1、 Question:

At present, there are many submersible pumps in rural areas of a certain county. How to collect electricity fees? The nameplate of the submersible pump indicates: rated power of 7.5 kW, voltage of 380 V, current of 18 A;

The public said, 'The nameplate of the submersible pump motor indicates 7.5 kilowatts, and electricity should be charged at 7.5 degrees per hour of operation.'. The electrician said, "The nameplate of the submersible pump motor indicates that the current is 18A, the voltage is 380V, the practical power is greater than 7.5 kW, and the working hour is about 9 degrees. Charging a fee of 7.5 degrees will result in losses.

Question: Is the electrician right? Is what the masses are saying right? Please explain the theoretical and practical basis.

2、 Answer;

（1） Theoretical analysis:

What does the 7.5 kW indicated on the motor nameplate mean?

7.5 kW refers to the rated power output on the motor shaft, which is the maximum limit value to ensure the long-term normal operation of the motor. When the load driven by the motor is 7.5 kW or less, it can operate normally. It provides a basis for users to choose electric motors and their supporting electrical equipment.

The rated power of submersible pump motors is different from that of incandescent lamps. It is not equal to the (input) power absorbed from the power grid, as it does not include the copper and iron losses of the motor; The rated power of the motor is also different from the rated power of the electric furnace wire. It is not equal to the power consumed by itself, as it does not include the mechanical wear power of the motor. The rated power of a motor represents only the ability of the motor to convert electrical energy into mechanical energy (the portion output through the shaft). Therefore, using it as a basis for calculating electricity consumption is unscientific and unreasonable.

What does the rated voltage and current indicated on the motor nameplate mean?

The rated working voltage and current values refer to the requirements and adaptability of the motor to the power supply equipment when outputting the rated power (full load). The indication of the rated voltage and current provides a basis for users to choose the matching of the motor and power supply equipment. In actual work, the measured voltage and current may not necessarily be consistent with the rated value *. Therefore, calculating electricity consumption based on the voltage value indicated on the nameplate is not very scientific.

What does the efficiency of an electric motor mean?

Efficiency of electric motor=output power/input power

The power loss of a motor includes electrical losses (copper and iron losses) and mechanical losses (such as bearing friction and fan air resistance pin losses), resulting in output power always being less than input power.

Different motors have different efficiencies, which can be found through experiments or from electrician manuals. For example, the commonly used JO2 type enclosed motor generally has an efficiency of around 0.85, while the commonly used submersible pump motor with the same capacity has an efficiency of only around 0.80. Taking a 7.5 kW motor as an example, the submersible pump motor requires an input power (P input=7.5/0.8=9.4 kW) that is 0.6 kW more than the input power of the JO2 type motor used in ordinary pumps (P input=7.5/0.85=8.8 kW).

Why did electricians not lose money when charging electricity fees based on kilowatt hours per kilowatt hour for irrigating land a few years ago?

In the past few years, centrifugal pumps were commonly used in rural areas for land irrigation, and the supporting motors were mostly JO 2 type. The commonly used method for charging was to estimate based on the rated power of the motor multiplied by the electricity consumption time. Due to the high groundwater level at that time, the efficiency of this type of motor was relatively high (generally=0.85), and the selected motor was relatively large, which concealed the shortcomings of this charging method. Electricians did not suffer from this method, so the contradiction was not prominent

In recent years, due to the implementation of the contract responsibility system in rural areas, many people have opted for compact and lightweight designs; A simple and flexible submersible pump, due to the low efficiency of the submersible pump motor (generally around 0.85), coupled with the decrease in groundwater level and the increase in power output, the submersible pump manufacturer designs and manufactures a pump body and motor that are very compatible, eliminating the phenomenon of large horses pulling small cars and allowing the submersible pump to operate at full load or near full load frequently. If the estimation is still based on the output power of the motor in kilowatt hours per kilowatt hour, there is no doubt that electricians will lose money in charging, thus creating new contradictions.

How should the electricity consumption of submersible pumps be measured?

(1) A reasonable method is to install an active energy meter.

For example, installing a three-phase three wire electricity meter or installing a single-phase electricity meter to measure and multiply by 3. However, due to the current risk of rural economic power, it is still difficult to install neutral lines or purchase electricity meters for the time being

(2) After measuring the voltage and current, calculate according to the following formula:

Electricity consumption=√ 3 x Line voltage x Line current x Power factor x Electricity consumption time

即： A＝√3×U×I×cos∮×T

Due to variations in voltage, current, and force, the calculation results may not be accurate. Although this method is not perfect, it is feasible in practical work ：

(3) Through experiments or formula calculations, obtain the actual electricity consumption data for one hour (as shown in the table below for reference), and then convert it according to the length of the user's electricity consumption time. This method is not very effective, but it is feasible and reliable in practical work.

Reference value of one hour electricity consumption of submersible pump under rated voltage, current and power conditions

Public electricity consumption=hourly electricity consumption of submersible pump according to the meter value multiplied by electricity consumption time (hours).

（2） Actual verification:

1. A user observed a submersible pump in a certain village, and the rated power of the submersible pump motor nameplate was 2.2 kilowatts, with a power factor of 0.8. A single-phase electricity meter was installed on site for actual measurement, with a line voltage of 380 volts and a load current of 5 amperes. The base code of the electricity meter is 0876.2, which will change to 0877.1 after one hour.

Verification: Electricity meter measures the degree of electricity consumption:

A=(877.1-876.2) × 3=2.7 (degrees)

Calculate the power consumption of the submersible pump according to the formula:

A '=√ 3 × 380 × 5 × 0.8/l000=2.63 (degrees)

2. At the XX County Foreign Trade Bureau, a set of test boxes has been installed for the electricity consumption of submersible pumps, and the parameters of the submersible pump motor are as follows:

Model: 200NQ Number of phases: 3

Connection: Δ Power: 13 kW

Voltage: 380 volts Frequency: 50 hertz

Speed: 2870 revolutions per minute Current: 30 amps

Temperature rise: 45 ℃ Efficiency: 79%

Power rate: 0.8 volts Number: 062

Date of manufacture: April 2013

Manufacturer: Pacific Pump Industry Group

① The experimental data using the electricity meter measurement method are listed in the following table:

② The calculation results of the formula calculation method are as follows

The known measured parameters are listed in the following table:

Electricity consumption A=√ 3 × 380, × 25.5 × 0.8 × 1.8/1000=24.17 (kWh)

The results are consistent with the measurements and formula calculations from the above electricity meter.

（3） Conclusion:

After theoretical analysis and practical verification, it has been found that the statements made by the public and electricians contain unscientific and unreasonable elements. Their calculation methods are only applicable to specific types of loads and operating conditions, and are not suitable for general application to submersible electric pumps. The correct measurement method is: ① equipped with a power meter; ② Calculate using formulas based on measured parameters such as voltage and current; ② Converted based on the hourly electricity consumption of submersible pumps. In short,; Starting from reality, specific situations should be treated on a case by case basis.

Company Name: Pacific Pump Industry Group

Address: Qingfeng Road, Anfeng Industrial Zone, Oubei, Yongjia County, Wenzhou