Electric Motor Drives Questions and Answers

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Motor Drives

Define and describe motor drives.

  • A motor drive is an electronic unit designed to control the speed of a motor using solid- state components.
  • Motor drives may be AC or DC drives.
  • In addition to turning motors on and off and providing overload protection, motor drives also provide speed control, timed acceleration and deceleration, motor-starting boost, fault monitoring, programmable set speeds, different stopping methods, and many other motor control functions.
  • The amount of motor control a drive provides depends on the drive used.
  • Basic drives provide a minimum of ON/OFF, speed control, and overload protection.
  • Advanced programming features include setting multiple preset speeds, limiting the motor maximum and minimum speeds, and providing motor braking.

Compare magnetic motor starter and motor drive control circuits.

  • When using a magnetic motor starter to control a motor, the starter is wired following a standard line diagram.
  • For example, a pump motor can be controlled by a magnetic motor starter that uses a control circuit. The control circuit includes a three-position selector switch (HAND/ OFF/AUTO) and a liquid level switch to control the motor starter.

• The magnetic motor starter can turn the motor on and off, but it cannot set the acceleration and deceleration times for the motor.

  • Also, the motor starter cannot be used to set the motor speed, provide circuit condition readouts, or display circuit or motor faults.
  • However, a magnetic motor starter does provide overload protection.
  • When using a motor drive to control a motor, the drive is wired following the control-circuit wiring diagram provided by the drive manufacturer.
  • For example, a pump motor can be controlled by a motor drive that uses a control circuit that includes a three-position selector switch (HAND/OFF/AUTO) and a liquid level switch to control a motor starter.
  • In a magnetic motor starter, the selector and level switches are connected to the motor starter coil. In the motor drive controller, the selector switches and level switches are connected to the input terminals.
  • Another difference is that the control circuit for the magnetic motor starter is normally powered by a step-down control transformer added between the power circuit and the control circuit. The control circuit for the motor drive does not require a control transformer because the motor drive provides the control circuit power at the terminals where the control devices are connected.
  • One of the major advantages of using a drive instead of a motor starter is that the drive offers motor speed control.
  • In addition to using a potentiometer for speed control, most motor drives allow a voltage or current input to control the speed of a motor.
  • Motor control and drive functions are set by placing switches in certain positions or by programming through a keypad.

Programming Motor Drives

Describe the parameter groups available in motor drives.

  • Some manufacturers’ list parameters in numerical order and other manufacturers arrange parameters by file and group based on function. Some assign a number to a parameter in addition to a file and group designation.
  • Some drive parameters are designated as display parameters and others as editing parameters.
  • Display parameters are used to give a visual display of operating conditions, which can be used when installing, testing, operating, and troubleshooting a circuit using a motor drive.
  • Although most motor drives include numerous parameters to customize a drive to a motor and application, normally most parameters do not need to be reprogrammed from the factory default settings. To simplify programming, some manufacturers group the most commonly programmed parameters together to make programming easier.
  • Although basic programs normally allow for good motor drive system operation, there are applications that require special operating conditions. Advanced programming parameters allow for customizing a motor drive application.
  • When drives are used in a system, troubleshooting can be more difficult and time consuming than when magnetic motor starters are used. To quickly identify a system fault, motor drives can detect and display common circuit faults.

DC Motor Drives

Describe DC motor drives and list the methods of braking a DC motor.

  • DC motor drives are normally used where a DC motor control circuit is in use and requires an upgrade from a mechanical motor control.
  • DC motor drives use electronic controls to vary the amount of DC power applied to a motor. The amount of applied voltage is proportional to motor speed and the amount of applied current is proportional to motor torque.
  • DC motor drives normally control the voltage applied to a motor over the range of 0 VDC to the maximum nameplate voltage rating of the motor.
  • If a DC motor drive can deliver more voltage than the rating of the motor, the drive should be set to limit the output voltage to prevent damage to the motor.
  • DC motor drives are designed to control the amount of voltage and current applied to the armature of DC motors to produce desired torque and prevent motor damage.
  • AC is used to power most DC motor drives because AC is readily available at most locations, except where portable equipment is used.
  • The two major methods of braking a DC motor faster than coasting are dynamic braking and regenerative braking.
  • Dynamic braking is a method of motor braking in which a DC motor is reconnected to act as a generator immediately after it is turned off.
  • Regenerative braking is a method of motor braking in which the regenerated power of a DC motor that is coming to a stop is returned to the input power supply.

AC Motor Drives

Describe AC motor drives and list their main sections.

  • An AC drive’s primary function is to convert the incoming supply power to an altered voltage level and frequency that can safely control the motor connected to the drive.
  • AC motor drives are designed to operate three phase AC motors regardless of whether the drive is designed for single phase power (115 VAC or 230 VAC), three phase power, or DC power.
  • The speed of an AC motor is determined by the number of stator poles and the frequency of the AC power supply.
  • AC motor drives control the speed of a motor by varying the frequency of the power applied to the motor.

The three main sections of an AC motor drive are the converter, DC bus, and inverter.

  • The converter (rectifier) receives incoming AC voltage and changes it to DC voltage.
  • The DC bus filters the voltage and maintains the proper DC voltage level. The DC bus may also deliver DC to the inverter for conversion back to AC.
  • The inverter controls the speed of a motor by controlling frequency and controls motor torque by controlling the voltage sent to the motor.

Describe the converter, DC bus, and inverter of an AC motor drive.

  • A converter is an electronic device that changes AC voltage into DC voltage.
  • Converters in electric motor drives are single phase full-wave rectifiers, single phase bridge rectifiers, or three phase full-wave rectifiers. Most electric motor drives are supplied with three phase power, which requires three phase full-wave rectifiers.
  • In order for a converter to deliver the proper DC voltage to the DC bus of an AC motor drive, the converter must be connected to the proper power supply. The power supply must be at the correct voltage level and frequency and must also provide enough current to operate an AC motor drive at full power.
  • Supply voltage to an AC motor drive must be checked when additional loads or drives are installed, serviced, or added to a system.
  • DC buses filter and maintain the proper voltage level.
  • DC buses (links) include DC filter components and are supplied with DC voltage by the converter. The capacitors and inductors in the DC bus filter and maintain the proper voltage level.
  • A bridge rectifier receives incoming AC supply power and converts the AC voltage to fixed DC voltage. The fixed DC voltage powers the DC bus of the AC motor drive.
  • To prevent damage to the diodes in the converter and to the AC motor drive electronic circuits, protection against transient voltages must be included in the drive. Protection methods include proper motor drive wiring, grounding, shielding for power lines, and surge suppressors. Normally, a surge suppressor consists of metal-oxide Varistors (MOVs) connected to the converter of a motor drive.
  • Capacitors in a DC bus are charged from rectified DC voltage produced by the converter. Capacitors oppose a change in voltage and, when DC bus voltage starts to drop, they discharge a voltage back into the system to stop the drop in voltage. The main function of capacitors in a DC bus is to maintain proper voltage levels when voltage fluctuates.
  • An inverter is an electronic device that changes DC voltage into AC voltage.
  • Inverters in an AC motor drive are the most important part of the drive because the inverter determines the voltage level, voltage frequency, and amount of current that a motor receives.
  • The main problem for manufacturers is to find a high-current, fast-acting solid-state switch that has the least amount of power loss (voltage drop).

Describe the relationship between voltage and frequency in an AC motor.

  • The voltage applied to the stator of an AC motor must be decreased by the same amount as the frequency. The motor heats excessively and damage occurs to the windings if the voltage is not reduced when frequency is reduced.
  • The motor does not produce its rated torque if the voltage is reduced more than required.
  • The ratio between the voltage applied to the stator and the frequency of the voltage applied to the stator must be constant. This ratio is referred to as the volts-per-hertz (V/Hz) ratio (constant volts-per-hertz characteristic).
  • The volts-per-hertz (V/Hz) ratio is the relationship between voltage and frequency that exists in a motor.

– The volts-per-hertz ratio for an induction motor is found by dividing the rated nameplate voltage by the rated nameplate frequency.

  • Above approximately 15 Hz, the amount of voltage required to keep the volts-per-hertz ratio linear is a constant value. Below 15 Hz, the voltage applied to the motor stator may be boosted to compensate for the large power loss AC motors have at low speed.
  • A motor drive can be programmed to apply a voltage boost at low motor speeds to compensate for the power loss at low speeds.
  • Motor drives can also be programmed to change the standard linear volts-per-hertz ratio to a nonlinear ratio.

Explain how to control motor speed and torque.

  • The primary function of all motor drives is to control the speed and torque of a motor.
  • To safely control a motor, an AC motor drive must monitor electrical characteristics such as motor current, motor voltage, drive temperature, and other operating conditions.
  • In addition to controlling motor speed and torque, an AC motor drive can include additional specialty functions that are built-in, programmed, or sent to the drive through on-board communication with a PC or PLC.
  • Controlling frequency (in Hz) to an AC motor controls the speed of the motor.
  • Controlling the V/Hz ratio applied to an AC motor controls motor torque.
  • Once an AC motor drive reaches the point of delivering full motor voltage, increasing the frequency does not increase torque on the motor shaft because voltage cannot be increased further to maintain the V/Hz ratio.

Describe motor frequencies and pulse width modulation (PWM).

  • The carrier frequency is the frequency that controls the number of times solid-state switches in the inverter of a motor drive with pulse width modulation turn on and off.
  • The higher the carrier frequency, the more individual pulses there are to reproduce the fundamental frequency.
  • The fundamental frequency is the frequency of the voltage used to control motor speed.
  • The fundamental frequency is the frequency of voltage that a motor uses, but the carrier frequency actually delivers the fundamental frequency voltage to the motor.
  • The carrier frequency of most motor drives can range from 1 kHz to approximately 16 kHz.
  • AC motor drives should be slightly derated or the size of heat sinks should be increased due to the increase in thermal loss. Derating a motor drive decreases the power rating of the drive.
  • Pulse width modulation (PWM) is a method of controlling the amount of voltage sent to a motor by converting the DC voltage into fixed values of individual DC pulses.
  • By varying the width of each pulse (time ON) and/or by varying the frequency, the voltage can be increased or decreased. The greater the width of individual pulses, the higher the DC voltage output.
  • When PWM is used with AC voltage, two IGBTs are used for each phase. One IGBT is used to produce positive pulses and another IGBT is used to produce negative pulses of a sine wave.

– Since AC drives are normally used to control three phase motors, six IGBTs (two per phase) are used to simulate three phase power.

List and describe different AC motor stopping methods.

  • An AC motor drive decelerates a motor at a controlled rate by placing an electric load on the motor. The advantage in using a motor drive to apply a braking force is that maintenance is kept to a minimum because there are no parts that come in contact during braking.
  • Common AC motor drive stopping methods include ramp stop, coast stop, DC brake stop, and soft stop (S-curve) stopping methods.
  • Ramp stop is a stopping method in which the level of voltage applied to a motor is reduced as the motor decelerates.
  • Coast stop is a stopping method in which the motor drive shuts off the voltage to a motor, allowing the motor to coast to a stop.
  • DC brake stop (DC injection braking) is a stopping method in which a DC voltage is applied to the stator winding of a motor after a stop command is entered.
  • Soft stop (S-curve) is a stopping method in which the programmed deceleration time is doubled and the stop function is changed from a ramp slope to an S-curve slope.

Describe electronic and programmed overloads.

  • An electronic overload is a device that has built-in circuitry to sense changes in current and temperature. An electronic overload monitors the current in the load (motor, heating elements, etc.) directly by measuring the current in the power lines leading to the load.
  • An electronic overload measures the strength of the magnetic field around a wire instead of converting the current into heat.
  • When motor drives are used to control motors, motors are protected from an overload by programming the motor nameplate current into the drive.
  • On basic motor drives, motor overload protection is set with an adjustment dial on the motor drive.
  • On advanced motor drives, motor overload protection is programmed into the motor drive using a keypad.
  • When setting motor overload protection using the dial on a motor drive, the maximum rated continuous output current of the motor drive and the full-load current (nameplate current) of the motor is used to determine the dial setting.
  • When programming motor overload protection, motor nameplate current is programmed into a motor drive. On advanced motor drives, motor nameplate current is entered into the motor drive as a drive parameter using a keypad.
  • Motor drives with advanced parameter programming may have customized overload protection for a given application.

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