⦁ Energy Savings with Variable Speed Drives
⦁ Variable Torque Versus Constant Torque
⦁ Why Variable Torque Loads Offer Great Energy Savings
⦁ Energy Consumption
⦁ Extended Equipment Life and Reduced Maintenance
Energy Savings with Variable Speed Drives
If you have an AC motor-driven application that does not need to be run at full speed, then you can cut down energy costs by controlling the motor with a Variable Speed Drive (VSD). Variable Speed Drives (Also known as Variable Frequency Drives) allow you to match the speed of the motor-driven equipment to the process requirement.
Variable Torque Versus Constant Torque
Variable speed drives, and the loads they are applied to, can generally be divided into two groups: constant torque and variable torque. The energy savings potential of variable torque applications is much greater than that of constant torque applications. Variable torque loads include centrifugal pumps and fans, which make up the majority of HVAC applications. Constant torque loads include vibrating conveyors, punch presses, rock crushers, machine tools, and other applications where the drive follows a constant V/Hz ratio.
Why Variable Torque Loads Offer Great Energy Savings
In variable torque applications, the torque required varies roughly with the square of the speed, and the horsepower required varies approximately with the cube of the speed, resulting in a large reduction of horsepower for even a small reduction in speed. A motor in a variable torque application will consume only 25% as much energy at 50% speed as it will at 100% speed. This is referred to as the Affinity Laws, which define the relationships between speed, flow, torque, and horsepower.
Variable Speed Drives (VSDs) allow you to consume less energy than other speed control techniques when load requirements are less than full speed, as is usually the case in HVAC applications.
Extended Equipment Life and Reduced Maintenance
Single-speed starting methods start motors abruptly, subjecting the motor to a high starting torque and to current surges that are up to 10 times the full-load current. Variable speed drives, on the other hand, gradually ramp the motor up to operating speed to lessen mechanical and electrical stress, reducing maintenance and repair costs, and extending the life of the motor and the driven equipment.
Soft starts, or reduced-voltage soft starters (RVSS), are also able to step a motor up gradually, but drives can be programmed to ramp up the motor much more gradually and smoothly, and can operate the motor at less than full speed to decrease wear and tear. Variable speed drives can also run a motor in specialized patterns to further minimize mechanical and electrical stress. For example, an S-curve pattern can be applied to a conveyor application for smoother decel/accel control, which reduces the backlash that can occur when a conveyor is accelerating or decelerating.