Examining adjustable speed drives

Adjustable speed drives (ASDs) are known by several other names: variable speed, variable frequency, or adjustable frequency. Variable infers a change that may or may not be under the control of the user. Frequency can only be applied to drives with an ac output. Adjustable speed drive system is the most correct and commonly accepted description.

Most ASD units consist of three basic parts: rectifier, inverter, and controls. A rectifier, also known as a converter, changes the fixed frequency ac input voltage to dc. The inverter switches the rectified dc voltage to an adjustable frequency ac output voltage. Controls direct the rectifier and inverter in producing the desired ac frequency and voltage to meet ASD system needs (Fig. 1).

ASDs are differentiated by their inverter sections, which use either current-regulating or voltage-regulating techniques to switch dc power to ac power. The most prevalent ASD classifications are:

• Current-source inverter (CSI)

• • Variable-voltage inverter (VVI)

  • • Pulse-width-modulated inverter (PWM).

    • Converters/rectifiers

      The converter section of an adjustable frequency drive is sometimes called the front end. It consists of power switching devices and logic controls. The converter/rectifier is usually used to control the voltage to the inverter section to maintain the constant volts/hertz ratio from the inverter.

      Rectifiers with diode converter

      The diode rectifier circuit (Fig. 2) is a three-phase, full-wave, bridge rectifier using diodes to produce constant voltage in the dc link. Voltage and frequency are changed in the inverter section. This type of converter is only used with a constant voltage, voltage-source PWM-type inverter.

      Rectifiers with dc chopper

      This circuit also rectifies using diodes (Fig. 3). An additional power-switching device is used to control the magnitude of dc voltage to the inverter section. For large horsepower motors, this converter is slightly more complex and costly than other types.

      Silicon-controlled rectifiers (SCRs)

      This circuit (Fig. 4) uses SCRs to control the magnitude of dc voltage to the inverter section. Logic circuits control the output voltage from minimum to maximum. This type of circuit is used in VVI, six-step inverters.

      Inverters

      The inverter section of an ASD consists of power switching devices and logic controls that are more complicated than a converter.

      Six-step inverter (variable voltage, voltage-source)

      This inverter section (Fig. 5) has six power switching devices and six diodes. Switches are turned on and off in a predetermined sequence to produce a six-step, three-phase voltage wave for the motor. To change the frequency to the motor, the conducting time of the switches is either increased or decreased for each step.

      Current-source inverter

      This inverter section (Fig. 6) has six power switching devices that are usually SCRs and used with large horsepower motors. The six SCRs, acting as switches, are turned on and off in a predetermined sequence to produce a six-step, three-phase current wave for the motor.

      PWM inverter (constant voltage, voltage-source)

      The inverter section has six power switching devices (Fig. 7). Insulated gate bipolar transistors (IGBTs) or gated turn off/ons (GTOs) are usually used because of their fast switching characteristics. Switches are turned on and off in a predetermined sequence to produce a series of pulses. These pulses vary in width to produce a square, three-phase voltage wave for the motor, which approximates a sine wave and minimizes harmonic heating. The power circuit is characterized by its simplicity, diode front end, capacitors in the dc link, and elimination of commutating circuits when using either IGBTs or GTOs.

      Controls

      Operation of the ASD can be as simple as off-on and a preset speed, or more complicated, including some degree of process control. The most common approach is proportional, integral, and derivative (PID) control. PID simply refers to the types of operation required to control modulating devices, such as dampers, valves, and ASD systems.

      With proportional control, an error signal is produced, resulting from the difference between the desired and actual conditions. The amplitude from a proportional-only controller is directly related to the amount of error.

      Proportional

      While ASD systems can be controlled by proportional-only logic, there are drawbacks. Proportional-only controllers require significant error conditions to produce an output. Therefore, a proportional-only system can never achieve and hold the desired condition or setpoint. A small amount of error or offset is always present (Fig. 8).

      Integral

      Integral action eliminates the offset caused by proportional-only control. The offset error is integrated by summing the offset error over time. This additional control signal is added to the proportional signal, eliminating the offset (Fig. 9).

      Derivative

      Derivative action minimizes overshoot problems. Overshoot occurs in many systems-when using ASD to drive a pump, for example. Maintaining a specific head pressure at the pump output is the object of the control system.

      Derivative control action anticipates overshoot and dampens or brakes the correction. Derivative action looks at the error signal’s rate of change by noting how quickly the actual condition is moving toward the desired condition or setpoint. It then produces a control signal based upon this change. The derivative signal counteracts the proportional and integral signal, resulting in much less overshoot (Fig. 10).

      ASD speed and function control can be accomplished by a variety of means. The most common control method includes conventional switches, pulse train signals, and digital human interface modules (Fig. 11). An ASD can be configured to perform as a unique device for any motor and load arrangement.

      P

      Plant Engineering acknowledges with appreciation the contribution made to this article by Wayne L. Stebbins, Perigon Engineering, Matthews, NC.

      Typical ASD starting conditions

      Most drives have a series of inputs that can be set to true or false prior to starting the motor. This setting ensures that proper operational and safety conditions have been met.

      Inputs are:

      • A momentary true input for the start function, which may be a normally open push button, logic from an interface with PLCs, or other solid state devices.

      • • A momentary false input for the stop function, which may be a normally closed push button or logic from other controls. There should be at least one hard-wired input to allow overide if other logic fails.

        • • A momentary true input for the enable function, which is usually a conditional input proving that all conditions have been met for a safe start. It may be internal to the drive or from an external source.

          • • A speed reference to set the operating frequency, which may be a potentiometer, analog signal, digital signal, or a pulse train input.

            • • An auxiliary, maintained true input or maintained closed contact to permit the drive to start, run, or jog. This device may be an external motor thermostat, overload element, or machine safety contact that can disable the drive. If this function is tripped, it generates an identifying fault code in the drive.

              • Questions to ask when applying ASDs

                What are the system benefits from an ASD installation?

                What reliability is required?

                What safety features are required?

                What is the speed-torque characteristic of the load?

                What operational overloads and starting conditions are required?

                What is the physical environment in the controller area?

                How will control commands be generated?

                What is the electrical supply rating? How is it configured?

                What levels of voltage distortion exist on the power system now? What harmonic current spectrum will be injected into the supply system by the ASD?

                What speed range is required? Will the load be operated beyond base speed?

                Is there enough space for the entire system?

                What waveform does the ASD produce? Are there any constraints on motor power lead length?

                Is the motor sized to provide enough load torque at reduced speed?

                Is the motor suitable for the application?

                What is the acceptable noise level in the vicinity of the motor?

                How much heat does the controller produce and how is it removed?

                What is the range of voltage and frequency on the supply which will permit full rated output of the ASD?

                How does the ASD operate under fault conditions?

                What motor protection does the ASD provide?

                Are installation and operating manuals available?

                What self-diagnostic tools are included or available?

                More info

                Additional sources of information on adjustable speed drives can be obtained from two IEEE publications: How To Select & Apply ASD Systems, Selected Readings (#HL5750), and ASD Applications – Self-directed Study Course with ASD Master Software (#SR113-QUO).

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