Motor Controls

Electrical Maintenance for Motor Drive Systems 348

Electrical Maintenance for Motor Drive Systems provides a comprehensive overview of the common power quality issues that occur in motor drive systems and the methods used to inspect and resolve these issues. Electrical maintenance involves inspecting input power, DC bus output, leakage current, and insulation resistance as well as checking for overloading, single phasing, electrical unbalance, transients, harmonics, and thermal abnormalities.

Many industrial applications rely on motor drive systems to power output devices. Motor drive systems consist of complex electrical components and require sufficient power quality to function correctly. Power issues in any system component can cause the entire system to malfunction and fail, leading to lost production time and increased costs. This class prepares users to effectively operate and maintain motor drive systems to minimize downtime and economic losses.

  • Difficulty Advanced

  • Format Online

  • Number of Lessons 23

  • Language English

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Course Outline
  • Power Quality
  • Electrical Testing Tools
  • Input Power Measurements
  • Diagnosing Input Power Problems
  • Motor Overloading
  • Single Phasing
  • Input and Motor Faults Review
  • Electrical Unbalance
  • Diagnosing Input Unbalance
  • Diagnosing Output Unbalance
  • DC Bus Output
  • Unbalance and Output Signal Review
  • Transients
  • Diagnosing Transients
  • Harmonics
  • Diagnosing Harmonics
  • Transients and Harmonics Review
  • Leakage Current and Insulation Breakdown
  • Insulation Inspection
  • Insulation Resistance Testing Methods
  • Thermal Inspection
  • Thermal Abnormalities
  • Insulation and Thermal Inspection Review
Objectives
  • Describe power quality in motor drive systems.
  • Describe common electrical testing tools used with motor drive systems.
  • Describe best practices for measuring input power in a motor drive system.
  • Describe input power inspection.
  • Describe motor overloading inspection.
  • Describe single phasing inspection.
  • Calculate electrical unbalance percentages.
  • Describe input unbalance inspection.
  • Describe output unbalance inspection.
  • Describe DC bus output inspection.
  • Describe transients.
  • Describe transient inspection.
  • Calculate harmonic values.
  • Describe harmonic inspection.
  • Describe leakage current.
  • Describe insulation inspection.
  • Distinguish between common insulation resistance testing methods.
  • Describe thermal inspection.
  • Describe common causes of thermal abnormalities in motor drive systems.
Glossary
Vocabulary Term
Definition

A

Ampere. A unit of measurement that indicates the amount of current flowing in a circuit. Amperes are also called amps.

AC

Alternating current. Electricity that reverses direction at regularly recurring intervals of time. AC switches direction 60 times per second, or 60 hertz (Hz), in the US.

AC coupling

An input option on an oscilloscope that displays only AC signals. The AC coupling option provides a more detailed look at the ripple in a waveform.

alternating current

AC. Electricity that reverses direction at regularly recurring intervals of time. Alternating current switches direction 60 times per second, or 60 hertz (Hz), in the US.

ammeter

A device that measures amperes in an electrical circuit. Ammeters that have built-in current clamps are called clamp meters.

amperes

A. A unit of measurement that indicates the amount of current flowing in a circuit. Amperes are also called amps.

amps

A. A unit of measurement that indicates the amount of current flowing in a circuit. Amps are also called amperes.

arcing

Overheating that occurs when electricity flows from one surface to another. Arcing is dangerous because it can cause injuries to operators and damage to equipment.

audible

Able to be heard by the human ear. Audible noise in a motor drive system may increase if the system experiences harmonics.

average

A number expressing the central or expected value within a set of values. The average of a set of values is equal to the sum of the values divided by the number of values in the set.

baseline

Initial data collected while a system or component is in working order. Baseline readings are compared to future readings in order to detect potential faults.

branch circuits

The part of a distribution system that consists of circuit conductors. Branch circuits are located between the final overcurrent protection devices in the distribution panel and the loads being powered.

capacitors

A device that stores electrical energy and releases it when needed. Capacitors are used to filter electricity in the DC bus of a variable frequency drive.

capacity

The amount of electricity that a conductor can carry safely. Capacity indicates how much current can flow through a device without it overheating.

circuit breaker

A safety device that detects overcurrent in a circuit. Circuit breakers open circuits to prevent overloading and overheating.

clamp meter

An ammeter or multimeter that includes a built-in current clamp. Clamp meters can be used instead of DMMs and clamp accessories to measure current.

cold spots

A point within a system or component at which the temperature is lower than the surrounding temperature. Cold spots may be caused by unbalance and other electrical faults.

conductors

A material or element that allows electricity to flow freely. Conductors are used to connect electrical components.

contactors

A type of relay that uses a small control current to operate contacts and energize or de-energize a load. Contactors are used in motor starters.

contacts

Conductive points in a circuit that allow current to flow when they are connected, or closed. Contacts that are open prevent electricity from flowing.

contaminants

Damaging foreign material, such as dirt or debris, that causes wear to machine components. Contaminants can lead to insulation breakdown.

current

The flow of electricity. Current strength is called amperage and is measured in amperes (A), also called amps.

current clamp

An accessory device used with electrical meters to measure current. A current clamp closes around a conductor to quickly measure current.

current draw

The amount of current a load demands from its power source. Current draw depends on the amount of resistance the load provides.

current unbalance

A condition in which the phases of three-phase power differ in current magnitude. Current unbalance is often caused by voltage unbalance.

cycle

The time it takes for alternating current to flow, pause, reverse direction, and then pause again. One cycle is considered to be one complete 360° waveform.

DC

Direct current. Electricity that flows in one direction. DC does not reverse the direction of flow.

DC bus

The part of a variable frequency drive that filters electricity flowing from the rectifier to the inverter. The DC bus uses capacitors to smooth out ripple and electrical noise.

DC coupling

An input option on an oscilloscope that displays both DC and AC signals. The DC coupling option measures ripple and the absolute voltage of DC.

delta

A connection of three conductors that forms a triangular series circuit. Delta connections are used in three-phase power systems.

digital multimeter

DMM. A device that can measure voltage, current, and resistance. Digital multimeters are the most versatile and common meters used today for electrical maintenance.

diode

An electronic device with two terminals that allows electric current to flow in only one direction. Diodes are used by VFD rectifiers to convert AC power to DC power.

direct current

DC. Electricity that flows in one direction. Direct current does not reverse the direction of flow.

distortion

The deformation of an electrical signal's waveform. Distortion may be caused by harmonics.

distribution panel

A grouping of electrical devices that divide upstream electricity into separate circuits to reach individual components downstream. Distribution panels consist of fused switches, circuit breakers, and other devices.

distribution system

An electrical circuit that provides power from the utility to specific end destinations. The distribution system sends power along the mains to the service entrance of a building.

DMM

Digital multimeter. A device that can measure voltage, current, and resistance. DMMs are the most versatile and common meters used today for electrical maintenance.

downstream

Moving in a direction away from the power supply in an electrical circuit. Power quality tends to suffer as it moves downstream.

downtime

A period of time when production stops, often due to mechanical failure or maintenance needs. Downtime can be planned or unplanned.

efficiency losses

The reduction of energy due to natural effects, which cause the energy output to be less than energy input. Efficiency losses can be minimized with good design, but no system is 100% efficient.

electric shock

The flow of electricity through the body. Electric shock can be fatal.

enclosure

The metal frame or case of an electrical device such as a motor or drive. The enclosure protects the wiring inside the device.

feed

A circuit conductor that provides power from a power supply to one or more branch circuits. One feed may serve several loads, and changing conditions on one load can affect other loads on the feed.

filters

Electronic devices designed to keep any internally generated noise within the electronic equipment from affecting adjacent equipment via the shared AC power lines. Filters often experience leakage current.

first harmonic

The waveform provided by the power supply. The first harmonic is also known as the fundamental.

flashover

An electrical explosion of heat and light that occurs when a short circuit forms between exposed conductors. Flashover may be caused by voltage transients.

flat-topping

A disturbance in a waveform that causes the peak to become overly rounded and resemble a flat line at the top. Flat-topping in the mains' supply can cause low DC bus voltages.

frequency

The number of complete AC cycles that occur in one second. Frequency is measured in hertz (Hz).

full load amps

FLA. The maximum current a motor uses when producing full torque and horsepower. The full load amps value can be multiplied by the service factor to approximate the motor's service factor amps.

fundamental

The waveform provided by the power supply. The fundamental is also known as the first harmonic.

ground

A source that absorbs stray electrical charge. Ground may refer to the earth or another large conducting body, such as a motor housing, that serves as a source of zero electrical potential.

ground fault circuit interrupters

GFCIs. A type of switch that is disabled if the electricity comes into contact with water. Ground fault circuit interrupters should be used whenever there is a risk of electricity contacting water.

grounding conductor

The wire that provides a low-resistance path to ground for fault currents. Grounding conductors are usually bare copper or covered with green insulation.

hertz

Hz. A unit of measurement indicating the frequency of alternating current. One hertz is equal to one cycle per second.

hot spots

A point within a system or component at which the temperature is higher than the surrounding temperature. Hot spots may be caused by high-resistance connections, unbalance, and other electrical faults.

housing

A protective cover designed to contain or support a component. The housing of an electric motor is a metal enclosure that protects the internal motor parts.

Hz

hertz. A unit of measurement indicating the frequency of alternating current. One hertz is equal to one cycle per second.

IEEE 519-2014

A standard that lists the acceptable levels of distortion for harmonics. IEEE 519-2014 was created by the Institute of Electrical and Electronics Engineers (IEEE).

impedance

The effective resistance to current flow in an AC circuit. Impedance should be the same in all phases of a motor.

induction motors

An AC motor in which power is connected only to the stator. Induction motors are the most common motors used for industrial applications.

infrared

IR. Invisible rays of light energy emitted by heated objects. Infrared light can be used to capture a visual representation of an object's thermal signature.

infrared cameras

A thermal inspection device that uses infrared technology to produce a visual representation of temperature conditions. Infrared cameras are also called thermal imagers.

infrared thermometers

A thermal inspection device that uses infrared technology to measure temperature. Infrared thermometers provide a numerical reading of an object's thermal signature.

Institute of Electrical and Electronics Engineers

IEEE. An international organization that creates standards for working with electricity and electrical components. The Institute of Electrical and Electronics Engineers defines the acceptable levels for different harmonics in IEEE 519-2014.

insulation

A material that does not conduct electricity and is used to cover electrical conductors. Insulation contains current to prevent excess leakage and other faults.

insulation resistance tester

A diagnostic tool that supplies a low DC voltage to a conductor to measure the current leakage rate. The insulation resistance tester uses the current leakage rate to calculate the resistance of the insulation.

intermittent

Occurring at periodic intervals. Intermittent electrical issues can be monitored with the logging function of a PQA.

inverter

A device that converts DC power into a signal that resembles AC power. Inverters often use pulse width modulation to output pulsed DC that the motor interprets as AC.

jaws

Movable parts that allow a device to open and close around an object. The jaws of a current clamp allow the clamp to close around conductors.

kilovolt

kV. A unit of measurement that indicates the amount of electrical force or pressure in a circuit. One kilovolt is equal to 1,000 volts.

leakage current

The current that flows through the insulation of a conductor to ground. Leakage current increases as insulation degrades.

leakage current clamps

A screening tool that measures the amount of current leaking from a conductor. Leakage current clamps can be used while equipment operates.

loads

The opposition to an applied force, such as a weight or resistance. 'Loads' also refers to the devices a motor powers since they provide resistance to the motor.

local distribution system

The circuits that deliver power to separate areas and components within a building. Local distribution systems include fixed equipment installations and distribution panels.

logging function

A feature that allows testing tools to repeatedly measure and record various parameters over time. The logging function of a power quality analyzer allows it to monitor systems for intermittent or long-term faults.

low-pass filter function

A setting that allows low-frequency signals to pass through and blocks high-frequency signals. The low-pass filter function on an oscilloscope emulates a motor's impedance characteristics and filters the waveform to give a realistic reading of the effective voltage at the motor terminals.

magnitude

The size of a wave. Magnitude is the highest positive point on the wave and the maximum positive value.

main disconnect switch

A heavy, spring-loaded switch on an electrical panel that cuts off power to a circuit within a facility. Main disconnect switches are used to ensure that a system is de-energized before maintenance.

mains

The power lines in a distribution system that supply electricity from the utility to buildings. Mains connect to the main switchgear in a building to provide power to a motor.

maximum deviation

The greatest existing degree of difference between a specific value and a set of values. Maximum deviation can be found by subtracting the specified value from the value furthest from it or subtracting the furthest value from the specified value.

mega-ohms

MΩ. A unit of measurement that indicates the amount of electrical resistance in a circuit. One mega-ohm is equal to 1,000,000 ohms.

megger

A device used to measure resistance in insulation. Meggers are also called megohmmeters.

megohmmeter

A device used to measure resistance in insulation. Megohmmeters are also called meggers.

milliseconds

One thousandth of a second. Transients last between a few milliseconds and a few nanoseconds.

motor disconnect

A device used to control when power is delivered to a motor. The motor disconnect can be used to isolate the motor from the greater electrical circuit.

motor drive

An electronic device that regulates the power supplied to a motor. A motor drive such as a variable frequency drive can be used to control the speed and torque of a motor.

motor drive systems

The combination of an electric drive and the motor it controls. Motor drive systems typically use variable frequency drives and three-phase AC motors.

motor starter

An electrically operated switch that starts a motor when actuated. Motor starters provide the startup current for a motor.

multi-channel oscilloscopes

An oscilloscope that can be used to measure multiple conductors at once. Multi-channel oscilloscopes are used to measure output unbalance.

nameplate

A label attached to a device, such as a motor, that displays manufacturer-provided ratings and information. Nameplates include information such as voltage and current ratings and full load amps.

nanoseconds

One billionth of a second. Transients last between a few milliseconds and a few nanoseconds.

neutral wire

The conductor that returns power back to its source and completes a circuit. The neutral wire is usually covered with white insulation.

nominal values

The specified operating level for an electrical variable. Nominal values determine the appropriate ranges for variables such as voltage, current, and frequency.

ohms

Ω. A unit of measurement that indicates the amount of electrical resistance in a circuit. Ohms can be measured by megohmmeters.

optimization

A systematic process of adjusting a device's operating conditions to maximize efficiency. Optimization helps prevent faults like electrical unbalance.

oscilloscope

An electrical testing instrument that produces a visual representation of a voltage or current waveform. Oscilloscopes are also called scopemeters.

output signal

The electrical current sent from one device to another. The output signal produced by the inverter of a VFD provides the voltage that the motor receives.

overcurrent

A situation in which excess current flows in an electrical component. Overcurrent conditions are also called overloading conditions.

overcurrent devices

A component that protects circuits from excess current flow to prevent overheating and fires. Overcurrent devices include fuses and circuit breakers.

overloading

A state in which the level of current exceeds the recommended level for a device or circuit. Overloading can cause overheating and equipment damage.

peak

The greatest magnitude of a waveform. Peaks can be positive or negative.

phase displacement

The separation of the three phases in a three-phase system. The phase displacement should always be 120 degrees.

point-in-time data

Information gathered at one instance by taking a single measurement. Point-in-time data reflect present conditions rather than long-term or intermittent conditions.

polarity

Having two oppositely charged poles, one positive and one negative. Polarity determines the direction in which current flows.

power quality analyzer

PQA. A testing tool that measures levels and characteristics of electrical power. A power quality analyzer can be installed in a system to act as a monitoring tool.

PQA

Power quality analyzer. A testing tool that measures levels and characteristics of electrical power. A PQA can be installed in a system to act as a monitoring tool.

probes

A device attached to an electrical testing instrument that uses a conductive tip to connect to an electrical component. Probes connect to conductors and terminals.

ramp voltage test

A method of measuring insulation resistance by applying increasing voltages and taking several readings. A ramp voltage test typically takes five readings in a five-minute period.

readout

A display that indicates the measurements taken by a testing tool. The readout may display measurements in numbers or in images.

real-time data

Information gathered over a period of time by taking multiple measurements. Real-time data reflect long-term and intermittent conditions and allows for trending.

rectifier

A device used in an electrical circuit to convert AC power to DC power. Rectifiers allow electrical current to flow in only one direction.

reflections

An unwanted electrical signal cast by an original electrical signal traveling in the opposite direction. Reflections may be whole or partial copies of the original waveform.

resistance

The opposition to current flow. Resistance is measured in ohms (Ω).

ripple

Fluctuation of voltage that results from the AC voltage supplied to a variable frequency drive. Ripple must be eliminated before the voltage reaches the inverter.

RMS

Root mean square. The square root of the arithmetic mean of a set of values. The RMS indicates the effective average of the values.

root cause

The true origin of a problem, as opposed to any resulting symptoms or effects. Identifying the root cause of a problem is an important aspect of troubleshooting and maintenance.

root mean square

RMS. The square root of the arithmetic mean of a set of values. The root mean square indicates the effective average of the values.

sags

A short decrease in voltage of more than 10% below the normal rated level. Sags last between one-half an AC cycle and one minute.

scopemeter

An electrical testing instrument that produces a visual representation of a voltage or current waveform. Scopemeters are also called oscilloscopes.

service entrance

The point where electricity enters a building. A service entrance switchgear has metering equipment and devices for overcurrent protection and electrical control.

service factor

SF. The amount of additional load above a motor's rated horsepower that the motor can withstand for short periods of time. The service factor can be multiplied by the full load amps value to approximate the motor's service factor amps.

service factor amps

SFA. The amount of current a motor will use when experiencing the amount of overload specified by the service factor. Service factor amps is approximately equal to full load amps multiplied by service factor.

short circuit

A situation in which current takes a shorter, unintended path between two conductors. Short circuits cause excess current flow.

single phasing

A condition in which one of the phases of a three-phase motor does not conduct electricity. Single phasing leads to 14% of all motor failures.

single-phase

1Φ. Alternating current power that consists of only one voltage. Single-phase power is typically used for simple applications like lighting and heating.

spot insulation test

A method of measuring insulation resistance by applying a voltage and taking a single reading. A spot insulation test typically takes one minute.

stator

The stationary part of a motor. The stator houses the motor's rotor and windings.

subpanel

An enclosed collection of electrical devices used to control electricity as it travels downstream to separate circuits and components. A subpanel contains devices such as switches, fuses, and circuit breakers.

superimposed

Placed over or above something else. Harmonics are additional waveforms superimposed over an original, or fundamental, waveform.

surge arrestors

A device that protects electrical circuits and equipment from transients by regulating voltage on the line. Surge arrestors react quickly to high-voltage conditions to protect against damages.

switching electronic loads

A power supply component that includes built-in devices that turn on and off to convert AC to DC. Switching electronic loads can cause transients within a building.

terminals

A connecting point in an electrical circuit or device to which a wire can be attached to connect a component. Electrical measurements are often taken at terminals.

THD

Total harmonic distortion. The combined effects of a fundamental's harmonics, from the second to the 50th. THD is expressed as a percentage.

thermal imagers

A thermal inspection device that uses infrared technology to produce a visual representation of temperature conditions. Thermal imagers are also called infrared cameras.

thermal signatures

The specific temperatures produced by a component. Thermal signatures can be used to identify abnormal temperature conditions.

three-phase

3Φ. Alternating current power that consists of three overlapping voltages. Three-phase power is used for all large AC motors and is the standard power supply that enters homes and factories.

threshold

A limit at which a given parameter is no longer within tolerance. Thresholds identify readings as acceptable or unacceptable.

time-resistance test

A method of measuring insulation resistance by applying a constant voltage and taking several readings. A time-resistance test typically takes readings every 10 seconds during the first minute of testing and then takes a reading every minute for 9 more minutes.

torque

A force that produces rotation. Torque is measured in pound-feet (lb.-ft.) in the English system and newton-meters (N-m) in the metric system.

total harmonic distortion

THD. The combined effects of a fundamental's harmonics, from the second to the 50th. Total harmonic distortion is expressed as a percentage.

transformer

An electromagnetic device that increases and decreases the voltage of electrical power. Transformers often increase voltage for power lines and then decrease voltage for the power to enter a facility.

transient

A momentary surge of power that causes a circuit to experience extremely high voltages for a short period of time. Transients can damage motors and other electrical devices.

transients

A momentary surge of power that causes a circuit to experience extremely high voltages for a short period of time. Transients can damage motors and other electrical devices.

unbalance

A condition in which the phases of three-phase power differ in current or voltage magnitude or do not have a phase displacement of 120 degrees. Unbalance is expressed as a percentage.

V

Volt. A unit of measurement that indicates the amount of electrical force or pressure in a circuit. Volts measure voltage, which is also known as electromotive force.

variable frequency drive

VFD. A device that converts incoming 60 Hz AC power into other desired frequencies. Variable frequency drives can be used to control the speed of AC motors.

vibration

A rapid, continuous, repetitive motion in a machine or other structure. Vibration can negatively affect mechanical operations.

voltage

The electrical force or pressure that causes current to flow in a circuit. Voltage is measured in volts (V) and is also called electromotive force.

voltage drop

A decrease in voltage that occurs as electricity passes through resistance. Voltage drops occur as voltage travels through a circuit.

voltage notches

A disturbance in a voltage that appears as an indentation in the normal waveform. Voltage notches last for less than one-half of an AC signal's cycle.

voltage suppression devices

An electronic component used to protect circuits and electrical systems from transient voltages. Voltage suppression devices work by weakening or diverting transients.

voltage unbalance

A condition in which the phases of three-phase power differ in voltage magnitude. Voltage unbalance often causes current unbalance.

volts

V. A unit of measurement that indicates the amount of electrical force or pressure in a circuit. Volts measure voltage, which is also known as electromotive force.

waveform

The shape of the wave produced by an electrical signal. A waveform's shape depends on the method used to produce the current.

wear

The gradual removal of material on a surface caused by contact and friction. Wear is both a cause and an effect of faults in motor drive systems.

windings

Wire wrapped around a core or into a coil that is used to conduct current. Windings create the magnetic field in a motor that makes the motor work.

wye

A connection of three conductors in which one end of each conductor is connected to a common central point. Wye connections are used in three-phase power systems.

harmonics

An extraneous waveform superimposed over the fundamental waveform. Harmonics occur in groups and create distortion.

harmonics

Extraneous waveforms superimposed over the fundamental waveform. Harmonics occur in groups and create distortion.