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1.5kW Induction Motor Speed Controller, Pt.2

Last month, we described the features of the 1.5kW Induction Motor Speed Controller and explained in detail how it works. This month we describe its construction and testing and give some guidelines for use.

By Andrew Levido

Before going any further, we must again remind readers that this project is intended only for experienced constructors. Most of the circuit operates at 230VAC mains potential and it has portions operating at 325-350V DC. Furthermore, the circuit can remain potentially lethal even after the 230VAC mains has been removed.

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Fig.8: install the parts on the PCB as shown on this layout diagram. Transformers T1 & T2, the three 470μF 400V electrolytic capacitors, bridge rectifier BR1 and IC1 (the IGBT module) are mounted on the underside of the board. Make sure that the electrolytic capacitors are orientated correctly.

Construction begins with assembly of the PCB. Fig.8 shows the component overlay. It is a fairly straightforward layout except that some components are mounted on the underside of the board and there are five surface-mount components to contend with. Fortunately, the surface-mount components are all passive (four 10μF capacitors in 2012/0805 packages and one 0.015Ω 2W resistor in a 6432/2512 package) and are quite easy to install with a conventional soldering iron, with a small tip.

Start by loading these SMT components, then move on to the rest of the components in reverse height order. Don’t install any of the parts that mount underneath the board at this stage.

Note that the 4N35 opto-coupler is mounted the opposite way to the two HCPL2531s. The 4.7kΩ 5W resistors should be mounted two or three millimetres proud of the board to ensure free airflow on all sides.

The input surge-limiting NTC thermistor TH1 should be mounted such that there is about 15mm of bare lead exposed above the surface of the board. This serves two purposes: first, it prevents the solder joints from overheating, since this component runs quite hot at full load. And second, it allows the thermistor to be bent down parallel with the PCB so that it will fit inside the IP65 case and not foul the lid. This can be seen in the photograph at the top of page 68.

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This view shows the underside of the PCB. Note the aluminium brackets attached to either side of the heatsink.

However, don’t bend the thermistor down at this stage because you need access to the screw hole for the bridge rectifier, BR1. The bridge rectifier must be secured to the heatsink and soldered to the PCB, before the thermistor is bent over.

You do not need to install the ICSP connector or the heatsink thermistor connector, CON7. We found it easier to first mount the thermistor on the heatsink with its leads poking upwards so that they can be later soldered directly to CON7’s pads.

Next you can begin mounting the parts on the bottom of the board. Leave the IGBT driver and bridge rectifier off for now. The polarity of the large electrolytic capacitors must be correct – a mistake here would be disastrous (not to mention messy and dangerous).


This circuit is directly connected to the 230VAC mains. As such, most of the parts and wiring operate at mains potential and there are also sections running at 325-350V DC. 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 230VAC 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 unless the power cord is unplugged from the mains socket, the on-board neon indicator has extinguished and at least three minutes have elapsed since power was removed (and the voltage across the 470μF 400V capacitors has been checked with a multimeter – see text in Pt.1).

This is not a project for the inexperienced. Do not attempt to build it unless you understand what you are doing and are experienced working with high-voltage circuits.

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