Variable frequency drives (VFDs) are widely used in industrial fan, pump and air compressor applications, which constitute a substantial portion of the electricity consumed by U.S. industrial operations. VFDs are used to get the most out of an ac motor. This month’s question is: What is the best variable frequency drive (VFD) design and installation plan considering noise, linearity, slip, reliability, rangeability and response time? The first answer is from Erik Cornelsen: When selecting a VFD for an application, I consider a few key factors to ensure optimal performance.
Understanding the relationship between torque and speed is fundamental. Applications generally fall into two categories: constant torque, like conveyors, material handling systems and hoists, or variable torque, where torque is proportional to the square of the speed, as in fans and pumps. Most VFD manufacturers provide models tailored to these categories, making it easier to choose the right one. Matching the VFD to the motor is equally important. Selecting a VFD with a power rating slightly above the motor's ensures it can handle the application without strain or performance issues.
During commissioning, I configure parameters such as motor nameplate details and acceleration/deceleration ramp times and then perform an auto-tune operation. This allows the VFD to account for actual installation factors like cable distances and fine-tune its settings for optimal performance. These are the basic steps to get the motor running, but fine-tuning is often necessary to tailor the system for the specific application.
I monitor and adjust parameters such as duty type (heavy or normal, selecting heavy for higher overload tolerance), motor control mode (e.g., vector control or V/f), maximum motor frequency, control method (e.g., Ethernet or hardwired signals), brake release configuration (if the motor has a brake) and IGBT switching frequencies.
Many of these adjustments are guided by field observations, alarms and warnings displayed by the system. The ultimate goal is to achieve smooth operation during both starting and stopping. Key parameters I monitor include motor current and motor speed to ensure the system is functioning efficiently and reliably.
Greg McMillan: Best practices
In an AC induction motor, the rotor and hence the pump shaft speed lag behind the speed of the rotating electrical field of the stator because a difference in speed is needed to provide the rotor current and consequently the torque to balance any motor losses and the load torque from pump operation. This difference in speed between the stator field and the rotor of the motor is called slip. There is a dynamic slip for large changes in the pump load (e.g., static head) or desired flow rate (speed signal).
There is also a steady-state slip for operation at a particular load and speed. It is important to note that VFD speed slip is not the same as valve stroke slip. In speed slip, the speed still responds smoothly to a change in the drive signal. At low speed, the loss in pump efficiency and an increase in slip cause a dip in flow. Slip affects the minimum controllable speed and hence the VFD rangeability, particularly for high static heads.
In a synchronous motor, the rotor is designed to inherently eliminate slip so the rotor speed is at the synchronous speed of the stator. Synchronous motors are significantly more expensive and complicated and are used only where inherent fast and precise speed regulation is needed. Synchronous motors have been used for ratio control of reactants or additives, where small transients or offsets in the speed could cause a significant variation in the product concentration. If there were no static head and no slip, and the motor and frame are properly designed to prevent overheating at low flows, the rangeability of a VFD would be impressive.
A drive with closed-loop slip control by the cascade of speed to torque control can achieve a rangeability of 80:1, which is comparable to the rangeability of a magnetic flow meter. When the pump head is operating near the static head, the minimum controllable flow is set by rapid changes in the static head and frictional loss. These rapid changes could be due to noise and sudden or large disturbances. The speed cannot be turned down below the amplitude of these fast fluctuations. The rangeability for a static head that is more than 30% of the system head at 100% speed is only 2:1, regardless of drive technology.
What is interesting is that a control valve's rangeability deteriorates for a valve pressure drop that is less than 30% of the system pressure drop. Thus, if you had a situation where the frictional losses in the piping are low, like in pH control, but the static head was high, a control valve with minimal stiction and lost motion would have much greater rangeability than a VFD. Here are some best practices for VFD design and installation:
- High-resolution input cards
- Pump head well above static head
- On-off valves for isolation
- Design B TEFC motors with class F insulation and 1.15 service factor
- Larger motor frame size
- XPLE jacketed foil/braided or armored shielded cables
- Separate trays for instrumentation and VFD cables
- Inverter chokes and isolation transformers
- Ceramic bearing insulation
- Pulse width modulated inverters
- Minimum deadband and rate limiting in the drive configuration
- Drive control strategy to meet rangeability and speed regulation requirements
- If tachometer feedback control is used, speed control should be in the drive, not the DCS
- External reset feedback (dynamic reset limit) using tachometer or inferential speed feedback to prevent PID output from changing faster than the drive can respond.
For more details, see Chapter 7 – “Effect of Valve and Variable Frequency Drive Dynamics” in my book, Tuning and Control Loop Performance, Fourth Edition . It is available as a free download . Another extensive resource is The Control Techniques Drives and Controls Handbook , edited by Bill Drury and published by The Institution of Electrical Engineers, London.
Michael Taube: Stop controllers from ‘fighting’
Both Greg and Erik have addressed aspects of VFD applications that I never considered—or would have thought of! Admittedly, my exposure to VFD application has been limited to just some pump applications and, invariably, the controls design (by others) was less than optimal: In most instances two separate controllers were implemented, one using the VFD and another manipulating a control valve to “control” the same variable (e.g., level, flow, etc.), which, of course resulted in the controllers “fighting” each other, or as the operators would say: “It just doesn’t work!”
When I’ve encountered such applications, I recommend that the VFD be used as a valve position controller that adjusts the motor speed to keep the control valve (which maintains the process variable of interest, e.g., level, flow, etc.) in some “optimal” range (nominally 60-70%). The VFD controller would be tuned to react to changes in the control valve position “slowly,” perhaps even using an error-squared algorithm, as well as a filtered value of the valve position.
There’s no need to have the VFD react to every tick, jerk or movement of the control valve. Thinking further on the topic, I have to question why have both a VFD and a control valve? Some years ago, when meeting with a prospective client about implementing pipeline automation, the client pointed out that it was far more energy efficient (meaning, lower in operational cost) to modulate the speed of the pipeline pump(s) rather than run them at constant power and then dissipate that energy across a control valve. So, if “energy efficiency” is the justification for using a VFD, then don’t bother with a control valve! And, as you point out, there is turn-down performance in addition to system hydraulics to consider if pursuing such a design.
Matthew Howard: Pulp and paper
My experience is in line with Michael’s last paragraph. We use VFDs with no valves for optimum energy efficiency. If we have a valve downstream, it is often due to the minimum pumping of the VFD being too large for the process, so the valve is used to backpressure, usually in a split-range control scheme. A “valve” position controller is better, but I would prefer to move the VFD more quickly than the valve. This is because the VFD has more precision and no stiction.
Also, my experience in pulp and paper is that there are “drives guys” and “controls guys.” My experience is limited, but drives seem to be a subset of controls that is very specialized and electrical in nature, similar to our quality control system (QCS) scanner systems. It is unlikely in my experience for a DCS manager to spec out and be an expert in VFD selection and installation.
Michel Ruel: Loop tuning with VFDs
I agree with the comments from my colleagues and would like to add my thoughts on loop tuning with VFDs. Proper VFD configuration is crucial, and a common mistake is using inappropriate parameters, particularly the current limit and acceleration/deceleration ramps. When a PID controller sends a signal to the VFD, if the change is within the configured limits, the control loop behaves as expected.
However, if the PID controller requests a large change, the VFD's limits (e.g., current limit or ramp time) will restrict its behavior. This restriction can cause the loop to appear as if it has a large time constant, which may lead to the mistaken conclusion that a higher proportional gain is needed. If the loop is tuned for large changes (which involve these limits), it will work well for those cases.
But when the process variable (PV) is close to the setpoint (SP) and the PID controller makes small adjustments, the dynamics change. The apparent time constant for these small changes becomes much smaller, which can lead to oscillations in the loop. It is common to see cycling loops with VFDs when the PV is close to the SP. The key is to focus on tuning for small changes, rather than large ones. Properly setting the VFD parameters is also essential to ensure stable loop performance.
Greg McMillan’s follow-up
As I previously noted, there is a serious decline in VFD rangeability when the static pressure is large compared to the system pressure drop. A possible option to extend the rangeability I have not tested, but was mentioned in an email to me eight years ago by ControlSoft Inc., is the option of installing a throttling valve that is normally wide open. Split-range control is used to start to throttle the valve when the VFD reaches its low-speed limit to move the intersection of the system curve with the VFD curve to a lower flow on the plot of pressure versus flow. The tuning for when the VFD speed is being modulated and when the throttle valve is being positioned is quite different, requiring scheduling of the tuning settings.
Directional move suppression offered by external reset feedback might be useful in suppressing unnecessary crossings of the split range point. Also, moving the speed control from the drive into the DCS can result in complications in coordination with speed to torque cascade control and a slower response due to DCS scan time and update rate. In the meantime, more information on variable speed drives can be found in my book , published by ISA, Essentials of Modern Measurements and Final Elements .
More advice on VFDs
ISA senior member Peter Morgan provides his insights on variable frequency drives in an extended version of this article available as a PDF resource from Automation.com . He discusses a modelling project he has just started and some of his previous and earliest experiences with pump control. He has experience, for example, using a valve directly to control flow and modulating pump speed to maintain constant differential pressure across the valve. “This and other possible alternatives for flow control using a variable speed pump, either as the only means of flow control or in combination with valve adjustment, will be the subject of a planned study by Greg McMillan and me, with an article forthcoming soon,” Morgan said.
This discussion is part of the Ask the Automation Pros series from the International Society of Automation. Find previous posts from this series on the ISA Interchange blog . Past Q&A videos are available on the ISA YouTube channel; view the playlist here . This feature originally appeared in the June/July issue of Automation.com Monthly.
