Understanding electrical starters

Contactors

Contactors control the electric current to the motor. Their function is to repeatedly establish and interrupt an electrical power circuit.

Overloads

Overloads protect motors from drawing too much current and overheating from literally “burning out”.

• They operate electromechanically and use a small control current to open and close the circuit
  
• They are used by electrical equipment that is frequently turned off and on (opening and closing the circuit), such as lights, heaters and motors
  

They function as follows:

• The E-frame, when energized by the coil, becomes an electromagnet 
  
• The armature is connected to a set of contacts. The armature is moveable but is held by a spring
  
• When the coil is energized, the moveable contacts are pulled toward the stationary contacts, because the armature is pulled toward the E-frame. Once the two sets of contacts meet, power can flow through the contactor to the load
  
• When the coil is de-energized, the magnetic field is broken, and the spring forces the two sets of contacts apart
  

When a motor is off, it requires no current because the circuit is open. When the circuit is closed, the motor draws a large amount of inrush current – as much as 6–8 times the running current – which if not protected against, can produce enough heat to burn up the motor. 

They consist of:

• A current sensing unit (connected in the line to the motor)
• A mechanism to break the circuit, either directly or indirectly
• To meet motor protection needs, overload relays have a time delay to allow harmless temporary overloads without breaking the circuit. 
• A trip capability to open the control circuit if mildly dangerous currents (that could result in motor damage) continue over a period-of-time. 
• Some means of resetting the circuit once the overload is removed.

• What is the motor voltage (V, single-phase or three-phase)?
• What is the hp of the motor?
• What is the Full Load Amps (FLA) of the motor?
• What is the service factor of the motor?

Application considerations:

• What type of starter is needed for application?
  • Full Voltage Non-Reversing
  • Full Voltage Reversing
  • NEMA
  • IEC
  • Definite Purpose
• What is the control voltage being used (coil voltage)?
• What auxiliary contacts are needed? (one normally open auxiliary contact comes standard with most starters)
• What type of overload protection is needed?
  • Bimetallic (Thermal)
  • Solid State (Electronic)
  • Advanced Monitoring Overload (Motor Insight)
• Does the customer need any other accessories?
  • Heater packs, extra auxiliary contacts, etc.
• Does the customer need an enclosure?

Choose the starter family

NEMA Freedom contactors and starters

• NEMA Sizes 00-8 (up to 900 hp)
• UL hp rated for single- and three-phase applications
• Tab 2 in the Eaton catalog, Volume 5: Motor Control and Protection

IEC XT contactors and starters

• Product selected by hp if being applied in a UL application (U.S.)
• Product available up to 2000 A
• Tab 1 in the Eaton catalog, Volume 5: Motor Control and Protection 

Choose the right starter