Patent Application
20180062544
The inventive subject matter relates to motor drives and, more particularly, to soft starters for motors.
Motor starters are commonly used with industrial electric motors. Typical solid-state motor starters control starting characteristics to meet application requirements, including acceleration and deceleration time, starting current and motor torque. Soft starters are commonly used to limit inrush current when the motor is first coupled to a power source, as large inrush currents may cause voltage dips that may negatively affect other loads coupled to the same source. Starters may also limit starting torque, as high starting torque may cause electromechanical shock that can damage windings and other components of the motor, as well as drive trains and other components mechanically coupled to the motor.
A reduced-voltage soft starter (RVSS) may use silicon-controlled rectifiers (SCRs) that are connected in series between an AC power source and the load. The SCRs may be phase controlled to apply a reduced RMS voltage to the motor during startup. Typically, the RMS voltage is ramped up to the normal operating RMS voltage at a preset rate using a series of pulses at the fundamental frequency of the AC power source. This may reduce the starting current at the expense of reduced starting torque, and thus limit the types of applications in which RVSSs may be used effectively. For example, applications that require high starting torque, such as motors driving conveyers or industrial equipment such as mixers, grinders or crushers, may have high starting torque requirements that cannot be met using a conventional RVSS. Such loads may require the use of a larger and costlier variable frequency drive (VFD).
Some embodiments of the inventive subject matter provide methods of starting a motor. The methods include operating a switch configured to couple an AC power source to the motor to modulate an AC voltage applied to the motor from the AC power source at a frequency that is less than a fundamental frequency of the AC voltage and bypassing the switch to couple the AC power source to the motor responsive to operation of the motor meeting a predetermined criterion. In some embodiments, operating a switch configured to couple an AC power source to the motor to modulate an AC voltage applied to the motor from the AC power source at a frequency that is less than a fundamental frequency of the AC voltage may include operating the switch to modulate an AC voltage applied to the motor from the AC power source at a frequency that is a subharmonic of the fundamental frequency. In some embodiments, operating a switch configured to couple an AC power source to the motor to modulate an AC voltage applied to the motor from the AC power source at a frequency that is less than a fundamental frequency of the AC voltage may include operating the switch to apply a voltage of a first polarity in a first half of a modulation cycle and to apply a voltage of a second polarity in a second half of a modulation cycle. Operating a switch configured to couple an AC power source to the motor to modulate an AC voltage applied to the motor from the AC power source at a frequency that is less than a fundamental frequency of the AC voltage may include providing a series of pulses of the AC voltage occurring at the fundamental frequency and having varying pulse durations.
In some embodiments, operating a switch configured to couple an AC power source to the motor to modulate an AC voltage applied to the motor from the AC power source at a frequency that is less than a fundamental frequency of the AC voltage may include modulating the AC voltage at a first frequency and modulating the AC voltage at a second frequency greater than the first frequency and less than the fundamental frequency. Modulating the AC voltage at a second frequency greater than the first frequency and less than the fundamental frequency may include changing from modulating at the first frequency to modulating at the second frequency responsive to meeting a predetermined criterion. The methods may further include sensing a speed of the motor using a speed sensor and/or back electromotive force (EMF) and changing from modulating at the first frequency to modulating at the second frequency responsive to meeting a predetermined criterion may include changing from modulating at the first frequency to modulating at the second frequency responsive to the sensed speed meeting a predetermined criterion. Modulating at the second frequency may apply a greater RMS voltage to the motor than modulating at the first frequency. The first and second frequencies may be subharmonics of the fundamental frequency.
Further embodiments of the inventive subject matter may provide a motor starter including a first switch configured to couple and decouple an AC power source to and from a motor, a second switch configured to bypass the first switch, and a control circuit configured to operate the first switch to modulate an AC voltage applied to the motor from the AC power source at a frequency that is less than a fundamental frequency of the AC voltage and to operate the second switch to bypass the first switch responsive to operation of the motor meeting a predetermined criterion.
Specific exemplary embodiments of the inventive subject matter now will be described with reference to the accompanying drawings. This inventive subject matter may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive subject matter to those skilled in the art. In the drawings, like numbers refer to like items. It will be understood that when an item is referred to as being “connected” or “coupled” to another item, it can be directly connected or coupled to the other item or intervening items may be present. As used herein the term “and/or” includes any and all combinations of one or more of the associated listed items.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the inventive subject matter. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless expressly stated otherwise. It will be further understood that the terms “includes,” “comprises,” “including” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, items, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, items, components, and/or groups thereof.
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this inventive subject matter belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
Conventional RVSSs use solid-state switches to sweep across an AC voltage source sine wave from its zero crossings to increase the motor voltage from zero to a rated value, thus developing a voltage output having the same dominant or fundamental frequency as the AC voltage source. In contrast, in some embodiments of the inventive subject matter, RVSS switches are used to control not only the motor voltage but also the dominant or fundamental frequency applied to the motor by pulse-width modulating the AC source voltage using the AC source frequency as the switching frequency. By turning-on the SCR bidirectional switches at the correct angles, an output voltage is produced having a dominant or fundamental component that is a subharmonic of the AC source fundamental frequency, which is filtered by the motor inductance and generate a current that can generate increased motor torque at lower speeds. Thus, a reduced voltage soft starter (RVSS) may be used to mimic a constant volts-per-Hertz (V/Hz) ramp using a combination of subharmonic pulse-width modulation sequences and phase gating control to control RMS voltage. The RVSS may ramp up the RMS voltage and the pulse-width modulation frequency through a series of steps to accelerate the motor sufficiently and allow transfer to the AC source. In some embodiments, an RVSS can pulse-width modulate the AC voltage at a frequency that is a subharmonic of the line frequency by selectively gating selected ones of pairs of antiparallel-connected SCRs to provided selected polarity and pulse width patterns.
A controller 130 is configured to control the first switch 110 and the second switch 120. The controller 130 may be configured to implement a sub-fundamental modulator 132 that causes the controller 130 to operate the solid-state first switch 110 to apply a pulse-width modulation to an AC voltage produced by the AC power source 10 at a frequency that is lower (e.g., a subharmonic) of a fundamental frequency of the AC voltage produced by the AC power source 10 to accelerate the motor 30. As explained below, according to some embodiments, such sub-fundamental modulation may be applied at a succession of increasing modulation frequencies to accelerate the motor 30 to a point at which the second switch 120 can be closed to operate the motor 30 directly from the AC power source 10. Similarly, a decreasing series of modulation frequencies may be used to decelerate the motor 30. It will be appreciated that controller 130 may be implemented using any of a variety of different analog and/or digital circuitry, including, for example, data processing devices, such as microcontrollers, and associated peripheral circuitry for controlling the first switch 110 and the second switch 120, and for interfacing with various sensors (e.g., current, speed and voltage sensors) that may be used to monitor the AC power source 10, the motor 30 and other system components.
According to further aspects, modulation of the form shown in
The pulse patterns may be generated in any of a number of different ways. For example, in embodiments using a microcontroller or other data processing device for controlling the modulation switch, the pulse duration may be determined from one or more lookup tables that associate pulse width with a phase of the modulation cycle in patterns that define one or more fundamental frequencies which are less than the AC source fundamental frequency. These patterns are swept by the controller, going up or down in fundamental frequency to accelerate or decelerate the motor. Other embodiments may use functionally similar analog circuitry. For example, a pulse width modulator may operate based on comparison of a saw tooth waveform to a reference sine wave signal that has a frequency corresponding to the desired subharmonic and an amplitude that controls pulse duration. Accordingly, selection of a modulation frequency and pulse duration pattern may include, for example, selection of entries in a lookup table or adjustment of a reference signal.
The modulation frequencies and pulse duration patterns used may be based, for example, on the nature of the load. Modulation frequencies and pulse duration patterns may be selected based on a priori knowledge of the motor characteristics and the load requirements and/or may be adaptively selected or modified based on performance feedback. For example, modulation frequencies and pulse duration patterns may be adjusted based on measurements of motor parameters (e.g., speed, current) and/or other parameters as the starter is operated, thus enabling optimization of starter operations.
It will be appreciated that the operations shown in
According to further embodiments, transitions between modulation frequencies may be conditioned upon the motor reaching a predetermined state. For example, the speed of the motor may be detected from a speed sensor (e.g., a Hall effect sensor or optical encoder) or back emf and, when the speed indicates that the motor is no longer sufficiently accelerating, the next higher subharmonic modulation frequency and associated pulse duration pattern may be selected. When the last modulation frequency is reached and the motor is no longer accelerating, the bypass switch (e.g., the second switch 120 of
It will be appreciated that further embodiments may variations on the criteria for changing modulations described above with reference to
According to further embodiments, similar operations may be used for decelerating a motor. Referring to
In the drawings and specification, there have been disclosed exemplary embodiments of the inventive subject matter. Although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the inventive subject matter being defined by the following claims.
