Will this combination really work?

Dave Wilson looks at motor/drive combinations and asks whether they have lived up to their initial promise.

Over the past couple of years, a host of reputable motor and drive suppliers have launched integral motor drives: products that closely integrate an inverter with an AC motor. The benefit is a variable speed motor which you can install as simply as a standard one.

Despite the advantages touted by the system vendors, some individuals question the whole rationale behind the idea.

‘If you look at the market, only 5% of all AC motors are inverter driven anyway,’ says Mark Hartley of Eurotherm, ‘While 95% use direct on-line starting. If making a drive integral to a motor is such a clever idea, why didn’t anyone go around and put direct-on line starters, which are most robust and unaffected by heat anyway, into the motor?’

According to Brian Banister at ABB, however, the integrated motor inverter solves a lot more problems in the field than a solution that just has an on-line starter – a technology, he claims, is already well understood. ‘Our unit is IP55 rated, so it can be used in applications where you couldn’t use a separate inverter,’ he says. ‘The new inverter/drives also have EMC filters built in, whereas in the past engineers have had to buy separate filters and screened cables that added to the cost of an installation.’

Andy Axtell of IMO, a company that represents the Fuji range of inverters in the UK, agrees. ‘Such products are ideal for certain applications, such as in the pump and fan industry’ he says. So perhaps it shouldn’t be surprising that, in addition to ABB, many other motor suppliers: Baldor, Brook Hansen and Danfoss, Siemens, Rockwell and LeRoy Somer have embraced the technique.

WHEN THE HEAT IS ON

Aside from the pump and fan market, most manufacturers also see a future for these drives in other applications. But there are fears – at least from some quarters – that placing electronics so close to the motor is asking for trouble. When motors get hot, they heat up the drive electronics and can cause failure.

In most motor/drive offerings, the drive electronics sits in an enclosure above the motor itself. ABB’s Banister says that heat generated in the motor comes from the core of the motor after high speed running when the motor is left in standstill mode. ‘With the inverter mounted right on top, the heat will rise directly into the high speed circuitry. It’s assumed that these units will be used in a horizontal foot mounted position. And if that’s the case, the heat will rise into the critical electronics.’

The exception to the rule, at least at present, is the ABB offering, where the drive has been placed on the back of the motor near the fan so it will be less affected by the heat dissipation from the motor. For heat dissipation reasons, most motor/inverter packages are currently rated at under 7.5kW. ABB, for example, is shipping a range of products from 0.75kW to 7.5kW. Detractors to the ABB approach claim an inverter on the back of the motor is a more expensive solution to the problem.

David Bates, the manager of standard drive products at Rockwell Automation says that his company hasn’t yet launched its range of motor/inverter drives into the European market, but will shortly have an equivalent offering to the other vendors in the market up to 5.5kW.

He agrees that heat dissipation is the biggest challenge for designers to wrestle with. ‘No matter how well you insulate the motor or package the inverter, you have a certain amount of heat to get rid of. Some of the designs to date have just bolted inverters onto motors. I don’t call that an integrated solution. An integrated solution would use the motor cage as a heat sink and dissipate the heat through the motor casing,’ he says. Bates confirms that Rockwell is still performing tests to ascertain the heat dissipation of its unit. ‘We don’t want to stick a product out into the market that isn’t quite ready,’ he adds.

ABB’s Bannister points out that the physical side of the inverter module itself also presents a challenge. ‘That’s where the limitations are at the moment. Once you go beyond 7.5kW, it doesn’t look like a truly integrated package,’ he says. ‘It looks like a huge inverter bolted onto the back end.’ ABB uses two sizes of inverter module that it uses on its drives. The first module can be used at ratings up to 3kW, the second covers the rest of the range up to 7.5kW. Beyond that size, any module would not be physically compact enough to fit into an integrated drive. Nevertheless, Bannister says that ABB has plans to go up to the next frame size taking the range to 11kW in 1998.

Aside from heat, there have been other problems with integrated motor/drives in the field. One such is vibration. Components on some vendors’ circuit boards have been known to crack due to it. Another issue is cold. Systems integrators should check carefully which integrated drive/motor combinations are rated lower than 0 degrees C if they need a motor for those conditions.

Like ABB, LeRoy Somer, has been marketing a range of integrated motors in the UK for several months now. Charles Raballand, the product marketing manager says that there are a wide range of applications for the integral motor drives, but LeRoy Somer is primarily targeting conveyor applications with motor, gear and drive combinations. ‘We have had a lot of interest, but people need to test out the solution first before committing to it,’ he says.

At Eurotherm Mark Hartley confirms Raballand’s view of the UK market. ‘We did a poll of 25 sales engineers recently and asked them whether they had ever lost an inverter sale to a combined motor/drive unit,’ he says. ‘Not one of them felt that they had lost an order, which tends to indicate that there isn’t a tremendous demand for such products in the marketplace at the moment.’

‘At the moment, we are in the early days of development of such products,’ says Steve Scales of Hitachi HID, a company that has built motor/drive combinations for the Japanese market. ‘Today’s products are a nonsense, really, offering only odd little benefits.’ Nevertheless, Scales is very optimistic about the future. ‘The next five years will see the technology of integrated motor drives take off,’ he says. ‘But not in traditional market places.’

Scales believes that the real opportunity will occur when inverter manufacturers can build an inverter so inexpensively that it can be fitted to any motor.

Many small pumps, for example, in factory applications could employ such a solution, conferring great energy savings to the user. At present, however, the cost of equipping such small motors to the cost of the inverter and the cost of integrating such units in the field. ‘But when the price of the inverter falls to a few pounds, and it is embedded, rather than simply bolted onto the motor, all that will change,’ says Scales.

The power rating of the Leroy Somer design, in which the inverter is mounted on top of the motor, presently goes up to 2.2kW, but soon will be increased to 4kW. The limitation on the rating is, again due to the heating of the inverter due to the motor.

Combination drive/motors are ideal for certain applications such as the pump and fan industry so it is not surprising that, many motor suppliers including: Brook Hansen, ABB, Baldor, Danfoss, Siemens, Rockwell and Leroy Somer have embraced the idea.

{{INFORMATIONABB. Tel: 0161 455 5555.EUROTHERM DRIVES. Tel: 01903 721311.HITACHI HID. Tel: 01908 325209.IMO. Tel: 0181 452 6444.LEROY SOMER. Tel: 01908 325209.ROCKWELL. Tel: 01908 838800.}}