Table of Contents
- Load Type for Variable Frequency Drives
- Motor information
- Supply Power
- Control Methods for Variable Frequency Drives
- Speed Reference Alternatives
- Efficiency and Power Factor
- Protection and Power Quality
Load Type for Variable Frequency Drives
Does your variable frequency drive application require a variable torque or constant torque drive?
if the equipment being driven is centrifugal, such as a fan or pump, then a variable torque drive will be more appropriate.
Energy savings are usually the primary motivation for installing variable frequency drives for centrifugal applications, and variable torque drives offer the greatest energy savings.
For example, a fan needs less torque when running at 50% speed than it does when running at full speed. Variable torque operation allows the motor to apply only the torque needed, which results in reduced energy consumption. Conveyors, positive displacement pumps, punch presses, extruders, and other similar type applications require constant level of torque at all speeds. in which case, constant torque variable frequency drives would be more appropriate for the job.
A constant torque drive should have an overload current capacity of 150% or more for one minute. Variable torque variable frequency drives need only an overload current capacity of 120% for one minute since centrifugal applications rarely exceed the rated current. if tight process control is needed, then you may need to utilize a sensorless vector, or flux vector variable frequency drive, which allow a high level of accuracy in controlling speed, torque, and positioning.
The following motor information will be needed to select the proper variable frequency drive:
if continuous operation is a must, then the following should be specified:
- 0% voltage for 1 cycle
- 60% voltage for 10 cycles
- 87% voltage continuous
if you need to supply a 3-phase drive with single-phase power, then the drive must be derated by 25% to 50% of its current-handling ability, which may require you to specify a larger-sized drive.
Control Methods for Variable Frequency Drives
With 2-wire control, only one switch is used to run the variable frequency drive. An open switch stops the drive, and a closed switch starts the drive. Two-wire control is predominately used in HVAC applications since t is able to maintain the RUN command to the drive during a loss of power, which enables variable frequency drives to automatically restart when power is restored. Plus, 2-wire control allows drives that have "power loss ride-through" to operate during a power drop that is 2 seconds or less in duration.
With 3-wire control, two switches are used to run the drive. One switch is needed to stop, and another to start the variable frequency drive. This allows an auxiliary contact from the start to "seal in" the RUN command, just like your more conventional motor starters.
Speed Reference Alternatives
Efficiency and Power Factor
A drive should have an efficiency rating of 95% or better at full load. Variable frequency drives should also offer a true system power factor of 95% or better across the operational speed range, to avoid penalties from the power utility, save on energy bills, and to protect you equipment (especially motors).
Protection and Power Quality
Current Distortion Limits for General Distribution Systems
(120V through 69,000V)
Maximum Harmonic Current Distortion in Percent of iL
|individual Harmonic Order (Odd Harmonics)|
|< 11||11 < h < 17||17 < h < 23||23 < h < 35||35 < h||TDD|
|20 < 50||7.0||3.5||2.5||1.0||0.5||8.0|
|50 < 100||10.0||4.5||4.0||1.5||0.7||12.0|
|100 < 1000||12.0||5.5||5.0||2.0||1.0||15.0|
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