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Constructional Project M k 2 Variable Speed Drive For Induction Motors Part 2 by Andrew Levido Last month, we introduced the Mk2 VSD and described its features, circuit and firmware. This month, we cover construction, testing and some hints for using it. E verything, including the heatsink and fan, is mounted on a single printed circuit board (PCB) that fits into an ABS plastic enclosure measuring 220 × 165 × 60mm, as shown in the accompanying photographs. Many of the components are surface-­ mount types, but they are all relatively easy to solder by hand. There are no fine-pitch chips, and the passives are all 2 × 1.2mm or larger, except for three diodes, which are a little bit smaller but should be manageable. Anyone with a modicum of SMT soldering experience should have no trouble putting it together successfully. That said, this is a complex build, and because of the high voltages and currents involved, it is recommended only for experienced constructors. Regardless of your skill level, if you build this, you must follow the safety instructions when it comes to the testing stage. It’s also a good idea to double and triple-check your work before powering it up. We’d hate for you to put a lot of effort into building this, only for it to blow up because something was installed backwards or in the wrong spot. Assembly We recommend assembling the VSD in two stages, as described below. First, 68 we will focus on the control circuitry, so we can test it safely at a low voltage and get it working. After that, we will move on to the power electronics. The VSD is built on a double-sided board coded P9048-C or 11111241 that measures 150 × 205mm. Start by fitting all the surface-mounting parts, using the overlay diagram (Fig.8) and close-up of the section near the microcontroller (Fig.9). Work methodically across the board, paying attention to the orientation of polarised components like ICs, diodes (including LEDs) and electrolytic capacitors. You can also refer to the silkscreening on the PCB. We won’t go into a great amount of detail here on how to solder SMD parts, as it is now pretty common, and many of our projects require it. However, we’ll give a quick overview and some tips. There are three main ways you could solder the SMDs: with a reflow oven, with a hot air rework station or with a soldering pencil/iron. Those with reflow ovens and hot air rework stations likely are already familiar with the required techniques, which involve adding solder paste to the board, placing the components on top and then heating the solder paste until it reflows. Manual soldering is best done with a syringe of good-quality flux paste. For each part, spread a thin layer of flux paste on the pads, then place the part on its pads, ensuring it is correctly orientated. One of the worst things you can do is solder an IC to the board backwards! For the microcontroller in a quad flat package, there are four possible orientations, but only one is correct (with the pin 1 dot as shown). With the part in place and a clean soldering iron, add a little solder to the tip and tack-solder one of the part’s pads. Check that all its pins are lined up with the other pads; if not, the joint can be remelted and the part gently nudged into position. Once in position, the remaining pins can be soldered and the initial one refreshed. Finally, for parts with closely spaced pins (like ICs), check for solder bridges between pins. If found, they can be cleared with the application of a little more flux paste and then solder-­ wicking braid. The braid can also be used to remove excess solder if there’s too much on some pins. Once all the surface-mounting parts are in place, clean the flux residue off the board, then add relay RLY2, DIP switch bank S1, trimpots VR1 & VR2, header CON17 and the input terminal blocks, CON8-CON11. Slot all four blocks together (in dovetail fashion) before soldering them in place. Practical Electronics | January | 2026 Variable-Speed Drive Mk2, part two 4.7nF Y2 CMC1 NC 4.7nF Y2 COM F1 10A NO 220nF X2 220nF X2 SLOW BLOW RLY1 NTC1 SL32 10015 Q3 DGTD65T15H2TF Q1 DGTD65T15H2TF 12W 330mF 400V 330mF 400V 12W Q5 DGTD65T15H2TF 330mF 40 0V 330mF 40 0V BR1 GBJ2506-F 100k 100k 12W 100k 100nF 5 .1 k 100k 1k 1 IC5 LM393 100nF 330mF 400 V + + + + 4.7nF Y2 + 4.7nF Y2 10mF NTC2 NRG2104F3435B2F 4.7nF Y2 40mm FAN TUNNEL HEATSINK 220nF X2 REG1 LD1117S50 + 4.7nF Y2 ZD1 12V CON8 100nF 5 .1 V 5 .1 V 2 .2 k 1 k 5 .1 V 1k CON9 CON10 COM Zettler ZP05S1200WB 4(–) 0V L(1) CON11 CON16 ICSP 1 CON17 12V GND 3 .3 V 10mF REG8 LD1117S33CTR + MOD2 IC4 ISO7721D 0W N(2) 3( +) +12V IC3 1 ISO7760D 1 L(1) 230V AC 10k 470 1 1 k 470k 2.2mF 4(–) 0V MOD1 12V DC 416mA 100p F IC7 STM32G030K6T6 2.2k 100nF 1 N(2) 3( +) +15V D4 10k Q11 10k Q10 LED3 2 .2 k 220 100nF 100nF 10mF 100n 10nF 470pF 82kW 1W D3 4.7k BSS138K BSS138K 2k 1k 1k 1k 1k K LED2 K 10k Q9 K NC NO D2 100n BSS138K ZD3 ZD4 Z D5 RLY2 IC6 1 2 3 4 5 6 220 IC2 6EDL04I06PT 100n 100n 220 1 3k 100nF 1k 18k LED1 1kW 82kW 1W VR1 10k VR2 10k S1 ON 2.2mF 2.2mF 2.2mF 12W Q8 Q7 BC847 AOTF4N60L 230V AC 12W 12W Q4 15mW Q6 DGTD65T15H2TF DGTD65T15H2TF 100n 12W Q2 DGTD65T15H2TF 15V DC 330mA 100m F 35V Zettler ZP05S1500WB 100mF 35V Fig.8: this component overlay shows where everything goes on the PCB. Fit the surfacemounting parts first, then the DIP switch, trimpots and relay RLY2. Test the control circuitry thoroughly, as described in the text, before moving on to the power electronics. WARNING: DANGEROUS VOLTAGES This circuit is directly connected to the 230V AC mains. As such, most of the parts and wiring operate at mains potential. Contact with any part of these non-isolated circuit sections could prove fatal. Note also that the circuit can remain potentially lethal even after the 230V AC mains supply has been disconnected! To ensure safety, this circuit MUST NOT be operated unless it is fully enclosed in a plastic case. Do not connect this device to the mains with the lid of the case removed. Do not touch any part of the circuit for at least 30 second after unplugging the power cord from the mains socket. Do not attempt to build this project unless you understand what you are doing and are experienced working with high-voltage circuits. Practical Electronics | January | 2026 69 Constructional Project Fig.9: this close-up of Fig.8 shows the most densely populated section, so that you can more clearly see the values of the resistors and capacitors there. At this stage, you will have installed all the parts in the low-voltage domain except for the AC-to-DC switch-mode power supply module, MOD2. We can now test this circuitry. Connect a bench supply to the +12V and GND pins of CON17. Make sure the polarity is correct and don’t accidentally connect it to the +3.3V pin! DuPont jumper leads are a good way to make this connection. Set the supply to deliver 12V DC, with the current limit set at around 200mA. When you switch it on, the power supply should not go into current limiting. If is does, there is a short circuit or incorrectly placed component somewhere, so switch off and check the components on the board carefully, including their solder joints. Initial testing The fully assembled PCB; it just needs the fuse cover added, to be mounted in the case and the wiring connected. 70 If the current draw is OK, check for 3.3V at the bottom pin of CON17 relative to GND. It should be in the range of 3.1-3.5V. If that is OK, and your microcontroller is not already pre-programmed, now is the time to connect an ST-Link programmer to CON16 and flash the code using the STM32Cube software (a free download). If yours is pre-­p rogrammed, you can skip this step. With the micro programmed, to check for the correct operation of the control circuit, first ensure all the DIP switches are in the off positions and both trimpots are wound all the way anticlockwise, then apply power. All three LEDs should flash briefly twice, then after about three seconds, the yellow LED (LED2) should come on. If you short the E-Stop & Run pairs of terminals with two wire links and advance the speed trimpot (VR1), the yellow LED should extinguish and the green LED (LED3) should flash while the speed ramps up to the setpoint, at which time LED3 will light steadily. If you turn the speed pot back to zero, the controller should ramp down with the green LED flashing until the yellow LED lights again. Increasing the ramp time using trimpot VR2 should prolong the ramp time. If you close the At-Speed DIP switch and repeat the above process, you should hear relay RLY2 close whenever the green LED stops flashing and lights steadily, then open when it begins to flash again. Don’t forget that you need Practical Electronics | January | 2026 Variable-Speed Drive Mk2, part two The finished VSD, all wired up, including the external control wiring (upper right). to cycle the power to read the new DIP switch configuration. You can try opening the Run switch or the E-Stop circuits while the speed controller is running (green LED on or flashing). If Run is opened, the green LED should flash while the speed ramps down to zero, then the yellow LED should light. If the E-Stop switch is opened, the yellow LED should come on immediately. Now you can check pool pump mode. Bridge the E-Stop and Run terminals again, set the speed and ramp potentiometers to about halfway and close the pool pump mode DIP switch (“POOL MD”). On reapplying power, the controller should start and ramp to full speed with the green LED flashing slowly. After about 30 seconds, the speed should ramp down (green LED flashing fast) to the preset speed (green LED on steadily). Trying again with the Pool-Time DIP switch (“POOL TIM”) also closed should extend the pool-pump period to about five minutes. You can check three-phase mode by closing that DIP switch. It should work as described earlier (ignoring the Practical Electronics | January | 2026 pool pump mode part). If you now short the Reverse terminals while it is running, the speed should ramp down (green LED flashing fast) then stop for two seconds (yellow LED on) and ramp up again to the preset speed. Finally, you can check fault operation by momentarily shorting out the thermistor terminals. The red and yellow LEDs should latch on. Opening and reclosing the E-Stop circuit should reset the fault. If you hit a snag at any point, stop, check the board carefully and fix the problem. Each step above tests a different part of the circuit, so consult the relevant part of the circuit diagram for components to check. Fix any problems and verify it has the correct operation before moving on. If you have an oscilloscope, you can take a look at the PWM motor drive signals on pins 2 to 7 of IC3. They can be a bit difficult to trigger on since the pulse widths are continuously varying, so consider using one-shot mode to capture a snapshot if your ‘scope supports it. There will only be signals on four of these pins if single-phase mode is selected. The switching frequency should be 15.625kHz (a period of 64µs) and the amplitude about 3.3V. Power electronics Start the assembly of the power components by preparing the heatsink. This is a 100mm length of 40 × 40mm heatsink ‘tunnel’ extrusion. Mine came cut to length from AliExpress. A total of 11 holes need to be drilled and tapped in accordance with the drilling diagram (Fig.10). There is a different arrangement of holes on each face, so take care to get them all in the right orientation with respect to each other. I recommend clearly labelling each face according to the diagram and marking the fan end. Mark the hole positions, but before drilling anything, offer it up to the board to check the marks line up with the IGBTs, Mosfet and diode bridge. Don’t forget to run the tap through the four extruded corner ‘holes’ on each end to make the mounting of the fan and finger guard easier. Use some wet & dry abrasive paper on a flat surface to ensure that the drilled faces are flat and free of burrs so that the power devices make good thermal contact. 71 Constructional Project Secure the fan to the appropriate end of the heatsink with four M3 × 25mm screws, making sure the arrow denoting the direction of airflow is pointing towards the heatsink. Orientate the fan so that the lead emerges at the corner shown in the photos. Now attach the finger guard together with its filter to the other end of the heatsink, using four M3 × 10mm machine screws. Mount the heatsink assembly to the PCB with two M3 × 10mm screws with spring washers under the heads. The rectifier bridge (BR1) and the discharge Mosfet (Q7) can be mounted next, with a smear of thermal compound between the devices and heatsink to ensure good thermal contact. Use M3 × 10mm screws with spring washers under the heads. Don’t solder the devices to the PCB just yet. Next, mount the six IGBTs (Q1-Q6) after carefully bending their centre pins to fit the footprint. Again, use thermal compound, M3 × 10mm screws and spring washers. Tighten all the devices down, making sure they don’t twist too much, then solder and trim all the leads (of Q1-Q7 and BR1). Give all the screws a final tighten – you can’t get to some of them once the DC bus capacitors are installed. Affix the thermistor to the top of the heatsink, again using thermal compound, an M3 × 10mm screw and spring washer. Orient the thermistor lead along the heatsink towards the fan as shown. Trim and strip the thermistor and fan leads, then solder them to the PCB pads provided. The thermistor is not polarised, but the fan is, so make sure the red lead goes to the pad marked by the plus sign. Now you can install all the remaining components. I suggest starting with the shortest and finishing with the five large electrolytic capacitors. Pay attention to the orientation of the filter capacitors – their positive leads must all go towards the top of the board! Be careful also with the AC-DC power modules; they look similar but have different secondary voltages. The 15V one is MOD1 and the 12V one is MOD2. You have finished the PCB assembly at this point, but it’s a good idea to take a bit of time to check your work thoroughly before moving on. Enclosure preparation The enclosure needs to have a square opening cut into the side to accommodate the heatsink exhaust, plus a series of ventilation holes in the top and opposite side and holes for the cable glands in the bottom end. The locations and dimensions of these are given in Fig.11. Making the square opening can be a challenge. It helps to screw the lid firmly onto to the case for this operation, as the opening overlaps both the base and the lid. I applied masking tape in the area of the cutout and marked its edges onto that. I created the opening by chain-drilling a series of holes near, but just inside the marked line and then filing carefully up to it. Fig.10: the heatsink requires a total of 11 M3-tapped holes. They are positioned differently on each face, so be careful to get them all correct with respect to each other. All dimensions are in millimetres, and the diagram is shown at actual size. 72 Practical Electronics | January | 2026 Variable-Speed Drive Mk2, part two Next, drill the 14 ventilation holes according to the diagram. I used masking tape as before to mark the centres, then drilled pilot holes with a 3mm drill bit, followed by a 10mm bit. You can then drill holes in the bottom end of the enclosure for the cable glands. Two of the glands are required: one for the mains input and one for the motor output cable, but the third one, for control cables, is optional. If you are using the VSD in standalone mode (see the applications section below), this hole may be unnecessary. The hole size should match the glands that you use. Make sure you get the correct sized glands for your cables – they will only provide good strain relief if they are matched to the cable diameter. The enclosure comes with a length of O-ring material which you should push into the slot in the lid, avoiding the area of the fan guard cutout. As a side note, you can get a set of mounting feet for the enclosure that allows it to be mounted on a panel or wall. If you are using those, now is a good time to screw them onto the bottom of the enclosure. Final assembly and wiring You can now mount the PCB assembly into the case with four self-­ tapping screws and wire it up to suit your application. For most singlephase applications, an input cable with a three-pin mains plug and an output cable with a matching mains socket should work. An easy way to create these cables is to sacrifice a low-cost extension cord by cutting it in half. Please use something that meets your local electrical standards, bought from a reputable supplier and not some random internet find. Feed the cut end of each cable through the appropriate gland, tighten, and then crimp female 6.3mm spade connectors to the conductors. Either use insulated spade connectors for the Active and Neutral (brown and light blue) wires, or add some insulating heatshrink tubing in the appropriate colours over the exposed metal after crimping. We need a direct 10A wire connection between the incoming and outgoing Earth wires to ensure the device can handle a high fault current if something goes wrong with the motor. Therefore, cut a 15cm-long piece of 10A green/ yellow striped wire (which can be Practical Electronics | January | 2026 Fig.11: the case needs a square opening for the heatsink exhaust, plus a total of 14 10mm ventilation holes as shown. The size of holes for the cable glands depends on the exact glands you are using. stripped from 10A mains flex or a spare 10A mains cord) and crimp piggyback spade lugs onto both ends. Plug the incoming/outgoing Earth wire spades onto the tabs on the piggyback connectors and then shrink some 10mm green/yellow striped heatshrink tubing over the piggybacked connectors. They will be close to the Active and motor output spades. While those are also insulated, it doesn’t hurt to have extra insulation! Plug the piggyback spade lugs onto both Earth connectors on the PCB, then connect up the Active (brown), Neutral (blue) and motor output wires. Double-check the wires are in the right places. The wire with the mains plug on the end (incoming power) must go to the A, EARTH and N spades near the fuse clips, while the one with the socket on the end goes to the EARTH, U and V motor connectors near IC2. Now is also a good time to insert the 10A 73 Constructional Project The fan and thermistor wires should be cable tied together preventing a loose wire from one of these straying onto any of the U, V or W terminals. We recommend that for safety, you strip back some of the insulation in the middle of the Earth wire (without cutting the conductors) and crimp the copper to an eyelet lug that’s attached to the heatsink via an extra tapped hole (the position isn’t critical) so the heatsink can’t become live if the PCB Earth tracks fuse. Make sure you don’t leave off the 10A Earth wire between the two Earth terminals as it’s vital for fault protection. Also fit an insulating cover over the fuse as seen here for safety. 74 slow-blow fuse into the F1 clips and slip the insulating cover over the top. If you are driving a three-phase motor, or building the VSD into another piece of equipment, you may need custom wiring. In any case, it is absolutely mandatory to wire in the mains Earth and to connect the motor Earth to the motor chassis with a proper wire between the two (not relying on the PCB to conduct Earth current!). The PCB Earth connections are for two purposes only: to Earth the heatsink for safety, and as part of the mains EMI filters that each have two Y2 capacitors between the phases and Earth. As mentioned in the adjacent caption, we recommend attaching the Earth wire directly to the heatsink as well. Control wiring This speed controller has been designed to be as flexible as possible. In the standalone configuration, no external controls are required. The E-Stop and Run terminals should be bridged by short lengths of hookup wire, and the internal speed pot selected on S1. In this case, as soon as power is applied, the motor will start and ramp up to the preset speed. The speed and ramp rate are set via the onboard trimpots, VR1 & VR2. When power is removed, the motor will coast to a stop just as it would if switched off when directly connected to the mains. This arrangement could be used to run a single-phase motor at a lower speed than usual, or to run a threephase motor at a fixed speed from a single-phase supply. It could also be used as a ‘soft starter’, to provide a gentle start for sensitive loads or to limit the initial starting current surge. Most pool pump applications will also use this configuration. At the other end of the spectrum, it is possible to use this controller as part of a more complex control system, such as for a machine tool. In such applications, the VSD would normally be mounted in an electrical cabinet, with external controls (run, emergency stop, speed control etc) located on a panel close to the operator. If the machine tool is numerically controlled, these control signals may come from a CNC controller or PLC. You can see from our photos that Practical Electronics | January | 2026 Variable-Speed Drive Mk2, part two we built a small ‘remote control’ box to test out the external control functions. It’s little more than three switches and a pot mounted to a Jiffy box and wired to a 9-core alarm cable, run through cable glands into the VSD case, where they connect to the EXT SPEED, ESTOP, RUN and REV terminals of CON8-CON10. We won’t go into details here, as we expect anyone who can build this VSD will be able to figure out the wiring from the PCB labelling. The cable gland outside nuts that are tightened to secure the mains input and output wires should be permanently fixed using super glue on the threads to prevent the glands from being undone from outside the box and the mains wires becoming loose. Using the VSD Using the VSD is straightforward. If the unit trips out when starting, you can extend the ramp rate and/or switch the BOOST DIP switch on. We tested it on a domestic pool pump and found that, with the correct settings, it had no trouble starting the pump under load. If you have one, you can use a current clamp meter around one of the motor power wires to monitor the motor current during startup. The VSD should be able to deliver its full rated current (9A in single-phase mode and 5.5A in three-phase mode) continuously and up to 18A/11A for a few cycles. You will need a clamp meter with a peak hold setting to measure this. If you are wiring the VSD directly to the motor, you will need to work out how to connect it. Single-phase PSC motors have notoriously confusing terminal housings with no discernible standard arrangement. There is usually a diagram inside of the terminal housing lid to help; otherwise, see if you can locate a wiring diagram for your motor online. Don’t forget to connect the Earth wire solidly to the stud provided in the terminal box. The only way to change the direction of rotation of PSC motors is to reverse the sense of the start winding with respect to the run winding. Many motors have an arrangement of relocatable bridges to allow this to be done without rewiring the whole motor. The terminal arrangement for threephase motors is usually a little simpler. The VSD can only supply a phaseto-phase voltage of 230V RMS, so it Practical Electronics | January | 2026 L1 L1 L2 L2 L3 L3 'STAR' CONNECTION 'DELTA' CONNECTION Fig.12: the windings of small 3-phase motors are normally connected in star configuration for use with the 400V RMS 3-phase mains supply. In this case, each winding is driven with the phase-to-neutral voltage of 230V. By changing how the windings are connected (which can usually be done by moving some jumpers), the motor can be changed to delta configuration, with just one winding between each phase. DUTY CYCLE 1 It can then be driven from a 230V RMS DUTY CYCLE 2 3-phase supply such as the output of this motor controller. PWM 1 Fig.13: this diagram illustrates the difference between traditional edgealigned PWM and centre-aligned PWM (also known as dualramp PWM). With centre-aligned PWM, the leading edge of each pulse moves as the duty cycle changes. This is an advantage because if all outputs switch high at the same time, as with edge-aligned PWM, the total current pulse is larger and so more EMI is generated. PWM 2 EDGE-ALIGNED PWM DUTY CYCLE 1 DUTY CYCLE 2 PWM 1 PWM 2 is suitable for motors with 230V or 240V windings (most small induction motors). The rating plate will normally quote the voltage rating as 230V/400V, 240V/415V or something similar. There are usually six terminals for the three windings, with bridges to connect the windings in star (Y) configuration for the higher voltage or delta (Δ) configuration for the lower (see Fig.12). For 230/240V operation, use the delta (Δ) option. Again, the inside of the terminal box lid should have a diagram to help. You can connect the VSD’s U, V & W outputs in any order, although this will affect the direction of rota- CENTRE-ALIGNED PWM tion. If the direction is not what you want, swap any two of the leads or use the Reverse control input, which does the same thing electronically. Again, connecting the Earth is mandatory for safety. A word of warning: induction motors often have a shaft-mounted fan that blows cooling air across the fins cast into the housing. This fan will be much less effective at low shaft speeds, so be careful if you intend to run a motor in this way for long periods of time or in very hot environments. If this is a concern for you, consider using an external cooling fan with a separate power source. PE 75