Variable Speed Drives (VSD) Notes

Variable Speed Drives (VSD) - Apprenticeship and Industry Training

Objective One: Application of Variable Speed Drives

Principles of Variable Speed Drives

• Adjustable Speed Drive (ASD), Variable-Speed Drive (VSD), and Variable Frequency Drive (VFD) are terms for equipment controlling motor speed.
• Controlling motor speeds based on load requirements increases efficiency and performance.
• VSDs can control torque output, provide ramped soft starts, and ramped deceleration.
• VSDs provide variable or constant torque output based on load requirements.
• VSDs are installed for:
  1. Energy savings
  2. Extended equipment life

Energy Savings

• Motors are often selected based on maximum system requirements, which may not always be needed.
• Variable frequency drives (VFDs) commonly control the speed of centrifugal pumps and fans.
• In liquid flow processes:
  • Constant speed motor drives the pump at full speed, and a throttling valve controls the flow.
  • Variable speed drive controls the motor's speed, which controls the pump's speed, varying the flow rate.
  • Using a VSD is more efficient as the motor runs only at the necessary speed.
• An upstream block valve is required to prevent back flow when using a variable speed pump.
• Pump Curves:
  • Constant Speed Pump: Operates at a fixed speed (e.g., 1780 RPM); flow is controlled by a throttling valve.
    • OP-1: 70% open valve provides the design flow rate.
    • OP-2: Closed valve provides no flow but maximum discharge pressure.
    • OP-3: 100% open valve provides maximum flow at reduced discharge pressure.
  • Variable Speed Pump: Varies speed to control flow rate.
    • OP-1: 1580 RPM provides the design flow rate; pump speed and discharge pressure are lower than constant speed.
    • OP-2: Increased pump speed results in higher flow rate and discharge pressure.
    • OP-3: Decreased pump speed results in lower flow rate and discharge pressure.
• Power is proportional to shaft rotational speed cubed:
  • $Power \propto (Shaft\ Rotational\ Speed)^3$
  • Decreasing pump speed reduces power requirements, increasing efficiency.
• Centrifugal fans: Inlet vanes or dampers are used to control pressure with constant speed motors. VSDs allow for varying fan speed to control pressure, saving energy.
• Energy savings are realized using variable speed fans compared to constant speed fans with inlet vane throttling.

Extended Equipment Life

• Sudden torque from full voltage motor starts can damage equipment.
• VSDs provide ramped soft starts to avoid inrush current and torque, which:
  • Reduces electrical stress on the motor and electrical system.
  • Reduces mechanical stress on bearings, belts, and materials.
  • Prevents product damage from sudden starts and stops on assembly lines.

Principles of Variable Speed Drives

Direct Current (DC) vs. Alternating Current (AC) Motors

• VSDs are available for both DC and AC motors.
• DC motors were once considered superior for variable speed applications requiring high starting torque.
• Variable Frequency Drives (VFDs) allow AC motors to perform tasks previously exclusive to DC motors.
• AC induction motors are simpler and more reliable than DC motors.

Direct Current Motor Drives

• A drive is an electronic power-control circuit that powers a motor.
• A variable speed DC motor drive controls the speed of a DC motor.
• Operating Principle (Figure 6):
  • Current (I) from a DC power supply passes through an armature (coil of wire).
  • The current creates a magnetic field that aligns with the stationary magnetic field, causing rotation.
  • The commutator breaks and re-energizes the wire in the opposite direction to maintain rotation.
• Electrical contact is made through carbon brushes, which wear over time and require periodic replacement.
• Practical DC motors have multiple loops on the armature for uniform torque.
• Stationary magnetic field produced by current through field wiring or field coils.
• Three Basic Types of DC Motors:
  • Shunt Motor (Figure 8A):
    • Armature and field windings in parallel with a common DC power source.
    • Provides good speed regulation but not high starting torque.
  • Series Motor (Figure 8B):
    • Armature and field windings in series with a common DC power source.
    • Provides very high starting torque for high inertia loads (e.g., trains, elevators).
  • Compound Motor (Figure 8C):
    • Combination of shunt and series configurations.
    • Combines characteristics of both shunt and series motors.

Direct Current Motor Speed Control

• Speed is directly proportional to armature voltage and inversely proportional to field current.

• Varying armature voltage is the main speed control technique.

• Large DC motors get DC power from converting three-phase AC voltage with a three-phase bridge rectifier.

• Silicon-Controlled Rectifiers (SCRs) are controlled to adjust the DC voltage to the armature.

• Kickback diode (freewheeling diode) eliminates inductive voltage spikes during motor stops.

• SCRs are turned on and off to control power to the motor.

• Duty cycle (on time vs. on + off time) determines the fraction of total power delivered:

• Most VFDs require motor speed feedback to adjust output voltage and maintain speed under changing loads.

• Feedback is typically provided by a tachometer (small DC generator) connected to the motor shaft.

• Tachometer output is a DC voltage proportional to shaft speed (e.g., 0-10V).

Alternating Current Motor Drives

• Induction AC motors are common in commercial and industrial applications.
• Speed control is achieved by altering the frequency of the supply voltage.
• Variable Frequency Drive (VFD) varies the frequency to control speed.

Alternating Current Motor

• A rotating magnetic field is generated in a three-phase AC induction motor.
• Electromagnets (stator) produce a rotating magnetic field.
• Three pairs of poles (A/A+, B/B+, C/C+) are energized by different phases of AC power.
• Each AC phase produces its strongest magnetic field at its maximum and minimum.
• Electromagnetic pairs are arranged to rotate the magnetic field clockwise.
• Synchronous AC Motors: Magnetized rotor precisely follows the rotating magnetic field.
• Induction AC Motors: Electrically conductive object (rotor) undergoes induction, with electric currents reacting against the rotating magnetic field, causing the object to be dragged along. Rotor rotates at a speed slightly less than the synchronous speed.
• Squirrel-cage induction motors are common that consist of steel with aluminum or copper conductors embedded in its surface.

Alternating Current Motor Speed Control

• Induction motor speed depends on supply frequency and number of stator poles:
  • $n_{syn} = \frac{120f}{N}$
    • $n_{syn}$ = synchronous speed
    • $f$ = frequency
    • $N$ = number of poles
• Number of poles is fixed; VFD varies the frequency of three-phase stator currents.
• VFD enables an induction motor to run at any speed from near standstill to maximum.
• VFDs control speed, torque, and direction by converting fixed voltage and frequency AC input to variable voltage and frequency AC output.
• Block Diagram of a Typical VFD (Figure 12):
  • Rectifier: Converts incoming AC power to DC.
  • DC Link: Filters DC voltage using capacitors and inductors.
  • Inverter: Produces the required three-phase frequency and voltage.
  • Control Logic: Generates pulses to control the inverter's power semiconductor devices.

Rectifier

• Typically a fixed diode bridge rectifier.
• For three-phase power, rectifiers can be 6-pulse, 12-pulse, or 18-pulse.
• Diode permits current flow in one direction when forward-biased.
• 6-Pulse Rectifier:

The rectified output is a pulsating DC voltage waveform with six pulses 60° apart.

• 12-Pulse Rectifier:
  * Uses 12 diodes plus circuitry.
  * Produces 12 pulses 30° apart
• 18-Pulse Rectifier:
  * Uses 18 diodes plus circuitry.
  * Produces 18 pulses 20° apart.
• Higher pulse numbers result in smoother DC waveform and fewer harmonics.
• Harmonics: Frequencies superimposed on a power system due to non-linear electronic loads.
• Rapid switching in VFD produces harmonics, which can harm equipment and interfere with electronics.

Direct Current Link

• Located after the rectifier section.
• Consists of an inductor (in series) and capacitor (in parallel).
• Filters surges/transients and smooths pulsating DC voltage.

Inverter

• Converts DC from the DC link to a pulsed DC voltage.

• Motor reacts as if it is AC power at a specific frequency.

• Pulse-Width Modulation (PWM): Modulates the time the DC voltage pulses are on or off and reverses the polarity of the DC voltage output.

• Insulated Gate Bipolar Transistor (IGBT) is a common switching device for pulse width modulation.

• Simulated AC power to the motor consists of pulsed DC voltages.

• Switching transistor Q1, connected to the positive rail, generates the positive half of the sine wave.

• Switching transistor Q4, connected to the negative rail, generates the negative half of the sine wave.

Pulsed DC Voltage

• The inverter produces a simulated sine wave, not a pure sine wave.
• Drive relies on motor winding inductance to filter the irregular current produced by pulsing voltages.
• Inverter outputs three simulated AC power waveforms 120° apart for the three-phase power supply.
• The drive produces a number of pulses depending on the carrier frequency. For example, with a carrier frequency of 5 kHz:
  • $T = \frac{1}{f} = \frac{1}{5k} = 200 \mu s$
• The inverter control logic can turn the DC output on and off once during each period of the carrier frequency. By varying the length of time that each pulse of DC voltage is on and then off, you modify the average voltage.

Control Logic

• A microprocessor controls the switching of IGBTs in the inverter section.

• Varying the frequency of simulated AC power to the motor controls the motor's speed.

• Voltage also needs to change with frequency for proper operation.

• PWM output techniques for motor speed control:

  1. Volts per Hertz (V/Hz):

• Requires a change in voltage to maintain a constant volts-per-hertz ratio as frequency changes.

  • For example, for a 460 V 60 Hz motor, the V/Hz ratio is 7.61:
    • $Ratio = \frac{V}{Hz} = \frac{460V}{60Hz} = 7.61 V/Hz$
  • At 10% motor speed (6 Hz), the voltage must be 45.66 V:
    • $6 Hz \times 7.61 V/Hz = 45.66 V$
  • Maintaining V/Hz ratio keeps current constant as motor speed changes, providing constant torque.
  • If voltage is not reduced as frequency reduces, the current rises to excessive values.

• Pulse widths are adjusted to control voltage value.

• Increasing on time and decreasing off time creates a higher average DC voltage output.

• Decreasing on time and increasing off time creates a lower average DC voltage output.

• At very low frequency, the motor with a constant torque load may stall. To prevent this, adjust the V/Hz ratio to provide a higher voltage, a higher current and higher torque at low frequency operation. This adjustment is called torque boost.

• The V/Hz VFD has become the most commonly used VFD because it works well with motors that range in size from fractured horse power (hp) to over 1000 hp, it is highly reliable, it is affordable and it reflects the least amount of harmonics back into its power source.

• Drives rated from 208 V to 600 V, 3 phase and output frequencies from about 0 Hz to 400 Hz in some units.

  2. Flux Vector:

• An improvement over V/Hz control.

• Optimizes motor operation by controlling stator and rotor flux angles.

• More exact speed control under changing load conditions.

• Uses mathematical algorithms to:
  * adjust frequency and magnitude of voltage (like V/Hz) and
  * control phase angle of voltage and current.

• Separates motor current into flux (magnetizing) and torque producing components, which are independently adjusted.

• Benefits: precise speed regulation and torque control.

Objective Two: Variable Frequency Drive Components

Variable Frequency Drive Components

• A VFD AC motor installation consists of an AC motor, the VFD, and auxiliary equipment.
• Power wiring supplies power to the drive and motor.
• Control wiring consists of inputs and outputs connected to the control terminal strip.
• Speed control input is a 0 V to 10 V signal or 4 mA to 20 mA signal.

Alternating Current Motors with VFD

• Standard three-phase AC induction motors operate at a fixed speed for their design power supply (voltage and frequency).
• Three-phase induction motors used with VFDs have special considerations and must be rated for VFD use.

Considerations for VFD Induction Motors

• Before using a standard three-phase AC induction motor with a VFD, consider the effects of:

  • Varying speeds
  • Voltage spikes
  • Induced voltages

• Varying Speed:

  • TEFC motors are cooled at rated speed.
  • VFD controlled standard motors operated at slower speeds may overheat.
  • Some motors may be approved for VFD operation, but only to a minimum speed.
  • Operational speed ratio (e.g., 4:1 for constant torque loads, 20:1 for variable).

• Voltage Spikes:

  • VFD outputs can produce voltage spikes higher than the supply line voltage.
  • These voltage spikes can damage standard motor winding insulation.
  • Smaller motors may be approved for 230 V, even if 460 V rated for non-VFD use.

• Induced Voltages:

  • High carrier frequency from VFD can induce voltages into the motor shaft.
  • This voltage can exceed the insulating value of the bearing lubricant and arc across the bearings, leading to pitting and premature wear.
  • High frequencies can also cause induced circulating currents in the rotor core, which causes overheating.

VFD Rated Induction Motors

• Motor and VFD must be compatible.
• Consult manufacturers for compatibility and recommendations for cabling and installation.
• Motors operated with a VFD must be marked for VFD use.

Ratings:

• Inverter Ready:
  • Better insulation and thermal capacity than standard motors.
  • Operational speed ratio may be 5:1 for constant torque or 20:1 for variable torque loads.
• Inverter Duty:
  • Specifically designed for VFD operation.
  • Can operate with constant torque loads with an operational speed ratio of 1000:1.
  • Features:
    • Thinner steel laminations in the rotor core to reduce increased iron losses.
    • Better winding insulation to withstand voltage spikes.
    • Shaft bonding rings to prevent induced voltage build-up
    • Insulated/ceramic bearings on the non-drive end in addition to the shaft bonding ring to prevent circulating currents.
    • Separately powered blower motor (TEBC) for cooling.
• Vector Duty:
  • Like inverter duty motors, but with shaft encoders to provide feedback to the drive about rotor speed and position.
  • Example of Inverter duty rated motor nameplate (Figure 21).

VFD Control

• VFDs control induction motor speed by controlling the frequency and voltage to the motor.

• Drives can run motors at different speeds, stop and start at specific rates, control torque, and respond to feedback.

• Speed Control:

  • Open loop control
  • Closed loop control

• Open Loop Control:

  • No direct feedback from the motor.
  • VFD outputs a frequency according to a programmed setpoint.
  • Setpoint can be adjusted locally or remotely.
  • With V/Hz control, the motor's actual speed may vary by 1% to 3% less than the synchronous speed due to rotor slip.
  • Some VFDs can estimate motor speed from programmed parameters and current load.
  • Flux vector control is used for more precise open loop speed control.
  • Sensorless flux vector control can maintain speed to 1% of synchronous speed regardless of load changes.
  • VFD responds only to the pre-set parameters and value of the setpoint. If the motor operates a pump, the VFD does not get feedback from flow-rate or pressure sensors; it runs the pump according to a frequency setpoint.

• Closed Loop Control:

  • Direct feedback from sensors allows the system to adjust for changing conditions.
  • The VFD outputs the frequency necessary to bring the motor's speed to a setpoint that is either locally or remotely programmed into the VFD by an operator.
  • Closed loop VFD speed control uses direct feedback from the motor to tell the VFD its exact speed.
  • Sensors (magnetic pulse generator or optical encoder) on the shaft provide feedback of motor speed.
  • The full flux vector drive controls percent speed regulation to approximately 0.01% from no-load to full-load conditions.

• PID feedback control: Most VFDs can be programmed for proportional integral derivative (PID) feedback control, which uses a remote or local speed setpoint.

• A process variable (temperature, pressure, or flow rate) may provide feedback.

• Pressure sensor in a pipe sends a signal to the VFD, which adjusts pump speed to maintain a specific pressure.

• Shaft encoders can be installed on inverter duty motors later for closed loop operation.

• Vector duty motors have shaft encoders and are used with vector type VFDs for closed loop operation.

Stop Control

VFD stopping or braking types include:

• Rheostatic Dynamic Braking:

  • Dissipates power generated as a motor slows through a resistor.
  • Motor becomes a generator when spinning faster than the supply voltage frequency indicates.
  • Voltage from regeneration can harm the DC-link filter section of the VFD.
  • Transistor controls current through the resistor.
  • Regenerative energy is dissipated as waste heat by the resistor.

• Line Regenerative Braking:

• Captures energy from a decelerating motor and sends it back to the electrical system.

• Requires a special rectifier section in the VFD that can invert excess DC voltage to AC voltage.

• IGBTs are incorporated into the bridge rectifier to do this inversion.

• Rectifier is bi-directional: AC to DC (normal operation) and DC to AC (regenerative braking).

• Direct Current Injection Braking:

  • Used at low motor speeds when little regenerative energy is produced.
  • Injects a DC voltage onto the motor stator windings to produce a steady flux.
  • Rotor spinning in this field produces a torque in the opposite direction, stopping the rotor.

• Some VFDs have both dynamic and DC injection braking for deceleration and final stopping, respectively.

Start Control

• VFD's provide ramped soft starts to avoid inrush current and torque.
• A controlled start
  • reduces electrical stress on the motor and electrical system
  • reduces mechanical stress on equipment bearings, belts and materials
  • prevents product damage by sudden starts and stops on assembly lines and conveyors
• Without using VFD, dc motors provide soft start and stop and speed control. However, the lower initial costs, maintenance costs and reliability of ac motor VFDs means that ac motors are now used more than dc motors.

VFD Auxiliary Components

Auxiliary components in the VFD motor circuit include line or load reactors and special cables.

Line or Load Reactors

• Reduce harmful effects of problem harmonics produced by the VFD.
• Harmonics distort voltage and current waveforms, creating problems for electrical systems.
• Harmonic filter techniques protect the motor and the utility's supply equipment.
• Solid state switching in the VFD drive produces harmonic distortion and is best described as electrical noise.
• A limit of 3% total harmonic distortion (% THD) is generally allowed.
• Above 3% requires remedial action.
• Input line reactor (large inductor in series with the drive on the line side) filters harmonics before they reach the supply line.

Special Cables

Consider the following characteristics of the output circuit of a VFD when selecting cables:

• Radio frequency interference

• Impedance matching

• Induced voltages and stray currents

• Cable installation

• Radio Frequency Interference:

• VFDs can generate high frequencies that cause problems with monitoring, metering, and sensing equipment.

• Shielding or continuous metal armour is required to prevent radio frequency interference (RFI).

• Shielding provides a low impedance path back to the VFD for induced RFI currents.

• RFI filter on input power cables to the VFD is often recommended.

• Impedance Matching:

  • Fast switching creates high voltage spikes that can damage cables, motor windings, bearings, and the VFD.
  • Voltage spikes can be twice the peak of the DC voltage of the VFD if cable and motor impedance are not matched.
  • High cable capacitance or length can cause impedance mismatch.
  • Cables must be of low capacitance and impedance and matched to the motor.

• Induced Voltages and Stray Currents:

• Effective bonding paths are important in VFD installations to deal with stray voltages.

• Voltages induced into the case and shaft of the motor can cause arcing in the bearings of a motor and cause premature bearing failure.

• Bonding conductor size should be no less than that of the circuit conductor.

• Three bonding conductors are often used in a VFD cable to provide extra bonding conductor area and reduce RFI.

• Proper bonding equalizes any voltage difference between non-current carrying motor parts and non-current carrying parts of the wiring system.

Cable Installation

Consider location and length when you install power cables for VFDs, including:

• Location:
  • VFD and cabling are sources of RFI.
  • Power cables must be kept away from sensors and equipment affected by RF noise.
  • Cables should be at least 300 mm away from control and signal cables and other RF sensitive devices.
  • Cables should not run parallel to control or signal cables.
  • Cables must cross control cables at an angle of 90º
• Length:
  • Long cable runs can increase cable capacitance and cause a cable-to-motor impedance mismatch, resulting in reflected waves with high voltage spikes.
  • Keep VFD to motor cables as short as possible (recommendations are no greater than 15 m).

Objective Three: Variable Speed Drive Software

Variable Speed Drive Software

• The most common variable speed drive used in the process industry is the VFD used with induction motors.
• Qualified personnel should plan or implement the installation, start-up, and software maintenance of a VFD system.

VFD Installation

• Installation involves wiring control circuits and power conductors.

  • The analog output can be a voltage or current by setting a switch.

• Control Wiring:

  • Remote stop-run switch or digital output from a control system to a VFD wiring: The source wiring the power comes from terminal 11. Input 02 must be active for the drive to run. When input 02 opens, the drive stops. * The sink wiring the power comes from terminal 01. Input 02 must be active for the drive to run. When input 02 opens, the drive stops.
    • Local pushbutton stop-start and reverse direction switch to a VFD: The source wiring the power comes from terminal 11. A momentary connection on input 02 starts the drive. A momentary disconnection on input 01 stops the drive. Input 03 reverses direction of the drive when it stops.
      • The sink wiring power comes from terminal 01. A momentary connection on input 02 starts the drive. A momentary disconnection on input 01 stops the drive. Input 03 reverses direction of the drive when it stops. Terminal 04 is the digital common.
        The power wiring from a power utility to a VFD and the power wiring from the VFD to the motor must follow the electrical code. Qualified electricians do this wiring. The installation must comply with specifications regarding wire types, conductor sizes, branch circuit protection and disconnect devices.
    • Remote setpoint to the VFD wiring:
  • If you use a 4 mA to 20 mA control signal, the positive is connected to terminal 15 and the common is connected to terminal 14.
    • If you use a 0 V to 10 V control signal , the positive is connected to terminal 13 and the common is connected to terminal 14.
  • Remote motor control with feedback from the motor programming:
    • The setpoint is a current signal
    • The feedback is a voltage signal
    • The VFD PID control controls the control algorithm controls the motor speed according to the supplied setpoint

• Recommendations for control wiring:

  • Use shielded copper wire; connect the shield to ground at only one end.
  • Use wire with an insulation rating of 600 V or greater.
  • Separate control and signal wires from power conductors by at least 0.3 m (1 foot).

• Power Wiring:

  • Must follow electrical code, including specifications for wire types, conductor sizes, branch circuit protection, and disconnect devices. Qualified electricians do this wiring.

• For remote motor speed control with feedback from the motor:
  The setpoint is a current signal
  The feedback is a voltage signal
  The VFD PID control algorithm controls the motor speed according to the supplied setpoint

VFD Start-Up

• VFD needs to be programmed with motor information.
• Minimum data input for a standard V/Hz VFD:
  • Rated motor voltage from nameplate.
  • Rated motor current from nameplate.
  • Rated frequency and maximum and minimum operational frequencies from nameplate.
  • Rated speed and maximum and minimum operational speeds from nameplate.
  • Motor magnetizing current (amperes): current that the motor draws when operating with no load at nameplate rated voltage and frequency
    • Motor stator resistance (ohms): dc resistance of the stator between any two phases.
  • Acceleration/deceleration rates.
    Other programming or VFD functionality that may need to be performed include:
    The control wiring and alarms signal wiring points may require programing for the specific application
    Local/remote operation, which tells the drive where to get the start, stop, reset, forward, reverse and speed setpoints.
    Voltage boost that allows the drive to adjust, starting torque based on the type of load that it is driving, such as constant speed, constant horsepower or constant torque.
    Direct current braking requirements that tell the drive the level of de injection and duration required in order to stop a load with inertia.

VFD Software Maintenance

• May require updating firmware.

Computer Connection

• Connect a laptop to the VFD via communication ports.
• The method with which you connect a computer to the VFD depends on the communication ports that the VFD has.
  A typical procedure when you use a serial communication port is as follows.
• Plug the appropriate serial cable connector into the VFD drive.
• Plug the other end of the serial cable into the interface unit if required or directlyinto the appropriate serial port on the laptop computer.
• It required connect the interface unit to the laptop computer using an appropriatecable.

Firmware Upgrade

• Once you have connected to the VFD, open the manufacturer's software on the laptop used to flash or upgrade the drive
  A typical procedure to flash a VFD firmware is as follows.
• Ensure that you have the proper firmware update version for the drive. Typically, you can download the firmware from the manufacturer's website. Save thefirmware update version to your hard drive.
• Open the communication software and select the appropriate communicationdriver, the communication serial port the cable is plugged into and configure thecommunication protocol. Select the highest possible baud rate to ensure efficient communication speed.
• Open the firmware update software and accept the license agreement.
• Select the VFD drive that you are connected to from the menu.
• Observe the current revisions, as well as the revision of the flash file, once youcan see the selected drives. The flash file firmware should be newer than thecurrent firmware revision number.
• Ensure the firmware revisions are what you are expecting, then initiate the update procedure. The update procedure may take up to 30 minutes depending on the communication speed.
• The VFD resets itself when the flash procedure is done.