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Improved Speed Controller for Universal Motors By JOHN CLARKE This new speed controller can be used with power tools and appliances rated up to 5A. Use it to control the speed of circular saws, routers, jig saws, electric drills, hobby lathes, lawn edgers and other appliances with universal “brush type” motors. 14  Silicon Chip www.siliconchip.com.au T his circuit is essentially a re- smoothness which you can obtain. that in mind if you want to use an vised version of the Speed Con- Some power tools and appliances electric drill as a power screwdriver with this control. You can do it for troller published in September don’t run smoothly at very low speeds short periods but if you want to do it & November 1992. It has been a par- when run from this type of control ticularly popular project and readers circuit and frequently display a be- for long periods you run the risk of have come up with countless appli- haviour known as “cogging” whereby severely over-heating the drill motor. it runs in short bursts. cations for it. In the middle range of the speed conIn fact, you might think that since So the practical minimum speed trol, the circuit gives good speed regthere are now so ulation. This means many battery-powthat if the motor is WARNING! ered and 240VAC loaded down, the power tools with applied voltage is (1) This Speed Controller circuit operates directly from the 240VAC inbuilt speed conautomatically inmains supply and is potentially lethal. Do not build it unless you trols that the uses creased to compenknow exactly what you are doing. for a circuit such sate. DO NOT TOUCH ANY PART OF THE CIRCUIT WHILE IT IS as this would have At the maximum declined. In fact, end of the speed PLUGGED INTO A MAINS OUTLET and do not operate the cirthis circuit is more control range, this cuit outside its earthed metal case. popular than ever circuit will not give and people are confull speed operation (2) This circuit is not suitable for use with induction motors or shaded stantly coming up when in variable pole motors used in fans – see “What motors can be controlled”. with new applicamode. This is betions for it. cause at the maxiApart from drills mum speed setting (3) This circuit must only be used with universal “brush type” (series and circular saws, it is essentially just wound) motors with nameplate ratings up to 5A. the speed control a high power diis particularly useode and this means (4) Power tools with inbuilt fans must not be operated at low speeds ful for controlling that it is feeding for long periods otherwise they may overheat and suffer damage. routers and jig half-wave rectified saws (when cutting 240VAC to the moplastics, for exam(5) This circuit must not be used to control the power to lamps or elec- tor or about 170V ple), hobby lathes RMS. tric radiators. To do so would contravene the regulations of (which use sewing So if do you want the NSW Energy Authority and affiliated state energy authorities. machine motors), full speed from the food mixers (where the in-built speed control has failed) and lawn edgers (where full speed operation tends to frequently break the Nylon line). The speed controller also offers much improved operation of sewing machines, in comparison to the conventional resistive pedal controller. The new 5A Speed Controller is housed in a diecast box which is much more rugged than the previous project which used a plastic case. The diecast case also improves heat dissipation from the Triac. The controls on the front panel comprise a full/control switch and a knob to set the speed. Power comes in via a 3-core mains lead and mains plug while the outlet is a flush-mount mains GPO (General Purpose Outlet) socket on the front panel. Speed control range This speed controller will enable you to set the appliance operating speed over a wide range down to a very slow rate. In fact, the minimum speed will mainly depend on the www.siliconchip.com.au for any appliance motor depends on freedom from “cogging”. There is another factor which limits the minimum speed that an appliance can be run at and that is the fact that most universal motors have an inbuilt fan for cooling. Below certain speeds that fan is largely ineffective and so there is no cooling at all. Bear A 10k 5W SPEED 4.7F 630V 2k D1 SCR A D2 G K 1k MOTOR N Fig.1: this circuit demonstrates the basic principle of motor speed control. The SCR feeds half-wave rectified AC to the motor and its trigger point is made sooner or later in each positive half-cycle to vary the power. motor, set the speed switch to “full”. Basic circuit operation The circuit is very similar in principle to simple SCR speed controls developed years ago except that we are using a Triac. So to explain the circuit principle, have a look at Fig.1 which is just about the simplest speed control you could get. The SCR device conducts in one direction only and feeds half-wave rectified AC to the motor. Essentially, the SCR is a rectifier diode which only conducts when it receives a trigger voltage at its gate. Once it starts conducting, from anode (A) to cathode (K), it stays conducting until the load current drops to zero or the circuit voltage is reversed. Because the SCR is a switching device, it can be used as a very efficient power controller, varying large amounts of current while itself dissipating very little power. The circuit of Fig.1 controls the AC power to the motor by triggering the SCR into conduction late or early in October 2002  15 So how does the circuit give speed regulation? The answer is that the circuit monA2 150k itors the back-EMF from the motor. TRIAC1 1W BTA41-600P Back-EMF can be defined as the D2 SBS1 G 1N4004 2N4992 A1 voltage developed by a motor which SPEED VR1 opposes the supply voltage. The higher 10k LIN the speed of the motor, the higher the GPO 47nF 1k back-EMF. This circuit monitors the 240VAC VR2 back-EMF in the following way. INPUT 5k A N SET MIN Notice that one side of the motor is SPEED connected directly to the SCR’s cathD3 ode while the other side is connected D1 R250H E to the cathode of diode D1 and to the 1N4004 6A 600V mains Neutral wire. This means that the gate-to-cathode voltage applied N to the SCR is the difference between the wiper voltage from VR1 and the E (CASE) back-EMF generated by the motor (disregard the voltage drop across diode Fig.2: this was the circuit of our speed control published in September 1992. It uses a silicon bilateral switch and 47nF D2). capacitor to give strong gate pulses for reliable triggering. Actually, in so-called universal motors (AC/DC series motors with each positive half-cycle of the 240VAC half cycles? commutators as used in most power waveform; being a diode, the SCR does The answer is that we could but then tools and appliances), there are two not conduct at all during the negative a fundamental advantage of this basic back-EMFs generated. The first is AC half cycles. circuit would be lost. The advantage a function of motor speed and the If the SCR is turned on early in each is speed regulation. A circuit with remanent magnetism (remanence) AC half cycle, the power fed to the mogood speed regulation will maintain of the field coils. It is generated durtor will be relatively high. Conversely, a selected motor speed regardless of ing the time when the SCR is not if the SCR is turned on late in each AC variations in the load. If you are using a conducting; ie, during the negative half cycle the power fed to the motor speed controller with an electric drill, half cycles of the AC waveform and will be relatively low and hence the you don’t want the motor to bog down during the first portion of the positive motor will run slowly. when you start to drill into the heavy half cycles before the SCR conducts. The trigger voltage for the SCR stuff, do you? The second back-EMF is generated comes from VR1, a during the time 2kΩ potentiometer when the SCR is connected in series conducting and What motors can be controlled? with a 10kΩ resistor since the there will Virtually all power tools and small appliances use so-called “universal” and diode D1. This now be current motors. These are “series wound” motors with a commutator and brush2kΩ potentiometer flowing in the field is fed with half-wave coils (and also in es. The “series wound” term refers to the fact that the motor armature rectified AC which is the armature). This and field windings are connected in series and this allows the motor to partly smoothed by back-EMF will be be run from AC or DC, ie, “universal”. the 4.7µF capacitor higher than the first. across it. The resultWe are only conInduction motors must not be used with this speed controller. So how ing ramp voltage cerned with the from the wiper is fed back-EMF generdo you make sure that your appliance is a universal motor and not an to the gate of the SCR ated while the SCR induction motor? via diode D2. is not conducting since it is this voltIn many power tools you can easily determine that the motor has brushSpeed regulaage which deteres and a commutator – you can see sparking from the brushes and that tion mines how late or settles the matter. But if you can’t see the brushes, you can also get a early in each posiNow you might tive half cycle that clue from the nameplate or the instruction booklet. ask: why just use the SCR begins conan SCR and allow duction. Hence, the conduction on only So how do you identify an induction motor? Most induction motors used motor applies negapositive AC half cyin domestic appliances (eg, washing machines, fridges, water blasters, tive feedback to the cles? Why not use a swimming pool pumps) will be 2-pole or 4-pole and always operate at gate of the SCR. Triac which can be This negative a fixed speed. Typically this is 2850 RPM for a 2-pole unit or 1440 RPM triggered into confeedback enables duction on both posfor a 4-pole unit. Bench grinders typically use 2-pole induction motors. the circuit to give itive and negative A F1 10A FUSE 16  Silicon Chip www.siliconchip.com.au 10A FUSE A F1 TRIAC1 BTA41-600P 100k 1W Fig.3: this is the revised speed control circuit which now uses a sensitive gate SCR as the trigger source for the high power Triac. D2 1N4004 SPEED VR1 10k LIN SCR1 C103B, MCR100 A A G 47k 240VAC INPUT VR2 10k G A1 100 K K A2 GPO 1 FULL A S1 2.2k 2 1nF N CONTROL 1k MIN SET E (CASE) 47nF A K D1 1N4004 K D3 R250H 6A 600V A N E 1N4004 (CASE) A MCR100 K MAINS CORD (CLAMPED TO CASE WITH CORD GRIP GROMMET) Now the basic circuit presented in Fig.1 will actually work. In fact, it was *USE 7.5A MAINS RATED WIRE VR2 R250H 1k A 100 47k 2.2k 1nF 47nF D1 N PC BOARD MOUNTED IN CASE WITH 6mm NYLON SPACERS, NYLON SCREWS AND NUTS CS D3 K RELLORTNOC ROTOM TRIAC1 (MCR100 MOUNTED UNDER BOARD) 12001101 NEUTRAL (*BLUE) D2 100k 1W 1 2 G Fig.4: Use this diagram when assembling the PC board and completing the wiring inside the case. *GRN/YELLOW SCR1 A1 A2 NOTE: IF USING C103B FOR SCR1, FLAT SIDE FACES BOTTOM OF PCB A A WS A the basis of most speed control circuits used about 30 years ago. However, it has a number of drawbacks. First, the power dissipation through the 10kΩ resistor is about 2.4 watts which means that it gets rather hot. Second, even though the current through the 10kΩ resistor and VR1 is relatively high, it won’t be enough for reliable triggering of higher power Better circuit 10A ACTIVE (*BROWN) A K K tive half cycle and hence more power will be applied to the motor. This will tend to correct the drop in motor speed. It’s not perfect but it’s a lot better than having no speed regulation at all. FUSE F1 BTA41-600P G G A 5A UNIVERSAL MOTOR CONTROLLER good speed regulation. Say a particular motor speed is set by VR1 and then the motor speed tends to drop because of an increase in loading. This reduces the motor back-EMF and therefore increases the voltage at the gate the SCR. More correctly, it means that the ramp voltage at the SCR gate will exceed the voltage at the SCR cathode earlier in the posi- C103B N *GRN/YELLOW * * *GRN/YELLOW * VR1 EARTH POT LOCATING PIN *BLUE Fig.5 (below): the mounting details for the Triac. It is an isolated tab device and does not need an insulating washer. 1 E 2 *BROWN *BROWN A N S1 (CASE LID) *BROWN MAINS OUTLET FIT HEATSHRINK SLEEVING OVER ALL SWITCH CONNECTIONS www.siliconchip.com.au PC BOARD CASE TRIAC 6mm TRIAC MOUNTING DETAIL October 2002  17 Scope 1: This waveform shows the Speed Controller set for maximum output when driving a resistive load. Note that the waveform is essentially a half-wave rectified sinewave with an RMS value of 170V. The early part of each positive half-cycle has been chopped out, due to the fact that the trigger circuit does not fire the Triac until about 2 milliseconds after the start of the cycle. SCRs. And third, the circuit is not particularly good at very low speed settings. Now fast-forward to September 1992 and have a look at the speed control circuit of Fig.2. You can see the similarities between it and Fig.1. Instead of an SCR, we have used a Triac and instead of feeding the gate directly from VR1 as in Fig.1, a trigger circuit consisting of a silicon bilateral switch (SBS1) and a 47nF (.047µF) capacitor has been used. While the Triac is capable of conducting on both positive and negative half cycles of the 240VAC 50Hz waveform, this circuit only enables it to trigger on positive half cycles, because of the rectifier action of diode D1. A silicon bilateral switch is a voltage breakover device; ie, at voltages below its breakover point it is essentially open circuit but once the breakover voltage is reached, it becomes a low value of negative resistance. Don’t worry too much about the Scope2: The same waveform now superimposed on the 240VAC 50Hz input waveform (in blue). Notice there is some small voltage loss across the Triac. The “flat-topping” of both waveforms is commonplace in areas where there are lots of fluorescent or gas discharge lights and/or PCs, which clip the peaks of the mains waveform. This is because capacitive-filter supplies take their power from the peaks of the AC waveform. “negative resistance” bit. All you have to remember is that it is used in conjunction with the 47nF capacitor. This charges up from VR1 via diode D2 until it reaches the break-over voltage of about 8V. At this point it dumps the capacitor’s charge into the Triac’s gate to trigger it into conduction and the cycle repeats for the next positive half cycle of the mains AC waveform. The energy stored in the capacitor is quite enough to trigger even insensitive Triacs, hence we are able to use a high power 40A device in this circuit. In this circuit, the motor back-EMF acts to reduce the charging voltage to the 47nF capacitor rather than reducing the SCR gate voltage as in Fig.1. But although the circuit arrangement is a little different, the speed regulation is just as good. The circuit efficiency is improved too, with only 200mW being dissipated in the 150kΩ resistor which feeds VR1. This resistor has a rating of 1W to ensure that it has an adequate voltage rating. Scope3: This waveform s Controller set for maximu driving an electric drill ( motor). Notice that there beginning of each positiv waveform, caused by the This caused the gross err measurement of 1.497kH The functions of the three diodes in the circuit need to be explained. Diode D1 is there to reduce the power dissipation of the series resistor string and to ensure half-wave operation of the circuit. D2 is there to protect the gate of the Triac when it is in the conducting state – terminal A1 can be above the potential of the gate. Diode D3 has been included as a flyback diode to quench the large inductive spike generated by the motor at the end of each positive half cycle. While the voltage spike does not cause any damage to the circuit, it does have the effect of disrupting the back-EMF monitoring system described above. VR2, the 5kΩ trimpot in series with VR1, is there to provide a minimum setting for the circuit. One question we have not answered so far is why we specified a Triac instead of an equivalently rated SCR. The reason is quite simple. We did it to avoid the need for parts stockists to have to order in another device. The completed PC board, reproduced same size. At right is the underside of the board showing the mounting of the triac. Its tab is isolated so no insulating washer is required – but give it a good smear of heatsink compound to help it keep its cool! 18  Silicon Chip www.siliconchip.com.au shows the Speed um output when (ie, with a universal is some hash at the ve swing of the e motor’s commutator. ror in the frequency Hz – should be 50Hz. Scope4: Now set for a lower speed from the electric drill, the Triac is on for a shorter time and the RMS value is reduced to 115V. Again, the “judder” in the waveform is caused by commutator hash. Again, this hash also caused the slight frequency measurement error. We have specified a 600V 40A device so that it can withstand the “locked rotor” current of any power tool with a nameplate rating of up to 5A. Note that a “locked rotor” condition may well blow the 10A fuse but the 40A Triac should not be damaged. Yes, we have done this test! Another reason for using the 600V 40A Triac is that it is an isolated tab device. This means that it can be attached directly to the metal case without any need for a mica washer or other means of insulation. Latest circuit version The circuit of our new 5A Speed Controller is shown in Fig.3. As already mentioned, this is a revised version of the design we featured in the September & November 1992 issues. Our new circuit replaces the now hard-to-get SBS with SCR1, a C103 sensitive gate SCR and this provides the same capacitor dump function as the SBS. The 47kΩ and 2.2kΩ resistors form a voltage divider between the anode and cathode of the SCR with the divided voltage applied to the gate. The SCR conducts when the gate voltage reaches 0.6V and is triggered by a mere 200uA of gate current. Because of the resistive divider, the voltage across SCR1 must rise to some 13.4V before the gate reaches the 0.6V sufficient to trigger the SCR. When the SCR fires, the charge on the 47nF capacitor is dumped into the gate of www.siliconchip.com.au Scope5: Now set for a very low speed from the electric drill, this scope waveform demonstrates motor “cogging”. As you can see, the Triac is no longer firing reliably on each positive halfcycle. The frequency measurement is also meaningless. This speed setting is too low to be useful as the drill will not run smoothly. Triac1 via the 100Ω resistor to fire the Triac. The 1nF capacitor between anode and gate of SCR1 is there is provide more reliable triggering when there is lot of commutator hash from the motor being controlled. Note that the revised circuit uses slightly different component values compared to Fig.2. Apart from the resistors and capacitors associated with the SCR, the original 150kΩ 1W resistor is changed to 100kΩ while the trimpot VR2 is now 10kΩ instead of 5kΩ. By the way, the 100kΩ resistor is a 1W type, not for disippation but to Parts List – 5A Universal Motor Speed Controller 1 PC board coded 10110021, 79 x 38mm 1 diecast box 120 x 66 x 38mm (Altronics H-0453 or equivalent) 1 panel label 119 x 65mm 1 mains flush-mount socket (Jaycar PS-4090 or equivalent) 1 SPDT 250V 6A rocker switch (S1; Altronics S-3215) 1 10kΩ linear 24mm potentiometer (VR1) 1 knob for potentiometer 1 7.5A three-core mains cord and moulded 3-pin plug 1 10A M205 fast blow fuse 2 M205 PC board mount fuse clips 1 cord-grip grommet to suit mains cord 2 crimp eyelets or solder lugs for earth connection 4 6mm Nylon spacers 2 M3 x 10mm csk screws 1 M3 x 10mm screw 2 3mm star washers 4 M3 x 15mm Nylon screws and nuts 4 stick-on rubber feet 1 200mm length of blue 7.5A 250VAC wire 1 200mm length of brown 7.5A 250VAC wire 5 100mm long cable ties 8 PC stakes Semiconductors 1 BTA41-600P Triac (TRIAC1) 1 C103B sensitive gate SCR (SCR1) 1 R250H 6A 400V diode (D3) 2 1N4004 1A 400V diodes (D1,D2) Capacitors 1 47nF (.047µF) 63V MKT polyester 1 1nF (.001µF) 63V MKT polyester Resistors (0.25W 1%) 1 100kΩ 1W 5% 1 47k 1 2.2kΩ 1 1kΩ 1 100Ω 1 10kΩ horizontal trimpot code 103 (VR2) October 2002  19 These two views inside the case show exactly how the wiring should be done. When we say exactly, we mean it: don’t take chances or shortcuts with 240VAC. Ensure that any hookup wire you use is 250VAC-rated. ensure that it has an adequate voltage rating. As mentioned above, switch S1 provides full power operation, bypassing the Triac so that the motor gets the full 240VAC applied to it. Note that the switch must be a changeover type to select either Active or the Triac A1 output rather than just using a single switch across the Triac. In the latter case, there would be a short circuit (which would blow the fuse) when diode D3 and the Triac conducts on negative half-cycles of the 240VAC mains. Construction All the components of the 5A Speed Controller are mounted on a PC board coded 10101021 and measuring 79 x 38mm. It is housed in a diecast box measuring 120 x 66 x 38mm. Begin construction by checking the PC board against the pattern in Fig.7. There should not be any shorts or breaks between tracks. If there are, repair these as necessary. Use the diagram of Fig.4 as a guide when assembling the PC board and completing the wiring inside the case. Start assembly by inserting the PC stakes at the external wiring connection points on the PC board. Then insert the resistors, using the table below as a guide to the values. When inserting the diodes, take care with their orientation. Note that in a kit you may be supplied with a C103B or MCR100 for SCR1. If so, not that the pinouts for the C103B are reversed to those of the MCR100, as shown on the circuit of Fig.3. Make sure you insert SCR1 into the PC board correctly, otherwise the circuit won’t work. The capacitors can be installed next. Use the table below to check the values. VR1 can also be installed at this stage. Fuse F1 is mounted in fuse clips which attach to the PC board as shown on Fig.4. Clip the fuse into the clips first, insert them into the PC board and solder in position. The Triac is mounted on the underside of the PC board with its leads protruding up through the holes in the PC board. Bend the leads so that the copper side of the PC board is 6mm away from the back of the Triac body, as shown in Fig.5. Insert the PC board into the case and mark out the mounting hole po- Resistor Colour Codes   No. Value ❐ 1 100kΩ ❐ 1 47kΩ ❐ 1 2.2kΩ ❐ 1 1kΩ ❐ 1 100Ω 4-Band Code (1%) brown black yellow brown yellow violet orange brown red red red brown brown black red brown brown black brown brown 20  Silicon Chip 5-Band Code (1%) N/A yellow violet black red brown red red black brown brown brown black black brown brown brown black black black brown sition for the standoffs and for Triac1. Remove the PC board and drill out the holes. You will also need holes in the end of the case for the cord-grip grommet and the earth lug screw. The cord-grip grommet hole is elongated but must be a tight fit to properly grip the mains cord. The hole for Triac1 must be deburred with a larger drill before it is secured in place. Attach the PC board to the case with Nylon standoffs and Nylon M3 x 15mm screws. Nylon screws are essential here, to avoid the possibility of arcing from the PC board tracks to the mounting screws. Use metal screws for the Triac and earth connections. Secure Triac 1 to the case with a metal M3 x 10 screw and nut after applying a smear of heatsink compound on the mating surfaces. Note that the specified Triac is an insulated tab device and does not require an insulating washer. Attach the mains cord wires to the PC board and lock the cord in place with the grommet. Mark out and drill the front panel for the mains outlet, speed control pot (VR1), the earth screw and the switch. You can use the front panel label as a guide to the positions. The cutting template for the mains socket is shown Capacitor Codes Value 47nF 1nF Old   Value .047µF .001uF IEC Code 47n 1n EIA Code 473 102 www.siliconchip.com.au Note that all of the circuit is connect- adjust VR2 and then try again. You ed to the 240VAC mains supply and may then want to try other power is potentially lethal. Do not touch any tools to get a compromise setting for part of the circuit when it is plugged the trimpot. into a mains outlet. Always remove the IMPORTANT: Do not operate the plug from the mains before touching circuit with the lid off the case. SC the circuit. In particular this applies to adjustA ment of trimpot VR2. SW A After testing the conA troller, then you need to adjust trimpot VR2. Plug SC in your favourite power N tool and note how it runs at the minimum setting MOTOR CONTROLLER of VR1. If it could run N slower, disconnect the circuit from the power, Fig.7: actual size artwork for the PC board. 10110021 in Fig.6.Note that it is important to drill a small hole for the locking tab on the potentiometer to prevent it rotating inside the case. Attach the front panel label and secure the mains socket. Attach the pot and switch. Wiring must be done using 7.5A 250VAC-rated wire. Earth connections are soldered or crimped to the solder lug using green/yellow mains wire. The lugs are secured to the case using a metal screw, nut and star washer. Tie the wires with cable ties to prevent them breaking from their terminations. Finally, attach rubber feet to the base of the case. MAX 240V - 5A MOTOR SPEED CONTROLLER Small hole 4.5mm in diameter MAX 240V - 5A MOTOR SPEED CONTROLLER CONTROLLED SILICON CHIP www.siliconchip.com.au For universal-type motors up to 5A nameplate rating Do NOT use on induction or shaded-pole motors 14mm 10.9mm Semicircular part 33mm in diameter 16.5mm (CASE LID) Fig.6: these are the panel cutout details for flush-mount AC socket. CONTROLLED SILICON CHIP www.siliconchip.com.au For universal-type motors up to 5A nameplate rating Do NOT use on induction or shaded-pole motors Fig.8: actual size artwork for the front panel label. “Universal” motors: the inside story This photo shows the construction of a typical double-insulated jig-saw which uses a universal motor with brushes and a commutator. The plastic case provides the “double insulation” construction and it also provides the alignment for the motor bearings, brushes and gears. Note the integral fan on the motor shaft to provide cooling. This jig-saw also has speed control built into the trigger switch. This motor is actually wound for operation from 180V so it gives a wide (no load) speed control range from 500 to 2900 RPM. www.siliconchip.com.au October 2002  21