Current Affairs: Ratings And Reality (September 1999)
- "Why does the electrical designer say we have a 480-V system and my motors are rated 460 V?" And,
- "Can I use a 230/460-V rated motor in a building with 208 V?"
In truth, these are excellent questions.
Standard VoltagesMost commercial and industrial buildings have three-phase power systems identified as 480/277 V or 208/120 V. In such a system, the three generator or transformer windings providing the voltages are wye-connected (see Figure 1). The three points labeled A, B, and C are called the line terminals, and the midpoint of the wye is called the neutral terminal.
This connection produces a three-phase set of line-to-neutral voltages and a three-phase set of higher line-to-line voltages. The phase voltages add vectorially, rather than algebraically, and we have that VL-L = v3 VL-N. Thus in a 480/277-V system, there is 480 V available between any two lines and 277 V between any line and neutral. A similar relationship holds between voltages in the 208/120-V system.
High-power loads, such as large motors and heaters, are connected line-to-line to take advantage of the reduced current required at the higher voltage (P = V I) and are usually three-phase. Most single-phase loads, such as small motors, lighting, and receptacles, are connected line-to-neutral. In most large buildings, both 480/277-V systems and 208/120-V systems are used, with the lower-voltage system derived from the 480-V system through transformers. The large motors associated with the hvacr systems are supplied at 480 V, the majority of the lighting is supplied at 277 V, and the 208/120-V system is used for receptacle circuits and small equipment loads.
Delta-connected three-phase systems and single-phase systems are occasionally encountered in existing facilities, but they are seldom used in new construction.
Voltage Drop (Again!)These standard service voltages are the nominal values provided at the building service by the utility company. Between the service and the point of utilization, however, the current passes through the conductors that make up the building power distribution system, and the resistance and inductance of these conductors results in voltage drop. To allow for this, standard utilization voltages are often established at lower values than the service voltages. On a 480-V system, a motor rated 460 V can accommodate nearly 5% voltage drop in the circuit feeding it and still be operating at nominal voltage. Similarly, it is typical for single-phase equipment intended for use on a 120-V system to be rated 115 V.
Motors rated at 230/460 V are three-phase motors with two sets of windings which can be reconnected in the field for either voltage. Prior to the introduction of the electrical service voltages we use today, utilities commonly served large power users with three-phase power at a nominal line-to-line voltage of 240 V. Many older facilities still have this service voltage and a 230/460-V rating permits a standard motor to be used on either 240-V or 480-V systems.
Living On The EdgeIndustry standards established by the National Electrical Manufacturer's Association (NEMA) require ac motors to operate properly and deliver rated horsepower at voltages from 10% below to 10% above their nameplate rating. Since 90% of 230 V is 207 V, it might seem that a 230/460-V motor connected for the lower voltage would be within acceptable tolerance at 208 V. In reality, however, both voltage drop and supply voltage variation must be expected, and the motor will unavoidably be required to operate below its design minimum voltage. This may be tolerated by a lightly loaded motor without ill effect, but in a properly sized and loaded motor it will cause longer acceleration time on starting, higher line current, and excessive heating of both the stator and rotor. The results can range from decreased motor life to inability to start and operate the driven equipment.
Three-phase motors designed for operation on 208-V systems are available; the appropriate nameplate voltage rating for these motors per NEMA MG-1 is 200 V or 200-230/460 V. Careful attention to this issue will reduce problems in the field. ES