Voltage The voltage at which the motor is designed to operate is an important parameter. Standard voltage for motors built to NEMA MG 1 (1987) are defined in MG 1-10.30. One common misapplication is of motors nameplated (rated) at one voltage but applied on a different voltage network using the + 10% voltage tolerance for "successful" operation. Nameplate-defined parameters for the motor such as power factor, efficiency, torque, and current are at rated voltage and frequency. Application at other than nameplate voltage will likely produce different performance. It is common for manufacturers to nameplate a wide variety of voltages on one motor nameplate. A common example is a motor wound for 230 and 460 V (230/460 V) but operable on 208 V. This 208-230/460 V motor will have degraded performance at 208 V. Another common misconception is to request a motor rated at network voltage; for example, at 480 V. The NEMA standard is 460 V.
The voltage rating assumes that there is voltage drop from the network to the motor terminals. Thus, the 460-V motor is appropriate on a 480-V network. Frequency Input frequency is usually 50 or 60 Hz. When more than one frequency is nameplated, other parameters that will differ at different input frequencies must be defined on the nameplate. The increasing use of adjustable frequency drives (AFDs) is also making it necessary to nameplate a frequency range, especially for hazardous-duty-listed applications. Phase This represents the number of ac power lines supplying the motor. Single and three-phase are the norms. Current Rated load current in amps is at nameplate horsepower (HP) with nameplate voltage and frequency. When using current measurement to determine motor load, it is important that correction be made for the operating power factor. Unbalanced phases, undervoltage conditions, or both, cause current to deviate from nameplate AMPS. Review both motor and drive for a matched system regarding current on AFD applications. Code A letter code defines the locked rotor kVA on a per-hp basis. Codes are defined in MG 1-10.37.2 by a series of letters from A to V. Generally, the farther the code letter from A, the higher the inrush current per hp. A replacement motor with a "higher" code may require different upstream electrical equipment, such as motor starters.
Type NEMA MG 1 requires manufacturer's type, but there is no industry standard regarding what this is. Some manufacturers use "Type" to define the motor as single or polyphase, single or multispeed, or even by type of construction. Type is of little use in defining a motor for replacement purposes unless you also note the specific motor manufacturer. Power factor Also given on the nameplate as "P.F." or PF," power factor is the ratio of the active power (W) to the apparent power (VA) expressed as a percentage. It is numerically equal to the cosine of the angle of lag of the input current with respect to its voltage, multiplied by 100. For an induction motor, power factor also varies with load. The nameplate provides the power factor for the motor at full load.