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Output cabling from a variable frequency drive (VFD) can act as a significant source of electromagnetic emission resulting from the cables’ characteristics in response to the high-frequency voltage pulses they send to the motor. In effect, the cables serve as antennae. For drives using IGBT’s to generate the voltage pulses – the most common method used in drives today – the emission spectrum coming from the output section of an AC drive may extend to 50 MHz; it is typically strongest in the 100 kHz – 10 MHz range. Below this range, electromagnetic interference (EMI) issues are rare, but in or above it the EMI emitted and its effects on cables and connected equipment can have a negative impact on drive system controls and any adjacent, sensitive equipment. Let’s examine more closely the role that cabling plays, and ways to mitigate EMI problems through cable specification and installation practices.

At high frequencies, current will flow primarily in the outer surfaces of the conductors due to skin effects, which raises the effective impedance of the cables and increases voltage and the potential for noise. Also, “stray” or “leakage” currents can arise via capacitive coupling and, if […]

Because of the high-frequency switching characteristics of the outputs of modern variable frequency drives (VFDs), additional attention should be paid to the cables connecting a VFD to its motor. Modern pulse-width modulated (PWM) VFDs use sets of controlled transistors turning on and off at frequencies from 2 – 20 kHz to generate voltage pulses which, taken together, approximate the sine wave an AC motor requires. These transistors, typically Insulated Gate Bipolar Transistors (IGBTs), switch very rapidly, and are capable of reaching 90% of rated output voltage in less than 0.1 microsecond. This results in very steep wave fronts on voltage pulses sent down the cables at very high frequency. In turn, this places additional design demands on cable capacitance, impedance, electro-magnetic (EM) shielding, and length in order to ensure a high-quality, EM-compliant installation that is safe for the equipment and dose not create interference with other connected loads.  In Part I of this series, we are going to discuss cable capacitance and impedance characteristics and their impact on the VFD and motor.

All cables have a characteristic capacitance determined by insulation type and thickness and shielding material, and influenced by conductor configuration. At fundamental frequencies […]

Adjustable speed drives are rated by their manufacturers to conduct a specified amount of current under specified conditions. While each manufacturer sets their own specifications, ratings generally share these common characteristics:

• Drive PWM carrier (switching) frequency: There is variation here among manufacturers, but rated current output is based on a set carrier frequency. If that frequency is adjusted upward, de-rating is required.
• Maximum ambient temperature: typically 40C, with a 1% drop in rated current for each degree from 41C to 50C. Most manufacturers will not rate their drives for operation above 50C. Ambient temperature is of course related to installation method, and so sometimes these two factors are taken together to generate a single de-rating factor. That is the case with Yaskawa drives in the examples below.
• Installation spacing: although there is more variation among manufacturers here than with other factors, to obtain full-rated current the drive will typically need to be installed with a clearance of 100mm above and below, to allow proper ventilation airflow (usually from bottom to top), and from 0 – 30mm side clearance. If the drive is self-contained, the type of enclosure will also factor into this.
• Altitude: due to the decreased cooling […]

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 […]