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Motor Operation Efficiency Under Abnormal Conditions

Motor Operation Efficiency Under Abnormal Conditions

Operation under unusual service conditions may result in efficiency losses and the consumption of additional energy. Both standard and energy-efficient motors can have their efficiency and useful life reduced by a poorly maintained electrical system. Monitoring voltage is important for maintaining high-efficiency operation and correcting potential problems before failures occur.

Preventative maintenance personnel should periodically measure and log the voltage at a motor’s terminals while the machine is fully loaded.

Motors must be properly selected according to known service conditions. Usual service conditions, defined in NEMA Standards Publication MG1-1987, Motors and Generators, include:

  • Exposure to an ambient temperature between 0°C and 40°C
  • Installation in areas or enclosures that do not seri- ously interfere with the ventilation of the machine
  • Operation within a tolerance of ± 10 percent of rated voltage
  • Operation from a sine wave voltage source (not to ex- ceed 10 percent deviation factor)
  • Operation within a tolerance of ± 5 percent of rated frequency
  • Operation with a voltage unbalance of 1 percent or less

Over Voltage

As the voltage is increased, the magnetizing current increases by an exponential function. At some point, depending upon design of the motor, saturation of the core iron will increase and overheating will occur. At about 10 to 15 percent over voltage both efficiency and power factor significantly decrease while the full-load slip decreases. The starting current, starting torque, and breakdown torque all significantly increase with over voltage conditions.
A voltage that is at the high end of tolerance limits frequently indicates that a transformer tap has been moved in the wrong direction. An overload relay will not recognize this overvoltage situation and, if the voltage is more than 10 percent high, the motor can over-heat. Over voltage operation with VAR currents above acceptable limits for extended periods of time may accelerate deterioration of a motor’s insulation.

Under Voltage

If a motor is operated at reduced voltage, even within the allowable 10 percent limit, the motor will draw in- creased current to produce the torque requirements imposed by the load. This causes an increase in both stator and rotor I²R losses. Low voltages can also prevent the motor from developing an adequate starting torque. The effects on motor efficiency, power factor, RPM, and current from operating outside nominal design voltage are indicated in the diagram below.

Voltage Variation Effects on Motor Performance

Voltage Variation Effects on Motor Performance

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Reduced operating efficiency because of low voltages at the motor terminals is generally due to excessive voltage drops in the supply system. If the motor is at the end of a long feeder, reconfiguration may be necessary. The system voltage can also be modified by:

  • Adjusting the transformer tap settings
  • Installing automatic tap-changing equipment if sys- tem loads vary considerably over the course of a day
  • Installing power factor correction capacitors that raise the system voltage while correcting for power factor

Since motor efficiency and operating life are degraded by voltage variations, only motors with compatible voltage nameplate ratings should be specified for a system.

For example, three-phase motors are available with voltage ratings of 440, 460, 480, and 575 volts. The use of a motor designed for 460-volt service in a 480-volt system results in reduced efficiency, increased heating, and reduced motor life. A 440-volt motor would be even more seriously affected.

Phase Voltage Imbalance

A voltage imbalance occurs when there are unequal voltages on the lines to a polyphase induction motor. This imbalance in phase voltages also causes the line currents to be out of balance. The unbalanced currents cause torque pulsations, vibrations, increased mechanical stress on the motor, and overheating of one and possibly two phase windings. This results in a dramatic increase in motor losses and heat generation, which both decrease the efficiency of the motor and shorten its life.
Voltage imbalance is defined by NEMA as 100 times the maximum deviation of the line voltage from the average voltage on a three-phase system divided by the average voltage. For example, if the measured line voltages are 462, 463, and 455 volts, the average is 460 volts. The voltage imbalance is:

voltage imbalanceA voltage unbalance of only 3.5 percent can increase motor losses by approximately 20 percent. Imbalances over 5 percent indicate a serious problem. Imbalances over 1 percent require derating of the motor, and will void most manufacturers’ warranties. Per NEMA MG1-14.35, a voltage imbalance of 2.5 percent would require a derate factor of 0.925 to be applied to the motor rating. Derating factors due to unbalanced voltage for integral horsepower motors are given in the diagram below. The NEMA derating factors apply to all motors. There is no distinction between standard and energy-efficient motors when selecting a derate factor for operation under voltage unbalance conditions.

Motor Derating due to Voltage Unbalance

Motor Derating due to Voltage Unbalance

Common causes of voltage unbalance include:

  • Faulty operation of automatic power factor connection equipment
  • Unbalanced or unstable utility supply
  • Unbalanced transformer bank supplying a three-phase load that is too large for the bank
  • Unevenly distributed single-phase loads on the same power system
  • Unidentified single-phase to ground faults
  • An open circuit on the distribution system primary

The following steps will ensure proper system balancing:

  • Check your electrical system single-line diagram to verify that single-phase loads are uniformly distributed
  • Regularly monitor voltages on all phases to verify that a minimum variation exists
  • Install required ground fault indicators
  • Perform annual thermographic inspections

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Power quality

Power quality

The term power quality seeks to quantify the condition of the electrical supply. It not only relates largely to voltage, but also deals with current and it is largely the corrupting effect of current disturbances upon voltage. Power quality can be quantified by a very broad range of parameters, some of which have been recognized and studied for as long as electrical power has been utilized. However, the advent of the term itself is more modern and it has created a useful vehicle for discussing and quantifying all factors that can describe supply quality. Power quality is yet another means of analysing and expressing electromagnetic compatibility (EMC), but in terms of the frequency spectrum, power quality charac- terizes mainly low-frequency phenomena. Perhaps because of this and because of the manner in which it affects electrical equipment, power quality has largely been dealt with by engineers with electrical power experience rather than those with an EMC expertise. In reality, resolving power problems can benefit from all available expertise, particularly since power quality disorders and higher frequency emissions can produce similar effects.

In 1989, the European Community defined the supply of electricity as a product, and it is therefore closely related to the provisions and protection of the EMC Directive (89/336/EEC), but in drawing a comparison between electricity and other manufactured product it is essential to recall a significant difference.

Electricity is probably unique in being a product which is manufactured, delivered and used at the same time. An electricity manufacturer cannot institute a batch testing process for example and pull substandard products out of the supply chain. By the time electricity is tested it will have been delivered and used by the customer whether it was of good quality or not.

Key parameters

The parameters that are commonly used to characterize supplies are listed in Table 1 together with the typical tolerance limits which define acceptable norms. Within Europe these power quality limits are defined by the EN 61000 series of standards in order to be compatible with the susceptibility limits set for equipment.

Table 1: Summary of power quality levels defined by EN 50160
.Power frequency (50Hz).Interconnected systems
.±1% (95% of week)
.+4% (absolute level)
.−6% (absolute level)
.Supply voltage variations on 230V nominal.±10% (95% of week based on
.10 min samples, rms)
.Rapid voltage changes.±5% Frequent
.±10% Infrequent
.Flicker.Pk=1.0 (95% of week)
.Supply voltage dips.Majority
.Few 10s
.Duration <1s
.Depth <60%
.Some locations
.Few 1000 per year of <15% depth
.Short interruptions.20–500 per year
.Duration 1s of 100% depth
.Long interruptions.10–50 per year
.Duration >180s of 100% depth
.Temporary power frequency overvoltage.<1.5kV
.Transient overvoltages.Majority
.<6kV
.Exceptionally
.>6kV
.Supply voltage unbalance.Majority
.<2%(95% of the week)
.Exceptionally
.>2%, <3%(95% of the week)
.Harmonic voltage distortion.THD <8%(95% of the week)
.Interharmonic voltage distortion.Under consideration
.Mains signalling.95 to 148.5kHz at up to 1.4Vrms (not in MV)

The more a supply deviates from these limits, the more likely it is that malfunction could be experienced in terminating equipment. However, individual items of equipment will have particular sensitivity to certain power quality parameters while having a wider tolerance to others. Table 2 provides examples of equipment and the power quality parameters to which they are particularly sensitive. Table 2 shows a preponderance of examples with a vulnerability to voltage dips. Of all the power quality parameters, this is probably the most troublesome to the manufacturing industry; and in the early 1970s, as the industry moved towards a reliance on electronic rather than electromagnetic controls, it was commonly observed how much more vulnerable the industrial processes were to supply disturbances.

Supply distortion (characterized by harmonics) is another power quality parameter that has received enormous attention, with many articles, textbooks and papers written on the subject. However, the modern practices that will be discussed later have reduced the degree to which this currently presents a problem. Other parameters tend to be much less problematic in reality, although that is not to say that perceptions sometimes suggest otherwise. Voltage surge and tran- sient overvoltage in particular are often blamed for a wide range of problems.

Table 2: Examples of sensitivity to particular power quality parameters
Equipment typeVulnerable power quality parameter Effect if exceededRang
.Induction motor .Voltage unbalance.Excessive rotor heating.<3%
.Power factor correction .capacitors.Spectral frequency .content.This is usually .defined .in terms of harmonic .distortion.Capacitor failure due to .excessive current flow or .voltage.Most sensitive if .resonance occurs.In resonant .conditions
.PLCs .Voltage dips.Disruption to the programmed .functionality.V tr
.Computing systems .Voltage dips.Disruption to the programmed .functionality.V
.Variable speed drives, .motor starters and .attracted .armature control .relays .Voltage dips.Disruption to the control system .causing shutdown. V
.Power transformers .Spectral frequency content of .load current.This is usually .defined in terms of harmonic .distortion.Increased losses leading to excessive temperature rise.At full load
.Devices employing .phase .control, such as .light .dimmers and .generator .automatic .voltage regulator .(AVRs) .Alteration in waveform zero .crossing due to waveform .distortion, causing multiple .crossing or phase asymmetry.Instability.Will depend .upon the r
.Motor driven .speed-.sensitive plant .Induction and synchronous .motor shaft speed are .proportional to supply .frequency. Some driven loads .are .sensitive to even small .speed variations.The motors themselves are .tolerant of small speed .variations.At high supply .frequencies (>10%) shaft .stresses may be excessive due .to high running speeds.Limits .depend on .the .sensitivity

However, very often this is a scapegoat when the actual cause cannot be identified. Even when correlation with switching voltage transients is correctly observed, the coupling introduced by poor wiring installations or bad earth bonding practices can be the real problem. Unlike the other power quality parameters, voltage transients have a high frequency content and will couple readily through stray capacitance and mutual inductance into neighbouring circuits. Coupling into closed conductor loops that interface with sensitive circuits such as screens and drain wires can easily lead to spurious events.

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ANSI Standards For Medium Voltage protection

ANSI Functions For Protection Devices

In the design of electrical power systems, the ANSI Standard Device Numbers denote what features a protective device supports (such as a relay or circuit breaker). These types of devices protect electrical systems and components from damage when an unwanted event occurs, such as an electrical fault.

ANSI numbers are used to identify the functions of meduim voltage microprocessor devices.

ANSI facilitates the development of American National Standards (ANS) by accrediting the procedures of standards developing organizations (SDOs). These groups work cooperatively to develop voluntary national consensus standards. Accreditation by ANSI signifies that the procedures used by the standards body in connection with the development of American National Standards meet the Institute’s essential requirements for openness, balance, consensus and due process.

ANSI standards (protection) – index
Current protection functions
Recloser
ANSI 50/51 – Phase overcurrentANSI 79 – Reclose the circuit breaker after tripping
ANSI 50N/51N or 50G/51G – Earth fault or sensitive earth faultDirectional current protection
ANSI 50BF – Breaker failureANSI 67 – Directional phase overcurrent
ANSI 46 -Negative sequence / unbalanceANSI 67N/67NC – Directional earth fault
ANSI 49RMS – Thermal overloadANSI 67N/67NC type 1
Directional power protection functionsANSI 67N/67NC type 2
ANSI 32P – Directional active overpowerANSI 67N/67NC type 3
ANSI 32Q/40 – Directional reactive overpowerMachine protection functions
Voltage protection functionsANSI 37 – Phase undercurrent
ANSI 27D – Positive sequence undervoltageANSI 48/51LR/14 – Locked rotor / excessive starting time
ANSI 27R – Remanent undervoltageANSI 66 – Starts per hour
ANSI 27 – Phase-to-phase undervoltageANSI 50V/51V – Voltage-restrained overcurrent
ANSI 59 – Phase-to-phase overvoltageANSI 26/63 – Thermostat, Buchholz, gas, pressure, temperature detection
ANSI 59N – Neutral voltage displacementANSI 38/49T – Temperature monitoring by RTD
ANSI 47 – Negative sequence voltageFrequency protection functions
ANSI 81H – Overfrequency
ANSI 81L – Underfrequency
ANSI 81R – Rate of change of frequency (ROCOF)

Current protection functions

ANSI 50/51 – Phase overcurrent

Three-phase protection against overloads and phase-to-phase short-circuits.
ANSI index ↑

ANSI 50N/51N or 50G/51G – Earth fault

Earth fault protection based on measured or calculated residual current values:

  • ANSI 50N/51N: residual current calculated or measured by 3 phase current sensors
  • ANSI 50G/51G: residual current measured directly by a specific sensor

ANSI index ↑

ANSI 50BF – Breaker failure

If a breaker fails to be triggered by a tripping order, as detected by the non-extinction of the fault current, this backup protection sends a tripping order to the upstream or adjacent breakers.
ANSI index ↑

ANSI 46 – Negative sequence / unbalance

Protection against phase unbalance, detected by the measurement of negative sequence current:

  • sensitive protection to detect 2-phase faults at the ends of long lines
  • protection of equipment against temperature build-up, caused by an unbalanced power supply, phase inversion or loss of phase, and against phase current unbalance

ANSI index ↑

ANSI 49RMS – Thermal overload

Protection against thermal damage caused by overloads on machines (transformers, motors or generators).
The thermal capacity used is calculated according to a mathematical model which takes into account:

  • current RMS values
  • ambient temperature
  • negative sequence current, a cause of motor rotor temperature rise

ANSI index ↑

Recloser

ANSI 79

Automation device used to limit down time after tripping due to transient or semipermanent faults on overhead lines. The recloser orders automatic reclosing of the breaking device after the time delay required to restore the insulation has elapsed. Recloser operation is easy to adapt for different operating modes by parameter setting.
ANSI index ↑

Directional current protection

ANSI 67N/67NC type 1
ANSI 67 – Directional phase overcurrent

Phase-to-phase short-circuit protection, with selective tripping according to fault current direction. It comprises a phase overcurrent function associated with direction detection, and picks up if the phase overcurrent function in the chosen direction (line or busbar) is activated for at least one of the 3 phases.
ANSI index ↑

ANSI 67N/67NC – Directional earth fault

Earth fault protection, with selective tripping according to fault current direction.
3 types of operation:

  • type 1: the protection function uses the projection of the I0 vector
  • type 2: the protection function uses the I0 vector magnitude with half-plane tripping zone
  • type 3: the protection function uses the I0 vector magnitude with angular sector tripping zone

ANSI index ↑

ANSI 67N/67NC type 1

Directional earth fault protection for impedant, isolated or compensated neutralsystems, based on the projection of measured residual current.
ANSI index ↑

ANSI 67N/67NC type 2

Directional overcurrent protection for impedance and solidly earthed systems, based on measured or calculated residual current. It comprises an earth fault function associated with direction detection, and picks up if the earth fault function in the chosen direction (line or busbar) is activated.
ANSI index ↑

ANSI 67N/67NC type 3

Directional overcurrent protection for distribution networks in which the neutral earthing system varies according to the operating mode, based on measured residual current. It comprises an earth fault function associated with direction detection (angular sector tripping zone defined by 2 adjustable angles), and picks up if the earth fault function in the chosen direction (line or busbar) is activated.
ANSI index ↑

Directional power protection functions

ANSI 32P – Directional active overpower

Two-way protection based on calculated active power, for the following applications:

  • active overpower protection to detect overloads and allow load shedding
  • reverse active power protection:
    • against generators running like motors when the generators consume active power
    • against motors running like generators when the motors supply active power

ANSI index ↑

ANSI 32Q/40 – Directional reactive overpower

Two-way protection based on calculated reactive power to detect field loss on synchronous machines:

  • reactive overpower protection for motors which consume more reactive power with field loss
  • reverse reactive overpower protection for generators which consume reactive power with field loss.

ANSI index ↑

Machine protection functions

ANSI 37 – Phase undercurrent

Protection of pumps against the consequences of a loss of priming by the detection of motor no-load operation.
It is sensitive to a minimum of current in phase 1, remains stable during breaker tripping and may be inhibited by a logic input.
ANSI index ↑

ANSI 48/51LR/14 – Locked rotor / excessive starting time

Protection of motors against overheating caused by:

  • excessive motor starting time due to overloads (e.g. conveyor) or insufficient supply voltage.
    The reacceleration of a motor that is not shut down, indicated by a logic input, may be considered as starting.
  • locked rotor due to motor load (e.g. crusher):
    • in normal operation, after a normal start
    • directly upon starting, before the detection of excessive starting time, with detection of locked rotor by a zero speed detector connected to a logic input, or by the underspeed function.

ANSI index ↑

ANSI 66 – Starts per hour

Protection against motor overheating caused by:

  • too frequent starts: motor energizing is inhibited when the maximum allowable number of starts is reached, after counting of:
    • starts per hour (or adjustable period)
    • consecutive motor hot or cold starts (reacceleration of a motor that is not shut down, indicated by a logic input, may be counted as a start)
  • starts too close together in time: motor re-energizing after a shutdown is only allowed after an adjustable waiting time.

ANSI index ↑

ANSI 50V/51V – Voltage-restrained overcurrent

Phase-to-phase short-circuit protection, for generators. The current tripping set point is voltage-adjusted in order to be sensitive to faults close to the generator which cause voltage drops and lowers the short-circuit current.
ANSI index ↑

ANSI 26/63 – Thermostat/Buchholz

Protection of transformers against temperature rise and internal faults via logic inputs linked to devices integrated in the transformer.
ANSI index ↑

ANSI 38/49T – Temperature monitoring

Protection that detects abnormal temperature build-up by measuring the temperature inside equipment fitted with sensors:

  • transformer: protection of primary and secondary windings
  • motor and generator: protection of stator windings and bearings.

ANSI index ↑

Voltage protection functions

ANSI 27D – Positive sequence undervoltage

Protection of motors against faulty operation due to insufficient or unbalanced network voltage, and detection of reverse rotation direction.
ANSI index ↑

ANSI 27R – Remanent undervoltage

Protection used to check that remanent voltage sustained by rotating machines has been cleared before allowing the busbar supplying the machines to be re-energized, to avoid electrical and mechanical transients.
ANSI index ↑

ANSI 27 – Undervoltage

Protection of motors against voltage sags or detection of abnormally low network voltage to trigger automatic load shedding or source transfer.
Works with phase-to-phase voltage.
ANSI index ↑

ANSI 59 – Overvoltage

Detection of abnormally high network voltage or checking for sufficient voltage to enable source transfer. Works with phase-to-phase or phase-to-neutral voltage, each voltage being monitored separately.
ANSI index ↑

ANSI 59N – Neutral voltage displacement

Detection of insulation faults by measuring residual voltage in isolated neutral systems.
ANSI index ↑

ANSI 47 – Negative sequence overvoltage

Protection against phase unbalance resulting from phase inversion, unbalanced supply or distant fault, detected by the measurement of negative sequence voltage.
ANSI index ↑

Frequency protection functions

ANSI 81H – Overfrequency

Detection of abnormally high frequency compared to the rated frequency, to monitor power supply quality.
ANSI index ↑

ANSI 81L – Underfrequency

Detection of abnormally low frequency compared to the rated frequency, to monitor power supply quality. The protection may be used for overall tripping or load shedding. Protection stability is ensured in the event of the loss of the main source and presence of remanent voltage by a restraint in the event of a continuous decrease of the frequency, which is activated by parameter setting.
ANSI index ↑

ANSI 81R – Rate of change of frequency

Protection function used for fast disconnection of a generator or load shedding control. Based on the calculation of the frequency variation, it is insensitive to transient voltage disturbances and therefore more stable than a phase-shift protection function.

Disconnection
In installations with autonomous production means connected to a utility, the “rate of change of frequency” protection function is used to detect loss of the main system in view of opening the incoming circuit breaker to:

  • protect the generators from a reconnection without checking synchronization
  • avoid supplying loads outside the installation.

Load shedding
The “rate of change of frequency” protection function is used for load shedding in combination with the underfrequency protection to:

  • either accelerate shedding in the event of a large overload
  • or inhibit shedding following a sudden drop in frequency due to a problem that should not be solved by shedding.

ANSI index ↑

Related book: Relay selection guide

Link: Register

Autor: Edvard Csanyi, CsanyiGroup

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