Variable Speed Drive FAQ
(Frequently Asked Questions)
Joliet Technologies is dedicated to providing our customers and website visitors with quality information about our products and services. As well as helpful information you can use to make well informed decisions regarding your drive needs.
Below is a list of questions that are common to our products and services. In addition we will make every effort to provide hard to find information you can use on a daily basis. If you are looking for specific information regarding AC variable frequency drives, DC variable speed drives and/or adjustable speed drives in general and did not find it, let us know and we will do our best to find it and publish it. All questions and comments are welcome.
Q. What makes Joliet Technologies different, compared to other variable speed drive integrators?
A. As you can tell from our website, we are committed to providing as much information as possible regarding the products and services we provide. We have done our best to give you the information about the drives we use in our systems, so you can make clear and informed decisions for your particular application.
From the moment you contact us (and you can contact us in a multitude of ways, by phone, fax, e-mail, RFQ form or Ask a Pro form) you will realize that our sales staff and engineers have the knowledge and resources to provide the drive system solutions you need. We will work one on one with you to make sure all your questions are anwsered and all application issues are addressed. Our goal is to give you the best products & services at the best price, it’s that simple.
Q. What’s the difference between a non-regenerative dc drive and a regenerative dc drive?
A. The difference between these two types of DC drives is explained on this page, DC Drive Types.
Q. What’s the difference between a variable frequency drive (vfd) and a variable speed drive (vsd)?
A. A variable frequency drive (VFD) refers to AC drives only and a variable speed drive (VSD) refers to either AC Drives or DC Drives.
VFD’s vary the speed of an AC motor by varying the frequency to the motor. VSD’s referring to DC motors vary the speed by varying the voltage to the motor.
Q. What is the difference between a VSD and a Soft Start Starter?
A. The difference is the speed that the motor can run, when at full speed. With a soft start starter, the motor is reduced voltage started, and then when the motor is at full speed, or a timing circuit has timed out, a “running by-pass contactor” pulls in and the motor continues to run at full base speed. With a VSD (Variable Speed Drive) or VFD (Variable Frequency Drive) (these are the same thing, just different names for the same unit), The motor will soft start, and you can vary the speed of the motor, by varying the output frequency from the VSD or VFD.
So, if you don’t need to vary the speed of the motor, once the motor is up to speed, then the correct solution is a soft start starter for that motor. If the process requires the motor speed to vary at anytime, the correct solution is a VSD / VFD.
The cost for a soft start starter is much less than a VSD or VFD.
Q. Can a variable frequency drive replace a softstart?
A. Yes, a variable frequency drive can replace a soft start starter, since most drives give you a current limiting ramp to start and stop your motor. BUT, if you do not need to vary the production speed of your motor, then the soft start starter is still the correct product for the application. This is because the cost of the soft start starter is less than the cost of a variable frequency drive.
Q. Are the variable frequency drives phase sensitive?
A. No, variable frequency drives are not phase sensitive. You can adjust the direction of rotation of the motor, through the software of the drive.
Q. Can a variable frequency drive be used for a fan application, blower application or a pump application?
A. This is one of the best applications for an AC drive. In most fan/pump applications, there is a need to vary the output of the fan/pump. This is very easy to achieve with an AC drive. Plus you get the benefit of reduced energy cost of running the motor!!!
Suitability of VFD for pumps.
VFD’s are well suited to pumping applications. Many are manufactured to include motor speed/torque profiles specifically intended to optimize pump performance. However, there are some issues which must be considered before deciding on the use of VFD’s:
- Suitability of the motors – the motors must be rated for inverter-duty to withstand the higher electrical stresses drives can impose. Some older motors were not built to withstand these stresses and may fail prematurely.
- Distance of motors from VFD’s – the VFD’s will induce voltage transients on the load connectors. If these connectors are too long (manufacturers often suggest keeping motor lead length to 50m or less), the voltage at the motor terminals may increase to several factors above nominal, which can also cause premature motor failure.
- VFD installation location and environmental conditions – this will determine the type of enclosure and conditions under which the VFD’s will operate. VFD’s which must withstand severe moisture, temperature swings, or dust can fail prematurely and must be protected by suitably rated enclosures.
Q. How can a variable speed drive improve our production?
A. The drive can be “tuned” to the application to achieve maximum output of the equipment. Sometimes this is slightly under the base speed of the motor, and sometimes, it is over the base speed of the motor. Since every application is different, contact us through an email or the phone to discuss your application in detail and discover how we can help improve your companies’ production output.
Q. What is the percentage of operating cost reduction after installing a VFD?
A. This question is a very open-ended one which depends on many factors – load, incoming supply voltage, utility costs, and several others – and so cannot be simply answered. A good place to start customizing the inquiry for your use would be at ABB’s web site; they have tools for rough calculations for energy savings using VFD’s.
Q. Can I run a 230V, 3Ph motor with 230V, 1Ph input?
A. Yes. Joliet Technologies is pleased to announce that we have the solution to run most 230V, 3Ph motors with only 230V, 1Ph input! Call for more information.
Q. What is the best drive for my application?
A. This is not an easy question to answer. Before we can give you a comprehensive answer to this question we will need to know more about your application and specifications. Simply call (815) 725-9696, toll free (866) 492-9888, fax (815) 725-9393 or e-mail us with your application and specifications and we will recommend the drive that’s right for you.
Q. Can I convert my DC Drive application to an AC Drive?
A. Yes, You can convert your DC drive system; to an AC drive system by replacing both the motor and the drive. Consideration must be given for the torque and speed range of the DC motor, when converting to AC. You will need to consider using a “Vector” AC drive for this conversion.
Q. Will a variable frequency drive or variable speed drive save me money?
A. Yes, the AC drive can save you money on operating cost, when the load and motor speed can be varied. By reducing the speed of the motor, you will consume less energy, which will reduce your electric bill.
Q. What is the cost of a VFD?
A. Cost varies widely based on manufacturer, voltage, ampacity, enclosure type, torque requirements, quantity of analog and digital inputs/outputs, and communications options. One example: a standard wall-mountable, NEMA 1 , 3-50/60-380/480 volt, 30Hp, variable torque/normal duty, general purpose drive, with 2AI/2AO, 6DI/3DO, and Modbus RTU protocol from ABB will cost approximately US$2,700 (Oct. 2011). We also sell drives from Siemens, WEG, Control Techniques, Schneider, and others.
Q. Should I replace my old analog equipment with new digital equipment?
A. DEFINITELY!!! For so many reasons. . . difficult and costly to get spare parts, Hard to find Tech. Support / serviceability, no communications, little flexibility, etc.
Q. Will Joliet Technologies train our staff to operate and maintain our equipment?
A. We can develop a training program that is tailored to your companies’ needs and location. So, when it makes sense, we can train your personnel in our facility, or bring the classroom to your factory, for hands on training.
Q. What is a line reactor and what will it do for my application?
A. A line reactor is an impedance device. Its benefits are reduction of drive cross talk, and interference.
Input reactors will limit the waveform distortion and harmonic voltages. This will help to improve the power factor due to a reduction of the RMS currents in the system.
Output reactors are generally good up to 100 feet is distance (drive to motor). The benefits are the damping of peak voltage overshoots, which will reduce internal motor heating and lessen the “noise” that a motor can produce when it is run by a drive. On installations where the distance is greater than 100 feet, filters are recommended for motor protection.
Q. What filters are used for the VFD?
A. Line-side filters are sometimes installed to reduce EMI, although modern PWM drives are much less likely to generate significant noise.
On the load side, output chokes can be installed to reduce drive generated harmonics in the motor leads. Many drives have some output filtration already installed and harmonics are generally not a problem as long as wiring is correctly segregated, properly grounded, and the motor leads are not excessively long (typically less than 100 meters).
Q. What is the best drive manufacturer for my application?
A. This is a difficult question to answer. Our sales engineers are knowledgeable of what products are available from well-known manufacturers. They will recommend the best drive for you based on your application and specifications. There maybe several products that will meet your specifications, we will recommend the one that is best based on price and performance. Simply call (815) 725-9696, toll free (866) 492-9888, fax (815) 725-9393 or e-mail us with your application and specifications and we will recommend the drive that’s right for you.
Q. What are the maintenance requirements of a VFD?
A. Under normal operating conditions, VFD’s generally require little maintenance once properly set up. Spare parts can be obtained readily, and some are replaceable by maintenance personnel. In other cases, qualified service technicians are required.
Also see How To Maintain a VFD?
Q. Is it possible to use the same soft starter to start two MV induction motors?
A. Yes, it is possible to start multiple motors from a single soft-start. Typically, the motors are started sequentially and are identically rated. In a typical MV application, additional contactor cabinets are connected and control-tied to the soft-start. Once the first motor comes up to speed, the soft-start “Switches” to control the next motor in the sequence.
In the case of only two motors, some soft-starts are manufactured with dual settings to permit starting of motors that have different starting characteristics, but actual motor output during ramp-up may not differ much, being limited by the inherent performance characteristics of the soft-start.
Multiple simultaneous starting of motors requires that the motors be identical and that the soft-start be sized for the total load and supplied with supplemental cooling.
Q. What is exactly the difference between AC and DC capacitor in functionality and manufacturing? Why a DC capacitor is required to connect across the alternator terminals?
A. In essence, a “capacitor is a capacitor”, although there are differences in design based on topology, type, and materials of manufacture. Sometimes capacitors will carry a dual rating, for AC and DC – the AC rating for a capacitor of the same capacitance is typically much lower than its DC rating (often at least 3-4x less since AC max voltage peak-to-peak is ~2.8x rms), to accommodate the significant heating internal to the cap caused by frequency/alternating polarity. Some manufacturers manufacture AC caps which are essentially two DC caps in series back-to-back; these limit current flow through either cap but their internal resistance is somewhat high, which can lead to eventual break down due to heating as well.
Use of a DC cap across alternator terminals is often done to filter alternator whine (noise) caused by voltage ripple, the “AC” component of rectified DC output. When the capacitance is properly installed and matched to the frequency of the noise, it provides a low-impedance path to “divert” the voltage ripple to ground.
Q. From a website visitor:
What is the problem if I use a 15hp vfd for a 1hp induction motor speed control?
I set data of the 1hp motor in the 15hp vfd, is there any problem using it?
A.The issue with a drastically over-sized drive is controllability, especially at lower speeds. Accuracy will suffer, because the drive thyristor firing angle will be so low that it will be difficult for the drive software to manage. And if you are concerned about motor protection you will have very little because the normal operating ranges of the motor protection parameters (overload, over-current, etc.) in a 15hp drive are centered on 15hp motor parameters; they are ranges, of course, but you’ll be way out on the low end of these ranges and response will suffer.
Q. Does the wiring between the VFD and motor require special cabling?
Can we just use standard 600V wiring such as THHN, THWN, etc.?
We come across cables that are noted as “VFD rated”. Are these cables required for all VFD installations?”
A. Drive manufacturers typically recommend shielded, symmetrically constructed cables to reduce the impacts of electromagnetic interference and capacitive coupling. In cases where compliance with European (CE) electromagnetic compatibility is required, these types of cables, properly installed and bonded, are mandatory. Where EU compliance is not a concern, the suitability of the cable used will depend on several factors, including cable length, sensitivity of nearby components and equipment to radiated interference, motor size, and installation methods. Typically, motors rated less than 40 hp, and less than about 100′ of lead length (per phase), can be fed with 600V single conductor cable as long as the conductors are encased in metallic conduit which provides a continuous bond and which is spaced to properly segregate the load conductors from line-side and signal wiring. However, care must be taken to ensure good grounding practices are followed, a properly sized earthing conductor (grounded conductor) is used, and any sensitive equipment nearby is shielded or separated by distance. For motors 30kW/40 hp and up, and/or more than about 100′ away from the drive, using unshielded and/or asymmetrical cable may result in noise problems.
Q. It was suggested that we could operate multiple chillers at 95% max rather than 100% and not lose much efficiency. Do you have an opinion?”
A. The answer depends in part on the design of the chiller equipment and the properties of the electrical system supplying the chillers. If the pumps were sized such that 100% output would require the motor(s) to operate at 100% of base speed, then assuming the chiller pumps were centrifugals a 5% reduction in speed would result in a 14% reduction in power required. This would be offset to some degree by a decrease in motor efficiency due to increased losses in a motor fed by the non-sinusoidal supply of a VFD. The extent of decrease in motor efficiency depends on several site-specific factors, including motor design, and so can’t be generalized. One generalization can be made, however – the closer to the motor’s base speed you operate a VFD, the less likely you will be to overcome motor efficiency losses and achieve significant savings. In this case, keep in mind also that if the process requires fluctuations in speed and the motors are not required to run at 95% all of the time, additional opportunities for savings exist.
Q. How do I sell my supervisors on using training programs and emphasizing proper VFD maintenance to ensure we do not have process interruptions?”
A. First of all, common sense would seem to dictate that the expenses related to proper VFD operation and maintenance are small compared to the capital costs required for VFD purchase and installation in the first place. Once the potentially high costs of downtime are figured in, the cost/benefit ratio of good training and maintenance drops even further. Looking at equipment or production downtime data will usually quickly reveal that the associated costs far exceed the cost of maintenance. If drive failure does not immediately involve production downtime but rather in facility systems losses, then compare the cost of drive repair or replacement to maintenance and you will likely still come out ahead by taking care of what you already have.
Q. How do servo drives differ from other drives?”
A. Servo drives are typically used in applications where accuracy of positioning and speed is of critical importance. When driving servo motors and coupled with closed loop feedback devices such as encoders, they are capable of very accurate positioning and rapid adjustment to wide variations in process operating speed. An example would be a bottle labeling application in a high-speed conveying operation. Servo drives differ from “standard” variable speed drives primarily in the firmware and software included with the drive. The servo drive is equipped with control schema to allow precise position control and rapid response.
As always feel free to contact us with any questions and/or comments you may have.
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