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ORIENTAL MOTORSpeed Control Methods of Various Types of Speed Control MotorsKazuya SHIRAHATAOriental Motor Co., Ltd. offers a wide variety of speed control motors. Our speed control motor packagesinclude the motor, the driver (controller), and a potentiometer which allows for easy speed control adjustment.There are three speed control motor product groups. The "AC speed control motor unit" that uses the mostpopular single-phase capacitor-run induction motor, the small and highly efficient "Brushless DC motor unit",and the "Inverter unit" that combines a three-phase induction motor with a small inverter. This article explainsthe structure, principle of speed control and the features of each product group, and our standard products areintroduced.1. IntroductionA large number of motors are being used for generalpurposes in our surroundings from house-holdequipment to machine tools in industrial facilities. Theelectric motor is now a necessary and indispensablesource of power in many industries. The function and theperformance required for these motors are wide-ranging.When focusing attention on the speed control segment ofthe motor market, servo and stepping motors controlFig. 1 Classification of speed control motorstheir speed with a pulse train, while the induction motorand the brushless DC motor control speed with anexternal resistor and/or DC voltage.This article explains the structure, the speed controlprinciple, and the features of the following three productgroups that can control the speed relatively easily byusing an analog input.x AC speed control motorx Brushless DC motor unitx Inverter unit2. Speed control methods of the various speed controlmotorsThe output control method of a speed control circuitcan be divided roughly into two groups: phase controland inverter control, which make up the product groupsshown in Fig. 1.12.1. AC speed control motors2.1.1. Construction of motorAs shown in Fig. 2, the construction of the singlephase and three-phase induction motors includes astator where the primary winding is wound and abasket-shaped, solid aluminum die cast rotor. The rotoris low-cost because the structure is simple and does notuse a magnet.

ORIENTAL MOTORFig. 2 Construction of induction motorcontrol mechanism (closed-loop control) that reduces thespeed error when compared to a set value.When the speed of this motor is to be controlled, atacho-generator is used to detect the speed and isattached to the motor as shown in Fig. 3. Thetacho-generator is made of a magnet connected directlyto the motor shaft and a stator coil that detects themagnetic poles, and generates an AC voltage at 12 cyclesper revolution. Since this voltage and frequency increasewith a rise of the rotational speed, the rotational speedof the motor is controlled based on this signal.Fig. 4 Rotational speed - torque characteristics ofinduction motorsFig. 3 AC speed control motor system2.1.2. Principle of speed controlRotational speed N of an induction motor can beshown by the expression (1). When the voltage applied tothe motor is increased and decreased, the slip s changes,then the rotational speed N will change.N 120xf x(1-s)/P x x x x x x x x x x (1)Fig. 5 Simple voltage controlAvailable voltage control methods include control by aN: Rotational speed [r/min]transformer or by phase control. Fig. 6 shows whenF: Frequency 〔Hz〕voltage is controlled by using a transformer. This methodP: Number of poles of a motorS: Slipis not so easy to do with an AC speed control motor.Alternately, the AC voltage can be adjusted by settingthe ON/OFF time of every half cycle of the AC voltage(50 or 60Hz) applied to the motor using a switchingIn the case of an induction motor as shown in Fig. 4, aelement (thyristor or triac) that can directly turn on andstable range and an unstable range exists in theoff the AC voltage as shown Fig. 7 and Fig. 8. SpeedRotational Speed - Torque curve. Since it is impossible tocontrol is obtained by the phase control method byreliably operate in the unstable range, simple voltagecontrolling the r.m.s. value of the AC voltage.control (open loop control) is limited to controlling thespeed in a narrow range like, N1 N3 in Fig. 5. To makeit possible to operate reliably even in theabove-mentioned unstable range, it is necessary to detectthe rotational speed of the motor and use a voltage2

ORIENTAL MOTORFig. 9 Block diagram of AC speed control motor systemFig. 6 Voltage change by transformerFig. 10 Waveform for each blockFig. 10 shows the waveforms of each block. The speedFig. 7 Voltage change by phase controlset value d and the detected voltage e of the speedgenerated by a tacho-generator is compared in thecomparison amplifier block. Then the level of the voltagesignal a is determined.The voltage signal a is low when the speed detectedvalue to the speed set value rises, and is higher when thespeed set value lowers. Since the trigger signal is outputat the point where the triangular wave b intersects withFig. 8 Triac control circuitThis AC speed control method can provide steadyspeed control by closed-loop phase control even in theunstable range.Fig. 9 shows the configuration of the speed controlsystem for an AC speed control motor in a blockdiagram.the voltage signal a, the timing (phase angle) when thetriac is turned on with a level of the voltage signal a isdetermined. When this timing is slow, the voltageapplied to the motor becomes low and the rotationalspeed of the motor decreases. The lowered rotationalspeed is fed back again and the control is repeated sothat the difference between the speed detected value andthe speed set value may always be constant.Fig. 11 shows the appearance of the above-mentionedspeed control. In Fig. 11, the operation point of the motordraws a loop of Q-R-S-T-Q centering on O, and therotational speed is maintained between N1 and N2. Thisloop is reduced as much as possible by increasing thespeed detection accuracy.3

ORIENTAL MOTOR2.1.4 Introduction of Oriental Motor productsOur typical AC speed control motors are shown below.ES01/ES02Designed for ease of whenoperating or wiring, with all ofthe functions required for speedcontrol. Conforms to safetystandards around the world.US SeriesPanel mounted design for ease ofuse and simple functions.Conforms to safetystandards around the world.Fig. 11 Operation of the speed controlThe AC speed control motor has the following featureswhen using this closed-loop phase control.1) Since the AC voltage is controlled directly, the speedcontrol circuit can be configured simply because asmoothing circuit is unnecessary, allowing for acompact design at a low price.2) In the same way, a long-life design is possible becausea large aluminum electrolytic capacitor isunnecessary.3) The switching is operated only one time each halfcycle of the commercial AC power supply, which keepsthe generated noise low.Fig. 13 Product group of AC speed control motorsThe AC speed control motor lineup includes manymotor output wattages, gear ratios and options in orderto meet a wide range of applications. In addition, the ACmotor product line up includes models that are equippedwith clutch brakes, a power-off electro-magnetic brake,and other options.2.2. Brushless DC Speed Control unit2.2.1. Construction of motorAs for the construction of a brushless motor, the coilhas a Y configuration with three-phases: U, V, and W2.1.3. CharacteristicsAC motor speed control motors generally have theRotational speed-Torque characteristics shown in Fig.12.and is located in the stator, and the rotor is made ofmagnets magnetized in a multi-pole configuration asshown in Fig. 14.Inside the stator, three hall ICs are arranged asmagnetic elements so that the phase difference of theoutput signal from each hall IC will be 120 degrees apartfor every rotation of the rotor.Fig. 12 Rotational speed-Torque characteristicsA "Safe-operation line" is included in Fig. 12. The"Safe-operation line" represents the limitation wherethe motor can perform at continuous duty withoutexceeding its maximum permissible temperature.4

ORIENTAL MOTORThis voltage control is realized by an inverter in theoutput part of the control circuit (driver). This invertergenerates a three-phase AC voltage from DC current byturning ON and OFF like the sequence shown in Fig. 16(b) by using the six switching elements (FET or IGBT)shown in Fig. 16 (a).Fig. 16 (a) Output part of control circuit (driver)Fig. 14 Construction of a brushless DC motor2.2.2. Principle of speed controlAs shown in Fig. 15, the Rotational Speed-Torquecharacteristics of a brushless DC motor show a negativesloping characteristic when its speed is not controlledwhich is similar to that of a brushed DC motor.Fig. 16 (b) Switching sequenceThe switching elements are connected to the motorwinding as shown in Fig. 16 (a), and the ON/OFF stateof the switching element determines which coil of thestator is energized and in which direction the currentwill flow, that is, which coil becomes a N pole or S pole.In fact, the position of the rotor’s magnetic pole isdetected by the hall IC, and an appropriate switchingelement is turned ON or OFF as shown in Fig. 16 (b).Fig. 15 Rotational speed-Torque characteristics of abrushless DC motorWhen no load is applied and the input voltage is set atV2 in Figure 15, the operation point of the motorbecomes P, and the rotational speed is N1. When theload torque T1 is applied, the operation point shifts to Q,and the rotational speed slows to N2, however, therotational speed returns to N1 if the voltage is raised toV3. Therefore, since the rotational speed changeswhenever the load torque changes, the speed controlmechanism will only have to change the input voltagewhenever a change in the speed is seen in order tomaintain a constant speed on the PR line.5For example, in case of step 1, the transistors Tr1 andTr6 are turned ON, and the current flows from theU-phase to the W-phase. At this time, U-phase is excitedas an N pole and W-phase becomes a S pole, and therotor rotates by 30 degrees moving to step 2. Onerotation of the rotor is made by repeating this operation12 times (Step 1 12).Fig. 17 shows the configuration for the speed control ofa brushless DC motor unit in a block diagram.

ORIENTAL MOTOR2.2.3. CharacteristicsThe Rotational speed-Torque characteristics of abrushless DC motor system have a limited duty region inaddition to the continuous operation area.The limited duty operation area is very effective whenstarting an inertial load. However, when operation in thelimited duty region is continued for five seconds or more,the driver's overload protection function is activated andthe motor decelerates to a stop.Fig. 17 Block diagram of a brushless DC motor systemThe switching sequence of the inverter is decided bythe signal from the hall IC in the positional detectionpart of the block diagram, and the motor rotates.Then, the signal from the hall IC is sent to the speeddetector to become a speed signal, and it is comparedwith the speed setting signal in the comparison amplifierblock, which then generates a deviation signal. Thevalue of the motor input current is determined by thePWM setting block based on the deviation signal.Brushless DC motor units have the following features.1) It has high-efficiency because a permanent magnetrotor is used and secondary loss is small.2) The rotor inertia can be reduced, and a high-speedresponse is obtained.3) It is possible to downsize the motor because it isFig. 19 Rotational speed-Torque characteristics2.2.4. Introduction of Oriental Motor productsOur typical brushless DC motor products are shownbelow.BX SeriesBX offer high performance andhigh function. Outputs up to400W. With a control module,positioning and torque limitingfunctions are available.BLF SeriesBLF offers high speed up to 4000rpm and conforms to safetystandards around the world and.With the digital operator, digitalspeed setting and display areavailable.FBLII SeriesFBLII is a standard brushlessmotor and driver package withsafety standards approval.BLU SeriesBLU is a panel mounted productdesigned for ease of use withsafety standards approval.highly efficient.4) Speed fluctuations with changing loads is low.Fig. 16 shows a typical switching sequence (120-degreeenergizing method). An even more efficient brushless DCmotor system uses a sine-wave drive method byobtaining high-resolution rotor position information fromsoftware from the hall IC signal. This method results ina low-noise drive method since the current that flows tothe motor does not change rapidly. (2)Fig. 20 Product group of brushless DC motorsThe brushless DC motor product lineup has a widerange of models that combine drivers and the brushlessDC motors.Fig. 18 Comparison of voltages applied by sine-wavedrive method and 120-degree drive methodThe high-performance BX and BLF series use thesine-wave drive method for low-noise operation.In addition, the BX Series can perform speed control in6

ORIENTAL MOTORvertical applications by combining a motor with anchanges the speed and closed-loop control that reduceselectro-magnetic brake. Position control and torquethe speed variation with load changes of the motor.limiting also are possible with optional system1) Open-loop controlenhancement tools.Fig. 22 shows a configuration of the open-loop controlin a block diagram.2.3. Inverter Speed Control unit2.3.1. Principle of speed controlThe inverter unit controls the speed of a three-phaseinduction motor by changing the frequency, f, of thevoltage applied to the motor. The inverter unit changesthe frequency, f, by changing the ON/OFF cycle of the sixswitching elements, and the rotational speed (N) of themotor changes in proportion to the expression in formula(1).Fig. 22 Block diagram of open-loop controlN 120xf x(1-s)/P x x x x x x x x x x (1)This method is used to change the input voltage andfrequency of the motor according to a set frequency. ThisN: Rotational speed [r/min]method is suitable for changing speed and can obtainF: Frequency 〔Hz〕high speeds (The frequency can be set up to 80Hz.)P: Number of poles of a motorS: SlipIn addition, to make the voltage appliedto thewindingCarrierSignalhave a sine-wave shape, the inverter controls theON/OFF duty cycle as shown in Fig. 21. The ON/OFFtime is controlled so that the average voltage applied tothe motor becomes a sine-wave shape by comparing thesimply when speed regulation with varying loads is notso much of a concern.The generated torque T of the motor is shown by theformula (2). From this relation, it can be said that thetorque will also be constant by making V/f, the ratio ofvoltage V to frequency f, constant.T K・I・V/f・・・(2)triangular wave called a carrier signal with thesine-wave shaped signal waveform. This method is calledPWM control.T: Torque [Nxm]V: Power supply voltage [V]I: Motor current [A]f: Frequency [Hz]K: ConstantHowever, the lower the speed is, the more difficult it isto keep constant the input impedance of the inductionmotor with the change in f. Therefore, to obtain a torqueFig. 21 Duty control of ON/OFFThe speed control method of our inverter units isdivided into the two types: open-loop control that simply7that is constant from low speed to high speed it isnecessary to adjust the V/f ratio at low speed inaccordance to the characteristics of the motor like thesolid line shown in Fig. 23.

ORIENTAL MOTORWith this characteristic table and the detected phasedifference time t, the inverter calculates an inverteroutput frequency that corresponds to the rotationalspeed command Nset set by the speed potentiometer,and outputs it as the inverter output frequency. Afterreceiving the output frequency, the V/f control blockcalculates the voltage applied to the motor correspondingFig. 23 V/f controlto the output frequency f, and performs the speed controlby driving the PWM inverter. As a result, when a load isapplied, the output frequency of the inverter is boostedso that the decrease in the rotational speed may be2) Closed-loop controlFig. 24 shows the block diagram configuration of theclosed-loop control system used in our BHF Series.compensated for. (3)2.3.2. CharacteristicsThe Rotational Speed-Torque characteristics of theinverter unit are shown Fig. 26 and Fig. 27. As explainedin the AC speed control motor section, a "Safe-operationline" is drawn on the torque characteristic. This linerepresents the limit for the continuous operation, andthe area under this line is called the continuousoperation area.Fig. 24 Block diagram of closed-loop controlThis method detects the phase difference between thevoltage of the inverter output block and the primarycurrent, which calculates the driving frequencycorresponding to the load using the characteristics datatable (Fig. 25) prepared beforehand, and controls theinverter frequency without the need for a speed sensoron the motor.Fig. 25 Characteristics data tableFig. 26 Rotational Speed-Torque characteristics foropen-loop controlFig. 27 Rotational Speed-Torque characteristics forclosed-loop control8

ORIENTAL MOTORTable 1 Speed control range and rate of variability ofeach product group2.3.3. Introduction of Oriental Motor productsOriental Motor offers the inverter controlled productsshown below.BHF SeriesFE100/FE200BHF is a safety standardsapproved inverter product.Use in vertical operationsis possible with anelectro-magnetic brake.FE is a panel mountedinverter designed for easeof use. For use with oursafety standard approved3-phase AC motorsFig. 28 Product group of invertersSince the constant for all of our motors is preset in thespeed control circuit of the inverter unit, the bestperformance of the motor can be obtained. In addition,the BHF is operated in closed-loop control mode and canbe used in vertical applications by combining a motorwith an electro-magnetic brake.3. SummaryOriental Motor offers three product groups (AC speedOriental Motor will continue to work on productcontrol motors, brushless DC speed control motors, anddevelopment so that we can offer the products that bestinverter units) for use in a wide range of speed controlmeet the various needs of our customers for in theapplications. The proper speed control product can befuture.selected according to the function, the performance, thecost, and the purpose desired for your application.ReferencesTable 1 shows the wide range of speed controlperformance (gear ratio and maximum speed) and thespeed regulation of typical models in these three productgroups.(1) AC Motor Technology Study Group: “Book to understand AC smallmotor”, Kogyo Chosakai Publishing (1998)(2) Kazuo Abe: “Low-noise drive technology of Brushless motor”,RENGA No.163, pp.19-25 (2003)(3) Koji Namihana, Masayoshi Sato: “New control method ofthree-phase induction motor”, RENGA No.159, pp.23-28 (1999)AuthorKazuya ShirahataTsuruoka Plant, ACIX OperationsCircuit Technology DivisionCircuit Development Section9

Oriental Motor Co., Ltd. offers a wide variety of speed control motors. Our speed control motor packages include the motor, the driver (controller), and a potentiometer which allows for easy speed control adjustment. There are three speed control motor product groups. The "AC speed control motor unit" that uses the most