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Energy Savings with Variable Speed Dives
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, aka variable frequency drive). Variable speed drives allow you to match the speed of the motor-driven equipment to the process requirement. There is no other method of AC motor control that allows you to accomplish this.
Example of a Decent Variable Speed Drive Candidate*
Example of a Poor Variable Speed Drive*
Variable Torque vs. 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 with the square of the speed, and the horsepower required varies with the cube of the speed, resulting in a large reduction of horsepower for even a small reduction in speed. The motor will consume only 25% as much energy at 50% speed than it will at 100% speed. This is referred to as the Affinity Laws, which define the relationships between speed, flow, torque, and horsepower. The following diagram illustrates these relationships:
Energy Consumption
As the table below shows, 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.Speed Control Techniques
Tighter Process Control with Variable Speed Drives
No other AC motor control
method compares to variable speed drives when it comes to accurate
process control. Full-voltage (across the line) starters can only run
the motor at full speed, and soft starts and reduced voltage soft
starters can only gradually ramp the motor up to full speed, and back
down to shutdown. Variable speed drives, on the other hand, can be
programmed to run the motor at a precise speed, to stop at a precise
position, or to apply a specific amount of torque.
In fact, modern AC
variable speed drives are very close to the DC drive in terms of fast
torque response and speed accuracy. However, AC motors are much more
reliable and affordable than DC motors, making them far more prevalent.
Most drives used in
the field utilize Voltz/Hertz type control, which means they provide
open-loop operation. These drives are unable to retrieve feedback from
the process, but are sufficient for the majority of variable speed
drive applications. Many open-loop variable speed drives do offer slip
compensation though, which enables the drive to measure its output
current and estimate the difference in actual speed and the setpoint
(the programmed input value). The drive will then automatically adjust
itself towards the setpoint based on this estimation.
Most variable torque
drives have PID capability for fan and pump applications, which allows
the drive to hold the setpoint based on actual feedback from the
process, rather than relying on an estimation. A transducer or
transmitter is used to detect process variables such as pressure
levels, liquid flow rate, air flow rate, or liquid level. Then the
signal is sent to a PLC, which communicates the feedback from the
process to the drive. The variable speed drive uses this continual
feedback to adjust itself to hold the setpoint.
High levels of
accuracy for other applications can also be achieved through drives
that offer closed-loop operation. Closed-loop operation can be
accomplished with either a field-oriented vector drive, or a sensorless
vector drive. The field-oriented vector drive obtains process feedback
from an encoder, which measures and transmits to the drive the speed
and/or rate of the process, such as a conveyor, machine tool, or
extruder. The drive then adjusts itself accordingly to sustain the
programmed speed, rate, torque, and/or position.
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.