Case study: Resistor choppers slow overspeeding fans

Fans don’t normally need a braking resistor and chopper but when they are operating in a closed system, a situation may arise where one fan will over-speed the next and the DC bus voltage on their motor drives will then rise. Here John Mitchell of CP Automation, reports on the case of GA Pet Food Partners in Leyland, Lancashire.

External dynamic braking choppers are useful to drive manufacturers, panel builders and end users alike. A seventh IGBT (Isolated-Gate Bipolar Transistor) for the chopper in a six-pulse drive is one way drive manufacturers can reduce the cost of the product by rating the IGBT to a given torque and duty level. Some six-pulse off-the-shelf AC drives manufactured today have a seventh IGBT built in as standard.

The calculation for sizing the internal brake chopper is not a difficult one, but getting accurate data in the first place can be. Some brake choppers on board a drive range from 25% to 100% ratings. And let’s not forget the fact that not all drive manufacturers fit a seventh IGBT above ratings of 22kW.

To this end, where it is too expensive to change the installed drive or where regenerative energy has been overlooked, CP Automation offers a range of external brake choppers that covers 99% of application needs up to 360kW peak, 100kW continuous.

Available in nine models across three frame sizes, there are several options to set the threshold to the desired switching level, along with fault output and master-slave options to chop higher power levels.

External brake chopper units should allow for a fit and forget procedure. During emergency stops the unit will still function – because it is powered by the DC bus and not by an external supply. These were key features for a fan application at GA Pet Food Partners.

GA Pet Food Partners works with some of the biggest pet food businesses in the world. However, it doesn’t manufacture an own label product because it doesn’t believe in having an own brand. It sees itself as a partner, not a competitor, to its customers.

From concept to launch and storage to dispatch, the company claims it partners with more private label pet food brands than any other pet food manufacturer, making it easy and profitable for pet food brands to launch and grow their business.

GA Pet Food Partners operates an environmental extraction system comprised of three parallel 185kW ABB Motors spinning the Halifax fans cascading into smaller fans, and was encountering difficulty in decelerating the biggest of these units. The effect of over-speeding becomes cumulative the more fans there are in close proximity on a factory floor.

The three large fans are controlled by Allen Bradley Powerflex 753 variable speed drives (VSDs) from Rockwell Automation, which count among their features predictive diagnostics to extend the life of cooling fans.

“In an environmental extraction system, if a fan runs too fast, it either pressurises air too much at one end or creates suction at the other,” explained Jayne Whittaker, engineering director at GA Pet Food Partners.

“Some machines on the shop floor have exhaust fans, which need to run at the appropriate speed. CP Automation supplied external brake resistors and retrofitted brake choppers within the existing panels that house the Allen Bradley drives.”

Advanced extrusion

The new GA Pet Food Partners factory is home to the most technically advanced extrusion plant in the world, making some of the finest super premium dry pet foods. GA Pet Food Partners has invested in the world’s first thermal twin extruder, which allows it to include very high levels of fresh meat in premium dry pet foods without the use of dry meat meals.

With four extruders, the company has a production capacity of 100,000 tonnes of super premium pet food and can make batches from 3-200 tonnes to suit small or large brands, delivering the same quality and consistency whatever the batch size.

A reliable electrical equipment provider should have a clear understanding of the application as a whole, the machine itself and the components. This way, CP Automation is able help make the manufacturing process more efficient, more cost effective and more sustainable in every sense.

The brake chopper size conundrum

Many drives manufacturers design and build their products with a seventh Insulated-Gate Bipolar Transistor (IGBT), in the form of a brake chopper, incorporated into the device. Here, John Mitchell, global business development manager of CP Automation, looks at the dangers of taking your IGBT for granted.

Manufacturers often include a brake chopper up to a certain size of inverter, but beyond that size, they will expect customers to use an external unit. However, to reduce the cost of the drive to the end user, the seventh IGBT isn’t necessarily rated to the full power rating of the drive itself.

Most inverter manufactures will argue that the cost benefit of not rating the IGBT to the full capacity of the drive is more than worth the loss of braking capacity. Furthermore, there is an argument that this is a legitimate design decision - manufacturers will claim that a very high percentage of end user applications don’t actually the full braking capacity.

In very light duty applications, such as emergency stop applications for instance, underrating the built in brake chopper wouldn’t be a problem. In fact, anything that is up to ten per cent duty will be very unlikely to suffer any kind of issue at all.

In contrast, any kind of application where the drive system has to stop and start regularly, such as an elevator or conveyor, needs to pay more attention to the brake chopper.

The real problem is not the manufacturer’s decision to install a low capacity IGBT but the decision not to make this information very clear in the manual and the accompanying documentation.

The key point in any application is that the mechanical stopping of the load is just as important as selecting the drive system.

For instance, a fifty-metre crane that lowers at half a metre per second will clearly take 100 seconds to lower fully. It may only do that twice a day, but when it does, it is operating at full capacity - not 30 per cent or 50 per cent. From a chopper and resistor point of view, that is a continuous period that the drive's onboard brake chopper couldn’t cope with.

The drive manufacturer might argue this only represents 20 per cent duty, but that’s where the application knowledge of the integrator or maintenance team becomes crucial. During the 100 seconds of descent, the application is dumping more and more energy and the resistors are getting hotter and hotter. It could be quite dangerous to attempt this kind of application with an inadequately sized internal seventh IGBT.

The bottom line is that brake choppers should be sized for the worst case, not the cumulative case. If you're worried you might be ignoring your drive applications, get in touch with your friendly automation maintenance and repair specialist today.