Doing a deal on drives

Variable speed drives have proliferated to provide the design engineer a bewildering choice of alternatives, not only of subtle variants in the technology but also in suppliers.

From the early days of thyristor DC drives in the 70s, when the choice was that or nothing, the electronic variable speed drive (VSD) has evolved to provide a drive for every application with greater functionality than ever before.

In deciding which drive to choose, the first criterion must be the dynamics of the application. The electronic drive is simply a device for controlling the torque production mechanism; the motor, and the choice of motor, both type and size, is critical to the success of the application.

A number of factors come into play, all from the perspective of the driven load. Starting torque, speed range, acceleration, deceleration, reversing time, steady-state speed holding accuracy, linearity of performance against input signal, load speed regulation, speed range and working environment are all factors to be considered.

Demise of dc?

So the first task is to choose your motor type and rating to suit the application. AC, DC or servo just about sums it up, with the synchronous reluctance (SR) motor standing in the background for specialist applications.

DC drives still represent a choice and although the demise of the DC drive has been predicted many times – and frequently by me – it simply refuses to die. DC drives now represent less than 20% of the UK market and are in decline, so what is keeping them alive and what dictates their selection?

DC converters still enjoy a market for application to existing motors and in some cases for larger power drives. At around 150kW the DC motor breaks even with the AC cage machine and as the DC controller is cheaper than the AC inverter, then the overall package price is attractive for larger power ratings. Offsetting this however, is the lack of availability of DC motors and their maintenance demands.

Servo’s home in high speed

Additionally, the control bandwidth of AC drives is now substantially greater than DC drives, with vector control offering bandwidths as high as 1000Hz. This represents a significant improvement in dynamic performance and smooth control, as evidenced by the lift industry’s adoption of AC drives for smooth take off and levelling. With their commutators and brushes, DC motors also demand regular maintenance otherwise they represent a reliability risk and for this reason the transition to AC drives in replacing DC drives is inexorable.

Before moving on to AC drives, it is perhaps logical to briefly consider servo drives. Servo drives find a natural home in high speed, high dynamic performance applications such as machine tools, and special purpose machinery. More expensive than either AC or DC drives, their application involves much deeper consideration of the mechanical aspects of the driven machinery, such as inertias, rates of change of speed and torque. Applying servo drives is not a simple off-the-shelf selection process and should only be considered if it is clearly identified that a standard DC or AC vector drive cannot cope with the application.

Call in the suppliers

Then it is time to call in a reputable supplier and discuss the application in depth.

When considering AC inverter drives, there is really only one driving reason for their adoption. Inverter drives utilise the standard AC squirrel cage induction motor.

Manufactured universally, they are robust, require little maintenance, are available off the shelf, offer a wide range of enclosures and mountings, and are relatively low cost. Unfortunately, for so many years, it was regarded as a fixed speed motor but controlled by variable frequency, variable voltage inverters, they can now offer variable speed and superb dynamic performance over a wide range, so it is the natural choice for the majority of applications. More recently, the evolution of digital electronic flux-vector control, in both open loop and closed loop variants, means that there are few application areas where the AC motor cannot be applied.

The development of flux-vector control, dating back to the early ’80s but made viable and cost effective by the microprocessor and its variants, radically changed the development of AC drives and spawned today’s proliferation of choice. More drive for your money than ever before is the general rule and cost per kW has never been lower. In addition, the current oversupplied state of the UK drives market means that very good deals can be done. Successive surveys of the market indicate that price considerations come well down the list after reliability, availability, specification etc. When you buy cheap – you buy dear is a good rule. So let’s look at some selection criteria for picking the right digital AC drive for the job.

Single phase AC drives:

These represent the lowest cost AC drive option, with the lowest functional specification, limited to 2.2kW and usually limited dynamic performance. Usually selected where only 230v single phase supplies are available, multiple single phase drives on a single phase can lead to high neutral currents and potential harmonics problems. Where a three phase and neutral supply is available, multiple drives can be distributed over the three phases to take advantage of the cost saving.

‘Intermediate’ three phase AC drives:

Most drives companies have a mid-range offering, ranging from fractional kW to around 15kW. These are usually open-loop or sensorless vector types requiring no feedback from the motor but offering high dynamic functionality and a wide operational specification. These are the biggest volume sellers in the UK, suitable to the widest range of applications. These will also offer a range of fieldbus options.

Full specification vector drives:

These usually range from around 4kW to hundreds of kW. Some suppliers offer different ranges for standard performance sensorless vector operation and high dynamic performance, closed loop vector operation although others make this level of functionality selectable within the drive. The selection rule here is to match the drive to your performance specification and don’t buy more drive than you need. Multiple functions do not always represent multiple benefits for your application, but the functionality within the drive, especially where a second processor is available, can create savings within the wider control system.

Fan and Pump rated drives:

Most of the larger drives either offer a selectable fan and pump rating, which enable you to trade off overload capacity, against full load rating, effectively uprating the drive by one motor frame size, or a specific fan and pump model. Where selecting a drive to run a centrifugal fan or pump, the principal selection criterion is efficiency. Modern drive control strategies control motor fluxing to optimise efficiency at all points over the operating range and even self-tune to the motor.

Shop around and haggle

When selecting a drive for your application, first be clear about the mechanical dynamics of the application and size the motor accordingly. Then look to the appropriate AC drive for that size and the dynamics of the application, considering DC or servo drives only if special circumstances demand it. Identify the control functionality required within the drive and buy to that specification.

Choose suppliers who provide a competent consultancy service as part of the buying process and involve their local sales engineer in the process. Keep reliability, warranty conditions, delivery and after sales service high on the list of discussion points. But most important – shop around and haggle, there has never been a better time to do a deal on drives.