Joe Evans is responsible for customer and employee education at PumpTech, Inc., a pumps and packaged systems manufacturer and distributor with branches throughout the Pacific Northwest.
NEMA MG-1 requires that certain information be included on the nameplate of all single- and three-phase motors. Typical nameplate information includes horsepower, volts, amps, Hz, phase, rpm, insulation class, enclosure, frame size, efficiency, service factor, power factor, duty, ambient, code and design. Most are straightforward but, there are several that require further explanation.
Efficiency (Eff) defines how well a motor converts electrical energy into mechanical energy. The motor's full load efficiency is shown on the nameplate, and it is often less than the actual maximum efficiency. Maximum motor efficiency typically occurs between 70 percent and 95 percent of full load, and most NEMA motors can be operated as low as 60 percent of full load without any significant loss in efficiency. This allows the flexibility of upgrading to the next higher horsepower when loading is at or very near the full load capacity of the lower horsepower motor.
Service factor (SF) is an often misunderstood piece of information. SF is a multiplier that indicates the actual horsepower that the motor can deliver over and above the nameplate horsepower. For example, if a 10 horsepower motor has an SF of 1.15, it is designed to deliver 11.5 horsepower without overloading.
SF is intended to handle small, intermittent overloads, occasional increases in ambient temperature and periods when the actual voltage is lower than the nameplate voltage. For example, a typical 10 horsepower, 230-volt motor draws approximately 24 amps at full load. If the voltage is reduced to 200 volts, the current increases to 28 amps, which is the normal current draw of an 11.5 horsepower motor.
Therefore, a 230-volt motor with a 1.15 SF can accommodate such a short-term voltage drop. However, it should not be operated on a true 200/208-volt circuit since there would be no remaining SF available to accommodate any additional drop in voltage.
Power factor (PF) is the ratio of active power in watts to the apparent power in volts/amps at full load. A motor can be designed for high efficiency or high PF but not both. Since efficiency cannot be enhanced in the field, motors are designed for high efficiency and the trade off is lower PF. Fortunately, PF can be easily increased in the field by adding an appropriate capacitor to the circuit.
Duty defines the length of time that the motor can operate while meeting its other nameplate ratings. Most industrial motors are rated continuous (or cont.) while certain special application motors will show intermittent run times in minutes.
Ambient temperature is the maximum allowable temperature of the air surrounding a motor when it is operated continuously at full load. A typical ambient rating is 40 degrees C (104 degrees F). The actual operating temperature is the sum of the ambient temperature and the internal temperature rise at full load.
For example, Class B insulation is rated at 130 degrees C and is designed to handle an internal temperature rise of 90 degrees C (assumes a 1.15 SF) when operating in a 40 degrees C ambient environment.
Code letters (A – V) provide the range of current required (locked rotor current—LRC) during across-the-line starting for a particular motor. The value indicated by the code letter is in units of KVA/horsepower. A typical industrial motor will require five to seven times full load current during starting. Motors 15 horsepower and above require a lower KVA/horsepower when starting than do lower horsepower designs.
The simple equation below will provide approximate LRC results at 460 volts (For 230 volts, change the constant to 2.5 and for 200 volts, change it to 2.9):
460 Volts LRC = Code Letter Value x horsepower x 1.25
Design letters (A-D) indicate the shape of the torque curve produced by a particular motor. Design B is the standard for industrial duty motors and provides excellent performance in most industrial applications. Design C provides a higher starting torque while Design D is a high slip motor that provides the highest starting torque. Design A is a special purpose motor that offers the highest pullout torque.
NEMA & IEC frame dimension charts can be found at http://www.baldor.com/pdf/501_Catalog/BackCover.pdf.
Next month's column will review the conditions that affect the life of an AC motor.
Pumps & Systems, June 2011