Design and rating standards developed by the National Electrical Manufacturer’s Association (NEMA) permit the comparison of motors from different manufacturers. Information on the nameplate may include any or all of the following:
· VOLTS, the proper operating voltage, may be either a single value or, for dualvoltage motors, a dual value.
· AMPS is the full-load current draw in amperes with the proper voltage supply. When a dual number is listed, the motor will draw the smaller amperage when connected to the higher voltage source.
· RPM is the rotor speed when the motor runs at the full-load point on the torquespeed curve (Figure 3.12).
· HZ is the design operating frequency of the electrical supply. In the United States, it is 60 cycles per second. A standard frequency of 50 cycles per second is used in some counties.
· FR is one of the standard frame numbers used by manufacturers to insure interchangeability. For motors with power ratings below 0.75 kW (1.0 hp), common frame numbers are 42, 48, and 56. The frame number divided by 6.3 (16) gives the height in cm (inches) from the bottom of the mounting to the shaft centerline. Letters may be added to specify the type of mounting, for example, T frame or the heavier U-frame. A replacement motor with the same frame number as the original motor will fit on the same mounting.
· DUTY indicates whether the motor is rated for continuous or intermittent; HOURS may be used to indicate the length of time the motor can be safely operated during intermittent duty.
· TEMPERATURE RISE (°C) may be stated as the allowable temperature rise above a 40°C (104°F) ambient temperature while the motor is operating at full load. Often, a motor can be operated at 10% to 15% overload without damage, but the motor temperature should never exceed 55°C (131°F). If, while operating, a motor is not too hot to touch, it is not overheated.
· As an alternative to temperature rise, the allowable AMBIENT TEMPERATURE may be listed. Then the motor can be operated at full load in environments with temperatures below the stated ambient temperature.
· SF, the service factor, is multiplied by the rated power to obtain the permissible loading. For example, a service factor of 1.10 means the motor could be operated at 10% overload without overheating. Service factors for farm-duty motors can be 1.35 or more.
· INSULATION CLASS is a temperature-resistance rating of the insulation on the wires in the motor. Typical classes are A, B, F, or H, where class A is the lowest temperature rating. Class A or B insulation is used in most farm-duty motors.
· The CODE LETTER is used to determine the maximum rating of the motor branch-circuit protection and is based on the locked-rotor current drawn by the motor. The following equation may be used to calculate the locked-rotor starting current from the code letter:
where
amps = starting current in amperes
kVA = rating from the National Electric Code (NEC)
hp = rated power from nameplate, in hp
volts = supply voltage in volts
Cph = constant = 1.0 for single-phase motor or 1.73 for three-phase motor
· A DESIGN letter may be given on the nameplate as an indication of their starting-to-rated currents and starting-to-rated torques. The five classes for squirrelcage motors are A, B, C, D, and F, with A and B being the most common. Design A has starting current 6 to 7 times rated current and starting torque 150% of rated. Design B has starting current 5.5 to 6 times rated current and starting torque 150% of rated.
· A THERMAL PROTECTION indication on the nameplate indicates the motor is equipped with such protection to prevent overheating the windings. Protection may be provided by sensing motor current or temperature in the windings and shutting off the motor when either becomes excessive. After shutdown, the motor must be reset manually unless it is equipped with an automatic reset.
