Variable speed drives (VSDs) are generally considered as vital contributors to energy efficiency, but they also have features that can reduce the operational disruptions caused by load shedding.
“Any industrial operation that relies on electric motors will face severe challenges to their continuous workflow when load shedding strikes,” says Pieter de Villiers, Gqeberha branch manager
for Zest WEG. De Villiers was previously the VSD Service Manager at the company. “Much of the disruption is related to the process of starting machinery up again after a power loss, which often
must be conducted as a series of sequential actions that an operator must oversee and implement.”
What many motor users are not aware of is the usefulness of WEG VSDs in automating and controlling this start-up, explains de Villiers. WEG VSDs can be programmed to initiate a sequence of
actions, so that this does not have to be done manually by the operator. A simple example to illustrate his point is where water is being pumped through pipelines which drain out during load
shedding. In most cases, the pumps cannot simply be started up again at full speed in a ‘dry’ condition without the risk of cavitation and other damage.
“In cases like this, the WEG VSD can be programmed to start the pump at a lower speed until the pipeline is full of water, after which it could resume full pumping duties,” he says. “Similarly, it is
important for a siren to be sounded on a mining operation before a conveyor system is re-activated after the start of load shedding for safety reasons. The WEG VSD can also be programmed to
automate the re-start process, and it initiates the siren to warn staff that the conveyor will be running again.”
He highlights that the full range of WEG VSDs have built-in PLC capability, allowing them to be programmed in this way – without the need for PLCs to be added to the system. Another function of VSDs that makes them valuable assets in times of load shedding is to prevent equipment from tripping out. This can happen when there is too much ‘dead time’ between grid power turning off
and a back-up generator kicking in.
“If dead time lasts more than a few seconds, many motor-driven applications can trip out and require a re-start which can be time consuming and labour intensive. A common way that the VSD
stays live during this dead time is through the stored energy in its capacitors, allowing it to then re- accelerate the motor when the generator kicks in,” he explains. “It is also possible to set up the drive to utilise the inertia from the load and therefore power from the motor itself – essentially using the motor briefly as a generator to keep the VSD alive.”
WEG VSDs also play a useful role when energy users want to harness renewable energy sources like solar power. In a hybrid power system where a motor is connected to both the grid and to solar
panels, a VSD can utilise the best source to feed the motor. If there is sufficient sunlight, the system will detect the power flowing in from the panels and optimise that power source.
“On the other hand, the VSD will switch to grid power at night or when it is cloudy,” says de Villiers. “This allows motors to be kept running while reducing the cost of energy, and also reducing the
user’s carbon footprint.”
