We build some pretty basic machines that have only limited need for variable speed control of 240/480 Vac motors from 5 to 20 hp. That's going to change as we expand the machines' capabilities and design them to integrate into systems. We're arguing about whether to switch from full-voltage starters to either VFDs or soft starters. We think we could save energy costs with VFDs because we sometimes run at faster speeds than we need to, but that's not a big deal at the moment. Soft starts would clearly be easier on the system at startup. Cost can't be ignored.
—from October '09 Control Design
Power Vs. Speed for Fans and Centrifugal Pumps
This machine builder has touched on a subject near to my heart and the basic mission of Vacon. That is the basic value statement of drives and why we say that every ac motor deserves a Vacon drive.
It is quite true that ac drives can save a tremendous amount of energy. That's particularly true when you reduce the speed on fans and centrifugal pumps. There, the relationship between speed change and power consumption is the most opportune for saving money. However, in all applications, the motor's energy consumption is at least related to rotational speed in a linear fashion.
The efficiency of every industrial process can be positively influenced by the variable speed capability of drives, some more so than others. Savings in material waste and the optimum use of an operator's skill set is the primary advantage here.
An electric motor starts very abruptly when powered from full voltage, often causing excessive wear and shortened life for mechanical components. This also is true when mechanical braking is employed. The ramp controls of the Vacon ac drive provide a subtle start and stop and extends the life of these mechanical components, saving maintenance time and money.
Whether you reverse the motor in your application or not, you still have needs to switch the motor on and off. All other motor-starting mechanisms, other than Vacon drives, have moving parts that wear and fail. Drives result in additional savings, particularly in applications where there's lots of starting and reversing, since this is where mechanical means wear out so quickly.
Finally, I don't know which market this builder appeals to, but the phase conversion capabilities of Vacon drives can extend the use of the machine to other markets. Vacon drives can allow three phase motors to be used effectively from single phase power. This can open residential and agricultural markets, where three phase power is not always available.
Tim Park, product marketing manager,
Full Vs. Reduced Voltage
With that said, there are a few things to consider when applying soft starters and VFDs.
An inverter-duty motor is required for VFDs, while any motor will do for a soft starter. This allows the soft starter to be a direct replacement for full-voltage starters and any reduced voltage starters.
Soft starters are less complicated to program and connect, compared to VFDs.
The soft starter almost always will be less expensive than the VFD.
The VFD converts ac to dc and then dc back to ac to control the frequency that controls the speed of the motor. This can lead to harmonic distortion (interference) in the power line.
The soft starter uses thyristors to control the voltage used to start and stop the motor, resulting in no harmonic distortion.
Unlike VFDs, soft starters can be bypassed while running to reduce heat.
The bottom line: If speed regulation is not required, a soft starter would be the best method of reduced voltage starting.
Lorenzo Di Maso, business development area manager, low voltage soft starters,
Weigh the Advantages
Soft start controllers are able to smoothly ramp up the motor and therefore able to reduce mechanical and electro-dynamic stresses in the system.
From both a mechanical and electrical standpoint, there are a variety of advantages to employing soft starts, rather than across-the-line starters. Overall, soft starting reduces downtime and lowers costs. Further, soft starting is able to:
- Reduce in-rush current, which decreases the peak demand and avoids utility company penalty
- Increase motor life by reducing the stress on the motor windings
- Minimize squealing and snapping of belts, which can extend belt life two to six times that of a traditional across-the-line starter
- Reduce or eliminate shock wave generation and propagation through the piping systems (also commonly known as the water-hammer effect), and therefore preventing damage to the joints in pumping systems
- Tune start performance to the specific application and load
- Minimize voltage dips at the main by reducing current peaks, which are present in across-the-line starting.
Additionally, advanced features can be embedded into the soft starts:
- Soft stop
- Pump control
- Extensive monitoring and protection
- Fault history logs
- Communications capability.
By increasing system life, reducing downtime and lowering maintenance costs, soft starters can quickly recoup the initial cost outlay.
Variable-frequency drives provide similar advantages to soft starters. When variable speed control is required, then the drive is the most appropriate controller. This also means that when rated RPM is not needed, the drive can provide tangible energy efficiencies. Further, variable frequency drives can provide higher initial torque.
When variable speed and high torque are not essential, soft starts may be a better option, compared to drives, because:
- Soft starters are smaller in size and do not typically necessitate a change in enclosure sizes or additional assemblies
- They are less expensive
- Heat losses are less
Ram Tenneti, product manager, soft starters and machinery drives,
When building a machine that has a basic version and an integrated version, it's better to have a common approach to hardware. More specifically, using VFDs for both makes sense from the design, build, support and operation of the machine. The basic version could use terminal controls; any speed setpoint changes must be made in the drive. The integrated units could use a fieldbus or gateway to enable full control from an upper-level controller. The integrated unit will require less wiring and PLC hardware than the terminal-controlled units, simplifying the electrical drawings, building and controls.
Energy costs for the system can be 90% of the follow–up costs for the machine. This should be considered and can be a selling feature for the OEM.
Energy savings will be found with VFDs running at speeds slower than 60 Hz. Using a drive's energy saving mode dynamically adjusts the motor magnetizing current for optimal efficiency in all operating speeds. Selecting the correct gearbox and mounting direct on the shaft, avoiding belts and chains, also will add to the efficiency and lower energy costs of the system.
Finally, costs can be kept to minimum by using the same VFD for the basic and integrated machine, adding the fieldbus/gateway. This keeps parts to a minimum and simplifies support by service personnel. While a VFD does cost more than the soft start, the features outweigh the differences. For example, with a VFD, you can see the actual load of the motor, both total current and the active current doing the work. This can be used to ensure the machine has been assembled correctly and is functional when compared to a baseline.
Stephen Sproule, motion control product engineer,
SEW Eurodrive, www.seweurodrive.com
Similar, but Different
There are usually several choices for starting motors. Two of these, ac variable-frequency drives and soft starters, seem to have similar characteristics. Terms and descriptions used in product literature are nearly the same. Even the list of possible applications is similar. However the technology and performance are significantly different. When these differences are understood, it becomes clear when and where to properly apply each of them.
In general, A solid-state, reduced-voltage starter (SSRVS) is used to get motors up to speed in a smooth transition. A smooth transition saves on energy demand and wear and tear on connected equipment. A VFD controls speed. If speed is needed for the application, then an SSRVS will not do the job, but if getting up to speed smoothly is the issue, then a SSRVS probably will cost less than a VFD.
Most manufacturers will not commit to exactly how much savings will be released as result of VFD or SSRVS application, compared to across-the-line starting, but clearly there is a savings.
Mike Flanagan, national marketing manager,
Sprecher + Schuh USA division, Rockwell Automation, www.sprecherschuh.com
Electric motors drive most production output and consume the most electricity in the plant, so improved motor-control performance and efficiency means greater overall production efficiency. Fortunately, today's advanced motor-management solutions can yield big results. Power-optimization tools such as variable frequency drives, energy-efficient motors and gears, motor controllers and software can deliver immediate, measurable bottom-line savings.
Variable speed drives can reduce the energy used in manufacturing processes significantly, particularly those that involve fans or pumps with changing flow rates. For example, using variable-frequency drives to lower speed or flow by just 20% might reduce energy use by 50%.
In any manufacturing process that requires less than 100% of the designed speed, manufacturers should consider integrating variable-frequency drives for both low- and medium-voltage applications. They can significantly reduce energy costs and when properly applied help eliminate valves, increase pump-seal life, reduce power surge during startup and contribute to a more flexible operation.
Potential energy savings offered by a VFD aside, from an application point of view, how do you choose between VFDs and soft starters? While the product descriptions and list of possible applications are similar between these two, some clear distinctions can be made.
The VFD works on the principle that the ac line voltage is converted to a dc voltage. This dc voltage is then inverted back to a pulsed dc whose RMS value simulates an ac voltage. The output frequency of this ac voltage normally varies from 0 up to the ac input line frequency. On certain applications the frequency actually could go above the line frequency. The most common VFDs manufactured today work using pulse-width modulation to create the output sine wave. The conducting components used in drives are diodes, SCRs, transistors and IGBTs.
The soft starter operates on a different premise. This principle is that by adjusting the voltage applied to the motor during starting, the current and torque characteristics can be limited and controlled. By using six SCRs in a back-to-back configuration, the soft starter is able to regulate the voltage applied to the motor during starting from 0 V up to line voltage. Unlike the VFD, line frequency is always applied to the motor. Only the voltage changes.
Motor speed is a parameter for which a VFD has an advantage over soft starters. First, and most obvious, is where the speed of the motor needs to be varied from 0 to line frequency and sometimes higher than line frequency. The soft starter applies line voltage and frequency and therefore the operating speed is fixed.
The second speed-related advantage is an inverter relates to processes that require a constant speed. If a fixed frequency is applied to a motor, the actual speed of that motor is not precisely regulated by the input frequency. The load applied to the motor regulates the output speed. So, if a process requires very tight speed regulation, the frequency applied to the motor must be changed in relation to the load that is applied. With the use of feedback to the VFD, this can be accomplished. Again, the soft starter only applies line frequency, so speed regulation is not possible.
On applications for which acceleration time needs to be consistent, an inverter should be used. Acceleration time for a soft starter is more dependent on the load than the selected ramp time. If acceleration time is not an issue and controlling the torque or current is what is needed, a soft starter is a good candidate.
If limiting current is the prime reason for not starting at full voltage, the first method to be considered today is usually soft starters. This is due to the cost differential between a soft starter and a VFD at the Ampere ratings that current limiting becomes a factor. In most instances, the soft starter is an appropriate choice.
There are applications where the additional cost of an inverter is appropriate, for example, if the motor cannot provide sufficient torque to start the load with the current limitations imposed by the distribution system. Unlike soft starters, drives can accelerate a motor to full speed at full load torque with line current that does not exceed the full load amps of the motor.
If starting torque is a concern when selecting a drive or starter, keep in mind the drastic difference in the amount of torque that can be developed for a given amount of line current. The drive has a much higher torque per amp ratio.
Here are some examples.
In Application 1, a pump is being started on full voltage. There is significant water hammer and the pipe bracing needs constant maintenance. Answer: A soft starter will fit the application. It provides controlled torque during acceleration and can minimize and in many cases eliminate water hammer. There is no concern about current limitations as the application is now being started on full voltage.
In Application 2, a new irrigation pump is being installed in a rural location. Because of this, the maximum current draw from the utility line without significant voltage drop has been calculated as 200% of the motor nameplate reading. Answer: An inverter is preferred over a soft starter. In some instances, soft starters can accelerate pumps with as little as 200% current. Application experience indicates that more often 250-300% current is required. The VFD can provide the torque required to accelerate the pump within the current limit restrictions of the distribution system.
In Application 3, an overland conveyor requires 100% torque to accelerate when starting fully loaded. The maximum current draw from the utility is limited to 500% of the motor full load amps. The conveyor will normally be started unloaded; however, on occasion it may need to be started when it is loaded. Rate of acceleration is critical to prevent the conveyor belt from being damaged. Answer: Initially a soft starter seems to be the correct choice. The soft starter can provide 101% torque with 450% current. However, the rate of acceleration, which equates to starting time is critical. The load also varies from unloaded to fully loaded. In this case, a VFD would be the correct solution.
In Application 4, a 20 hp motor drives an overhead plastic chain conveyor through a gearbox. It starts and stops frequently. Full voltage starting could be used but if the conveyor starts too quickly the product will swing and may be damaged or the chain may break. Answer: A soft starter would fit the application. There is no time constraint and no limitation on current. Ramp start would typically be used to allow for minor load variations reflected back to the motor. If the gear reduction is high enough, a current limit start could provide a smoother start.
Sharon James, application engineer,
Rockwell Automation, www.rockwellautomation.com
Questions Beget Questions
There are many things to consider when trying to determine if soft starters or VFDs provide a better solution than full-voltage starters for a given application. Both can provide significant savings when used in the right application. These savings can be realized through reduced energy usage and through reduced wear and tear on the mechanical system. It's important to evaluate if return on investment, as a result of these savings, is acceptable to offset the initial cost of using a VFD or soft starter (listed in order of initial hardware cost from lowest to highest: full-voltage starter, soft starter, VFD). There are a few simple questions, that when answered, can help determine which solution would work best.
Will the motors be run at different speeds or at a constant speed? If different speeds or variable speed capability is required, a VFD is the way to go. VFDs enable a motor when the cooling method is specified correctly to be run over a wide speed range. If required by the application, speeds other than base speed can be maintained continuously. This could significantly enhance the machines capabilities.
Based on the problem description above, it appears the base speed operation is acceptable and variable speed operation is not required. As a result, additional questions must be asked before the right solution can be chosen.
Is the load a constant torque or variable torque load? Variable torque loads, such as fans and pumps, can offer excellent opportunities for energy savings with a VFD. Due to the effinity laws (power being proportion to the cube of the speed), even a small decrease in speed can result in significant energy savings. Many free software tools are available to help calculate the savings that can be realized by using VFDs in these applications. Constant torque applications such as conveyors can be a bit more challenging. For these types of applications, additional questions need to be asked.
Are the motors operated close to their rated power? If the motors are being run close to their power rating, a full-voltage starter can be a pretty efficient solution. This is particularly true of motors over 15-20 hp, the larger the better. In the case described above, we are at or below this level, so there could be efficiency gains with either a soft starter or VFD.
Another way to look at this is by looking at the resultant power factor of the solution. Both VFDs and soft starters have ways to improve the solutions power factor over a full-voltage starter. In fact, VFDs do a good job of improving power factor over the entire speed range, primarily displacement power factor.
Will the motor be stopped and started many times in an hour, shift or day? If the machine will be started and stopped several times per hour, shift or even day, a soft start or VFD could provide energy savings. When an ac induction motor is started at full voltage, the inrush current can be as high as 10 times the rated motor current, found on the motor nameplate. A soft starter or VFD can be used to limit this starting current. In the problem above, the motors are relatively small, but, in volume and if several motors are started simultaneously, this could account for a significant amount of energy—maybe even enough to impact a customer's peak demand charges.
Also, ramping the motor up to speed can significantly reduce the wear and tear on the machine's mechanical parts. Although this does not translate into energy savings, this can result in savings in other areas such as machine maintenance, reduced downtime and overall equipment longevity.
Ramp (acceleration/deceleration) times and how much load is present when starting the motor can help determine if a VFD or soft starter is a better fit. If very long ramp times are required or if certain speeds need to be avoided during acceleration and deceleration, due to mechanical resonances, a VFD may be the better choice. Additionally, if there is a large load on the motor when starting, VFDs can provide more options with better energy characteristics for these cases.
Corey Morton, engineering manager
B&R Industrial Automation, www.br-automation.com