Motor Control 115 116 Control and Automation Solutions Guide Overview Introduction Motor control design for industrial industries. Furthermore, electric motors savings will soon offset those initial applications requires attention consume about 45% of the world’s expenses. The return-on-investment to both superior performance electricity according to the International calculations are often straightforward. and ruggedness. Maxim’s feature Energy Agency (IEA) report of May Interfacing to the Motor integration and superior specifications 2011 on global energy consumption enhance motor controller equipment by electric motor driven systems. By Controller precision while improving robustness comparison, lighting is a distant second A very important aspect of every in harsh industrial environments. consuming 19%. With the cost of energy motor controller in the industrial rising steadily, plant operators look for control and automation setting is the Motor controllers either control variable ways to reduce energy consumption communications interface between power supplies to the motor or to while maintaining throughput. the factory control system and the electronic switches between the power Furthermore, with the availability of individual motor controller. All the supply and the motor. These switches reasonably priced and highly capable block diagrams in the individual motor are precisely timed to open and close to motor controllers for all types of motors, controller sections show a control panel make the motor rotate most effectively. plant engineers are free to choose motor that provides a direct user interface The timing is often governed by complex types that are less expensive, more at the controller and a standard mathematical equations based on motor efficient, and require less maintenance. separately wired communications architecture and electromagnetic theory. interface that connects to the fieldbus. Depending on the application, a motor To put the energy savings opportunity The fieldbus ultimately runs back to a controller can be as simple as a variable- in perspective, compare motor power PLC (programmable logic controller) voltage generator, a pulsed-DC voltage consumption vs. speed when driving that sends commands to the motor source, or a complex signal generator fans and centrifugal pumps. The torque controller such as motor start, motor requiring sophisticated digital signal needed rises with speed, resulting in acceleration, speed adjustment, motor processing algorithms to generate the power draw that is proportional to stop, etc. An additional option exists: correct timing. For large motors, those in the cube of the speed! In other words, powerline communications (PLC, not to the multihorsepower range with multiple reducing the speed to one-half of full be confused with programmable logic power phases, precise control is essential. speed drops the power to one-eighth controller). This technology gives the At a minimum, the wrong timing can of full power. Even dropping the speed option of sharing command and control result in extreme power use. In the to 75% of full speed drops the power connections with power connections worst case, wrong timing can destroy consumption to 42% of full power (0.75 between the PLC (programmable logic the motor and the installation itself. cubed = 0.42). It is clear that significant controller) and the motor controller. savings in energy use can be realized Many electric motors have maximum by even small reductions in speed. This Motor Types torque at zero RPM, so these large fact, in turn, justifies the use of VFDs in Brushed DC Motors (BDCs) motors must be soft-started. To reduce applications that can tolerate the speed maintenance to a minimum, the Brushed DC (BDC) motors are among reduction. Of course, speed reduction mechanical mechanisms (clutches) the first motor types put to practical equates to performing the work more that traditionally provided this soft- use and they are still popular where low slowly, which, in some cases, directly start capability are rapidly being initial cost is required. These motors impacts throughput. Nonetheless, replaced by electronic soft-starters or have a wound rotor armature and either there are numerous applications where variable frequency drives (VFDs). In a permanent magnet stator or field motors do not need to run at full speed some applications motors must supply wound stator. Brushes slide across the to accomplish the work quickly enough. both forward and reverse tension to segments of the commutator on the Pumping out a tank of fluid may not the load; optimally, braking energy rotor to switch the DC power source to need to be done as fast as possible. from overhauling loads is fed back the appropriate windings on the rotor. Venting a room may need a full-speed into the AC line using regenerative fan at first, but once the air is moving a BDC motors have their place for two VFDs instead of being wasted as heat slower speed may suffice. The EIA report important reasons: low initial cost and in large braking resistors or in high- (May 2011) states that it is feasible and ruggedness, because no electronics are maintenance mechanical brakes. cost effective to save 20% to 30% of total needed inside the motor. Because the Motor control is a very significant portion motor power consumption worldwide. motors suffer from wear of the brushes, of the Control and Automation market. brush springs, and commutators, they Certainly adding variable-speed According to U.S. Department of Energy, require high maintenance in intensive- controllers adds cost to the installation; motor driven equipment accounts for use applications. Sparking also occurs however, the forecasted energy 64% of the electricity consumed by U.S. between the brushes and the commutator Motor Control 117 as a part of normal motor operation. Controllers for BDC Motors designers are abandoning linear voltage This, in turn, creates EMI/RFI and small The only variable available to control regulation because of its inherently amounts of ozone. Where system cost the speed of a BDC motor is the supply low efficiency. One way to realize a is a priority, BDC motors are a low- voltage. The voltage can be varied variable-voltage power supply from cost solution. While their efficiency or a fixed voltage can be pulsed with any switch-mode voltage regulator is to is generally lower than brushless variable duty cycle. For high efficiency inject or extract current into or out of DC (BLDC) motors, they approach in a variable voltage approach, a switch- its feedback node using a current sink/ equality under high-load conditions. mode power supply is required. Most source DAC. See Figure 1. When the VDC VARIABLE VOLTAGE TECHNIQUE CONTROL PANEL VCC VDC BUCK BUCK START STOP SHDN FB VCC CURRENT- MOTOR SENSE AMP FASTER SWITCH IOUT DEBOUNCER DAC MOTOR TEMPERATURE SLOWER ADC SPEED, SENSOR POSITION µP SENSOR SENSOR INTERFACE VCC DISPLAY DISPLAY SUPERVISOR DRIVER UART ISOLATION TRANSCEIVER FIELDBUS VDC PWM TECHNIQUE CONTROL PANEL VCC BUCK START VDC STOP VCC TEMPERATURE SENSOR MOTORMOTOR FASTER SWITCH GATE DEBOUNCER DRIVER SLOWER MOTOR SPEED, SENSOR POSITION µP INTERFACE SENSOR VCC OP AMP ADC DISPLAY DISPLAY SUPERVISOR DRIVER UART ISOLATION TRANSCEIVER FIELDBUS Figure 1. Two control techniques for BDC motors. The upper diagram shows a variable voltage technique that is high efficiency due to the switching power supply. The lower diagram shows the PWM technique that can be lower cost if the motor is rated for the full supply voltage. 118 Control and Automation Solutions Guide user adjusts the speed control or when speed signal (usually a pulse frequency rotating part through a commutator. The the microcontroller receives a command proportional to the motor rotation rate) brushes and mechanical commutator are through the electronic interface, the must be fed into a controller that will replaced by electronic commutation of microcontroller then instructs the current respond by either adjusting the motor the stator windings. This increases motor sink/source DAC (e.g., DS4432) to change voltage or the PWM duty cycle. With life significantly. The initial cost of a BLDC its output current value. This forces the sufficient switching frequency, the motor is higher than an equivalent BDC regulator to change the output voltage inductance of the motor windings act motor, although the cost of permanent to the motor up or down, respectively, to as a lowpass filter that keeps the motor magnets has decreased significantly over keep the feedback pin’s voltage constant. current close to constant with only minor the past years. With precise commutation ripple, thus producing low torque ripple. and rotor position sensing, efficiency is Alternatively, if the motor can handle generally higher than equivalent BDC the high-DC voltage, one can convert To reverse the direction of the BDC motors. They also produce more torque the input control to a pulse-width- motor, current must flow through the per unit weight. Another significant modulated (PWM) duty cycle applied motor in the opposite direction. This advantage for industrial applications is to a power switch between the power can be accomplished using power that since there are no brushes, there are supply and the motor. By varying the MOSFETs or IGBTs in an H-bridge no sparks generated, so the BLDC motors duty cycle, the average power to the configuration (Figure 2). These MOSFETs can be used in explosive environments. motor is adjusted, as is its output power can be either voltage controlled or and speed. If constant speed is needed PWM controlled for speed control. Due to their higher efficiency over under a varying load, then motor Brushless DC (BLDC) Motors a wide range of speeds and loads, speed detection is needed. This motor BLDC motors are seeing wider use in A BLDC motor spins the magnets heating, ventilation, air conditioning, instead of the windings—the inverse of VDC and refrigeration (HVAC&R) systems. a BDC motor. This has advantages and disadvantages. A BLDC motor has neither Q1 Q2 commutator nor brushes, so it requires less maintenance than a BDC motor.