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Inertial loads, and/or those in processes requiring rapid deceleration, present special challenges for variable speed drives (VSDs) and the motors they control. As power is removed or frequency reduced at the VSD output, a load with high inertia will prevent its motor from slowing down as quickly as it would under light- or no-load conditions. In such cases, the motor will change from its normal operation as a current “consumer” and become a current generator, wherein the motor shaft’s mechanical movement produces a negative current flow before the motor’s flux degrades. During this process, called regeneration, the power flow is back-fed into the DC bus of the VSD. The rectifier bridge upstream of the DC bus blocks the flow of this current, and unless the drive is properly equipped the DC link risks over-heating and damage.

So how does a VSD compensate? There are several techniques used depending on process requirements, capital cost, and other factors. Of these, two of the more common are brake choppers and the use of rectifiers with regenerative diode bridges. Brake choppers are essentially switches which close when the DC bus voltage reaches a defined limit and divert the back-flowing […]

NEMA MG 1 assigns code letters to AC induction motor designs to indicate relationships between speed and torque. These relationships reflect the torque capabilities of various motor designs from zero (locked rotor) to synchronous speed. Why is knowing the torque characteristics of a motor important? Well, the most obvious reason is to ensure that the motor’s rated torque can supply the force needed to drive the load. This must be considered at all operating speeds – at start-up, the motor’s starting torque (also called “locked rotor” or “breakaway” torque) must be sufficient to move the load in order to avoid stalling the motor; and while running, the load torque requirements must not exceed the motor’s breakdown torque (also known as maximum or “pull-out” torque) or else the motor will see a steep drop in speed and rapid current and temperature rise.

For 3-phase motors up to 500 hp, there are basically three classes of speed/torque designs: A & B; C; and D. Let’s examine each in more detail; you can find representative curves and data in the figures below:

• A & B: these are similar in characteristics, differing principally in terms of […]

Here’s an essential truism – knowing what to ask for is key to getting what you want. What follows is a checklist, with comments as appropriate, intended to help you define your variable speed drive (VSD) needs and ensure you get exactly the drive your application requires.

Motor Data:

• Full load amperage (from the motor nameplate)

This will typically be labelled as “F.L.A.” on AC induction motors; “ARM Amps” and “FLD Amps” on DC motors. Rated amperage, NOT horsepower, is used to properly size VSD’s, because the power electronics are sized to conduct a certain amount of current for a certain amount of time, which can vary for a given motor horsepower depending on torque required.

• Rated phase, frequency and voltage

This information can again be found on the motor nameplate. Often AC motors are rated for dual frequency (50/60 Hz); if so, this information should be included on the nameplate. If you intend to operate such a motor above rated speed, take care to note any changes to Service Factor; the dual frequency rating usually means a lower Service Factor when supplied […]

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 […]