US-20260128694-A1 - SAFE START OF AN AC MOTOR

US20260128694A1US 20260128694 A1US20260128694 A1US 20260128694A1US-20260128694-A1

Abstract

A method for safely starting an alternating-current, AC, motor from a pre-loaded stationary condition, the method including: applying a brake to immobilize the AC motor's axle; feeding the AC motor with a predefined drive signal waveform configured not to generate torque and sensing resulting stator currents; comparing the sensed stator currents with reference currents associated with the drive signal; and enabling release of the brake in response to finding that the sensed stator currents match the reference currents.

Inventors

Joakim Lindgren
Erik Thenander

Assignees

ABB SCHWEIZ AG

Dates

Publication Date: 20260507
Application Date: 20221011

Claims (19)

1 . A method for safely starting an alternating-current, AC, motor from a pre-loaded stationary condition, the method comprising: applying a brake to immobilize the AC motor's axle; feeding the AC motor with a predefined drive signal waveform configured not to generate torque and sensing resulting stator currents; comparing the sensed stator currents with reference currents associated with the drive signal; and enabling release of the brake in response to finding that the sensed stator currents match the reference currents.
2 . The method of claim 1 , wherein the predefined drive signal waveform has a zero Q component in a rotor-synchronous DQZ reference frame.
3 . The method of claim 1 , wherein the motor is a permanent-magnet synchronous motor, PMSM.
4 . The method of claim 3 , wherein the predefined drive signal waveform is field-strengthening in relation to a rotor of the PMSM.
5 . The method of claim 3 , wherein the predefined drive signal waveform is field-weakening in relation to a rotor of the PMSM.
6 . The method of claim 1 , wherein the motor is an induction motor.
7 . The method of claim 1 , wherein the release of the brake is enabled in response to finding that the absolute error between the sensed stator currents and the reference currents is below a threshold.
8 . The method of any of claim 1 , further comprising: providing an error indication in response to finding that the sensed stator currents do not match the reference currents.
9 . The method of any of claim 1 , wherein said comparing the sensed stator currents with the reference currents includes converting the stator currents from a stationary reference frame into a rotor-synchronous reference frame.
10 . The method of claim 1 , wherein said feeding the AC motor with a predefined drive signal waveform includes performing a phase-width modulation, PWM.
11 . The method of claim 1 , wherein the AC motor is installed in an industrial robot.
12 . A motor controller arranged to control an electric drive unit configured to feed an alternating-current, AC, motor, the motor controller comprising processing circuitry configured to perform the method of: applying a brake to immobilize the AC motor's axle; feeding the AC motor with a predefined drive signal waveform configured not to generate torque and sensing resulting stator currents; comparing the senses stator currents with reference currents associated with the drive signal; and enabling release of the brake in response to finding that the sensed stator currents match the reference currents.
13 . A computer program comprising instructions to cause a motor controller arranged to control an electric drive unit configured to feed an alternating current, the motor controller having processing circuitry configured to perform the method of: applying a brake to immobilize the AC motor's axle; feeding the AC motor with a predefined drive signal waveform configured not to generate torque and sensing resulting stator currents; comparing the senses stator currents with reference currents associated with the drive signal; and enabling release of the brake in response to finding that the sensed stator currents match the reference currents, to execute the steps of the method.
14 . The method of claim 2 , wherein the motor is an induction motor.
15 . The method of claim 2 , wherein the release of the brake is enabled in response to finding that the absolute error between the sensed stator currents and the reference currents is below a threshold.
16 . The method of any of claim 2 , further comprising: providing an error indication in response to finding that the sensed stator currents do not match the reference currents.
17 . The method of any of claim 2 , wherein said comparing the sensed stator currents with the reference currents includes converting the stator currents from a stationary reference frame into a rotor-synchronous reference frame.
18 . The method of claim 2 , wherein said feeding the AC motor with a predefined drive signal waveform includes performing a phase-width modulation, PWM.
19 . The method of claim 2 , wherein the AC motor is installed in an industrial robot.

Description

TECHNICAL FIELD The present disclosure relates to the field of electric motors and more precisely to a method for safely starting an alternating-current (AC) motor from a pre-loaded stationary condition. BACKGROUND Various types of internal failures can render an electric motor unable to provide its nominal torque. The seriousness of such a failure is dependent on the nature of the applications where the electric motor is installed. For example, if the failing motor drives a fan, pump or blower, the failure may have limited consequences. In contrast to such use cases, the failure of an electric motor in a robot, elevator or crane motor could be very destructive and even lead to bodily injury. In a robotic use case, potential sources of failure could include: broken insulated-gate bipolar transistor (IGBT) used for voltage generation,wrong motor cable connected between controller and robot,broken motor cable between controller and robot,broken motor windings,broken current measurement sensors,excessive stator resistance,leakage inductance. US20180254729A1 discloses a method for ensuring that an electric motor provides sufficient starting torque before a brake is released. This is achieved by compensating an insufficiency in starting torque, to the extent it is due to a voltage drop, by applying a compensation voltage. Similarly, JP2011254596A discloses a method for improving the rise of magnetic flux in an induction motor, and initiating the start-up of the induction motor in an optimal state with a sufficient torque produced. While these prior art methods propose ways of ensuring that a desired torque is generated in a braked state of the electric motor, it would be desirable—especially with regard to some use cases in robotics—to carry out a health check on the motor while in a passive condition. SUMMARY One objective of the present disclosure is to propose a motor control method and a motor controller suitable for safely starting an AC motor from a pre-loaded stationary condition. It may be considered safe to start the AC motor, in this sense, after it has successfully passed a health check. A further objective of this disclosure is to propose a motor control method and motor controller by which the correct functioning of the AC motor can be verified while the AC motor is in a torque-free condition. A still further objective is to propose such a motor control method and motor controller suitable for starting an AC motor installed in an industrial robot. At least some of these objectives are achieved by the invention as defined by the independent claims. The dependent claims relate to advantageous embodiments of the invention. In a first aspect of the present disclosure, there is provided a method for safely starting an AC motor from a pre-loaded stationary condition. The method comprises: applying a brake to immobilize the AC motor's axle; feeding the AC motor with a predefined drive signal waveform configured not to generate torque and sensing resulting stator currents during such feeding; comparing the sensed stator currents with reference currents associated with the drive signal; and enabling release of the brake if it is found that the sensed stator currents match the reference currents. According to the first aspect, because the AC motor is fed with a drive signal waveform configured not to generate torque, the comparison as to whether the sensed stator currents match the reference currents associated with the drive signal can be carried out in a rotation-free and substantially torque-free condition of the AC motor. The applied brake therefore does not have to absorb any electrically induced torque, and it is not exposed to the mechanical wear that would result. Accordingly, the braking serves primarily to immobilize the axle against the action of torques exerted by the self-weight of machinery in which the AC motor is installed, and/or from external forces on such machinery. When the machinery is a robot arm, the external forces could include a gravity force that acts on a load or various elastic forces during the gripping of a workpiece. As used herein, the act of “enabling release of the brake” could correspond to granting a (human or automated) operator of the AC motor the ability to release the brake at the operator's discretion. Enabling release shall not be understood as implying that the brake is necessarily released upon a positive finding, which could be unsafe unless the AC motor is controlled to apply a suitable starting torque. A starting torque value could be considered suitable in this sense if it corresponds approximately to the load on the motor axle that the brake is currently holding. If it is found that the sensed stator currents match the reference currents, some important classes of failures in the AC motor can be ruled out, and it may therefore be considered safe to start the motor. The qualifier “safe”, as used in the present disclosure, is not to be understood in an absolute