Faceplate Starter Components

Faceplate Starter

• A faceplate starter is a type of manual starter, which consists of essential components:
  • Current-limiting resistors (r1, r2, r3, and r4)
  • Conducting arm 1 with insulated handle 2
  • Dead contact M (where the motor circuit is open)
  • Electromagnet 4 in series with the shunt field
  • Spring 3 (which pulls the arm to the dead position)

Working of Faceplate Starter

• The electromagnet holds the arm at the close position, allowing the motor circuit to be closed
• If the supply voltage fails or the field excitation is lost, the electromagnet releases the arm, which returns to the dead position under the pull of spring 3
• This safety feature prevents the motor from:
  • Restarting unexpectedly when the supply voltage is re-established
  • Being energized when the excitation is lost

Characteristics of Faceplate Starter

• The faceplate starter is now obsolete, except for small motors up to 5 kW
• It provides a total resistance of 7 Ω to the motor during the starting period, as calculated in the example

Example: Calculating Total Resistance of Starter

• A 240-V dc shunt motor has a full-load speed of 750 rev/min and a full-load armature current of 20 A
• The armature resistance is 1 Ω
• The maximum armature current during the starting period is 30 A
• The total resistance of the starter is calculated as: Ra + Rst = V/Ia = 240/30 = 8 Ω, and Rst = 8 – 1 = 7 Ω

Starting of DC Motor

• For larger machines, the armature resistance is small, and starting them direct-on-line results in a highly excessive armature current.
• Consequences of a highly excessive armature current include:
  • Armature winding burning out
  • Damage to the commutator and brushes due to heavy sparking
  • Overloading of the feeder
  • Shaft snapping off due to mechanical shock
  • Damage to the driven equipment due to the sudden mechanical hammer blow

Limiting Starting Current

• The starting current must be limited to 1.5 to 2.5 times the rated current.
• Limiting the starting current can be achieved by connecting resistors in series with the armature.
• A liquid rheostat provides a smooth variation in resistance, but robust metallic resistors are usually preferred.
• Metallic resistors are arranged in series sections which are cut out successively by manual or automatic operations.

Electronic Methods for Speed Control

• Electronic methods are often used to limit the starting current and to provide speed control.
• Changing the flux is a method used to control the speed of a motor.
• A variable resistor termed a field regulator is connected in series with the shunt field winding in the case of shunt and compound motors.
• A variable resistor termed a diverter is connected in parallel with the field winding in the case of series motors.

Speed Control by Changing the Flux

• The speed can only be raised above its base value.
• For shunt motors, a high-speed/base-speed ratio of 3 to 1 is possible.
• However, when this speed range is exceeded, instability and poor commutation result.
• The method is economically sound.
• The main motor characteristics are similar for all settings of the field strength.

Speed Control by Changing the Motor Terminal Voltage

• The conventional method is known as the Ward-Leonard method.
• In this method, the field winding of the motor is supplied from a constant-voltage source, and the armature from a variable supply.
• The control is effected by varying the current of the generator and hence its supply voltage.
• Modern installations use high-power electronic converters to vary the armature terminal voltage.
• The disadvantage of this method is its high initial cost.
• The advantages of this method include:
  • A wide range of speed from standstill to high speeds in either direction.
  • The main motor characteristics are similar.

Applications of Ward-Leonard Method

• The Ward-Leonard method is used for large reversing motors found in:
  • Steel mills
  • High-rise elevators
  • Mines
  • Paper mills