TechArchive

Original article date: February 1998

Cost has kept the 400V three-phase drive in the shadow of its more popular 230V single-phase cousin. Not any more, perhaps, argues MARK HARTLEY Drives Marketing Manager of Eurotherm Drives, anticipating a radical rethink in low power AC motor control.

The last 10 years has seen a rapid increase in the market for low power (0.18-2.2kW) AC variable speed drives. Although motors in this power range are invariably three phase machines, it is normally possible to control them with either 230V single phase or 400V three phase drives.

230V single phase drives have always been popular with users, as traditionally they have been much cheaper than 400V three phase equivalents. Three phase drives do though have distinct advantages.

The majority of low power AC motors are manufactured as six terminal machines: both ends of each of the three phase windings is available in the terminal box. This allows the motor to be star-connected for (three phase) 400V supplies, or alternatively delta-connected for (three phase) 230V (Fig 1). Certain motors are, however, internally connected in star configuration and such motors cannot be reconfigured for 230V.

Inverters control the speed of AC motors by rectifying the fixed voltage and frequency AC supply to DC. This then acts as the power supply for a three phase transistor bridge which supplies the motor with variable voltage and frequency (see Fig 1). The inverter therefore can be single phase 230V supplied or three phase 400V supplied – all that is required is for the motor to be delta or star connected appropriately. There are, however, significant benefits to employing three phase 400V supplies.

Reduced motor and inverter input current: the most immediate advantage gained by controlling motors from 400V supplies is the reduced current (see Table 1). This reduction gives the user potential savings in cable size and any associated switch gear and fuses. It can be seen from the table that the motor currents are about twice and input currents about three times higher on 230V supplies.

Reduced harmonics and balanced supplies: site electrical engineers are always careful to balance the load equally across the three phases. Any imbalance (a greater load on one phase) will cause currents to flow in the neutral conductor. If this is taken to extremes, then the neutral can become overloaded. Employing three phase drives guarantees the load is equally shared.

The problem is potentially further exacerbated by the harmonic currents induced onto the supply (see Table 2). All AC drives will draw a certain level of harmonic currents, but three phase drives do not produce any at the third multiples (3, 9, …. times fundamental frequency). This is important because these harmonics sum in the neutral rather than cancel out. So if, for example, three equally powered single phase AC drives were connected one to a phase, appreciable current would still flow in the neutral. If three phase drives had been used, this would not be the case.

DOL drive by-pass circuits: it is common practice to employ a DOL (direct on line) drive by-pass circuit in certain critical applications. If the drive should fail then it is by-passed and the motor connected direct to the supply in order to keep the process running (see fig 2).

If a three phase 400V drive were employed then the motor will already be star-connected. If however a 230V single phase drive were used, then the motor would be delta connected. This greatly complicates the by-pass circuit, as not only must the drive be isolated, but also the motor must be reconfigured (using contactors) to change from delta to star connection.

Neutral availability: many installations – and certain geographical regions – do not have a neutral conductor available. In these situations, it will only be possible to employ a single phase 230V drive if a step-down transformer is used – generally an undesirable addition due to cost, extra panel space and installation time/cost.

A significant volume of AC drives are fitted in applications which previously used DOL fixed speed motors. Such motors are invariably three phase 400V (not requiring a neutral), fitted because of the superior starting torque available compared to a single phase capacitor start equivalent. A three phase 400V drive is much more convenient in such retrofit situations.

Table 1 Comparison of motor and input current at 230V and 400V

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Table 2 Example of harmonic currents at 1.5kW

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It is always preferable to operate AC drives from 400V three phase supplies. But at lower powers, cost has often been prohibitive compared to 230V single phase supplied units.

The new 400V 601 series inverters from Eurotherm Drives offers all the advantages of three phase control but at prices previously only associated with 230V units.

This cost benefit has been achieved by optimising the drive design for non-complex lower power applications only. The range is aimed primarily at the OEM market where designers are looking for the advantages of three phase fed drives but have previously been put off by the price.

Historically low power 400V AC drives have been just the lower end of ranges up to several hundred kilowatts. As such they carry a large control overhead, necessary for the higher power more sophisticated applications but an overkill for 90% of low power installations. Eurotherm’s new 601 series has been limited to powers up to 2.2kW in order to avoid this cost burden.

Each unit also includes an internal dynamic braking switch for applications requiring rapid deceleration of the motor or involving high inertia loads. The operator and programming controls are included and the set-up configuration of the inverter can be stored or cloned to other 601s via a smart card reader.

The series covers the range 0.37 to 2.2kW (powers previously associated with 230V inverters) and can operate on AC supplies from 380 to 460V. They are extremely simple to set up, having only 15 adjustable parameters of which only a few will require adjustment in the majority of applications.

• Mark Hartley
• Eurotherm Drives
• 01903 721311

Market Analysis

Growth has returned to the Motors and Drives market and is forecast to continue at an average rate of about 3% in the years leading up to 2001, according to the Reed Electronics Research Profile of the European Motors and Drives Industry (second edition, price £675).

Whilst the market shows modest overall growth, different segments will vary in fortune, according the predictions. There is an increasing preference for AC motors and drives for industrial applications, whilst DC motors and drives are becoming more common for small FHP motors.

Over the last few years, there has been a considerable amount of merger and acquisition activity in an increasingly competitive market. Advances are being made in the design of electric motors to improve efficiency and provide the output power of a standard motor in smaller frame sizes.

There will be continuing technical advances in AC drives to provide improved performance, reliability and ease of use, with reduced costs. Intense competition and reduced costs are forming drives prices downwards, but the introduction of sub-$200 inverters is seen by suppliers as a way of enlarging the market with a new generation of simple, cheap inverters for new applications.

February 1998