In some motor applications the inertia profile can be such that once power is removed the motor effectively becomes a generator, feeding power back into the control system at a gradually decreasing magnitude until motor speed decreases. For adjustable speed drives (ASDs) this presents a primary problem – the risk of DC bus over-voltage as the capacitors charge in response to the “reverse” power flow. Modern ASDs resolve this problem differently depending on the configuration of their power components (typically uncontrolled or controlled rectifier arrays referred to as “bridges”). Regenerative drives (often referred to as 4-quad), made for handling reverse-torque applications such as elevators, utilize anti-parallel bridges – one set conducts from drive to motor, and the other from motor to drive – to dump the generated power back into the supply line. Non-regenerative ASDs (sometimes referred to as single-quad) do not have this capability and so must use other means of dissipating the back-fed power, namely a brake chopper circuit. So as to avoid confusion with injection braking technology, let’s examine the brake chopper in more detail. Much of what follows is taken from ABB’s “Technical Guide no. 8: Electrical Braking”, which can be found in the […]
Proper installation of adjustable speed drives (ASD’s) hinges in part on compliance with the requirements of National Fire Protection Association (NFPA) 70 – the National Electrical Code (NEC). Beginning with the 2005 Code, requirements for ASD’s were consolidated in a single section, Part X, under Article 430 (Motors, Motor Circuits, and Controllers). This consolidation was made with the explicit intent that other relevant parts of Article 430 would also apply unless specifically addressed in Part X. A brief overview of these requirements will assist in understanding correct installation, and increase the likelihood of a trouble-free drive application.
Branch circuit and feeder conductors supplying ASD’s are to be sized for 125% of the drive’s rated input current, unless a bypass is part of the drive system (more on that shortly). Note that the rated input current is based on the drive electronics’ ability to handle amperage without over-heating and as such factors in over-load capacity. For a given application, the input current also varies when a drive is de-rated for single-phase supply; manufacturers typically require a 50% de-rating, which means that, for a given load, a single-phase drive will draw approximately twice the input current of […]
It will surprise absolutely no one that the power grid in many parts of the US is stressed, particularly when more extreme weather events occur. Power sags, brown-outs, and even black-outs can occur. Normally, variable speed drives (VSD’s) are quite capable of handling these low-voltage conditions before they become damaged; worst case, the VSD shuts down and the process is interrupted, which admittedly can be an expensive problem. On the other hand, power surges, depending on their causes and characteristics, can be a much more serious problem for VSD’s.
Surges can be transient or sustained, and result from internal or external sources. Their magnitude is a consequence of the duration of the surge and the power available to feed it, which in turn results from the impedance available in the supply system. Transient surges, defined as occurring for less than one millisecond, are of greatest concern because they are harder to detect and interrupt before equipment damage occurs. Even high-speed fuses may not clear quickly enough to prevent electronic component damage if the current let-through is high enough, and coordination with upstream over-current devices is more difficult, meaning the extent of process interruption is harder […]
In processes where variable speed is not an absolute necessity, but where users are concerned about energy savings and the impacts on electrical and mechanical systems of full-voltage starting, the choice between a variable speed drive (VSD) and a reduced voltage (i.e. “soft”) starter can be a difficult one. There are a number of factors which should be considered when making this choice. Several of these factors are described below, together with recommendations for the starting/control method to be used.
- Overall system design efficiency: consider two piping systems: one sized to utilize pumps designed to operate at their “best efficiency point” (BEP); and one designed and built with excess capacity available, perhaps for future expansion. In the former instance, regulating process flow by controlling motor speed will typically not lead to enhanced process efficiency, and choosing a VSD which will only serve to operate its motor at base (rated) speed will not gain you much in terms of energy use, either. However, a system built with excess capacity will typically suffer in efficiency when that capacity is not needed, so regulating process output by reducing pump speed may actually enhance efficiency, and can certainly reduce […]
Please note: the following information is derived from the NEMA MG 1-2007 condensed standard. More information, as well as the full NEMA MG 1 standard, can be found at www.NEMA.org.
When applying adjustable speed drives for the control of induction motors, several operating impacts should be considered in order to ensure proper operation and equipment life. What follows is a brief overview of some common considerations:
- Motor torque, speed, and temperature: Many modern adjustable speed drives (ASD’s) are capable of controlling torque by directly manipulating motor flux, such that torque is maintained constant across the full zero-to-base speed range. That said, when operating a self-cooled motor at reduced speeds, temperature rise must be factored in. This means that in many cases it is advisable to de-rate a self-cooled motor to ensure temperature rise is maintained within the range dictated by insulation classification. It is generally stated that each increase of 10 degrees Celsius in winding temperature above rated levels reduces winding insulation life by 50%, so proper cooling is essential. In addition to de-rating, there are several other ways to address this issue, including auxiliary cooling (such as […]
Editor’s Note: Much of the information which follows is taken from engineering information provided by Siemens AG in their “Sinamics DCM Converter Units” catalog D 23.1 – 2010. The Sinamics DCM is the line of industrial DC drives in the Sinamics family, which forms a part of Siemens’ “Totally Integrated Automation” concept; learn more at www.siemens.com. However, the concepts discussed herein can generally be applied to any drive application.
Because of the high switching frequencies of their electronic components, variable speed drives are by their nature radiating devices. This radiated energy is termed electromagnetic interference (EMI); measures to reduce EMI during design and installation are intended to ensure electromagnetic compatibility (EMC), which is essentially the ability of a device to function satisfactorily in an electromagnetic environment without itself causing interference unacceptable to other devices in the environment.
In the typical industrial environment, EMI occurs in the range of 150kHz – 30 MHz and can have adverse consequences on the operation of nearby sensitive equipment. When considering measures to ensure EMC, the drive must be looked at as forming part of a system, the other components being minimally the cables and motor. Mitigation involves […]
Many of you have written in with questions regarding installation methods, drive technologies, operating parameters, and other issues. Since it is often time-prohibitive to respond individually to all of them, I thought I would consolidate and address some of the more common – and commonly applicable – questions that have been submitted.
- “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?”
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 […]
Modern variable speed drives (VSDs) are equipped with a multitude of features to provide programming flexibility, enhance efficiency and increase the accuracy of control. Let’s dissect some of these features, typified in this case by the A1000 series of industrial AC drives from Yaskawa (www.yaskawa.com).
Control methods: the A1000 provides up to seven different control methods to suit specific motors and applications. Methods range from basic scalar (volts/hertz, or V/f) control, which adjusts frequency and voltage output in direct proportion based on command reference, through open-loop vector control, to closed loop vector control. Vector control essentially “splits” the stator current into separate torque and field components, analogous to the separate armature and field components of a DC motor, and controls VSD output by regulating voltage magnitude, angle of displacement, and frequency. In open loop systems, modeling is used to calculate vectors and adjust output based on measured output current, while in closed loop configuration, sensors such as encoders or tachometers directly measure rotor position and speed and are able to control output even more tightly. In the A1000, V/f control can provide a typical speed control range of 40:1, while open-loop vector can achieve 200:1 […]