Single-Phase Meter Components

Transcription

1 Single-Phase Meter Components S T U D E N T M A N U A L March 2, 2005

2 2 STUDENT TRAINING MANUAL Prerequisites: None Objectives: From memory, you will be able to explain the parts and function of a single-phase meter. Rationale: The measurement of a customer s consumption is calculated by an energy meter. This module will help in the installation, maintenance and troubleshooting of single-phase metering in order to avoid loss of utility revenue. Learning Objectives Explain the requirements to measure electrical energy. Describe the parts of an energy meter. Describe the operating principle of an energy meter. Describe the five types of single-phase meters and their application. Describe the function of the inherent meter multiplier. Learning Methods Self-learning + On-the-job Self-learning + On-the-job Self-learning + On-the-job Self-learning + On-the-job Self-learning + On-the-job EVALUATION METHODS Written test Written test Written test Written test Written test.

3 SINGLE-PHASE METER COMPONENTS 3 STUDENT RESOURCES Learning Steps None 1. Read the Learning Guide. 2. Follow the steps outlined in the Learning Guide. 3. Clarify any questions or concerns you may have. 4. Complete the Practice and Feedback. 5. Complete the Evaluation.

4 4 STUDENT TRAINING MANUAL Lesson 1: Requirements to Measure Electrical Energy Learning Objective:Explain the requirements to measure electrical energy. Learning Method:Self-learning + On-the-job Evaluation Method:Written test Watt-Hour Meter Watt-hour meters are used by power companies to determine the amount of energy used by a customer. This is essential since the customer is charged for the amount of energy used. A watt-hour meter measures the amount of power consumed (P = E x I) and multiplies this value by the amount of time it has been consumed. Energy = Power (watts) x Time (hours) Therefore, a watt-hour meter measures watt/hours or kwh.

5 SINGLE-PHASE METER COMPONENTS 5 Lesson 2: Components of an Energy Meter Learning Objective:Describe the parts of an energy meter. Learning Method:Self-learning + On-the-job Evaluation Method:Written test Cover The cover is made of clear glass or Lexan type plastic and serves three functions: encloses the meter seals the meter to prevent moisture from entering the metering components prevents tampering Terminals Frame ---Note--- A meter with a cracked or broken cover should be changed immediately upon discovery and sent in to meter repair for testing and reparations. Aluminum Disc Lugs found at the rear of the meter are used to make the connections between the line and load, as well as the internal parts of the meter. The frame is simply used for mounting the different components of the meter securely. An aluminum disc can be seen through the glass cover of the meter, revolving in a left to right direction. This disc is connected to a register by means of a gear train and records the number of kilowatt-hours consumed over a specific period of time.

6 6 STUDENT TRAINING MANUAL Register The register records the number of aluminum disc turns. For every revolution, the disc represents a definite number of watt-hours consumed. The amount of energy (watt-hours) per revolution of the disc is called the watt-hour constant and is represented by the symbol kh. This kh factor is established when calibrating the meter and is identified on the register of the meter itself. If a meter had a kh of 4/5, this would indicate 4 watt-hours consumed for every five revolutions of the disc. As it is not practical to measure energy in watt-hours due to the average consumption of our customers, the meters are engineered to record consumption in kilowatt-hours (kwh). The register consists of dials from which the kwh are read and holds the gears connecting the disc to the dials. Generally, the type of register used is the clock type.

7 SINGLE-PHASE METER COMPONENTS 7 Lesson 3: Operating Principles of an Energy Meter Learning Objective:Describe the operating principle of an energy meter. Learning Method:Self-learning + On-the-job Evaluation Method:Written test Introduction A watt-hour meter is identical to that of a wattmeter consisting of a potential coil and a current coil. However, in a watt-hour meter, the magnetic fields of the current and potential coils are used to drive a small electric motor rather than move a pointer across a scale. The rotor

8 8 STUDENT TRAINING MANUAL of this miniature motor is the aluminum disc. ---Note--- The disc is driven by opposing magnetic fields from the potential and current windings.

9 SINGLE-PHASE METER COMPONENTS 9 Lesson 4: Types of Single-Phase Meters Learning Objective:Describe the five types of single-phase meters and their application. Learning Method:Self-learning + On-the-job Evaluation Method:Written test Single-Phase 2-Wire Circuit Meter The single-phase 2-wire circuit meter consists of a single current coil and a single potential coil that uses a single line voltage (usually 120V) to measure the amount of power consumed. Since current and voltage are in phase with each other, there is no need to illustrate the relationship with vectors. As can be seen from the above circuit diagram, the current coil of the meter measures the current flowing through the customer s load. The voltage coil of the meter measures the voltage across the customer s load.

10 10 STUDENT TRAINING MANUAL Example: ---Note--- A red warning label may be on the face of the meter to identify that the meter is a 120 volt and is to be used only on a 2-wire circuit. Single-Phase 3-Wire Circuit (120/240V) Meter Single-phase 3-wire circuit meters measure the line to line voltage of

11 SINGLE-PHASE METER COMPONENTS volts using a coil connected line to line. The load current is measured using two half coils. Half coils are implemented to gain accuracy in averaging the current flow on each hot leg. Example In this case, the customer s load would be 2,400W. Meter Registers For an unbalanced condition, the customer load is connected across 120 volts (phase-to-neutral). The meter is still connected across 240 volts; however, the load passes through only one of the meter s half coils. Example: Customer: Meter:

12 12 STUDENT TRAINING MANUAL Types of Meters There are various types of meters used to measure consumer usage. Two common meters used in conjunction with instrument transformers are the instrument rated meter and the network meter. Instrument Rated Meter Network The instrument rated meter is used for 120V circuits with loads of 50kVA or greater. The current coils of the meter are replaced with a current transformer, also containing half-coils which deliver a measured portion of load current to the meter at the 5 amp level. These current transformers are sized in ratios such as 200:5 and 400:5 (400 amps of line current to 5 amps of maximum measured current in the meter). Because of the ratio effect, a multiplier external to the meter is required to calculate the actual amount of power consumed. Instrument rated meters can be identified by a yellow box with a black X in it. The network meter is a specialty meter used where 120/240V singlephase service is not available, but 120/208V single-phase is available and acceptable to the customer. This meter is distinguished by a fifth jaw or terminal lug located on the left-hand side of the meter facing the front. It is between the top and bottom terminals and runs horizontally instead of the traditional vertical terminal. CAUTION Do not use this meter on 120/240V services due to potential metering inaccuracies.

13 SINGLE-PHASE METER COMPONENTS 13 Lesson 5: Inherent Meter Multipliers Learning Objective:Describe the function of the inherent meter multiplier. Learning Method:Self-learning + On-the-job Evaluation Method:Written test. Introduction A four dial kwh meter will read up to 9,999 kwh and will start over at zero once it has cycled. Meters are read once every month or every three months. If such a meter were connected to a load where consumption was more than 10,000 kilowatt-hours, meter readers would be left confused, to say the least. To avoid this problem, an inherent or built-in multiplier is used to slow down the dial movement on the meter. This is possible by way of a gear, mounted between the disc and the register. For example, if an inherent meter multiplier is ten, the meter would record only one kilowatt-hour for every ten kilowatt-hours consumed. For all meters where an inherent multiplier is used, the multiplier is marked on the face of the meter or, quite often, on the glass meter cover. In order to obtain the correct number of kilowatt-hours used in a period of time, multiply the dial readings by the meter multiplier. ---Note--- A meter with five dials can also be used to accomplish the same thing as a meter with an inherent multiplier of x10. For example, a meter has an inherent multiplier of 10: Present reading: 8810 Previous reading: 8767 Consumption = 43 x 10 (inherent meter multiplier) Consumption = 430 kilowatt-hours

14 14 STUDENT TRAINING MANUAL Summary To summarize this module, you have learned: The requirements to measure electrical energy. The parts of an energy meter. The operating principle of an energy meter. The five types of single-phase meters and their application. The function of the inherent meter multiplier. Practice Feedback Review the lesson, ask any questions and complete the self test. Evaluation When you are ready, complete the final test. You are expected to achieve 100%.

15 SINGLE-PHASE METER COMPONENTS 15 Review Questions 1. The requirements needed to measure electrical energy are: (a) Resistance and power. (b) Power and time. (c) Amperage and voltmeters. (d) Time and resistance. T / F 2. A wattmeter consists of a current coil connected in series with the circuit, and a potential coil connected in parallel across the circuit. 3. A wattmeter reads: (a) Apparent power. (b) Reactive power. (c) True power. (d) None of these T / F 4. Electrical energy is the product of the rate measured in Vars multiplied by the current measured in amps. 5. The meter used by power companies to calculate the amount of energy used by a customer is: (a) A voltmeter. (b) A watt-hour meter. (c) A wattmeter. (d) An ammeter. 6. The power factor of a circuit is 95% and the apparent power is 560kVA. The true power of the circuit is: (a) (b) TP = 532kW. TP = 532kVA. (c) TP = 560kW. (d) TP = 577kW. T / F 7. The amount of energy (watt-hours) per disc revolutions is called the watt-hour constant and is represented by the symbol kh.

16 16 STUDENT TRAINING MANUAL T / F 8. T / F 9. The magnetic field of the current and potential coils are used to drive a small electric motor in a watt-hour meter. The aluminum disc and potential coils are used to drive a small electric motor in a watt-hour meter. 10. A 120/208V single-phase service is metered using a: (a) Network meter. (b) Instrument rated meter. (c) A-base meter. (d) All of these 11. A 120V single-phase service with 110A of current is metered using a: (a) Network. (b) 2-wire single-phase meter. (c) 3-wire single phase meter. (d) Instrument rated meter. 12. A 100 amp residential 120/240V service is metered using a: (a) 2-wire single-phase meter. (b) Instrument rated meter. (c) 3-wire single-phase meter. (d) All of these 13. A 60kVA single-phase 120V service is metered using a: (a) 2-wire single phase meter. (b) A base meter. (c) An instrument rated meter. (d) None of these T / F 14. An inherent meter multiplier is used to slow down the dial movement on the meter.

17 SINGLE-PHASE METER COMPONENTS A meter with an inherent meter multiplier of 5 has a present reading of 0015, the previous months reading was How many kilowatt-hours has this service used? (a) (b) 77kWh 385kWh (c) 9923kWh (d) 7700kWh

18 18 STUDENT TRAINING MANUAL Review Question Solutions 1. Power and time. 2. T 3. True power. 4. F 5. A watt-hour meter. 6. TP = 532kW. 7. T 8. T 9. F 10. Network meter wire single-phase meter wire single-phase meter. 13. An instrument rated meter. 14. T kWh