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Q - Why is the design of SSAC's new 3 phase line monitor an improvement over the competitor's designs? A - A. The "Easy Set" design assures accurate adjustment of fault trip points. B. Separate circuitry senses low voltage, voltage unbalance, and phase loss; therefore, phase loss and unbalance voltage protection are not lost when regenerative voltages (back EMF) are present, or during periods of large fluctuation in the average line-to-line voltage. C. For the first time, the OEM design engineer or plant engineer is able to select the maximum percentage of voltage unbalance his 3 phase motor will be subjected to. Knowing the maximum voltage unbalance is crucial to accurately calculate load derating, and for selecting and adjusting overload devices (relays) for any given application. Q - Which model(s) of SSAC's 3 phase line monitors contains this new integrated circuit design? A - All models - PLM, RLM, WLM Series, provide protection against: Phase reversal Phase loss (single phasing) Low voltage Unbalanced voltages Q - How is percentage of voltage unbalance calculated? A - The 1978 NEMA Standard MG1-14.34 defines voltage unbalance as: Example #1: If the line-to-line voltage readings were 22, 215, and 210, the maximum deviation is volts and the average is 215. Example #2: If the line-to-line voltage readings were 224, 208 and 213, the average voltage again is 215. The maximum deviation is 9 volts. Example #3: If the line-to-line voltage readings were 231, 215 and 199, the average voltage again is 215. The maximum deviation is 16 volts. *Note: A comparison of Example #1, #2, and #3 should illustrate why line monitors designed to detect voltage faults by monitoring the average line-to-line voltage, cannot sense moderately unbalanced voltages. In each example, the average line-to-line voltage is the same; however, the voltage unbalance varies from 2.3 to 7.4%. 7.4% voltage unbalance is too large for safe operation of 3 phase motors. 7.4% voltage unbalance will cause the motor windings to overheat by over 100 percent. NEMA Standard MG1-14.34: "Operation of a (3 Phase) motor above 5% voltage unbalance is not recommended." SSAC's 3 phase line monitors can reliably sense voltage unbalance by measuring the actual deviation of the voltages. Many other approaches do not provide adequate protection. Q - How does the unbalanced voltages cause the motor winding's temperature to rise? A - During periods of moderately unbalanced voltages, the motor will draw more current without increasing its output torque. This extra energy is converted to captive heat within the windings. Operating at normal speeds, a small percentage voltage unbalance will result in a much larger current unbalance. In most cases, the current unbalance percentage will be 6 to 10 times the voltage unbalance percentage. A 2% voltage unbalance can mean a current unbalance of 12 to 20%; therefore, a very small increase in voltage unbalance will create a large increase in the motor's winding temperature. THE PERCENTAGE OF OVERHEATING in the motor windings can be calculated directly if the percentage voltage unbalance is known. (See Graph Below). % Overheating = 2 (% Voltage Unbalanced2) Example: Voltage Unbalance = 4.0% % Overheating = 2 (42) = 2 (16) = 32% Q - I have an overload relay attached to my disconnect contactor. Why should I consider 3 phase voltage monitoring? A - First let's review the purpose of the overload disconnects (relay). A. To prevent fires caused by excessive high current levels, insulation failure, and then flash over. B. They are usually sized or adjusted to resist locked rotor currents for 10 to 20 seconds, and overload currents for 8 to 16 minutes. C. In the time it takes for overloads to respond, insulation damage will have already occurred. D. Overload relays cannot provide protection against: Phase reversal Moderately unbalanced voltages Overload relays may not provide protection against Phase loss (single phasing) Low or high voltages E. Overload relays do not protect the rotor, only the stator windings. Both the overload relay and the 3 phase voltage monitor provide necessary and complementary protection. SSAC suggests both types of protection be included in motor control designs. Q - I have temperature sensors embedded in the stator windings of my motors. Can the 3 phase voltage monitor provide extra protection? A - The answer is yes; let me explain how: A. The 3 phase voltage monitor provides fast disconnection of the motor windings when voltage faults occur. Over-temperature sensors will not disconnect the motor until the winding is in an overheated condition. Insulation damage will have already occurred near hot spots found within the windings. B. The 3 phase voltage monitor protects both the stator and rotor windings from voltage faults. Winding temperature sensors protect only the stator windings. C. Over-temperature protectors cannot provide protection against: Phase reversal Over-temperature protectors may not provide adequate protection to prevent insulation deterioration caused by: Moderately unbalanced voltages Phase Loss (single phasing) Low or high voltages Since insulation deterioration is permanent and cumulative, elevated winding temperature should be avoided. Disconnection of the motor windings can help prevent premature failure. D. Both the over-temperature sensors and the 3 phase voltage monitors provide necessary and complementary protection. SSAC suggests both types of protection be included in motor control designs. Q - How long is the payback period for 3 phase line monitors? A - When you consider downtime costs, motor rebuilding and replacement in most installations, a motor failure will cost the operator from $1,000 to $100,000. 60% or more of this figure is downtime and lost productivity costs. The line monitor will pay for itself at least tenfold for each motor failure it prevents. Although the cost for rebuilding or replacement are the most readily recognized costs of motor failure, these costs represent only the smallest portion of the total cost. Each time a prospect attempts to convince you (and themselves) that it is foolish to pay $100 to protect a $200 motor, you need to question: 1. How the motor is used? 2. How long it would take to get a new motor installed if the breakdown occurred Friday at 4:30 p.m.? 3. What is the output (in dollars) of the machine the motor powers per work hour? 4. Will employees be sent home early (with pay) waiting for this machine to be repaired? 5. Will the ship date of the product this machine manufactures be delayed because of downtime? 6. Will a customer find a new vendor for this component or raw material? THESE HIDDEN COSTS are important parts of the total cost of a machine (motor) failure. Three phase line monitors are protective devices similar to an insurance policy. The cost of installation is somewhat like a single payment premium. |
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