A guide to smart motor controls
While there is no specific definition to the phrase "smart motor controls," there is no question that ac variable-speed drives are becoming more sophisticated, offering an increasing amount of intelligence.
This intelligence is displayed in features such as removable and interchangeable keypads that help plant engineers program drives quickly, sophisticated diagnostics and motor protection, and networking capability.
Keypad interface modules
Drive setups can be performed quickly using detachable and/or interchangeable keypad interface modules. Most keypad modules can be removed, whether the drive is powered or not. Most companies offer keypads that can be mounted on the drive, as well as used as handheld devices (Fig. 1.).
Keypad modules allow users to program speed, start, stop, acceleration, deceleration, and parameters into the drive. Some modules permit uploads and downloads so that multiple drives requiring the same setup parameters can be programmed using only one keypad. The parameters for one drive are set with the keypad. Then those parameters are copied into the keypad, which is taken to other drives for subsequent setups (Fig. 2.). The user then downloads the setup parameters from the keypad into each drive in succession.
These devices are available in either light emitting diode (LED) or liquid crystal display (LCD) readouts. The readouts display speed information, setup parameters, or error codes.
Many drive manufacturers offer software that monitors, diagnoses, configures, and archives information and parameters concerning drives in the plant (Fig. 3.). This feature is useful for plant engineers with many drives to maintain. Setups are done within the software, then downloaded to the appropriate drives. Drive setup information is archived for future retrieval. Typical drive software can also interface with CMMS and EAM systems.
|Fig. 1. This flow chart represents a typical startup menu using a keypad interface module.|
Most drives can diagnose problems internally or with the motor. Available for some time, drives display codes that alert users when an error occurs. Faults are conditions that trip the drive, while alarms warn the user of a problem while allowing the drive to stay connected to the motor.
Some fault monitoring parameters are programmed by the manufacturer and cannot be changed by the user. However, most drives provide user-selectable fault monitoring. Some intelligent drives log faults in a buffer with time markers for a history of events. The status of the drive is usually saved when the fault occurs.
Most drives have fault indication capabilities. Some have the ability to troubleshoot themselves, virtually to the component level. When taken offline, they can scan their own circuitry, then tell you which components are defective.
Motor braking and drive protection
Typical ac drives use insulated gate bipolar transistors (IGBTs) as the current-supplying devices for the motor. Pulse-width modulation circuitry varies the duty cycle applied to the input of these IGBTs. Upon deceleration, the motor tries to act like a generator, using its motion to actually induce a current back into the circuitry that supplied current to the motor.
The three major categories of electrical braking for ac induction motors with variable frequency drives are dynamic, regenerative, and dc injection. The braking method used depends on cost, regeneration energy magnitude, and specific installation requirements.
Typically, dynamic braking is sufficient when coasting is not applicable. However, regenerative braking is recommended for fast duty cycles or when a relatively large energy loss is undesirable.
Dynamic braking directs the regenerative energy from the motor into a resistor in the drive circuits, providing an electrical load, or retarding torque, to the motor. The energy is dissipated as heat. The thermal capacity required for the resistor is determined by the stopping duty cycle for the load and the energy dissipated for each deceleration.
The dynamic braking feature consists of an electronic switch, such as an IGBT with a comparitor, placed across the drive’s dc bus. A resistor (or several resistors), with sufficient wattage rating to dissipate the regenerative energy, is in series with the IGBT. The regenerative energy from the motor tends to cause a rise in the drive’s dc bus voltage. Therefore, dynamic braking circuits are usually set to turn on at a specific voltage, and turn off at some lower voltage. The standard dynamic braking feature is designed to absorb six times the stored energy of a motor running at full speed.
Dynamic braking cannot operate during periods where power is lost, and cannot maintain holding torque when the drive is stopped. A mechanical brake must be used when the application requires a holding torque at zero speed.
Regenerative braking directs the regenerative energy from the motor back into the ac line, thus saving energy. Current limiting adjustments in the drive’s regenerative circuits control the level of energy returned to the line, and consequently the braking torque.
Regenerative braking is preferred over dynamic for applications with a relatively fast duty cycle and when large amounts of energy losses are undesirable. As with dynamic braking, regenerative braking is not effective during power outages. A mechanical brake must be used with the motor when the application requires a holding torque at zero speed. Regenerative braking cannot maintain a holding torque unless the drive is capable of operating at zero speed. To achieve this condition, sophisticated control circuits using field-oriented control (vector control) are required.
DC injection applies dc power to an ac induction motor, producing a braking torque. The energy is dissipated in the motor. This feature has limited use for deceleration from full speed because of motor heating considerations.
Like dynamic and regenerative methods of electrical braking, dc injection braking is not fail-safe. Loss of dc power results in loss of braking capability. A mechanical brake should be used with the motor when the application requires holding torque at zero speed.
Some manufacturers use internal protection that operates like an overload relay. Circuitry within the drive senses the current and disconnects the power from the motor if it exceeds a value set within the drive. This current sensing can be accomplished by measuring the voltage drop across a load resistor in small drives or using current transformers (CTs) in larger drives, then using this signal to fire a comparitor circuit that removes drive current from the motor or otherwise adjusts the output.
Some drives incorporate smart motor protection that knows how long a motor has been running under what loading conditions. The drive can be programmed to trip the motor if it senses an overload condition for a certain amount of time, based on overload class. Also, if a motor becomes jammed — creating a locked rotor condition — the drive trips the motor immediately.
Choosing an intelligent variable speed drive
Drive and motor choices are primarily application dependent. Application-specific variables that affect drive and motor choices include ambient conditions, types of loads, duty cycle, maintenance accessibility, horsepower range, and sequencing. Both ac and dc drives and motors offer intelligent features. The following paragraphs describe the characteristics of ac drives and motors.
AC drive characteristics
An ac drive uses a solid-state adjustable frequency inverter, which adjusts frequency and voltage to vary the speed of an otherwise conventional fixed-speed ac motor. Pulse-width modulation (PWM) of the drive output controls the speed of the motor. Voltage and frequency are maintained at a constant relationship at any motor speed to maintain a constant torque . This relationship is known as the volts-per-hertz (V/Hz) ratio.
Sensorless vector ac motors operate in an open-loop mode because they do not use tachometers or encodors. Flex vector ac motors provide tighter speed regulation than sensorless vector motors. The use of a tachometer or encoder allows the drive to provide a higher degree of control over speed precision of the motor. Sensorless vector provides near-flex vector performance, while flex vector provides high-performance motor speed and output torque.
Knowing ac drive and motor criteria help you decide which is better for the application. The table and sidebars provide further selection criteria.
— Jack Smith, Senior Editor, Plant Engineering
Variable speed drive and motor comparison
Standard dc V/Hz ac Vector ac – sensorless Vector ac – flex Speed regulation 0.01%(2) 1% 0.5% 0.01% Speed range 100:1 10:1 120:1 >1000:1 Encoder/tachometer desired? Yes/No No No Yes/No Constant hp range 4:1 2:1 4:1 4:1 Starting torque 150% 110% 150% 150% High-speed capability(1) &3000 &6000 &6000 &6000 Regeneration Line Snubber/line Snubber/line Snubber/line Dynamic braking without regulation Yes No No No (1) Speed rating is in rpm with standard motors.
(2) Regulation depends on the encoder or tachometer used.
Acknowledgements PLANT ENGINEERING magazine extends its appreciation to ABB Automation, Baldor Electric, Cutler-Hammer, Motortronics, Rockwell Automation (Allen-Bradley and Reliance Electric), Square D/Schneider Electric, and Toshiba Intl. Corp. for the use of their material in the preparation of this article.
How ac PWM vector drives work
Flex type ac vector drives
The drawing shows an ac PWM vector control block diagram. The following list provides an overview of how this technology works.
As with standard units, the user supplies a speed reference to the drive. The reference can also be supplied from a network interface if that option is included with the drive.
As with open-loop PWM drives, the speed reference may be conditioned for many parameters
The conditioned speed reference is compared to a speed feedback signal supplied by the motor encoder or tachometer (unless this is a sensorless vector system). An error signal between the reference and the feedback signal is amplified in the outer loop via proportional and integral gains (PID control). Stability adjustments change the gain of the loop to achieve the desired dynamic response. The resulting signal is applied as a reference to the current regulator. Motor current provides torque, which changes the motor speed to satisfy the outer loop, driving its error signal to zero.
The inner loop receives a reference from the speed loop. This reference is compared to the current feedback from the motor. The error signal is amplified and used to change the relative on and off times for gate pulses to the IGBTs. The relative on-to-off times for pulses to be successively fired is continually alternating to create a sine wave voltage at the IGBT output. The wave frequency and its amplitude are altered to respectively produce variable speed and torque outputs.
The rectified dc voltage (dc bus supply voltage) is the source for the IGBTs. As with standard open-loop drives, the IGBTs are arranged in three pairs with wave patterns 120-deg apart. The wave frequency changes to change the motor speed.
Vector drives provide a way to regulate field supply voltage or current. These drives have the capability to limit the flux or field command and affect a field weakening condition to allow extended speed operation. This can be seen as an optional ac drive operating range in typical motor speed-torque curves.
Sensorless type ac vector drives
Sensorless vector operation, which is used to achieve dynamic speed and torque control without a speed feedback device works as follows.
Current feedback from a motor can be broken into its components: magnetizing and torque producing currents. This relationship can be illustrated by allowing these currents to be represented by the legs of a right triangle with the actual current being the hypotenuse.
Magnetizing current can be shown as equivalent to the motor’s no-load current (less windage and friction losses). This case is a function of the motor’s rotor design and is constant.
Torque-producing current can be measured based on knowing the magnetizing current. Motor slip and torque are related. This factor means that a particular load current measurement at a given commanded speed will be used to cal-culate the motor’s slip at that moment. This will provide an accurate determination of actual motor speed.
Based on the actual vs desired speed, the reference is continually adjusted to get the proper speed given the calculated slip.
This method of control provides speed regulation to approximately 0.5% on a steady-state basis. Operation at or near zero speed is possible, but accuracy falls off because slip measurement at low frequencies becomes more difficult to measure.
Choose vector ac drives when:
Applications require full-load torque at zero speed
The load changes rapidly
There is a requirement for coordinated speed control, such as multiple drive axes
Increased starting torque is required
Precise closed-loop speed regulation (0.01%) is required
High dynamic response is necessary.
How standard open-loop ac PWM drives work
The drawing shows a standard open-loop ac PWM control block diagram. The following list provides an overview of how this technology works.
User supplies a speed reference to the drive from an analog reference or keypad selection.
Speed reference may be conditioned for acceleration, deceleration, maximum and minimum speed settings, and many other parameters.
Speed regulator section provides gate pulses to the IGBTs. The duty cycle is continually alternating to create a sine wave at the IGBT output. The wave frequency is altered to produce variable speed outputs.
Rectified dc voltage (dc bus supply voltage) is the source for the IGBTs, which are arranged in three pairs with wave patterns 120-deg. apart. The wave frequency changes to change the motor’s speed.
Choose standard ac drives when:
The environment surrounding the ac motor is corrosive, potentially explosive, or wet; this application requires special enclosures, such as explosion-proof or washdown
Motors are likely to receive little regular maintenance
The motor must be small or lightweight
Motor speeds can reach 10,000 rpm
Multiple motors are operated at the same speed by a single drive
Speed regulation of 1% is acceptable
Existing fixed-speed ac motors can possibly be used.