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Motor protection


It covers protection of low voltage and medium voltage motors through the use of contactors, switches, fuses, MCC, reduced voltage motor starting techniques and motor protection relays.

It covers protection of low voltage and medium voltage motors through the use of contactors, switches, fuses, MCC, reduced voltage motor starting techniques and motor protection relays.

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  • 1.
      Protection & Coordination
    Session 12: Motor protection
  • 2. Protection & Coordination Standards: NEMA & NEC
  • 3. Protection & Coordination
  • 4. Protection & Coordination
  • 5. Protection & Coordination
  • 6. Protection & Coordination
  • 7. Protection & Coordination Conductors for a single motor must have an ampacity of not less than 125% of the motor full-load current rating. Conductors that supply several motors must have an ampacity of not less than: (1) 125% of the highest rated motor FLC , plus (2) The sum of the FLCs of the other motors (on the same line). . The highest rated motor is the motor with the highest FLC from the standard. Determine the “other motors in the group” value by balancing the motor FLCs on the feeder being sized, then select the line that has the highest rated motor on it The motor branch-circuit short-circuit and ground-fault protective device must be capable of carrying the motor’s starting current. A branch-circuit short-circuit and ground-fault protective device protects the motor, the motor control apparatus, and the conductors against short circuits or ground faults Size overload protection devices based on the motor nameplate rating (this protects locked-rotor current damage to motor windings) . Branch-circuit conductors are protected against overloads by the overload device.
  • 8. Protection & Coordination Motor nameplates: To define the basic performance and mounting parameters of a motor, the National Electrical Manufacturers Association (NEMA) defines some basic design and dimensional parameters in NEMA Standard MG 1. These parameters are then coded onto the motor nameplate to give you a basic definition of what you have received. Manufacturers often include additional information to further define some key motor features. Section MG 1-10.40, "Nameplate Marking for Medium Single-Phase and Polyphase Induction Motors," of the NEMA standard requires that "The following minimum amount of information shall be given on all nameplates of single-phase and polyphase induction motors. For abbreviations, see MG 1-1.80." * Manufacturer's type and frame designation * Horsepower output.
  • 9. Protection & Coordination * Time rating. (intermitent, for how long or continuous.) * Maximum ambient temperature for which motor is designed. * Insulation system designation. * RPM at rated load. * Frequency. * Number of phases. * Rated load current. * Voltage. * Code letter for locked rotor kVA. * Design letter for medium motors. * NEMA nominal efficiency when required by MG 1- 12.55 * Service factor if other than 1.0. * For motors equipped with thermal protectors, the words "thermally protected" if the motor provides all the protection described in MG 1-12.52. * For motors rated above 1 hp equipped with over-temperature devices or systems, the words OVER TEMP. PROT.-." A type number as described in MG 1- 12.53 inserted in the blank would identify the protection type. The information on a motor nameplate can be arranged in categories. They are electrical inour, mechanical output, performance, safety and reliability
  • 10. Protection & Coordination Motor control centers: They come in various configurations. Their main purposes are to control (turn On & Off) the motors and provide protection to motors & connecting cables. The basic construction consists of an enclosure 90" high with widths & depths varying to accommodate the different components & devices packed into the enclosure. Such enclosures have: the 3-phase horizontal main bus, the risers, the barriers, the main incoming compartment (that would accommodate the lugs, the main switch or the main breaker), the different compartments complete with the back-plates & front compartment doors. The number of compartments per cubicle varies with the size of starters installed, the maximum is 6 or 7 (function of the design) & the minimum is 1. Motor control centers (MCCs) can be built back to
  • 11. Protection & Coordination back. The starters included in the MCCs may be wired according to any of the following standard designations: 1)EEMAC class I type A (no terminal blocks). 2)Class I type B (control wiring are brought to terminals in the starter unit). 3)Class I type C (no starter inter-wiring with the control & load wiring brought out to terminals at top or bottom of each section - cubicle- or to a master control section). 4)Class II type B (the control wires from each starter are brought to terminals in each unit with interlocking & inter-wiring between line-up units connected). 5)Class II type C (all wiring is brought to a master terminals section as class I type C but with interlocking & starters inter-wiring connected). The motor type, application and location affect the selection of the starters complete with their associated overload and short circuit protective devices. The full load current of a motor is the current required to produce the full load torque at rated speed. The locked-rotor current (inrush or starting) is the current drawn (from the power source) when the motor is first connected to the line (energized). It lasts over the acceleration period & it magnitude varies from 4 to 10 times the full load current.
  • 12. Protection & Coordination The speed of the SCIM depends on the number of poles of the motor & the frequency of the power supply. The output torque of the motor (which is the turning force) in lb-ft is equal (motor horsepower/motor revolution per minute) times 5252. The currents passing through the stator as well as the rotor windings of the motor cause an increase in the motor temperature (temperature rise over & above the ambient temperature). In general, the temperature rise produced at full load is not harmful, provided that the motor ambient temperature does not exceed 40oC. For each increase of 10oC above the rated (allowable) temperature of the motor (function of the insulation used for the windings & the iron laminations), the motor’s life is halved. Most motors have a continuous duty rating. Intermittent duty ratings are based on a fixed operating duration (5, 15, 30 or 60 minutes), after which the motor has to be stopped & left (allowed) to cool. If the motor is given a service factor, the motor is allowed to develop more than its rated HP. This increase in the output horsepower is equal the service factor (eg. 1.25, 1.15) minus 1 times the rated HP. Under such condition, no damage or deterioration of the insulation will occur. Jogging describes the repeated starting & stopping of the motor at frequent intervals (for short periods of time). Plugging of a motor means when a motor running in one direction is momentarily disconnected & reconnected to the source (in such a manner as to reverse its direction of rotation). This action will cause the motor to come to rest faster (than if one tries to brake it while running in the forward direction). The 3 common types of enclosures that a motor might have are the open drip proof (O.D.P.), the totally enclosed fan cooled (T.E.F.C.) and the explosion proof ones.
  • 13. Protection & Coordination A contactor is a 2-state device for repeatedly establishing & interrupting an electric power circuit. A magnetic contactor is a contactor activated by electromagnetic means (applied in ac circuits). The basic components of a contactor are: contact system, arc extinction devices, mechanical linkages between the contacts & the drive unit, the drive unit, the conductor connections & the frame. The electromagnet (for electromagnetic contactors) consists of, basically, a coil of wire placed on an iron core. The armature is mechanically connected to the contacts. When a contactor's armature is sealed-in, it means that it is held closely against the magnet yoke. When the coil is de-energized, thanks to the small air gap in the iron circuit, the armature drops out (opening the contacts). The contacts pressure corresponds to the rated current of the contactors.
  • 14. Protection & Coordination In order to reduce the heating produced by the changing of the magnetic flux, the magnets are built up of thin iron laminations. The hum is reduced through the use of shading ring (lag coil) in the pole; it is made of copper. The contacts make, break & carry the rated currents corresponding to the rating of the contactor. As mentioned previously, the most widely used industrial drives are the ac poly-phase induction motors having an average starting currents of 6 times their full load currents. In general, the starting motors currents have hardly any effect on the life of the contactor. The interrupting medium of low voltage contactor is air. The quality of contacts depends on the contact pressure as well as the properties of the contact material. An example for contact material is silver with additives like cadmium oxide. The rating structure of low voltage contactors (used in motor starters) is per EEMAC, the common sizes are 00 to 6 at 575 V, 3-phase supply, 2 HP to 400 HP motors. An example for a contactor continuous current rating for size 00 is 10 amperes (open) & 9 amperes (enclosed). Another example would be for size 6, where the continuous current rating would be 600 A (open) & 540 A (enclosed). The most common types of conrol diagrams used with motor control & protection circuits are the wiring (or connection) diagrams and the schematic (or the elementary) diagrams. In a 3-phase, 3-wire system, the motor circuit consists of 3 power lines (L1, L2 & L3). The motor terminals are identified as T1, T2 & T3. A manual starter consists primarily of a set of manually operated contacts & a motor overload-ensing device (melting alloy, bi-metallic or electronic type). Electro-magnetically operated ones are basically manual ones plus a solenoid for closing the power contacts. The magnet frame & armature assembly of the low voltage contactors are 1 of the following: clapper, plunger (vertical action) or bell crank.
  • 15. Protection & Coordination The combination motor controller consists of an externally operable circuit disconnecting means (a switch in series with Form II fuse for only short circuit protection or a circuit breaker with magnetic element protection only) in series with a magnetic motor controller (contactor plus overload or a relay that matches the motor heating curve), refer to sketch on page 50. The methods of starting a squirrel cage induction motor are: across the line (full voltage starting that provides the highest starting torque as well as the highest starting current & accelerates the load to full speed in the shortest possible time), auto- transformer, primary resistor, wye-delta starting & part winding.
  • 16. Protection & Coordination
  • 17. Protection & Coordination The second to the fifth methods, inclusive are termed reduced voltage motor starting methods. The selection of the reduced voltage method involves a co-ordination between the required starting torque & allowable starting current (this current overloads the circuit & may reduce the voltage - causing a sag - to other loads in the power distribution system). Certain starting methods necessitate that the motors would be built in certain ways. The starting torque varies with the square of the starting current (when reduced voltage methods - applying less than full line voltage to the motor during starting - are used, the starting current & torque will be lower). Contactors have to undergo certain tests to be qualified or approved by the bodies having jurisdiction like UL, CSA, IEC. The principle tests are: spacing between live parts & ground, temperature rise of coils, contacts & terminals, operating voltage range, overload capability, mechanical/electrical endurance & dielectric strength tests. The power (source connected) to the control circuit can be classified into: common control, separate control or control circuit transformer. It can come directly from the power feeding the (power) terminals of the motor, from an external source (d.c.- battery or a.c. station service transformer) or from a step-down control potential transformer connected to the supply line of the motor.
  • 18. Protection & Coordination
  • 19. Protection & Coordination Control panels can be built in different constructions depending on the environment and may include any or all of the following devices: contactors, breakers, switches, relays, push-buttons, selector switches, meters, pilot lights (filament or LED). These panels incorporate the logic (decision making), input & output devices. Motors are too sensitive to negative sequence currents generated when the voltage supplied to the motor is unbalanced or when a single-phase condition occurs. A sequence relay is used and is installed in the control panel. This relay can be of the negative sequence voltage sensing or the current sensing type. The type of the relay sensing technique dictates the location of the relay i.e. upstream or downstream the fault (to be sensed) for current sensing relays and downstream the fault for voltage sensing ones. The fused switch, the moulded case current limiting circuit breaker or the mccb plus the current limiter is used whenever the available short circuit current in the system is beyond the circuit breaker interrupting capacity. When it is required to limit the short circuit current (downstream) to a lower level than the available in the system, a switch/fuse combination is used with class J fuses (high rupture capacity current limiting fuses, 200 KA interrupting rating). Other fuse classes found in MCC line-ups are C & R. Programmable logic controllers (PLC) are used often to control motors and make decisions regarding starting, running & stopping the motors. They are usually in stalled in the MCC line-up.
  • 20. Protection & Coordination Protection of medium voltage motors:
  • 21. Protection & Coordination Medium voltage controllers: They consist of one or more contactors, each with 3 current limiting fuses, the draw-out mechanism, instrument transformers, meters, protective low voltage devices (relays), bus bars, barriers, shutters, operating coils, control transformers, auxiliary switches, activating levers & springs, the frame and the steel enclosure housing all these components. The enclosures may be front access (to be built back to back or to be installed against the wall) only or front & back access. Typical applications would include controlling & protecting the following equipment: squirrel cage induction motors (SCIM), including non-reversing, reversing & multi-speed, reduced voltage starters including autotransformer & reactor starting types, synchronous motors including brush-less & brush types, wound rotor motors, transformer & capacitor banks. For decades, medium-voltage motors were switched exclusively by air-break contactors. The design of air-break contactors has to allow for the large contact gap required (approximately 1.5 ") & for the arc chutes (as the arc has to be drawn & forced into the chutes to get elongated, cooled, having its resistance increased & extinguished). The arc is, generally, maintained for several 1/2 cycles, it restrikes after a current zero because the dielectric strength of the break (gap) recovers relatively slowly. As the arc is driven into the chute, its voltage rises sharply & the current amplitude decreases. At the instant of arc extinction, the phase angle between the current & voltage becomes zero and the voltage stressing the break becomes less severe. The second type of interrupting medium, used in motor controllers contactors, is vacuum. With vacuum contactors, the arc is confined to the
  • 22. Protection & Coordination contact gap (which is only a few millimeters as vacuum has a dielectric strength of 30 KV/mm). Arcs in vacuum can be classified into diffused (for currents less than 10 KA ) & constricted (for higher currents). When the short circuit current exceeds 10 KA, a transition occurs from the diffused to the constricted form of arc in vacuum. Overheating (which is one of the side effects of constricted arcs) can lead to restrikes. The two most commonly used contact shapes, of vacuum interrupters, are the disc & the hollow cylindrical ones. The latter would have inclined slits (to divert the current flowing away from the axial direction). The arc is driven along the circular end (contact end faces). The contact rings are beveled to push the arc outwards (toward the rim or edge of the
  • 23. Protection & Coordination circular contact). After contact separation, the metal vapour plasma fills the entire contact gap. Short recombination time of the charge carriers (in the contact gap zone) is one important property of the plasma. As the current decreases (on the short circuit current cycle heading toward zero), the carriers’ number decreases. After arc extinction, the remaining charge carriers recombine on the metal surface (contacts) and the gap (contacts break) de-ionizes pretty fast. After current zero, restriking is eliminated. The current carrying contacts experience minimum erosion. High number of operations (2 million) with up to 300 amperes motor load is possible. The arc energy is low as well as the arcing time (1/2 cycle). The heat dissipation is reasonable as the arc duration is short & the amount of heat generated is small i.e. no possibility of contact welding. Contact surface oxidization and contamination is not possible as the interior of the vacuum bottle is isolated from its surroundings (environment). Filler material (for the contact) is used in order to achieve lo current chopping levels (in the range of 0.3 ampere). To calculate the superimposed peak overvoltage due to current (arc chopping), use the following formula overvoltage = surge impedance times chopped current magnitude (where surge impedance = (Inductance / Capacitance) .5 ohms). The use of motor starters to control medium voltage motors, placed an emphasis on the ability of the series fuse to withstand current pulses (due to motor starting, inrush, currents). A fuse used in protection of electrical motors must fulfill the following requirements: 1)It should have a good a good withstand ability, in the 10 to 60 seconds zone, and it should act fast in the.1 to .01 seconds zone. 2)It should possess minimum values for the cut-off currents as well as for the current let through (I2t).
  • 24. Protection & Coordination 1)It should have the ability to interrupt all fault currents beyond the capability of the associated devices (the contactor) up to the maximum fault level of the system. 2)It should have the highest possible current rating (for a given fuse size) to allow for maximum utilization of space within associated equipment. The use of contactor/fuse combination may lead to smaller size cables (than with circuit breakers due to the fact of the limiting capability for the let through energy of the fuses). The fuse elements (used in motor protection fuses) have a straight through element with stress relief points (rather than a spirally wound filament on a core). In general, surge arresters are not required with vacuum contactors due to the fact that the cable distance between the contactor & the terminal box of the motor is quite short and shot cable lengths are used. Though, if multiple-reignition is expected, then surge arresters must be used and located close to the motors. Vacuum contactors are not suitable for use on direct current systems. De-rating of the motor controller units may be deemed necessary under abnormal operating conditions like altitude, frequency of power supply, temperature, exposure to fumes, dirt, humidity, etc. The table defining a medium voltage controller (contactor and fuse) will have the following data as a minimum: the contactor maximum continuous current, the interrupting capacity (at the specified KV), the designation, the voltage rating and range, the interrupting capacity of the fuse (in KA and MVA @ the rated voltage), the maximum HP motor rating (for the motor design, power factor, voltage, controller current rating), the dielectric withstand voltage (including BIL), the controller-type (full
  • 25. Protection & Coordination voltage, reversing vs. non-reversing or reduced voltage, auto transformer vs. reactor type). The medium voltage fuses for controllers can be defined when the following values are given: the motor locked rotor current, motor full load current & service factor, the maximum continuous current rating of the fuse inside the compartment, the fuse size, the peak current let through characteristics. The standard ratings for the medium voltage open (i.e. not installed in an enclosure) contactors are 400 & 800 amperes. The contactor rating is derated when installed in its compartment in the cubicle. For example, with one-high controller cubicle, the current ratings for the 800 & 400 amperes would be 720 & 360 amperes.
  • 26. Protection & Coordination
  • 27. Protection & Coordination
  • 28. Protection & Coordination Motor protection relays: A relay offering comprehensive protection may have but not limited to the following set of features: a. thermal protection (49) b. extended start protection /monitoring (accelerating time) (48) c. stalling protection (mechanical jam) d. number of starts limitation (66) e. short circuit protection (50P) f. ground (earth) fault protection (50/51G) g. winding RTD measurement/trip (38) h. negative sequence current detection (46) i. undervoltage protection (27) j. loss-of-load protection or under current (37) k. out-of-step protection (for synchronous motors only) (40) l. loss of supply protection (for synchronous motors only) (56) m. auxiliary supply supervision (59K) Motor thermal protection (protection against overload conditions/overheating): The majority of winding failures are either indirectly or directly caused by overloading (either prolonged or intermittent/cyclic), operation on unbalanced supply voltage (causing negative sequence current through the motor negative resistance), or single phasing (has the same effect as unbalance source), which all lead through excessive heating to the deterioration of the winding insulation until an electrical fault occurs.
  • 29. Protection & Coordination The generally accepted rule is that insulation life is halved for each 10° C rise in temperature above the rated value, modified by the length of time spent at the higher temperature. As an electrical machine has a relatively large heat storage capacity, it follows that infrequent overloads of short duration may not adversely affect the machine. However, sustained overloads of only a few percent may result in premature ageing and insulation failure. Furthermore, the thermal withstand capability of the motor is affected by heating in the winding (which function of the motor loading) prior to a fault. It is therefore important that the relay characteristic takes into account the extremes of zero and full-load pre-fault current known respectively as the 'Cold' and 'Hot' conditions. The relays, in general, offer the following choices to the user: • Thermal limit curves per NEMA standard, voltage dependent and customized • IEC 60255-8 Thermal overload curves • Current unbalance biasing (including the effect of negative sequence current in heating the motor) • Independent running and stopped exponential cooling curves • Optional RTD biasing of the Thermal Model to adapt to real-time temperature measurements • Compensation for Hot (when the motor is restarting and its temperature is higher than under cold starting condition)/cold motor conditions. In general, the relay thermal model integrates both stator and rotor heating and cooling into a single model. Extended start time protection:
  • 30. Protection & Coordination The relay function protects the motor from overheating in cases of abnormal loading during motor starts. The motor can be tripped if the motor does not reach a running condition within the programmable Motor Acceleration time. Stalling protection (mechanical jam): This function senses increased loading associated with process or load related faults causing the motor to jam or stall. A programmable delay setting can be used to allow the process to attempt clear itself before issuing a trip. Number of starts limitation (start inhibit): This function prevents starting of a motor when the motor is too hot and does not have a sufficient amount thermal capacity available to allow a start without being re-tripped. Short Circuit Protection: Motor short-circuit protection is often provided to cater for major stator winding faults and terminal flashovers. Short Circuit Overcurrent protection protects damage to the motor during a locked rotor condition. The relay provide for instantaneous overcurrent protection, the elements can be configured for protection, alarming, and control during locked rotor conditions. For motors fed by circuit breakers, a single definite time overcurrent relay element is all that is required for this purpose, set to about 125% of motor starting current. The time delay is required to prevent spurious operation due to CT spill currents, and is typically set at 100ms. Ground fault protection: One of the most common faults to occur on a motor is a stator winding fault. Whatever the initial form of the fault (phase-phase, etc.) or the cause (cyclic overheating, etc.),
  • 31. Protection & Coordination the presence of the surrounding metallic frame and casing will ensure that it rapidly develops into a fault involving earth. Therefore, provision of earth fault protection is very important. The type and sensitivity of protection provided depends largely on the system earthing. It is common, however, to provide both instantaneous and time-delayed relay elements to cater for major and slowly developing faults. Overtemperature protection: The crelay ould support RTD inputs that allow for configuring the Alarming and Tripping Temperature of each RTD, detecting RTD shorting conditions, and selecting RTD voting that requires more than one RTD to detect an over-temperature condition before it will issue a Trip command. The programmable RTD inputs can be used in monitoring the Stator, Bearing and the ambient temperatures. Each RTD input has 2 operational levels: alarm and trip. The relay could support RTD trip voting and provide open RTD failure alarming Negative sequence current detection: Unbalanced currents give rise to a negative sequence component in the stator current. The negative sequence current produces an additional flux which rotates at synchronous speed in the opposite direction of the rotor. The eddy currents which are induced in the rotor parts will have the double network frequency. During such sustained conditions, the temperature of the rotor may reaches high levels which accelerate the ageing of the insulation and cause mechanical stress on the rotating components. The relay calculates negative sequence current
  • 32. Protection & Coordination and compared it with the set operate value for the alarm and the trip function. Undervoltage protection: Motors may overheat or even stall when subjected to prolonged undervoltage conditions. Transient undervoltages will generally allow a motor to recover when the voltage is restored, unless the supply is weak. Motors fed by contactors have inherent undervoltage protection, unless a latched contactor is used. Where a specific undervoltage trip is required, a definite time undervoltage element is used. If two elements are provided, alarm and trip settings can be used. An interlock with the motor starter is required to block relay operation when the starting device is open, otherwise a start will never be permitted. The voltage and time delay settings will be system and motor dependent. They must allow for all voltage dips likely to occur on the system during transient faults, starting of motors, etc. to avoid spurious trips. As motor starting can result in a voltage depression to 80% of nominal, the voltage setting is likely to be below this value. Re-acceleration is normally possible for voltage dips lasting between 0.5-2 seconds, depending on system, motor and drive characteristics, and therefore the time delay will be set bearing these factors in mind. Loss-of-load protection or under current: The Undercurrent Protection element provides the ability of tripping the motor due to external conditions that can cause the load being pulled by the motor to drop below a pre-set level. This element is useful when the loss of the load results in a loss of cooling which will cause the asset to overheat. Loss-of-load protection has a number of possible functions. It can be used to protect a pump against becoming unprimed, or to stop a motor in case of a failure in a mechanical transmission (e.g. conveyor belt)/
  • 33. Protection & Coordination Out-of-step protection (for synchronous motors only): A synchronous motor may decelerate and lose synchronism (fall out-of-step) if a mechanical overload exceeding the peak motor torque occurs. Other conditions that may cause this condition are a fall in the applied voltage to stator or field windings. Such a fall may not need to be prolonged, a voltage dip of a few seconds may be all that is required. An out-of-step condition causes the motor to draw excessive current and generate a pulsating torque. Even if the cause is removed promptly, the motor will probably not recover synchronism, but eventually stall. Hence, it must be disconnected from the supply. The current drawn during an out-of-step condition is at a very low power factor. Hence a relay element that responds to low power factor can be used to provide protection. The element must be inhibited during starting, when a similar low power factor condition occurs. This can conveniently be achieved by use of a definite time delay, set to a value slightly in excess of the motor start time. The power factor setting will vary depending on the rated power factor of the motor. It would typically be 0.1 less than the motor rated power factor i.e. for a motor rated at 0.85 power factor, the setting would be 0.75. Loss of supply protection (for synchronous motors only): If the supply to a synchronous motor is interrupted, it is essential that the motor breaker be tripped as quickly as possible if there is any possibility of the supply being restored automatically or without the machine operator’s knowledge. This is necessary in order to prevent the supply being restored out of phase with the motor generated voltage.
  • 34. Protection & Coordination Two methods are generally used to detect this condition, in order to cover different operating modes of the motor. Auxiliary supply supervision (59K): This supervision may include any or all of the following: • Integrity of coils (monitoring continuously their resistance). In case coils are in a good condition, the output COIL OK will be activated. • Control voltage (supervision of low voltage). If voltage is correct, the output BREAKER VOLTAGE OK will be activated. • Checks the breaker control circuit. If both control voltage AND coils integrity are correct, then the output BREAKER CIRCUIT OK will be activated. Other protection functions that may be found in motor protection circuits : Stator Differential This relay provides Stator Differential Protection for fast clearing of stator phase faults. Advanced CT saturation detection algorithms as well as dual slope characteristic are incorporated for increased security during heavy faults. Underfrequency function: The underfrequency relay element will operate in the case of the supply failing when the motor is on load, which causes the motor to decelerate quickly. Typically, two elements are provided, for alarm and trip indications. The underfrequency setting value needs to consider the power system characteristics. In some power systems, lengthy periods of operation at frequencies substantially below normal occur, and should not result in a motor trip. The minimum safe operating frequency of the motor under load conditions must therefore be determined, along with minimum system frequency.