Fullscreen HTML5Med Res (??MB)HTML4

Super Smooth, FullSpeed Controller for by John Clarke T his 10A electronic speed controller provides an impressively smooth running universal motor that can be adjusted from very slow up to full speed. Using the feedback control, the motor can be set to maintain its speed even under load. A similar 230VAC 10A Full-Wave Brush Motor Speed Controller was published in May 2009. This controller worked well but this latest controller has additional features which give a significant improvement over the earlier version. This includes improvements to the motor control along with added protection to the controller circuitry such as cycle-by-cycle over-current limiting and soft starting. We need to mention here that this controller is not suitable for use with induction motors, such as are typically used with compressors, bench grinders, lathes and pumps. For more information see the panel entitled: What motors can be controlled? Why is it so good? So why is this controller so good at driving brush-type motors, particularly at slow running speeds and for full speed operation? It is all to do with the type of voltage waveform that is use to provide speed control. Typically, brush motor speed controllers use a simple phase-control circuit. We published such a phasecontroller for brush motors in the February 2009 issue. Shortcomings of phase control are immediately apparent when using this design. One is that the maximum speed from the motor when under full speed control adjustment is significantly reduced – up to 25% or more – compared to running directly from the mains. Why build this when you can buy a power tool with inbuilt controller? Many hand power tools these days have inbuilt (trigger or dial) speed controllers. And many cost less than this stand-alone controller kit. So why would you build this one? Quite simply, this is better! That’s no idle boast – everyone who has tried this out has been very pleasantly surprised. You won’t believe how smooth the control is, nor how much “grunt” you get at low speed. Or any of the other features this new 230V 10A Universal Motor Speed Controller offers! It’s not just better than any previous controller – it’s significantly better . . . Or perhaps you have an existing favourite power tool that doesn’t have speed control: build this and enjoy new versatility! 14  Silicon Chip siliconchip.com.au -range, 10A/230V Universal Motors Most mains motor speed controllers aren’t very good! They often have very poor low-speed control or won’t allow control right up to the motor’s maximum speed. Or both! Here’s one that is exceptional: a microcontroller-powered full wave circuit that overcomes both these problems and gives extremely smooth control as well. It’s ideal for use with electric drills, lawn edgers, circular saws, routers or any other appliance with universal (ie brush-type) motors, rated up to 10A. So for an electric drill that normally runs at say 3000 RPM, the maximum speed might be reduced to around 2200 RPM. This is inevitable with a controller circuit that effectively half-wave rectifies the 230VAC mains waveform to give a maximum output voltage of around 162V RMS. The second drawback of the February 2009 phase control design has to do with low speed control. While the circuit does allow your drill or other appliance to run at quite low speeds, the result is that there isn’t much torque available and the speed regulation is poor. This means that if you’re operating the drill at a low speed and you put a reasonable load on it, its speed will drop right away or it may stall completely. Worse still, the motor will tend to ‘cog’. Cogging is caused by erratic firing of the main switching device (a TRIAC) within the Drill Speed Controller, so that the motor receives intermittent bursts of power. An electric motor that is cogging badly is virtually useless and the only cure is to increase the speed setting, defeating the purpose of a speed controller if you want to operate at low speed. What’s the alternative? Both of these drawbacks are basically eliminated with the new SILICON siliconchip.com.au CHIP Motor Speed Controller. ishing tools and even electric whipper The design does not use phasesnippers – they’re less likely to snap control circuitry but uses switch-mode their lines when slowed down. power supply techniques to produce Phase control an outstanding controller for all types of universal brush motors. (Virtually Before we continue, we should exall mains-powered [handheld] power plain what we mean by phase control tools and many appliances use uniso we can illustrate the benefits of this versal motors. These are series wound new design. motors with brushes.) As you know, the mains (AC) voltage It has very low speed control with closely follows a sine wave. It starts excellent maintenance of speed under at zero, rises to a peak, falls back to load. Additionally, it will run the mozero, then does the same thing in the tor over its full speed range, even at opposite direction. This repeats at 50 full speed if required. times each second (50Hz). Most power tools will do a better job A motor connected to the mains if they have a speed control. For example, electric drills should b e s l o w e d d o w n • Extremely sm ooth and precise motor sp when using larger • eed control Speed can be controlled from zero to maximum drill bits as they make • Superb speed regulatio a cleaner cut. n under load Similarly, it is use- • Adjustable speed regula tion with feedback control ful to be able to slow • Excellent low-speed motor operatio n down routers, jig• 2300W (10A) rating saws and even circular saws when cut- • Cycle-by-cycle current overloa d protection ting some materials, • Over-current limitin g particularly plastics • Soft starting (many plastics actu• NT C Thermistor for initial surge ally melt and then current limiting meld if the speed is • Fused protection too high). • Rugged case with interf erence suppression includ The same comed • For 230VAC brush (un ments apply to iversal) motors sanding and pol- Features February 2014  15 These waveforms illustrate the operation of a typical phase-controlled SCR when driving a typical electric drill. In Fig.1 (above) the SCR is triggered early in the positive half-cycle, so the motor voltage is 138V RMS and it runs at a relatively high speed. The motor can never run at maximum speed for another reason: half of the energy is unavailable because only one half of the cycle is used. (Even if the whole half cycle could be fed to the motor, it could only ever be about 162V RMS). Also notice that there is considerable hash at the beginning of each positive half-cycle, caused by interaction between the drill’s commutator and the Triac. Compare this with Fig.2 (below) where SCR is triggered much later in the halfcycle, meaning less power is available to the motor – the voltage being fed to the motor here is just 45V RMS. While it does run much slower – the aim of the exercise, of course – it suffers from low torque and is also liable to “cog”. Note the frequency error in both these screen grabs, which is caused by hash on the waveform and the fact that the SCR triggering is erratic. 16  Silicon Chip makes full use of the energy from each cycle so that it runs at its maximum speed. But if we were only to supply a portion of the waveform, with less energy available to power it, the motor would not run so fast. By varying the time during each half cycle when power is applied, you would have a variable speed control. This then is the basis of phase control: feed power very early in the cycle and it runs fast; delay power until much later in the cycle and it runs slowly. The term ‘phase control’ comes about because the timing of the trigger pulses is varied with respect to the phase of the mains sine wave. Phase control has in the past been the basis for incandescent lamp dimmers and even heater controls. By the way, phase control is not generally suitable for fluorescent and compact fluorescent lamps. The oscilloscope waveform of Fig.1 shows the chopped waveform from a phase-controlled circuit when a motor is driven at a fast speed. Fig. 2 shows the waveform from the phasecontrolled speed control at a lower setting. At the low setting the motor has 45V RMS applied, while at the higher setting, the motor has 138V RMS applied to it These examples show only the positive half of the mains waveform being used, as is the normal case with a phase-controlled circuit. This automatically limits the amount of energy which can be delivered to the motor – the power available from the negative waveform cycles is not used. It means that in a half-wave phasecontrol circuit, the range of control is limited to a relatively small range of speeds. For the motor to run at full speed, it would need to be fed with both the positive and negative half-cycles of the 50Hz mains waveform. Normally this is not possible with a phase-control circuit that uses an SCR (Silicon Controlled Rectifier), which is, effectively, a controlled diode that only conducts in one direction. While a TRIAC could be used to switch the full 50Hz mains for phase control (ie, both positive and negativegoing half cycles), it is difficult to achieve and still incorporate constant speed control under load without a complex circuit. Additionally, another big problem siliconchip.com.au This series of scope screen grabs, taken with the controller driving a typical handyman electric saw, show the voltage wave-forms applied to the motor at progressively higher speed settings. Fig.3 (above) is the lowest setting with very short pulses from the IGBT delivering just 77.4V RMS to the motor. The yellow trace shows the output from IC1 (as applied to the IGBT driver), while the green trace is the output from that driver. The top (blue) trace shows the voltage actually applied to the motor via the GPO. You can see that it follows the follows the full-wave-rectified mains “outline” but the pulses themselves are very narrow. Fig.4 shows a significantly higher speed setting (114V RMS) with the IGBT being switched on with longer pulses. The yellow and green traces remain constant in their amplitude but of course the pulses are wider, therefore delivering more energy. By the way, the spikes on the leading edges of the motor waveform (blue trace) mainly appear to be an artefact of the measurement method (ie, they are not actually present!). 18  Silicon Chip with conventional phase-controlled circuits is that the trigger pulse applied to the TRIAC or SCR is very short. If this corresponds with the instant when the brushes hit an open circuit portion of the commutator, no current will flow and the motor will miss out on a whole cycle of the mains waveform. Similarly, even if the TRIAC or SCR has been correctly triggered on, the SCR or TRIAC may switch off again as current falls to zero when a brush passes an open circuit on the commutator. This problem is more critical at low speed settings and is one of the reasons for the ‘cogging’ behaviour referred to earlier. Incidentally, the sparks you see when you look into a universal (brush type) motor are mostly caused by brushes passing over the open circuit section of the commutator. Typically, a power drill might have a dozen or more open circuit sections on the commutator. These open circuit sections or gaps in the commutator are necessary to keep motor windings separate. Speed regulation Most phase-controlled SCR or TRIAC speed control circuits claim to include a form of feedback that is designed to maintain the speed of the motor under load. They rely upon the fact that a motor can be used as a generator when it is spinning with no power applied. When the motor is loaded and the motor speed slows, the back-EMF (electromotive force) produced by the motor drops and the circuit compensates by triggering the SCR earlier in the mains cycle. This earlier triggering helps to drive the motor at the original speed. In practice, however, the back-EMF generated by most series motors when the SCR or TRIAC is not conducting is either very low or non-existent. This is due in part because there is no field current and the generation of voltage is only due to remanent magnetism in the motor core. If there is any back-EMF produced, it is too late after the end of each half-cycle to have a worthwhile effect on the circuit triggering in the next half-cycle. So while phase control is simple and cheap, it is not an ideal method for controlling motor speed. Instead siliconchip.com.au we use a different method as follows: Pulse-width modulation The new Silicon Chip speed control circuit uses Pulse Width Modulation (PWM) and a different feedback method for speed regulation that effectively solves the above problems associated with phase control. Fig.3 to Fig.6 show the voltage waveforms applied to the motor at progressively higher settings from very low to full speed. What happens is that we rectify the mains voltage and then chop it up at a switching rate of about 980Hz using a high voltage IGBT (Insulated Gate Bipolar Transistor). For the lowest speed setting (Fig.3), the pulses are very narrow and for the higher speed settings the pulses applied to the motor are progressively wider. There are between 9 and 10 pulses during each half cycle, so the motor receives a more continuous stream of current compared to when driven via phase control. As a result, the motor operates very smoothly over the whole of its speed range. For speed regulation, the circuit does not rely upon the back-EMF from the motor. Instead, it monitors the current through the motor and adjusts the pulse width to maintain the motor speed. When a motor is idling, it draws a certain amount of current to keep itself running. When the motor is loaded, the motor speed drops and the current drawn by the motor increases. The motor controller senses this and then compensates for this speed drop by widening the pulse width to maintain motor speed. Similarly, Fig.5 shows an even higher speed setting – very close to 50% duty cycle – with now 170V RMS being delivered to the motor by the IGBT. Motor speed would already be higher than that capable of a phase-controlled circuit and shows how good this circuit is. Incidentally, all the waveforms displayed in this series of figures have been measured using high voltage differential probes on the oscilloscope. Do not attempt to make any of the measurements using conventional probes and an isolating transformer – as there is a risk that you will blow the IGBT, the fast recovery diode, D1 and the gate driver chip, IC3. We write this from bitter experience! Block diagram Fig.7 shows the basic circuit arrangement. The 230VAC input waveform is fed through a filter and fullwave rectified. An NTC thermistor in series between the full-wave rectified supply and the motor limits the initial surge current drawn by the motor. The thermistor has a relatively high resistance when cold; as it heats up, the resistance drops allowing full power to be applied to the motor when necessary. The NTC thermistor is ideal for use with heavy current appliances to reduce the start up current. The resulting positive-polarity waveform is fed to one side of the motor. The other motor terminal is switched on and off via IGBT Q1. siliconchip.com.au Fig.6: here the IGBT is virtually full-on delivering maximum voltage to the motor. The drive pulses are virtually at 100% so the motor would be running at the same speed (or very close to it) as it would if plugged directly into the 230V AC mains. However, the RMS voltage reads a little lower than expected, due to the fact that the spikes which were present in the earlier waveforms are no longer there to confuse the scope. February 2014  19 230V N INPUT ~ A A FUSE (F1) AND FILTER NTC THERMISTOR BR1 + – q N E ~ 220nF K 220nF – MOTOR D1 A ~ ~ BR2 +5V + REG1 OUT IN +15V K GND C ZD1 GATE DRIVER (IC3) A VR1 VR2 VR3 E SNUBBER PWM SPEED FEEDBACK Q1 G R1 MICRO– CONTROLLER (IC1) SAMPLE & HOLD, AMPLIFIER (Q2, IC2a) CURRENT MONITOR (IC2b) OVERCURRENT COMPARATOR CURRENT MONITOR Fig.7: microcontroller IC1 produces a PWM signal proportional to the speed setting of VR1. Higher speed settings will produce wide pulses while a lower speed setting will reduce the pulse width. Switching of the IGBT is under the control of the gate driver, IC3. An IGBT is a hybrid of a Mosfet and flow through the motor. This current peak over-current protection provided bipolar transistor. It has the high im- measurement is used for two purposes. by IC3. pedance gate drive of a Mosfet but high Firstly, the current is monitored by IC3 It works as follows: whenever the current handling at high voltages, like and this IC will reduce drive to the average current exceeds 15A, IC1 a power transistor. IGBT should the current go beyond a begins to reduce the duty cycle of the The IGBT we are using has a 40A, peak of about 23A. This IC monitors PWM drive until the comparator out1200V rating (120A peak) and can the peak current during each switching put switches low, indicating a lower even withstand a short circuit for 10µs. cycle to protect the IGBT from damage current. It thus provides an overall Switching of the IGBT is under the due to over current. current limit. control of the gate driver, IC3, which For speed regulation, the voltage VR2 and VR3 are for the feedback in turn is controlled by the microcon- across R1 is filtered, sampled and am- control. VR2 is a potentiometer that’s troller, IC1. plified. Sampling of the current occurs externally adjustable as it is mounted IC1 monitors the speed potentiome- only whenever Q1 is switched on to on the lid of the controller. Alternater VR1 and produces a PWM signal drive the motor. The current feedback tively, if you prefer not to have VR2 that is proportional to the speed set- is held at this sampled voltage level mounted on the case lid, then VR3 ting. So for higher speed settings of when the motor is switched off. The can be used to set the degree of feedVR1, the PWM output from IC1 will amplified current measurement is back. VR3 is a trimpot installed inside be wide pulses while a lower speed monitored by IC1 and averaged over a the controller. The feedback control setting will reduce the pulse width. 10ms period thus capturing a full half adjusts by how much the duty cycle The PWM output is fed to IC3 that mains cycle of current. of the PWM motor drive is increased then drives the high voltage IGBT An over-current comparator is under load. (Q1). Diode D1 is a fast-recovery type included and is also monitored by One of the advantages of using a to conduct the motor current when Q1 IC1. It differs from the cycle-by-cycle, microcontroller is that the feedback is switched off. control can include features The “snubber” not possible with convenacross Q1, consisttional circuitry. Rating:...........................................10A, 230VAC ing of a 33Ω resistor Firstly, when starting the Speed adjustment:.........................Zero to motor’s maximum and 10nF capacitor, motor from stopped, any suppresses excesfeedback control is inactive PWM frequency:............................980Hz sive voltage excuruntil the motor reaches the Cycle-by-cycle current limiting:.....23A peak sions. speed that it is set to run at Average current limiting: ...............15A The very low by the speed control. This value resistor, R1, motor-start operation can Soft start rate: ...............................Up to 2.54s from zero to full speed is included for be activated by turning the NTC thermistor:.............................10Ω at 20°C, <0.1Ω <at>10A monitoring current speed control up (from fully Specifications 20  Silicon Chip siliconchip.com.au Parts List – 10A 230V Motor Speed Controller 1 PCB, code 10102141, 112 x 141mm 1 metal diecast case, 171 x 121 x 55mm (Jaycar HB-5046) 1 front panel label, 168 x 118mm 1 10A single switched mains power outlet (GPO) (HPM CDXL787WEWE or equivalent) 1 240VAC 10A PCB mount EMI filter (Jaycar MS-4000) (or Schaffner FN 405-10-02 or equivalent) 1 NTC Thermistor (SL32 10015) (Element14 Cat.1653459) 1 10A IEC mains lead (3-pin mains plug to IEC line female connector) 1 IEC male chassis connector with fuse (Altronics P 8324, Jaycar PP-4004) 1 10A M205 fast blow fuse (F1) 2 knobs to suit potentiometer shafts 2 2-way PCB mount screw terminal blocks with 5.08mm spacing (CON1) 5 6.35mm PCB mount male spade connectors with 5.08mm pin spacing (Altronics H 2094) 5 6.35mm insulated female spade quick connectors with 4-8mm wire diameter entry 2 5.3mm ID insulated quick connect crimp eyelets with 4-6mm wire diameter entry 1 18-pin DIL IC socket 1 M4 x 10mm pan head or countersunk screw (Earth to case) 1 M4 x 10mm countersunk screw (Earth to lid) 2 M4 x 15mm pan head screws (GPO Mounting) 1 M4 x 20mm pan head screw (BR1 mounting) 5 M4 nuts 4 4mm star washers 2 M3 x 10mm countersunk screws (for IEC Connector) 2 M3 x 15mm pan head screws (for Q1 and D1) 8 M3 nuts 2 3mm star washers 2 M3.5 x 6mm screws (supplied with case) (for mounting PCB to case) 4 stick-on rubber feet 6 100mm cable ties 2 TO-3P Silicone insulating washers 1 400mm length of blue 10A mains wire 1 400mm length of brown 10A mains wire 1 400mm length of green/yellow 10A mains wire 1 200mm length of brown 7.5A main wire 1 200mm length of blue 7.5A mains wire 1 70mm length of black 5mm heatshrink tubing 1 10mm length of red 5mm heatshrink tubing 1 40mm length of 2.5mm Vidaflex heat resistant sleeving Semiconductors 1 PIC16F88-I/P microcontroller programmed with 1010214A.hex (IC1) 1 LMC6482AIN dual CMOS op amp (IC2) 1 IR2125 PDIP current limiting single channel Mosfet/IGBT driver (IC3) 1 LP2950ACZ-5 5V regulator (REG1) (Jaycar ZV1645) 1 STGW40N120KD 1200V 40A NPN IGBT (Q1) (Element14 Cat. 2344080) 1 2N7000 N-channel Mosfet (Q2) 1 STTH3012W 30A 1200V TO-247 ultra fast recovery diode (D1) (Element14 Cat.1295262) 1 1N4148 general purpose diode (D2) 1 15V 1W zener diode (ZD1) 1 35A 400V or 600V bridge rectifier (BR1) (PCB mount; Altronics Z 0090) or (with quick-connect terminals; Jaycar ZR-1324 with additional components required. See below) 1 W04 400V 1.2A bridge rectifier (BR2) Capacitors 2 100µF 16V PC electrolytic 5 1µF 50V monolithic multilayer (MMC) 1 470nF 63V or 100V MKT polyester 2 220nF 250VAC X2 class MKT polyester 1 100nF 250VAC X2 class MKT polyester 5 100nF 63V or 100V MKT polyester 1 15nF 63V or 100nF MKT polyester 1 10nF 250VAC X2 class MKT polyester 1 470pF ceramic Resistors [0.25W 1%] #=1W, 5% 2 1MΩ # 1 1MΩ 3 10kΩ 1 4.7kΩ 2 2.2kΩ 1 1kΩ 2 470Ω # 1 330Ω 3 100Ω # 1 10Ω 1 4.7Ω 0.25W 5% 2 24mm 10kΩ linear single gang potentiometers (VR1,VR2) 1 0.010Ω 3W low ohm shunt resistor (TT Electronics, Wellwyn OAR3 R010) (Jaycar RR-3420) 1 10kΩ miniature trimpot (horizontal mount with 5mm pin spacing) (VR3) Extra parts required for BR1 with quick-connect terminals: 4 6.35mm PCB mount male spade connectors with 5.08mm pin spacing (Altronics H 2094) 4 6.35mm insulated female spade quick connectors with 4-8mm wire diameter entry 1 80mm length of 10mm diameter heatshrink tubing We have been advised that Jaycar Electronics will be producing a kit for this speed controller: Cat KC5526 will sell for $149.00 and should be available from all Jaycar stores next month. siliconchip.com.au February 2014  21 Ready for next month’s construction details, here’s the inside view of the new Speed Controller. It’s fully self contained within a rugged diecast box. anticlockwise) or by switching on the motor. The lack of feedback control prevents the motor giving a large overshoot in its speed when it first starts up. A stopped motor is detected as being each time the average motor current drops to zero. Secondly, the microcontroller can “dial out” the idle (no load) motor current so motor speed is not increased markedly with increased feedback settings. If this is dialled out, only the extra current drawn by the motor under load is used by IC1 to adjust PWM to maintain motor speed. This feature is especially useful with higher-current motors. The motor idle current is dialled out by running the motor at the speed required with the speed control and 22  Silicon Chip with the feedback control set to its minimum setting. The motor’s idle current will then be recorded by IC1 and feedback will only operate when motor current exceeds this current. Any changes that increase the motor speed, either through a change in position of the speed control or starting the motor, the PWM signal is varied at a slow rate with small increases made each 10ms. For a complete ramp-up in motor speed over the possible 255 speed settings, full PWM duty is only available after ramping up over 2.54s. Circuit description The circuit for the Motor Speed Controller is shown in Fig.8. It comprises three ICs, several diodes, resistors and capacitors plus the high voltage IGBT, Q1. Power for the circuit is derived directly from the 230VAC mains. The entire circuit floats at mains potential and is therefore unsafe to touch whenever the circuit is connected to the mains. Also note that the circuit ground is floating at mains potential (it is not connected to mains earth which connects only to the metal case). Mains power supplied to the controller circuit is via a fuse, F1, that’s integral to the IEC input connector. This fuse protects the circuit against excessive current flow such as can occur with a short across the motor. An Electromagnetic Interference (EMI) filter reduces switching artefacts from the IGBT and motor being radiated back to the mains wiring. This is a commercially-made filter that consists of a pair of 2.2nF to 3.3nF capacitors from Active and Neutral to Earth, followed by a 0.3 to 0.4mA current-compensated series choke for each line, then a 15nF to 100nF capacitor across the load terminals (actual values depend on manufacturer). BR1 is a 35A bridge rectifier with a 400V or 600V rating. The bridge provides the circuit with the positive fullwave rectified mains voltage to power the motor. This supply is filtered using a 100nF 250VAC capacitor. The capacitor does not provide a smoothed DC supply. Instead the capacitor just filters out much of the high frequency switching noise on the supply due to the motor and also helps to reduce the voltage induced when the IGBT is switched off and D1 becomes forward biased. A separate supply arrangement is used for the low voltage circuitry. Instead of just using high wattage resistors to limit current to a zener diode, we use a capacitor-coupled separate bridge rectifier in order to reduce power and more importantly heat dissipation inside the controller case. The second rectifier (BR2) is fed via two 220nF capacitors and series 470resistors. The 220nF capacitors are used to provide an impedance limited current to the 15V zener diode, ZD1. For 50Hz, the impedance of each 220nF capacitor is 14.5kΩ . This, plus the 470Ω limits the current through ZD1. A 100µF capacitor across the resulting 15V supply smooths the voltage to a near-constant value. The 470Ω resistors in series with siliconchip.com.au siliconchip.com.au February 2014  23 E F1 10A 100nF 100nF 2.2k FEEDBACK OSC1 OSC2 AN3 AN2 AN1 5 Vss RA0 RB0 RB1 RB2 AN4 RB3/PWM RB4 RB5 RB6 RB7 14 Vdd IC1 PIC16F88 PIC1 6F88 RA5/MCLR 4 10k 17 6 7 8 3 9 10 11 12 13 +5V 7 100mF 4 IC2b 8 – CON7 6 5 10k MMC 1mF GND OUT + 1M 1W 100nF 250VAC X2 1M 1W MMC 1mF A K ZD1 15V 1W 220nF 250VAC X2 CON6 100mF MMC 1mF 470W 1W – BR2 W04 470W 1W +3.4V MMC 1mF 1 AMPLIFIER 100nF 10k IC2a 2 3 IC2: LMC6482AIN 330W 0V 3 2 Err In 1M K COM 4 G S A A ZD1 IN4148 SAMPLE & HOLD Q2 2N7000 D 1k Vs 5 6 K K 15nF Cs 8 Vb 7 Hout IC3 IR2125 A q NTC THERMISTOR SL32 10015 CON9 CON8 325V D1 STTH3012W 1 Vcc A K D2 1N4148 + 470pF +15V ~ ~ 100nF ALL COMPONENTS AND WIRING IN THIS CIRCUIT OPERATE AT MAINS POTENTIAL. DO NOT OPERATE WITH CASE OPEN – ANY CONTACT COULD BE FATAL! 4.7k IN REG1 LP2950ACZ-5 BR1 35A 600V ~ ~ OVER-CURRENT COMPARATOR METAL CASE EARTH (NOT CONNECTED TO CIRCUIT GROUND) CON5 CON4 10A 230VAC INTELLIGENT SPEED CONTROLLER 16 15 2 1 18 CON3 10A EMI FILTER Fig.8: the complete circuit diagram. Note the warning – it is not safe to work on an open case when connected to the 230V AC mains! SC Ó2014 CON1 CON2 MMC 1mF 100nF 2.2k *ALTERNATIVE TO VR2 FOR FEEDBACK ADJUSTMENT VR3* 10k VR2 10k VR1 10k SPEED FUSED IEC INLET N A 220nF 250VAC X2 E N 0V D S 3W ~– + ~ W04 0.01W G 2N7000 G 470nF 10W 4.7W Q1 STGW40N120KD IGBT METAL CASE EARTH (NOT CONNECTED TO CIRCUIT GROUND) A GPO (MOTOR) G IN A OUT E STGW40N120KD C K 10nF 250VAC X2 LP2950 GND STTH3012W E C 3x 100W 1W 325V What motors can – and cannot – be controlled? We’ve noted elsewhere in this article that this controller suits the vast majority of power tools and appliances (which use universal motors – series-wound motors with brushes). Incidentally, they’re called universal motors because they can operate on both AC and DC. But how do you make sure that your power tool or appliance is a universal motor and not an induction motor? As we also said before, induction motors must not be used with this speed controller. One clue is that most universal motors are quite noisy compared to induction motors. However, this is only a guide and it’s certainly not foolproof. In many power tools you can easily identify that the motor has brushes and a commutator and you see sparking from the brushes and that shows that the motor is a universal type. But if you can’t see the brushes, you can also get a clue from the nameplate or the instruction booklet. So how do you identify an induction motor? Most induction motors used in domestic appliances will be 2-pole or 4-pole the 220nF capacitors are there to limit surge current when power is first applied to the circuit. The surge current could be high should power be switched on at the peak voltage of the mains waveform. 1MΩ resistors across the capacitors are there to discharge any stored voltage when the power is switched off. Without these, the capacitor could have high voltage stored ready to provide an electric shock to anyone touching the capacitor when say for example, trouble shooting the circuit (even when 230V AC power is disconnected). The 15V supply powers the IGBT driver IC3 directly, while a low-power 5V regulator derived from the 15V line supplies both IC1 and IC2. The 100µF and 1µF capacitors at the regulator’s output and input ensure the regulator remains stable and that it can provide transient current without losing regulation. IC3 is a dedicated Mosfet (or IGBT) driver used as a low-side driver where the output produces a 15V gate drive with respect to the circuit ground. Apart from providing gate drive for the IGBT, IC3 also protects the IGBT. 24  Silicon Chip and always operate at a fixed speed, which is typically 2850 rpm for a 2-pole or 1440 rpm for a 4-pole unit. The speed will be on the nameplate. Bench grinders typically use 2-pole induction motors. Controlling induction motors If you do need to control this type of motor use the 1.5kW Induction Motor Controller published in April and May 2012. Note that there are important modifications published in December 2012. And a reminder: You cannot control the speed of any universal motor which already has an electronic speec control built in, whether part of the trigger mechanism or with a separate speed dial. This does not include tools such as electric drills which have a twoposition mechanical speed switch. You can use our speed controller with the mechanical switch set to either fast or slow. It does this in several ways. Firstly, the gate drive is a high current pulse to minimise the time that the IGBT is in its unsaturated state to minimise power dissipation. Secondly, current is monitored across a 0.01Ω resistance between the emitter and the circuit ground. Whenever the voltage across this resistor rises above 230mV, representing a 23A current, the IGBT will be current-limited. Current limiting is done by reducing the gate drive output voltage to maintain the 23A. This limiting occurs within 500ns of the over current and this is well within the 10µs required for the IGBT to be protected. Thirdly, under-voltage protection provided by IC3 prevents any gate drive if the supply is below about 8V. Note that while IC3 is powered from WARNING! This is NOT a project for the inexperienced. Do not attempt to build it unless you are familiar with working with high voltage circuits. 15V, the input at pin 2 can be as low as 3.3V logic level. In our circuit a 0V to 5V signal is applied to IC3 from the PWM output of the IC1 microcontroller. IC2a also monitors the current across the 0.01Ω shunt via a 10Ω and 470nF low-pass filter and Mosfet Q2 is used as a sample and hold buffer. Q2 is switched on when the PWM signal being applied to its gate is high. The Mosfet then conducts and passes the voltage that’s across the 470nF capacitor through to IC2a’s pin 3 input. When the PWM signal goes low, the Mosfet is off and so the sampled voltage is stored in the 100nF capacitor. The 15nF capacitor at the gate of Q2, in conjunction with the 1kΩ gate resistor, slows down the switch-on speed of Q2. Diode D2 switches off the Mosfet more quickly when the PWM goes low. The slow switching of Q2 is needed to reduce voltage feed-through from the gate to the drain and source. Feedthrough occurs each time the gate is switched and the sudden voltage change is capacitively coupled to the drain and source. This effect is minimised by reducing the switch on rate and also having a siliconchip.com.au low impedance source to the Mosfet. Low impedance is ensured using the 0.01Ω shunt, the 10Ω series resistor and 470µF capacitor. Note that internal to Q2 is an intrinsic diode that allows conduction of current from the source to the drain. While Q2 could be connected in this circuit with the drain and source reversed, connecting this way would allow the 100nF capacitor at pin 3 of IC2a to discharge via the diode, when the shunt resistance voltage is lower than the 100nF capacitor’s voltage. IC2a amplifies the sampled voltage by about 31. The resulting voltage is read by IC1 via its AN4 input. IC1 effectively averages the voltage at AN4 over a 10ms period so as to capture a full half-wave portion of the mains cycle for current measurement. The averaged current measurement is multiplied by the feedback setting of VR2 (which can be regarded as optional) or VR3. This multiplication value is then used to apply PWM adjustment for maintaining motor speed. IC1 determines if VR2 is connected at each power up. If it is not, monitoring is redirected to VR3. Initially, AN2 is configured as an output that is set siliconchip.com.au low (0V). Then AN2 is reconfigured as an analog input and the voltage level is measured. If the level is much higher than 0V then VR2 must be connected to be able to change the level. If the level is essentially unchanged, the pin is configured as an output again but this time the output is set high (5V). Then AN2 is set as an input and the level measured. If it remains high, then the input is open. If the input is at a lower level, then VR2 must be connected. If VR2 is not detected, pin 1 is set as a low output and VR3 is used as the feedback input. The 2.2kΩ resistor in series is there to prevent the output being shorted during testing. The 100nF capacitor is to hold voltage during testing. The 2.2kΩ resistor and 100nF capacitor are also included to filter out noise from associated mains wiring that could be coupled in through the potentiometer’s wiper wiring. The same filtering is also included for potentiometer VR1. Over-current IC2b compares the voltage from IC2a’s output (pin1) against a reference set at 3.4V by the 4.7kΩ and 10kΩ resistors connected across the 5V supply. The output (pin 7) goes high when IC2a output is higher than 3.4V. Output from IC2b is ignored by IC1 unless the averaged current as detected at the AN4 input exceeds 15A. IC1 then begins to reduce the duty cycle of the PWM drive until the comparator output switches low. Physical details The motor speed controller is housed in a rugged diecast aluminium case, and has separate rotary speed and adjustable feedback controls. The controller plugs into the mains via a standard IEC mains lead, while the motorised appliance plugs into a switched mains socket on the controller’s case lid. Next month: That completes the technical description of our new Super-Smooth Full Range Universal Motor Speed Controller. We’re sure you’ll agree that this one really delivers the goods. In our next issue, we’ll get into the exciting part: building it! SC February 2014  25