to increase the speed of the motor M in the reverse direction, as illustrated at (3) on FIG. 2.

[0049] During backspin, the electric motor M may accelerate under the action of the load torque. The motor M may generate electrical power, which may be stored by the bus capacitor C.sub.bus, resulting in a rising DC bus voltage. An uncontrolled backspin may be undesired for a number of reasons. For example, such a reason may be reaching backspin speeds beyond a rated speed of the motor M, which could be damaging to equipment or unsafe to personnel. Some of the power generated by the electric motor M may be used by the variable speed drive 10 to control the backspin speed of the motor M. The control of the backspin speed may be achieved by the motor M producing mechanical power to oppose the action of the load. The mechanical power may form a resistive torque applied to maintain the backspin speed of the motor M at a desired value. This is illustrated at (4) on FIG. 2. However, some excess power may be generated, corresponding to the electrical power produced by the motor M during backspin that is not being stored or used by the variable speed drive 10 to control backspin speed.

[0050] FIG. 3 illustrates an example of a method 100 according to this disclosure. The method illustrated in FIG. 3 is carried out for example by the variable speed drive 10 to control the electric motor M during backspin.

[0051] As illustrated in block 101, the method 100 comprises determining a mechanical power value P.sub.mech. By mechanical power value P.sub.mech, it is to be understood the power produced by the electric motor M. The mechanical power value may be calculated from a motor backspin speed ω and an estimated load torque T.sub.est.

[0052] As illustrated in block 102, the method 100 comprises determining a specific electrical losses profile P.sub.ref. By electrical losses, it is to be understood the power losses occurring at the motor M and the variable speed drive 10. Electrical losses may comprise motor losses and variable speed drive losses. Electrical losses may be calculated from motor characteristics. Electrical losses may vary with motor speed, torque and flux level. Thus, an electrical losses profile P.sub.elec may represent electrical losses occurring at a flux level for any motor speed and torque. The specific electrical losses profile P.sub.ref is the electrical losses profile for which the mechanical power value P.sub.mech may coincide with the electrical losses occurring. Thus, power produced by the electric motor M is comparable to that required by the variable speed drive 10 to stay running and control the backspin speed of the motor M.

[0053] As illustrated by block 103, the specific electrical losses profile P.sub.ref may be used to determine a flux reference φ.sub.ref and a speed reference corer. The flux reference φ.sub.ref may correspond to the flux level associated to the specific electrical losses profile P.sub.ref. The speed reference ω.sub.ref may correspond to the backspin speed at which the mechanical power value P.sub.mech coincides with the specific electrical losses profile P.sub.ref.

[0054] As illustrated by block 104, the backspin speed of the motor M may be controlled to maintain the coincidence with the specific electrical losses profile P.sub.ref. By controlling the backspin speed of the motor M, it is to be understood that the variable speed drive 10 may regulate the motor M to the achieve coincidence between the mechanical power value P.sub.mech and the specific electrical losses profile P.sub.ref.

[0055] The method 100 illustrated in FIG. 3 enables the motor to backspin in a state where the mechanical power produced by the motor M is comparable to the electrical power required by the variable speed drive 10 to run and control the motor M. Little or no excess power may be produced, reducing or even suppressing the need for power dissipation.

[0056] In some examples, the load torque applied to the motor by the load may remain constant in time. In such cases, the mechanical power value P.sub.mech and the electrical losses occurring may also remain constant. The speed reference ω.sub.ref and the flux reference φ.sub.ref may, in such examples, remain constant.

[0057] In some examples, the load torque may vary with time. In such cases, the mechanical power value P.sub.mech and electrical losses occurring may also vary with time. As a result, the mechanical power value P.sub.mech may not continuously coincide with the specific electrical losses profile Pref.

[0058] FIG. 4 illustrates a method 200, which may be carried out within block 104 as discussed in FIG. 3. Method 200 consists in updating the flux reference φ.sub.ref. In the present disclosure, a same block may appear in different Figures, in which case such block is numbered in the same manner in the different Figures.

[0059] The method 200 comprises, at block 201, updating the mechanical power value P.sub.mech to account for a change in estimated load torque T.sub.est. As illustrated at block 202, the specific electrical losses profile P.sub.ref may be redetermined. The redetermined specific electrical losses profile P.sub.ref may coincide with the updated mechanical power value P.sub.mech. As illustrated at block 203, an updated flux reference ref may be determined from the redetermined specific electrical losses profile P.sub.ref. Updating the flux reference φ.sub.ref can allow to maintain coincidence between the mechanical power value P.sub.mech and the specific electrical losses profile P.sub.ref even while the load torque applied to the motor M during backspin varies. The speed reference ω.sub.ref, in such examples, may remain constant.

[0060] FIG. 5 illustrates a method 300, which may be carried out within block 104 as discussed in FIG. 3. Method 300 consists in updating the speed reference ω.sub.ref.

[0061] The method 300 also comprises block 201 of updating the mechanical power value P.sub.mech. Method 300 further comprises, at block 301, extracting, from the specific electrical losses profile P.sub.ref, an electrical losses value occurring at the estimated load torque T.sub.est. The electrical losses value corresponds to the electrical losses occurring at the estimated load torque T.sub.est while following the specific electrical losses profile P.sub.ref. At block 302, the backspin speed at which the electrical losses value is realized may be determined. The determined backspin speed may indicate a new speed reference ω.sub.ref at which the updated mechanical power P.sub.mech may coincide with the specific electrical losses profile P.sub.ref. As illustrated by block 303, the speed reference ω.sub.ref may be updated from the determined speed of block 302. A single specific electrical losses profile P.sub.ref may be used to control the backspin speed of the motor M. Updating the speed reference ω.sub.ref can allow to maintain coincidence between the mechanical power value P.sub.mech and the specific electrical losses profile P.sub.ref even while the load torque applied to the motor M during backspin varies. The flux reference φ.sub.ref may, in such examples, remain constant.

[0062] In some cases, both the flux reference φ.sub.ref and the speed reference φ.sub.ref may be updated. The speed reference ω.sub.ref may be updated when the flux is below a threshold. The flux reference φ.sub.ref may be updated to limit or suppress speed oscillations at low backspin speeds. The combination of updating both the flux reference φ.sub.ref and the speed reference ford may increase robustness and performance of backspin speed control.

[0063] FIG. 6 illustrates a method 400 comprising blocks 101-104 as discussed in FIG. 3, wherein a maximum electrical losses profile P.sub.max is determined, at block 401. Electrical losses may be limited by the current which can be applied to the electric motor M. Thus, by maximum electrical losses profile P.sub.max, it is to be understood the maximum achievable electrical losses at any given speed and torque, and for any flux level.

[0064] At block 402, the electrical losses value, comparable to the mechanical power value P.sub.mech through the application of method 100, may be compared to the maximum electrical losses profile P.sub.max. The electrical losses value may be below the maximum electrical losses, suggesting that a specific electrical losses profile P.sub.ref can be found to coincide with the mechanical power value P.sub.mech. The electrical losses value may be above maximum electrical losses, suggesting that no electrical losses profile P.sub.elec can coincide with the mechanical power value P.sub.mech. The electrical losses value may be reduced, for example by lowering the speed reference ω.sub.ref. Thus, the mechanical power value P.sub.mech may also be reduced following method 100 and coincide with a specific electrical losses profile P.sub.ref.

[0065] In some examples, block 104 of controlling the backspin speed of the motor may be carried out until a predefined threshold is met.

[0066] The predefined threshold may be a minimum load torque T.sub.min. At the minimum load torque T.sub.min, the backspin speed required to maintain coincidence with the specific electrical losses profile P.sub.ref may exceed the rated speed of the system. In other words, producing sufficient mechanical power to maintain the variable speed drive running may require an excessive backspin speed. In such a case, it may be preferable to remove control from the variable speed drive 10 and let the variable speed drive 10 power off.

[0067] The predefined threshold may be a minimum mechanical power value P.sub.min. At the minimum mechanical power value P.sub.min, a negligible load may be applied to the motor M. Backspin speed may remain at a speed below the rated speed of the motor M without control from the variable speed drive 10.

[0068] The predefined threshold may be a maximum backspin speed ω.sub.max. Reaching the maximum backspin speed may indicate that a low load torque is applied to the motor. At the maximum backspin speed ω.sub.max, the backspin speed required to maintain coincidence with the specific electrical losses profile may exceed the rated speed of the system. In such a case, it may be preferable to remove control from the variable speed drive 10 and let the variable speed drive 10 power off.

[0069] In some examples, the methods described herein may comprise selecting whether controlling the backspin speed of the motor updates the speed reference ω.sub.ref, according to method 300, or the flux reference φ.sub.ref, according to method 200. In some cases, the selection may be done by an operator. Selection may depend on the application. Selection may improve the flexibility of the methods described herein.

[0070] In some examples, the methods described herein may comprise detecting the motor M backspinning. Detection of backspin occurrence may trigger controlling of the backspin speed of the motor M. Detection of backspin may be achieved by a measured speed from speed or current measurements at the motor M.

[0071] FIG. 7 illustrates a control system configured to carry out the methods described herein.

[0072] As illustrated, the mechanical power value P mech may be determined at block 101 from the backspin speed ω and the estimated load torque T.sub.est. The estimated load torque T.sub.est may be determined from measurements at the motor. The estimated load torque T.sub.est may be determined by taking current measurements and calculating the estimated load torque T.sub.est. The estimated load torque T.sub.est may be determined from taking torque measurements at the motor. Measurements may be taken by sensors at the motor.

[0073] In some examples, the backspin speed ω may be a predefined backspin speed ω.sub.assigned. The predefined backspin speed ω.sub.assigned may be assigned depending on a desired backspin speed of the motor M. The predefined backspin speed ω.sub.assigned may be chosen depending on the application. Predefined backspin speed ω.sub.assigned may be chosen by an operator. Predefined backspin speed ω.sub.assigned may, for example, be chosen between a medium, faster and slower backspin speed. In such cases, the predefined backspin speed ω.sub.assigned may be comparable to the speed reference ω.sub.ref. The motor may operate at the predefined backspin speed ω.sub.assigned. Motor control at block 104 may be achieved by applying method 200.

[0074] In some examples, the backspin speed ω may be an estimated backspin speed. The estimated backspin speed may be established by measurements taken at the motor M. Measurements may be taken by sensors at the motor.

[0075] As illustrated, determining the specific electrical losses profile P.sub.ref at block 102 may comprise identifying the specific electrical losses P.sub.ref profile from a plurality of electrical losses profiles P.sub.ref. The electrical losses profiles P.sub.elec may correspond to the electrical losses profiles occurring between 10% and 150% of a nominal flux. The electrical losses profiles P.sub.elec may correspond to the electrical losses profiles occurring between 20% and 120% of a nominal flux. The nominal flux may correspond to a rated flux of the electric motor M. The electrical losses occurring between 10% and 150% of the nominal flux may correspond to likely mechanical power values. The electrical losses occurring between 10% and 150% of the nominal flux may also correspond to achievable motor flux levels to preserve the electric motor M.

[0076] In some examples, determining the specific electrical losses profile P.sub.ref may comprise calculating the specific electrical losses profile P.sub.ref from the determined mechanical power value P.sub.mech, motor data and the estimate load torque T.sub.est.

[0077] In addition, block 500 illustrates the calculation, by the variable speed drive 10, of the voltages to be applied to the motor M. Voltages may be calculated using vector control laws. The speed reference ω.sub.ref and the flux reference φ.sub.ref may be converted to a flux producing voltage and a torque producing voltage. The flux producing voltage and a torque producing voltage may be converted to 3 phase voltages to be sent to each motor winding. Current measurements at the motor M may be used in voltage calculations to improve the accuracy of voltage calculations.

[0078] FIG. 8 illustrates an example of calculating the mechanical power value P.sub.mech. The backspin speed ω may be estimated or assigned, in this example, as −10 Hz, or approximately 62 rad/s. The estimated load torque T.sub.est may be estimated, in this example, as 80% of a nominal torque. If the motor M nominal torque is 135 Nm in this example, the estimated load T.sub.est may be 108 Nm. Thus the mechanical power value P.sub.mech may be calculated as 6.7 kW.

[0079] FIG. 9 illustrates an example identifying a specific electrical losses profile P.sub.ref and using the maximum electrical losses profile P.sub.max. As illustrated, six electrical losses profiles P.sub.elec have been determined, as a function of speed and torque, for flux levels ranging from 20% of the nominal flux to 120% of the nominal flux. The maximum electrical losses profile P.sub.max illustrates the maximum achievable electrical losses. In this example, the maximum electrical losses occurring at the backspin ω speed of −10 Hz are 9.3 kW. In this case, the mechanical power value P.sub.mech of 6.7 kW is inferior to maximum electrical losses. The specific electrical losses profile P.sub.ref coinciding with the mechanical power value P.sub.mech at the backspin speed of −10 Hz is, in this case, the electrical losses profile P.sub.elec at 80% of the nominal flux. The flux reference of 80% the nominal flux may be applied to the motor M. In addition, in this example, the speed reference ω.sub.ref may be set as 10 Hz, comparable to the backspin speed ω.

[0080] FIGS. 10 and 11 illustrate examples wherein the estimated load torque T.sub.est is reduced from 80% of the nominal torque to 25% of the nominal torque. The change in estimated load torque T.sub.est leads to an updated mechanical power value P.sub.mech-new. The updated mechanical power value P.sub.mech-new may be calculated from the speed reference ω.sub.ref and the reduced estimated load torque T.sub.est. According to this example, the new mechanical power value P.sub.mech-new may be found to be 2.1 kW. However, the electrical losses occurring at the speed reference value ω.sub.ref remain at 9.3 kW. The motor M may be generating an excess power of 7.2 kW.

[0081] FIG. 10 illustrates an example wherein the flux reference ω.sub.ref is updated. The variable speed drive 10 may select a new specific electrical losses curve P.sub.ref. In this example, the new specific electrical losses profile P.sub.ref coinciding with the new mechanical power value P.sub.mech-new is the electrical losses profile P.sub.elec at 20% of the nominal flux. The speed reference ω.sub.ref may remain as 10 Hz, comparable to the previous backspin speed ω. The updated flux reference of 20% the nominal flux may be applied to the motor M.

[0082] FIG. 11 illustrates an example wherein the speed reference ω.sub.ref is updated. The variable speed drive 10 may determine a new speed reference ω.sub.ref-new. According to the specific electrical losses profile P.sub.ref at 80% of the nominal flux, electrical losses of 2.1 kW may occur at a new speed reference ω.sub.ref-new of −5 Hz. The flux reference may remain as 20% the nominal flux. The updated speed reference of −5 Hz may be applied to the motor M.