Overvoltage and Undervoltage View Overvoltage Demonstration
View Undervoltage DemonstrationOvervoltage is an average line-to-line voltage that is greater than the maximum acceptable voltage for the equipment in use. Per NEMA standards, induction motors should be able to deliver their nameplate load rating when subjected to (Balanced) overvoltage of +10% of the motors nameplate value. Voltage variations beyond 110% may cause winding insulation failures due to excessive heat during startup while surge voltages can cause arching across the first turns of a winding. Overvoltage breaks down motor insulation causing the winding to prematurely fail. Undervoltage is an average line-to-line voltage that is lower than the minimum acceptable operating voltage for the equipment in use. Per NEMA standards, induction motors should be capable of delivering their nameplate load rating when subjected to (Balanced) undervoltage of -10% of the motors nameplate value. Voltage variations below 90%, of the nameplate rating, may cause overheating and winding insulation failures. When the voltage is low, the starting current increases dramatically. Undervoltage increases motor slip and difficulties in motor startup. |
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| Single Phasing Or Phase Loss Single phasing or phase loss is the operation of a three phase motor on only two phases to due the loss of voltage on one phase. Phase loss is the maximum condition of voltage unbalance. This occurs when one fuse blows, when there is a mechanical failure within the equipment, a broken power line, open supply transformer winding, or a lightning strike. |
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Phase Loss Motor Stalled or Phase Loss on Startup
View Phase Loss Motor Stalled Demonstration
View Phase Loss on Startup DemonstrationWhen one phase is lost, a loaded three phase motor cannot start or may stall under load. The electrical resistance (impedance) of a stalled motor is considerably less than a rotating motor. The current flowing, in the remaining winding(s), will increase to 600% of the nameplate rating. These current levels are called "Locked Rotor" current. Winding insulation subjected to locked rotor current may fail in as little as 15 to 90 seconds. This winding insulation damage is permanent and cumulative. Motors should not be started during phase loss and if they stall, they must be disconnected immediately. |
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Phase Loss Motor Running
View Phase Loss Motor Running DemonstrationDuring phase loss, a motor can regenerate more than 92% of the supply voltage in the open phase. This phenomenon occurs because the disconnected winding acts a generator. This regenerative voltage can fool "voltage averaging" and "phase shift detecting" voltage monitors into believing that a phase loss is not occurring. For proper protection, a voltage monitor MUST sense 5 to 8% unbalance otherwise, phase loss may not be detected. SSAC voltage monitors are designed to provide this protection. |
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The supply fuse on phase #1 has blown. Readings 3-1 and 1-2 are the result
of regenerated voltage. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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During phase loss, the current in the connected winding(s) will increase to 150% of full load current. The extra heat created causes the winding insulation to breakdown and fail. |
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Unbalanced Voltage
View Unbalanced Voltage DemonstrationThe measured line-to-line voltage values of the three phase supply are not equal. Per NEMA Standard MG1 14:35, voltages should be as evenly balanced as practical. Voltage unbalance results in a current unbalance that is 6 to 10 times the voltage unbalance. This causes the overheating that deteriorates the winding insulation. If an unbalanced condition exists, the nameplate horsepower rating must be derated to prevent overheating of the windings. A three phase motor operating in an unbalanced circuit, cannot deliver its rated horsepower. With 3% voltage unbalance, a motor can produce 90% of its rated horsepower without overheating. Operation of a motor when the unbalance exceeds 5% is not recommended. When unbalanced voltages exceed 5%, the temperature rise is so fast that protection by derating is not practical. Example: 3% Unbalance = 18% Overheating 5% Unbalance = 50% Overheating 7% Unbalance = 98% Overheating  Additionally, the life of a 1.0 service factor motor operating with a 3% continuous voltage unbalance (at full load) will be cut in half.  When the voltages are unbalanced, the line-to-line and the average voltages are different. Deviation is the difference between any one line-to-line voltage and the average of all three voltages. When the voltages are balanced, the line-to-line voltages and the average voltage are equal. This difference in voltage magnitude will cause an eddy current to flow. Eddy currents are blamed for the overheating of the windings. |
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Voltage Unbalance Example:
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The largest deviation is used to calculate the percentage of voltage
unbalance: Voltage Unbalance = Largest Deviation Average Voltage x 100 Voltage Unbalance = 16 208 x 100 = 7.7% Voltage unbalance is the most difficult to detect and therefore, often overlooked. Motor failures caused by voltage unbalance and are often misdiagnosed as being damaged by phase loss. Unless moderate unbalance can be monitored, phase loss with motor running, may not be detected. |
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Phase Reversal (Incorrect Phase Sequence)
View Phase Reversal DemonstrationWhen the connection of any two phases in a three phase power supply are reversed. This phase reversal will not affect heater or resistive loads but, a three phase motor will run backward. This condition is destructive to rotation sensitive equipment such as elevators, screw and scroll compressors, centrifugal pumps, and conveyors. The National Elevator Code ASME A17.1 Rule 210.6 requires phase reversal protection on all personnel transportation equipment such as moving walkways, escalators, elevators, and ski lifts. |
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Power Outage (Short Cycling, Overcycling)
View Power Outage DemonstrationMost power outages are 1 to 3 seconds long and are often accompanied by low voltages or brownouts. The utilitie's re-closing breaker will make three attempts to clear the lines. This process can result in short cycling. Short cycling is the repeated act of turning on and off a device in a short duration of time. Remember, during startup, a motor draws 600% of nameplate current. The motor must operate or sit idle until the normal temperature is reached. Most motors are designed to be started less than 10 times per hour. Short cycling can cause excessive heat and winding failure. |
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Overloads An overload is caused by the application of excessive load to a motor. This excessive load exceeds the FLA (Full Load Amps) rating of the motor causing it to draw more current and therefore, produce damaging excessive heat in the windings. When a motor has failed due to overloading, troubleshooters can often observe the even blackening of all of the motor windings. This blackening is caused by the motor's slow destruction over a long period of time. No obvious damage or isolated areas of damage to the insulation are visible. SSAC's ESTM Series provides full protection against the overloading of a three phase motor. |
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