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1.5kW Induction Motor Speed Controller, Pt.1

You've asked for it many times and we have always said NO! It's too complex, too difficult, too expensive, whatever. Now we're saying YES. This Induction Motor Speed Controller is suitable for motors up to 1.5kW (2HP) and can be used to control speed over a wide range. It will save big dollars with swimming pool pumps and will be great for running machinery at different speeds. Even better, it will control 3-phase motors as well!

By Andrew Levido

WE HAVE PUBLISHED quite a few speed controllers over the years, some suitable for DC motors and others for universal AC motors. Up until now, we have not published a design suitable for the most common type of AC motor of all – the induction motor.

Controlling the speed of induction motors is not easy; you cannot simply reduce the voltage and hope that it works, for two reasons. First, an induction motor’s speed is more or less locked to the 50Hz frequency of the 230VAC mains supply; so reducing the supply voltage doesn’t work. Second, induction motors don’t like reduced supply voltage; it makes them difficult to start and there is the risk of burnout.

No, the only reliable way of controlling the speed of an induction motor is to vary the drive frequency. As we shall see, it is also not enough to simply vary the frequency; as the frequency drops below 50Hz, the applied voltage must be reduced proportionally to avoid magnetic saturation of the core. This makes the electronic circuitry complex and its design is made more difficult by the wide variety of induction motors.

Fortunately, advances in power semiconductors have reached a point where such a project is now viable. But our previous objections still apply. It is complex, relatively expensive and potentially dangerous.

This project is only recommended for experienced constructors. Most of the circuit is at 230VAC mains potential and worse, it has sections running at 325-350V DC. Furthermore, the circuit can remain potentially lethal even after the 230VAC mains supply has been disconnected.

We envisage the main application of the speed controller will be in reducing the energy consumption of domestic pool pumps – one of the biggest single contributors to the power bills of pool owners. We’ve previously published a review of a commercial unit that does this but at a price tag of over $1000. You should be able to build this unit for a couple of hundred dollars, making it a much more attractive proposition.

That said, we have tried to make this unit fairly versatile. It will drive virtually any modern 3-phase induction motor or any single-phase motor that does not contain a centrifugal switch, rated at up to 1.5kW (2HP).

In this first article, we describe the features of the controller and explain how it works. A following article will detail the construction, testing and installation.

Features

• Controls single-phase or 3-phase induction motors

• Runs from a single-phase 230VAC, 10A power point

• Over-current, over-temperature, under-voltage, over-voltage and short circuit protection

• EMI (electromagnetic interference) filtering for reduced radio interference

• Inrush current limiting

• Isolated control circuitry for safety

• Can be mounted in an IP65 sealed enclosure

• Adjustable speed ramp up/down

• Pool pump mode

• Can run 3-phase motors in either direction

• Optional external speed control pot with run, reverse and emergency stop switches

• Motor run/ramping and reverse indicator LEDs

• Fault indicator LED

• Open collector output provides either fault or up-to-speed indication

Specifications

Motor power: up to 1.5kW (2 horsepower)

Maximum output voltage (single or 3-phase motor): ~230V RMS

Continuous output current: 8.5A RMS (single-phase), 5A RMS (3-phase)

Short-term overload current: 13A RMS (single-phase), 7.5A RMS (3-phase)

Switching frequency: 16kHz

Quiescent power: 28W

Speed ramp period adjustment: 3-30s to full speed

Continuous input current: up to 8.7A RMS

Speed control range: 1-100% or 1-150% (0.5Hz to 50Hz or 75Hz) in 0.05Hz steps

Efficiency: up to 96%

Speed control signal: 0-3.3V

Up-to-speed/fault output sink: 12V/200mA

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