Face Plate Starter for Shunt Motors Quiz & Flashcards

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Questions and Answers

What is the primary function of the conducting arm in a faceplate starter?

To cut out the resistors successively (correct)

To provide a path for the current to flow

To hold the arm at the close position

To regulate the supply voltage

What happens to the arm when the electromagnet releases it?

It remains in the same position

It returns to its dead position under the pull of spring 3 (correct)

It breaks the circuit

It moves to the close position

What is the purpose of the current-limiting resistors in a faceplate starter?

To hold the arm at the close position

To provide a path for the current to flow

To limit the current during the starting period (correct)

To regulate the supply voltage

What would happen if a 240-V dc shunt motor were started direct-on-line?

The starting current would be 12 times the full-load value Signup and view all the answers

What is the function of the electromagnet in a faceplate starter?

To hold the arm at the close position Signup and view all the answers

Why is the faceplate starter considered obsolete?

It is only suitable for small motors up to 5 kW Signup and view all the answers

What is the safety feature of the faceplate starter?

It prevents the motor from restarting unexpectedly Signup and view all the answers

What is the value of the total resistance of the starter for a 240-V dc shunt motor?

7 Ω Signup and view all the answers

What is the purpose of spring 3 in a faceplate starter?

To pull the arm to the dead position Signup and view all the answers

What is the total armature and series field resistance of the motor in Example 5?

0.2 Ω Signup and view all the answers

What is the current when the variable resistance R is 0 Ω in Example 5?

25 A Signup and view all the answers

What is the relationship between the field flux and the current in a series motor?

Φ ∝ I Signup and view all the answers

What is the speed of the motor when the variable resistance R is 2 Ω in Example 5?

1128 rev/min Signup and view all the answers

What is the field resistance of the shunt motor in Example 6?

200 Ω Signup and view all the answers

What is the armature current when the shunt motor is running at 500 rev/min on full load in Example 6?

25 A Signup and view all the answers

What is the purpose of the external resistor in Example 6?

To control the speed Signup and view all the answers

What is the new speed of the shunt motor when the external resistor is connected in Example 6?

417 rev/min Signup and view all the answers

Why is the starting current of a dc motor limited by the armature resistance?

Because the speed is zero Signup and view all the answers

What is the advantage of using a small motor with direct-on-line starting?

It simplifies the starting circuit Signup and view all the answers

Study Notes

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

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Description

This quiz covers the essential components of a faceplate starter, including current-limiting resistors, conducting arm, contact, electromagnet, and spring.