Lesson 3: Full Voltage Motor  Control

Full-Voltage Starters

 Full-voltage starters, also referred to as across-the-line starters or direct-on-line (DOL) starters, are full-voltage non-reversing (FVNR) and connect the motor to a power line. Manual starters are limited to single-phase motors through about 5 HP at 320 VAC and three-phase through 10 HP at 460 and 575 VAC. Starters are generally designed to meet either NEMA (U.S.) or IEC (Europe) standards. The two starter types differ in ratings, life, and overload types. 

Frame Ratings 

NEMA standards define 11 magnetic starter sizes (00 – 9) for low-voltage starters and specify horsepower ratings for each size. Ratings for IEC starters provide 15 sizes and the physical size may be smaller.

Contactor Life 

NEMA standards require manufacturers to design all contactors for heavy service; so they are typically larger than corresponding IEC contactors. IEC standards define various levels of service called utilization categories. NEMA starters generally exhibit longer life. 

Overload Relays 

The industry has essentially discontinued the use of heater-element overloads in favor of electronic solid-state overloads, which provide greater protection. Electronic overload monitors actual motor current and takes it offline in three seconds or less when it exceeds the preset rating. They also protect against phase-loss, phase imbalance, and short circuit. 

NEMA standards call for overload relays to have interchangeable heaters or electronic overloads to exhibit Class 20 trip characteristics at 600 percent of full load current. Most electronic overloads have field-selectable trip classes from 5 through 30. 

Reversing Starters 

Three-phrase motors are reversed by interchanging any two of the three power leads to the motor. Full-voltage reversing (FVR) starters have two contactors (run-forward and run-reverse). When a motor is running in one direction and the contactor for the opposite direction is energized, this is called plugging. The motor decelerates rapidly and accelerates in the opposite direction. When an application calls for rapid deceleration but not the subsequent reverse rotation, a motor can be equipped with a plug-stop switch. A plug-stop switch is a centrifugal switch that applies opposite rotational power to the motor for fast deceleration but drops out completely when the motor speed nears zero.