In terms of primary function, today’s AC drives are very similar to the drives of a decade ago: they control motor speed, provide motor protection and allow users to customise things like acceleration and deceleration time. So what is different? By examining the attributes of new drives from Rockwell Automation – namely the Allen-Bradley PowerFlex family of AC drives – users can see how much more efficiently today’s drives meet the need of speed control, motor protection and user application requirements. While the end goal of drives is the same today as it was a decade ago, today’s means to reach it are significantly more advanced.
The most obvious advancement in drives is their smaller size, driven by the advanced level of integration in drives' electronic circuit boards. Allen-Bradley PowerFlex drives are, in some cases, more than 60% smaller than the previous generation. This decreased size is mostly attributed to the reduction in size of its electronic components and circuit boards. Both end users and OEMs benefit from the smaller size because drive installation requires less panel space - and often there is not a lot panel space to spare.
An automotive manufacturer, for example, looking to retrofit an existing line can now install twice as many drives in a panel, or avoid the time and cost of installing an entirely new panel section to accommodate line expansions.
Beating the heat
With new, smaller packaging designs, innovative heatsink configurations to release heat also are needed. Drive manufacturers have typically offered one-piece extruded aluminium heatsinks with heat-dissipating fins, but this technique restricted how closely the fins could be positioned next to each other, inhibiting space savings. Newer heatsink designs use more heat-dissipating surface area in a smaller space. Today, some drive manufacturers utilise heatsinks with grooves in a block of aluminium, then bond fins into these grooves. This allows for more heat dissipating surface area in a smaller volume. Additionally, an air duct - or wind tunnel - design better concentrates air flow to maximise the transfer of heat away from heat-producing drive components.
In the case of the PowerFlex 700 - this wind tunnel contains the power device heatsink, DC link chokes and optional internal dynamic brake resistor. This fan and duct design also contributes to efficient, bookshelf packaging. The cooling layout eliminates space formerly needed between drives when mounted side by side and creates Zero Stacking alternatives, or side-by-side mounting (drives touching), to save panel space.
Some drive heatsink designs still use traditional aluminium castings to provide better cost savings, but these castings can now serve multiple purposes, such as mounting points for internal components and the drive itself.
Advancements in internal drive components also reduce heat. For example, many drives now include an active thermal regulator algorithm that uses temperature feedback. This regulator resides within the power devices mounted on the heatsink and it constantly measures the drive's temperature. If it detects an unacceptable increase in temperature - for example, from a clogged fan - the active thermal regulator is smart enough to protect the drive. Drives with this feature first try to keep the motor running with corrective action, such as reducing the carrier frequency - which will reduce heat. Then they will reduce the current limit to a lower level in a secondary attempt to keep the devices cool. Eventually, if they cannot correct the overheated condition, it will shut down the motor, preventing serious damage.
Easing operation
Today's drives are easier to operate. Drive users have stressed the importance of designs that lower operator training time and provide an intuitive, comfortable operator interface. As a result of this feedback, operator interface modules for new PowerFlex drives look more like calculators with silicone keys. Its liquid crystal display (LCD) option resembles television remote controls or personal game displays. Larger screen display areas make it easier to display and read more drive data, allowing the user to simultaneously display multiple variables such as motor amps, output frequency and voltage. Other packaging improvements include more ergonomic operator interfaces with tactile feedback (the customer can be assured the button has been pressed), light pipes to make status and communication light emitting diodes (LEDs) visible through the cover and designed for aesthetic form as well as operating function.
Spelling it all out
Drives are more interactive and intuitive. Ten years ago, drive users needed to configure speed potentiometers, switches and jumpers to operate the drive. Drive operations were not spelled out in plain English (or any other language, for that matter). Using a potentiometer to set minimum speed and acceleration time, the only operator feedback was viewing the knob to determine its position. If users had LED displays, their messages were very cryptic. In the event of a fault alarm, for example, a message such as 'E17' might appear, meaning 'error 17', and the users would have to look up its meaning in a reference manual.
The PowerFlex drives with a seven-line, backlit, graphical LCD display, tells users what the settings are and actually gives recommendations as to which parameters users might consider setting. In a fault condition, users get clear messages, such as 'over-voltage condition.' This clarity eliminates the time it would take to decipher fault codes. The PowerFlex 70 and PowerFlex 700 drives actually engage users in a dialogue with a Windows-like wizard, prompting users through start up and further easing operation.
Engineers can now monitor variables like voltage, frequency and current at the same time. In the past, users had to look at each item separately because there was insufficient display space. Now it is easier to have more information at the users' fingertips.
Simplified drive commissioning
New menus and prompts ease start up compared to previous drives. A graphical LCD display that puts more information on the drive screen makes it easier to prompt a user what to do next. And multiple-language features (in the case of PowerFlex - Dutch, English, French, Italian, German, Portuguese and Spanish) on some newer drives support global-user understanding - a necessity for OEMs shipping products around the world
Since the 80/20 rule - 80% of users only use 20% of the features - applies to drive users just as it applies to VCR users, with a new feature called 'S.M.A.R.T. Start', Rockwell Automation engineers have reduced the drive parameter set selection to show only parameters that the majority of users need to set up. This feature saves operators time by allowing them to focus on the most critical parameters.
Quick fixes
If a drive needs to be replaced, new I/O terminals on some units are 'pluggable' to expedite the switch-out. With pluggable I/O, the terminal block can be pulled out and plugged into a new drive, compared to a decade ago when users would commonly need to completely unwire all the terminals. In the case of the PowerFlex 700, the control cassette - the brains of the drive - can be replaced without time-consuming dismantling.
Virtually trip-free performance
On the manufacturing floor, trips (faults) mean downtime. To decrease downtime caused by a drive trip, today's drives operate smarter. For example, if users installing a drive in an application first set up acceleration time for three seconds but in reality the motor would need five seconds with the available motor torque to accelerate the load, this would potentially create a trip. In the past, drives would allow the motor to pull too much current because they were not smart enough to control the acceleration rate and limit the current in order to prevent the trip. Today's drives have the intelligence to preclude a 'nuisance trip' to help keep operations running.
A thermal regulator can monitor both the temperature of the insulated-gate bipolar transistor (IGBT) power structure and the overall loading on the drive based on current and time to determine excessive overload conditions. If either reaches the first threshold or 'warning' level, the drive will automatically reduce heat by reducing the pulse-width modulation (PWM) switching frequency. If the condition continues to rise to a second threshold level, the drive will attempt to reduce load via current limiting. If neither corrective action is successful, the drive will generate a fault.
Maintaining target speed
In a conveyor application, for example, if a heavy load suddenly drops on the conveyor, the motor will pull more current. If workers on the line do not make necessary load size or motor control adjustments, the conveyor is going to slow down. With slip compensation in the drive, the drive will recognise the extra load situation and increase its output frequency to keep the end load speed the same.
Energy created in an overdrive condition, such as a conveyor moving material from a second floor to a lower floor, makes the motor behave like a generator. In this overhauling load condition, the motor creates three-phase electricity, which is then rectified through the drive's fly-back diodes and charges the DC bus. If the electrical charges are not dissipated, they will ultimately raise the DC bus voltage too high and trip the drive. When the DC bus voltage gets to a predefined level, it then turns on a seventh IGBT acting as a brake transistor and converts the energy from electricity to heat by dissipating it into a brake resistor. If the internally mounted brake resistor does not offer enough energy dissipation, larger, externally mounted resistors or, in some cases, completely separate brake modules and resistor banks can be added.
Additionally, a bus regulation feature within some of today's drives can manage line over-voltage or power-regenerative situations to prevent nuisance tripping that interferes with continuous process control. Integrated electronic motor overload software can also protect the motor from overload and overheating. This feature adjusts the drive output frequency in order to regulate the DC bus voltage level.
Communications
With networks becoming more and more important and the option of remotely monitoring and diagnosing equipment growing in popularity, communication options within drives are becoming a necessity. Typically, users looking to install drive communications had to use a separate communications card that mounted externally to the drive, taking additional panel space. PowerFlex drives, for example, offer a dedicated internal communications option that helps users assemble highly integrated applications that link drives to the manufacturing process through open networks without taking more panel space. Network cards now mount inside the drive under the cover.
Today's drive users reap energy-saving, performance-enhancing and system-protecting benefits. In addition to regulating speed and, in some cases, torque control in diverse applications, some of today's drives contain many features that used to be purchased as add-on options, giving these motor enhancing devices more up time for greater productivity. Advancements described above are achieved in a smaller package and delivered to users with a lower price.
For further information contact Jeff Sandison, Rockwell Automation, 011 654 9700, [email protected], www.ra.rockwell.com
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