Some commercial customers have converted their building services from 240 volts 3-wire to 208Y/120 volts 4-wire. Seeing nameplates on equipment (especially HVAC) reading “208- 230” or “208/230” volts, they’ve assumed “no problem.” When motor failures then occurred, the complaint was “low voltage.” What’s the story?
First, motors are sold and warranted for voltages no lower than 10% under the value on the motor nameplate. This is a NEMA requirement. Standard motors are designed and nameplated “230” (or 230/460) volts. That means the minimum permissible terminal voltage (not back at the controller, or service entrance) is 90% of 230, or 207 volts. Second, an American National Standard (ANSI C84.1) prescribes the lowest allowable power system or supply voltage. For a “240” volt system, the service entrance voltage can be as low as 220; at motor terminals, the minimum is 212. Thus, the 207 volt motor minimum will always be met.
But suppose the same motor is connected to a “208 volt” system. The ANSI standard then allows minimum service entrance voltage of 191; 184 is permitted at the motor. Those values are well below the 207 volt level. That forces the motor current way up, so that the motor is likely to overheat, and is no longer warranted for normal performance or life expectancy. So what’s the meaning of equipment nameplates reading “208-230” or “208/230” volts? No such markings appear in any standard. All they mean, in commercial practice, is that the basic motor design (most often made for 230 volts) is satisfactory at a terminal voltage of 208 – no less. That’s not the same as saying “OK for use on a 208 volt system,” where supply voltage can dip well below what the motor will tolerate.
Because of the confusion this has caused over the years, NEMA motor standards are now being revised to make this distinction clearer. But the changes may not appear for some time. Meanwhile, remember this: The motor truly intended for use on any 208 volt system is one designed and nameplated for “200” volts — an existing NEMA standard motor nameplate voltage intended for this purpose.
Too low a voltage does more than raise motor current. It also sharply decreases accelerating torque. The motor may no longer be able to start its load.
Why don’t starter overloads or other protective devices shut the motor down if the supply – voltage is dangerously low? Most of them will – provided the motor’s horsepower output (and therefore the incoming line current) is high enough. However, choice of conventional starter overloads is based on available discrete steps and on electrical code rules, which often leave some gaps in the protection. The further away from its rated voltage that any motor must operate, the more difficult it is to ensure full protection. An oversized motor, one in a cool location, or one on a 208 volt system that remains well above its undervoltage tolerance, may not be in trouble. But don’t count on that.
Richard L. Nailen, P. E.