UNIT11

E3106/11/15

ELECTRICAL MACHINERY & CONTROL                           

UNIT 11

 

ELECTRIC MOTOR CONTROLLER (PART II)

 

             

                            OBJECTIVES

 

 

 

GENERAL OBJECTIVE

 

To know the concept of motor protection

 

 

SPECIFIC OBJECTIVES

             

At the end of this unit, you would be able to:

• define the over current protection
• define the under-voltage protection
• recall IEE rules

 

 

 

 

 

 

                            INPUT

 

 

 

11. INTRODUCTION TO MOTOR PROTECTION

 

Emmm….. What is a PROTECTION????

  rotection as used in the electrical industry  is no different to that in everyday    use. People protect themselves against personal or financial loss by means of insurance and from injury or discomfort by the use of the correct protective clothing. They further protect their property by the installation of security measures such as locks and alarm systems. In the same way, electrical systems need the following; (1) to be protected against mechanical damage, the effects of the environment and electrical over currents, (2) to be installed in such a fashion that a person and/or livestock are protected from the dangers that such an electrical installation may create. Despite all these, we should know the over current protection and under voltage protection (Figure 11.1) to protect the motor.  

 

 

 

 

 

 

9.1              OVER CURRENT PROTECTION

 

An over current is a current greater than the rated current of the circuit it may occur in two ways:

1. as an overloads current
2. as a short circuit or fault current

Because of these, each motor must have an over load and short circuit protections. 

 

Figure 11.1: A typical across the line starter for an induction motor

(Source: Electrical Machinery Fundamentals (3rd edition) Stephen J. Chapman)

 

 

 

 

 

 

 

 

 

 

 

Overload Protection

 

11.1.1              Over Load Protection

 

Over load protection for the motor is provided by devices labeled OL in the Figure 11.1. These overload protection devices consist of two parts, an overload heater element and overload contacts.

 

Under normal conditions, overload contacts are shut. However when the temperature of the heater elements rises far enough, the OL contacts open, deenergizing the M relay, which in turn opens normally open M contacts and removes power from motor.

 

 

 

Devices for protection motor???

 

Typical rewireable fuse assembly

internal arrangement of single phase isolator

11.1.2              Short Circuit Protection

 

Short circuit protection for the motor is

provided by using fuses F1, F2 and F3. If a

sudden short circuit develops within the

motor and causes a current flow many times

larger than the rated current, these fuses will

blow, disconnecting the motor from the

power supply and preventing it from

burning up.

 

However, these fuses must not burn up

during normal motor starting, so they are

designed to require currents many times

greater than the full load current before they

open the circuit. This means that short

circuits through a high resistance and/or

excessive motor loads will not be cleared by

the fuses.

 

 

 

HRC fuse

Cartridge fuse

 

 

 

 

 

 

 

 

 

 

 

Test your UNDERSTANDING before you continue to the next input

ACTIVITY 11 A

 

 

 

 

 

 

 

mechanical damage	environment	electrical over current
person	livestock	dangers
installation

 

Define the protection in electrical by using the words given below.

 

 

 

 

 

 

2. Label each part of picture below

 

(ii) ……………... (i) ……………...

 

 

 

 

 

 

FEEDBACK TO ACTIVITY 11 A

 

 

1. to be protected against mechanical damage, the effects of the environment and electrical over currents, to be installed in such a fashion that person and/or livestock are protected from the dangers that such an electrical installation may create.

 

2.  

             

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                            INPUT

 

 

 

2. UNDER VOLTAGE PROTECTION

Figure 11.2: Hold-on Circuit

The basic concept for under voltage

protection is the hold-on circuit (Figure

11.2). When the start button is pushed,

the coil becomes energized and its

normally open (N/O) contacts close.

When the start button is released the coil

remains energized via its own N/O

contacts. These are known as the hold-on

contacts. The coil can be only de-energized by opening the circuit by the used of the stop button or by a considerable reduction or loss of voltage. When this happens, the N/O contacts open, and even if the voltage is restored or the circuit is made complete again, the coil will remain de-energized until the start button is pushed again.

 

Notice from the Figure 11.1 that the control power for the M relay comes from directly across the line to the motor. If the voltage applied to the motor falls too much, the voltage applied to the M relay will also fall and the relay de-energized. The M contacts then open, removing power from the motor terminals.

 

 

 

 

 

 

Test your UNDERSTANDING before you continue to the next input

ACTIVITY 11 B
 

 

 

 

 

 

 

11.3              Based on the circuit given, explain the operation of the circuit.

 

11.4              What is the device used to protect the system from under voltage fault.

 

 

 

 

 

 

 

 

 

 

FEEDBACK TO ACTIVITY 11 B

 

 

3. When the start button is pushed, the coil becomes energized and its normally open (N/O) contacts close. When the start button is released the coil remains energized via its own N/O contacts. These are known as the hold-on contacts. The coil can be only de-energized by opening the circuit by the used of the stop button or by a considerable reduction or loss of voltage. When this happens, the N/O contacts open, and even if the voltage is restored or the circuit is made complete again, the coil will remain de-energized until the start button is pushed again.

 

4. isolator

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                            INPUT

 

 

 

11.3              IEE RULES

 

1. Every consumer’s installation must incorporate means of over current protection which should comprise either a fuse or circuit breaker inserted in each phase conductor of the supply.

 

2. Every method of over current protection should be based on the prospective short circuit current.

 

3. Fuses and circuit breaker must be arranged in an installation discrimination is ensured.

Protective devices must be inserted only in the phase conductor of two wire system in which one wire is connected to earth (i.e neutral is earthed at the supply transformer).

 

 

 

 

 

 

5. Every circuit conductor must be protected by a fuse or circuit breaker installed at the origin of the circuit. The protective device must have a setting not less than the design current of the circuit (e.g a 5A lighting circuit is protected by no less than 5A protection). The setting should not exceed the lowest current-carrying capacity of any conductor in the circuit, and the value at which the protection operates must not exceed 1.45 times the lowest current carrying capacity of any circuit conductor.

 

6. Effective and accessible means of isolation must be provided at the origin of the installation to cut off all voltage as may be necessary to prevent danger.

 

When installation serves two or more detached buildings (i.e house detached garage) means of isolation must be provided in each building.

7.  

 

 

 

 

 

 

8. If the purpose of a switch or circuit breaker is not clear, it should be labeled to show which apparatus it control.

 

 

 

 

 

 

 

 

 

 

             

 

 

 

 

 

 

 

 

 

Test your UNDERSTANDING before you continue to the next input

ACTIVITY 11 C
 

 

 

 

 

5. Give two IEE rules in protection system.

 

6. What must you do if you want to make installation to two or more detached buildings?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

FEEDBACK TO ACTIVITY 11 C

 

 

5. refer notes 11.3

 

6. when installation serves two or more detached buildings (i.e house detached garage) means of isolation must be provided in each building

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

SELF-ASSESMENT

 

 

 

If you face any problem, discuss it with your lecturer

You are approaching success. TRY all the questions ini this self-assessment section and check your answers with those given in the feedback on Self-Assessment given on the next page. 

 

Question 11-1

 

A.              What are characteristics that show motors are in good condition?

 

B.              How can over current occur in a motor?             

 

C.               What is the component used to protect motor from over loaded?

 

 

Question 11-2

 

A.              How to protect a system from over current?

 

Question 11-3

 

A.              If you are an electrical contractor, what must you know about the IEE rules if you want in installing the power system.

FEEDBACK TO SELF-ASSESMENT

 

Question 11-1

Refer from the note 11.1

 

Question 11-2

Refer from the note 11.2

 

Question 11-3

Refer from the note 11.3