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VFDs: Frequently Asked Questions

  • January 19, 2007
  • Feature

Summary

Q: What is sensorless vector control?A: Sensorless vector control is a technique used in variable frequency drives to rotate the force vector in the motor without the use of a shaft position sensor. The goal of AC sensorless vector technology is to give the user "DC-like" control while making traditional speed or shaft position feedback from the motor unnecessary. Sensorless vector control removes a major source of complexity and potential for failure, such as tachometer feedback, while simplifying many AC drive installations. The sensorless vector algorithm must be tuned to match the characteristics of the particular motor being controlled by the drive. This improves torque performance at very low speeds as compared to typical volts per hertz control.Q: Does a motor need a chassis ground connection?A: Yes, for several reasons. Chassis grounding is used for protection in the event of a short in the motor that puts a live voltage on its housing. Motors and other components exhibit leakage currents that increase with aging and a grounded chassis generally emits less electrical noise than an ungrounded one.Q: What type of motor is compatible with inverters?A: Inverter motor type must be a three phase AC induction motor. Preferably, you should use an inverter-grade motor that has 800V insulation for 200V class inverters, or 1600V insulation for 400V class inverters. For motor size, in practice it is much better to find the right size motor for your application; then look for an inverter to match the motor.Q: How will I know if my application will require resistive (dynamic) braking?A: For new applications it may be difficult to tell before you actually test a motor/drive solution. In general, some applications can rely on system losses such as friction to serve as the decelerating force, or otherwise can tolerate a long decel time. These applications will not need dynamic braking. However, applications with a combination of a high-inertia load and a required short decel time will need dynamic braking. This is a physics question that may be answered either empirically or through extensive calculations.Q: What is ringing and what happens to the motor when I place it more than 50 feet away from the drive without a line reactor?A: High voltage ringing (reflective voltage) occurs on all VFDs. With IGBTs replacing SCRs on smaller, more inexpensive drives, ringing has become more pronounced. This capacitive-coupling effect is caused by high speed switching (commutation). Distance greatly enhances the effect. Cables on the output side of the drive act like capacitors. When the IGBTs switch or produce the PWM output, the higher frequency part of the current will find a path through that capacitance. Think of the cable as a capacitor that increases in size as the cable length increases. Ringing is very detrimental to motors; it weakens the first stage rotor windings and shortens the life expectancy of a motor. This is especially true for motors under 10 hp. Typically they are machine wound, have thinner coats of varnish, and have neither phase nor end paper (i.e., aggregate insulation properties). That’s why we always recommend a drive rated motor. Vintage and non-drive rated motors that are used with VFDs have a lifespan of unknown length. You simply cannot know what to expect from them. Long cable runs with ringing can add 10 to 15% to the drive’s current rating, causing the drive to trip out on excessive current. The capacitance will also cause a voltage drop that may cause speed performance problems on the lower end of the speed scale (increased current demand for demanded torque). A few general guidelines: 1. Try to reduce distances as much as possible2. Use an output line reactor on cabled distances greater than 50 feet3. Always use a drive-rated motor.Q: Is there a standard for inverter rated motors?A: NEMA MG-1, Section IV, Parts 30 and 31. Part 30 pertains to fixed supply motors; part 31 pertains to inverter supply motors. Always talk to your motor manufacturer for additional details. Note: the international standard is IEC 60034-17.Q: What drive power cable do you recommend and why?A: Belden 29500 – 29507 cables are specifically designed for use with VFDs. Using the wrong cable can increase the detrimental effects from ringing due to capacitive coupling. Thermoplastic insulation, found in standard THHN cable, will break down over time if used to connect inverters to motors. The standing waves caused by pulse width modulation (PWM) will cause high voltage potential on single conductors. This renders corona (reflective/ringing voltage) in the air gaps between the conductors, which could break down and cause a system failure.Q: Can a drive replace a softstart?A: Yes. Softstarts are used for reduced torque starting and stopping of standard 3-phase induction motors (such as centrifugal pumps, compressors, ball mills, jar mills, fans, blowers and saws). If the motor is not adequately protected from sudden changes in rotational torque associated with starting and stopping, the current will be excessive to the system and the motor will surge, causing damage to all the equipment linked to it. Over the long term, this leads to increased over-current tripping and increased mechanical wear of gearboxes, clutches, transmission and conveyor systems. ANSI/IEEE Std. 141-1993 (Red Book) provides a comparison of different reduced voltage starting methods. An electronic softstart of VFD, although not listed, would have similar characteristics to the autotransformer starter.Q: Can I run my drive at extremely low speeds?A: Three major problems exist with both conventional methods of motor control and VFD control: 1. When a motor is directly coupled and run at a low speed, it becomes very inefficient. You could go with a high torque motor that is significantly larger than the standard and would have the same results most of the time. 2. Typical motors do not cool themselves well at extremely low speeds. The added heat buildup in the windings can cause premature motor failure. Check with the motor manufacturer for more details. 3. The third issue is load inertia, which is larger than that of the motor. The effects of all of these things increase greatly with a sloppy transmission system. A speed reducer or gearbox should be employed to solve this problem. It will match the inertial changes and increase efficiency. Speak with a technical resource about sizing the gearbox to meet your application needs.Q: Are variable frequency drives phase sensitive?A: The input wiring is not sensitive to phase. If you were to change one set of input leads, the rotation would not change. The opposite is true for the output of the drive. It is sensitive to phase, so changing one set of leads to the motor changes the direction of rotation.Q: What is a line reactor and what will it do for my application?A: A line reactor is a special type of inductor used on the line side or the motor side of a drive application depending on the specific circumstances. They are used on the line side of the drive to smooth inrush current, reduce noise, and to act as a buffer and protect the drive system. The line reactors have a 3% voltage drop based on impedance. This drop can be beneficial for drives on systems exceeding the rated input voltage of the drive. In this regard, the line reactor has been used to replace the much larger drive isolation transformer. Line reactors are used on the motor side of the drive to protect the motor by smoothing the drive output waveforms and by reducing ringing and capacitive coupling, especially with long cable runs between the drive and the motor.Q: What is grounding and what reference material is available?A: Equipment or conductor-enclosure ground refers to connecting the non-current-carrying metal parts of the wiring system or equipment to ground. This is done so the metal parts which a person might come into contact with are at or near ground potential. The grounding of motors is referenced in NEC article 430 part M and methods are described in article NEC 250.Q: What are some of the common reasons why motors fail?A: The EPRI Power Industry Study by General Electric in 1985 offered the following causes based on 6,000 utility motor failures: 41% were bearing related, 37% were stator related, 10% were rotor related and 12% were other causes. Other institutions like DOD and the IEEE came up with similar results.Q: My pump is losing speed; does the drive have a problem?A: Pumps are divided into three basic categories – centrifugal, rotary, and reciprocating. Take any of the three types and break them down further into four parts: the rotating element, the casing, the motor, and the drive. The rotating element has a shaft, sleeves, bearings and an impeller. The casing has a pump shell, wear rings, and shaft seals. The motor and drive are intuitive. Here is a basic remediation list: 1. the belts are slipping (if applicable)2. the impeller is worn3. the impeller is loose on the shaft or the shaft is sheared4. the casing is worn5. the original motor was replaced with a slower model or with a greater amount of motor slip6. the requirements have changed beyond the design specifications of the system. Using a manual tachometer, verify if there is a speed problem. If all of the above have been investigated and a problem still exists, then you may need to increase the maximum frequency of the drive. Note that anything beyond 60 Hz will result in a shift from a constant torque to a variable torque situation. There is a low probability that the drive will detrimentally affect the intrinsic speed of the system.Q: Is there a reference for application help?A: NEMA standards publication “Application Guide For Adjustable Speed Drive Systems.”Q: What is the difference between torque control and torque limiting?A: Torque control can be done with any of AutomationDirect's PID-capable drives. This would be a closed loop system using torque as the process variable. There should be both a torque reference and torque feedback signal. The user would establish a torque setpoint and configure the drive to maintain that setpoint. Many customers have used torque sensors, load cells, and current transducers to establish an analog input to the drive. Ideally, a vector drive would work better than a volts/hertz drive. A vector drive allows tighter speed regulation and better control in the lower speed range. Torque limiting can be done with AutomationDirect's Hitachi SJ300 series drive. Instead of using an external torque sensor, torque limiting uses the drive's internal current sensor. The operator would set the torque limit, and when that level is exceeded, the drive would act like a governor on a generator. The speed would be restricted until the appropriate level is again maintained. Mechanical devices, in addition to drives, are also employed for torque limiting, such as clutches, shear pins, gearboxes, etc. Learn More

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