Variable Frequency Drives vs Variable Speed Drives

The use of motors in modern society is staggering.  It is estimated that motor systems alone account for as much as 47% of the world’s electricity usage.  Further, within manufacturing environments, industrial motor systems account for 70% of all electricity used. 

Intersecting this reality is the fact that as technology in the form of software, AI, cloud-based systems and other advances continues to grow, opportunities exist to improve the performance and efficiency of industrial motor systems through monitoring, automation and more precise management of the entire system.  Today, the norm is to link the control of these motors into entire ecosystems to run production lines on scale with better performance for the system overall.

Two Approaches to Motor Speed Control

To make these complex systems work and to leverage the benefits of an integrated control system, it is important for users to be able to control the most integral cog in the equipment itself – the motor.  Performing the mechanical work done by humans in the past requires control of torque, ramp up and ramped down speed, braking and many other measures managed by the control system.  But it is the ability to accurately and efficiently control speed that makes everything work.

To control motor speeds across a range of applications, designers long ago created two different approaches to manage motor speed.  One is the Variable Frequency Drive (VFD).  In a VFD, the speed of the motor is changed by changing both the voltage and frequency.  Because of this, VFDs can only be used on AC motors.  The second is the Variable Speed Drive (VSD).  Here, the VSD controls motor speed by changing only the input voltage, allowing VSDs to be sued bot for AC and DC motors.

There are other differences between the two drive’s capabilities.  And in addition to the distinction between AC and DC motors, application plays the largest role in choosing the right drive.   Here is a deeper look into the differences between VFD and VSDs.

VFD and VSD

Speed

VFDs can allow speed control at startup, motor stop and during the time of the run.  However, motor efficiency is lost via heating due to resistance (I2R), by reversing magnetic polarity of the stator (Hysteresis) and current flow in both the rotor and stator (Eddy Currents).  VSDs are used in cases where the speed of the equipment running needs to vary while maintaining a constant motor speed. 

Torque

Depending on application, a motor may require either constant torque or variable torque.  If a motor requires constant torque, torque loading is not considered a function of speed.  Even at increasing and decreasing speed the load torque will be constant.  The motor will also draw higher current at lower speeds.  Constant torque is used for application such as conveyors, hoists and mixing equipment.

In applications requiring variable torque, lower speeds call for lower load torque.  The load performance can be measured in such decrease with the load torque decreasing by a value equal to the square of the speed.  Variable torque is used in applications such as centrifugal pumps and fans.

VFDs are associated with variable torque applications.  Because the torque measures the amount of work done, in equipment where the load torque is different for different speeds, VFDs can adjust the torque to prevent damage or injury to the machine or operator using the motor.

VSDs are generally used for applications with constant torque.  In the example of a conveyor, the speed of the conveyor may be changed, but the torque on the system can remain constant, unaffected by the changing speed without damaging the equipment or user.

Horsepower

VFDs and VSDs are different in how the use horsepower.  VFDs are used in situations where constant horsepower loads are required.  These applications include machines such as grinders and lathes. In constant horsepower loads, higher torque is required at low speeds while lower torque can be used during high-speed operation.  This requires that the horsepower generated remain constant, thus justifying the use of the VFD.

In situations where constant horsepower is not required, VFDs can control HP in relation to speed through a change in frequency.  For example, at one frequency, torque can be maintained at 100% while HP is reduced.  However, changing the frequency above 60Hertz can reduce torque while leave HP at 100%. 

Compare the two use applications above with a fundamental fact of a VSD.  In a VSD, torque is independent of machine speed.  This means that horsepower is linearly related to RPMs.  Increasing RPMs increases the HP while decreasing the RPMs decreases HP.

Energy Efficiency

The efficiency of a motor and its impact on the type of system chosen will depend again on what the application requires.  In general, VFDs are more energy efficient and result in more energy saved than VSDs.  VFDs can connect to more than one motor.  This allows them to control other elements in a system such as a pump system or an HVAC.  By controlling motors on the elements of the system such as valves, actuators, dampers, etc., the load on the motors is optimized and requires less energy.

Another way VFDs save energy over VSDs lies in their ability to bring system component motors up to speed gradually.  This places less strain on all the motors in the system and reduces energy in ramp ups or accelerations.  Generally, VFDs can achieve efficiencies as high as 97% under full load.  This is somewhat related to HP as the HP of a motor drops.  Those over 10HP can still achieve as high as 90% efficiency while lower HP motors will degrade significantly based on size and load.

In general, with the advent of smart motor control, using a VFD for control of numerous motors is a more efficient solution.  Other energy saving aspects of VFDs over VSDs include regenerative enhancements to put power back in the grid and the fact that power usage in a system using a VFD uses only the energy required by the demand of the driven equipment.

Process Control

Variable speed drives control speed independent of torque.  As a result, the ability to affect change on the system in which they operate is limited to the speed adjustment for that motor.  VFDs have several advantages over VSDs in optimizing process control to improve efficiency and overall operational excellence.  These benefits include:

  • System Optimization – Because a VFD is linked to the entire system, it can send and receive data on components of the system.  It can also send instructions and control behavior of those elements as well.  As mentioned previously, a VFD can control dampers, actuators and a host of other machine components and other motors to always set the system to optimum performance.
  • Communication – Because VFDs can use common communication protocols, they can collect and analyze data that allows them to change operating parameters over a programmed set of responses.  This may include things such as slowing or stopping the equipment for safety reasons, to prevent damage, or to identify the exact location of a break of fault and send an alert to the correct technician.
  • Connectivity and IIoT – As the concept of a connected factory grows through improvements in technology, additional software can be added to optimize for vibration reduction or to identify maintenance issues before they arise.  These and many other benefits make VFDs a stronger choice over simple VSDs in process control.

Types of VFD

Pulse Width Modulation

Pulse Width Modulation (PWM) VFDs are the most common today.  They have a versatile range that can be used from as low as one half HP all the way to 500 HP.  PWMs also transmits the least amount of harmonics into the power source.  The Hz range can cover from 2Hz on the low end up to 400Hz and they are easily adapted to most power services at 230V or 460V.

PWMs operate the highest efficiencies compared to other types and range from 92%-96% efficient.  They are also lower in cost and can work in multi-motor systems.  PWM types may have some heat issues depending on use and location.  And they do not have onboard regenerative capability.

Current Source Inverter

Current Source Inverter (CSI) VFDs are used where system requirements dictate that current should be regulated.  They regulate power as it comes in to produce a variable DC bus voltage.  CSI VFDs are the only type of VFD with regenerative capabilities and are preferred over VSIs. 

The ability to regulate current renders CSIs extremely reliable.  They are often used when a drive system requires high power but where dynamic response is not as important or is not required.  On the downside, they have higher resistance and can be unstable at low speeds.  They may also encounter high torque ripple.

Voltage Source Inverter

Voltage Source Inverters (VSI) also regulate DC voltage but differs on inverter output.  The VSI uses a six-step output.  It is used as a voltage regulator and can be used in conjunction with a PWM.  The design of a VSI is simple and they can be operated independent of load.  They are also able to be used in multi-motor situations.  Negatives include power harmonic feedback to the power source and poor input power. 

Types of Variable Speed Drives

While their design and purpose are simpler, VSDs are versatile in their ability to control the speed in both AC and DC motors. They are excellent for standalone applications as they consist primarily of the motor and a control unit. 

DC Motor Drives

In a DC motor drive, speed is controlled by relationships within the armature voltage.  There is a direct proportional relationship between the motor and the armature voltage and an inverse relationship between motor and motor flux.  Because of the commonality of AC power, DC motors are being used less and less and have therefore become more expensive.

AC Motor Drives

AC motor drives are less expensive than DC motor drives.  They can be standalone, operating in control of a single motor where torque is not related to speed.  However, even in those cases, if the motor is part of a system, it can be added to a VFD system and subject to control of the VFD as a system component.

Eddy Current Drives

This special type of VSD uses a motor running at a fixed speed with a clutch that utilizes naturally generated eddy currents from the motor.  This field of eddy currents can be used in conjunction with a field coil to create a magnetic field.  This helps determine the amount of torque needed on the rotors and by closing the clutch within the field, torque can be transmitted to control speed.  Eddy current controllers have an internal tach to monitor speed.  Eddy current clutch drives are nearing obsolescence.  Having been an important part of machine automation and control in the late 20th century, they are rapidly being replaced by AC VSDs or brought into a control system controlled by a VFD.


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