Perhaps the first thing that springs to mind when we think of VSDs is their ability to rein in energy use by matching the speed of the process to the demand. Pumps and fans are the classic example here, which food production plants make widespread use of in tasks such as heating, cooling, refrigeration and the pumping of liquids.
If you use a VSD to reduce the speed of a conveyor belt from 100 percent to 80 percent, then you save 20 percent of the energy. But if you do the same with a pump or a fan, then you save about 50 percent of the energy. How is this possible?
The answer is that a conveyor belt is a constant torque application, where the torque requirement throughout the speed range is the same. The current to produce torque is relatively constant and just the voltage, and so the power, varies in proportion to the speed. All of which means that 80 percent speed requires 80 percent power.
By contrast, centrifugal fans or pumps are variable torque applications. The requirement for torque (and hence current) increases with the square of the speed. The voltage again varies in proportion to the speed, so power actually varies in proportion to the cube of the speed. Hence, by reducing the speed by a certain percentage, the power reduces by the cube of the speed change.
This means that operating at 80 percent speed results in (0.8)3 = 51 percent power. So, if flow is reduced by a third by slowing down the pump, we reduce power consumption to 35 percent of the initial figure - a 70 percent reduction in power used. Such savings are of immense benefit in the highly competitive food and beverage industry, where margins are squeezed by the large retailers on behalf of increasingly cost-conscious consumers.
Freeze refrigeration costs
Refrigeration is one of the major costs of any food producer. Since refrigeration systems typically run 24 hours, most fan and pump applications are potential candidates for VSDs. Essentially any refrigeration system with a wide variation in operating hours or with a heat load that is less than the peak load can benefit from a VSD.
Many refrigeration systems which use pumps, fans and compressors are still using inefficient on/off or two speed controls for the condenser fans. Significant savings can be made by using a VSD to directly control the speed of the electric motor. Where drive belts are used on the fan systems, the VSD reduces belt stress and maintenance costs.
An example here is Ginsters bakery, which asked ABB to carry out an energy appraisal at its plant. The appraisal focused on the refrigeration system, in which chilled water pumps ran constantly with a full unrestricted flow. These pumps provide chilled water to the cold storage and production areas of the plant.
The power consumption of the pumps was measured over 24 hours and analysis showed that using a VSD to achieve a speed reduction of only six percent would reduce energy consumption, and therefore, operational costs by some 17 percent.
A trial installation of an ABB drive showed improved stability of the system and again reduced power consumption. Following this, permanent ABB drives were installed on all pumps.
VSD displays can now present an array of information, for example, showing exactly how much energy they are using on the drive’s screen. These measurements can help ensure that a plant or process is working to its optimum performance, while avoiding the extra cost of external energy measuring equipment.
Not just energy saving
Although VSDs are mostly thought of as contributing to energy saving, many food and beverage companies are using them to improve the performance of their processes, saving material, achieving greater control of conveyor systems and contributing to safe operations.
Because they control a process, VSDs can readily contribute to safe working in the food and beverage industries. By controlling the speed of motors and the fans, pumps and conveyors driven by those motors, VSDs can ensure that an oven is operating at the right temperature or that a conveyor is moving at the right speed. In processes such as milling, VSDs can be used to run the fans that extract the dust produced, preventing the risk of explosions while also reducing the risk of respiratory complaints caused by air borne particles. They can also help pumps transport liquids safely by maintaining a safe pressure in a pipe.
Within the drive itself, increasing use of software is leading to more intelligent VSDs that can replace many of the functions of traditional PLCs, timers and counters, reducing the complexity of control systems and cutting costs. Some drives include a PID controller, eliminating the need for an external PID controller for flow or pressure control.
An example of VSDs improving the productivity of an application was a project for tomato grower John Baarda, which has used VSDs to double the packing rate of a tomato handling machine.
Using ABB machinery drives, the speed of the tomato handling conveyors are controlled to match the speed of the packing machine and ensuring that the tomatoes are packed quickly and accurately. Another ABB machinery drive runs rollers on the conveyors, allowing the tomatoes to be turned automatically and inspected for quality.
The two feeder conveyors are each driven by an ABB machinery drive in master-slave configuration, with the master receiving an encoder signal from the wrapper. This ensures that the drive knows where the wrapper is in its cycle and can control the speed of the conveyor precisely to ensure the tomatoes arrive at the wrapper at the correct time. If the speed of the wrapper changes, the drive can alter the speed of the conveyor accordingly to maintain the correct timing.
The new system has allowed far higher production rates than with previous mechanical systems, giving an average of 70 to 80 packs per minute compared to around 60 packs per minute.
From keeping energy costs down to maintaining competiveness to aiding safe working and giving better, more efficient production, VSDs have a wealth of uses. For food and beverage plants, variable-speed drives have much to offer.