AD300 series AC drives in Atlas Air compressor retrofit

I. Introduction:

1.1 An air compressor is a device that uses an electric motor to compress a gas in a compression chamber and to have a certain pressure on the compressed gas. It is widely used in metallurgy, machinery manufacturing, mining, power, textile, petrochemical and other industries. Air compressors account for 15% of the power consumption of large industrial equipment (fans, pumps, boilers, air compressors, etc.).

 

After investigation, most traditional air compressors have the following shortcomings:

(1) When the output pressure is greater than a certain value, the discharge valve is automatically opened to make the asynchronous motor idling, which wastes energy;

(2) Asynchronous motors are required to start and stop frequently, reducing the service life of the motor;

(3) Automation degree is low. The adjustment of the output pressure is achieved by manually adjusting the valve. The adjustment responses slow and fluctuation is large with instability and low pressure precision.

(4) The air compressor directly using grid power has very large starting current and thus give great impact to the grid. The motor bearings wear easily and maintenance work is much.

1.2 Traditional air compressor energy waste analysis:

Traditional air compressor has 2 working states: loading state and no-load state.

1) Loading process energy consumption

In loading state, when the air pressure reaches the minimum value, the pressure is required to keep rising until it reach Max value. During pressure rising, more heat is radiated outside and cause the energy loss. On the other hand, the air whose pressure is higher than Max. value needs go through a pressure-reducing valve before entering pneumatic device, which is also a energy consuming process.

(2) Power consumption during unloading

In the no-load state, when the pressure reaches the maximum value, the air compressor is decompressed and unloaded by closing the intake valve to make the motor idle, and venting excess compressed air in the separation tank through the vent valve. This method of adjustment requires a lot of energy waste. According to our calculations, the energy consumption of the air compressor unloading accounts for about 10% to 25% of the air compressor's full load operation, which is only the case when the unloading time is not long. In other words, the air compressor is idling for about 20% of the time, making no physical work. Obviously, air compressor motor has a large energy saving space when the motor is idling.

The pressure control of the traditional air compressor is the upper and lower limit control. Firstly, set the pressure lower limit according to the production equipment requirement. The air compressor starts to load from lower limit. Normally the upper limit is 1Pa higher than the lower limit, that is, the pressure at which the unloading starts. The output working pressure will fluctuate between the upper and lower limits. The power consumption of the air compressor is proportional to the output pressure. The higher the output pressure, the more the power consumed. From the lower limit to the upper limit, the 1Pa pressure difference will consume 7-10% of the total power. In the traditional air compressing system, if there are multiple air compressors running at the same time, each compressor output pressures fluctuate between lower limit and upper limit and thus consuming extra 7-10% of rated power.

Different parameters setting in traditional air compressor will also cause different amount of power consumption. It must be set according to the gas working conditions to achieve the most economical state. Conventional air compressors do not allow the motor’s frequent stop and start. So when the demand gas amount is smaller, the motor is normally idling which waste the energy. And the frequency loading and unloading makes the gas network not stable and could not be kept at a constant pressure.

ii Frequency inverter transformation retrofit plan

2.1 Introduction of the original gas supply system:

The factory has 2 sets 110kW air compressors in a polypropylene gas supply system.

The problem is that the air compressors are running with grid power supply and the motors are always running at full load. When the gas demand is at peak time, the gas supply pressure gets lower; while gas demand is at low peak, especially in holidays, the gas supply pressure is too much. This easily damages the venting valves and gas pipes, and at the same time it is wasting electricity and reducing the equipment service life. So constant pressure gas supply system is called for and the 3sets air compressors need balanced running.

AD300 series frequency inverter retrofit design:

Each air compressor is controlled by each VFD and the grid supply bypass is reserved for use. Taking the polypropylene gas pipe network pressure as the control target, the gas tank pressure P is collected by the pressure transmitter SP and converted into an electric signal to the PID self-tuning controller built in the AD300 VFD. P value will be compared to the preset pressure reference value SV. Frequency inverter PID mode will work according to the difference value between P and SV and then output signal to control the frequency and the rotational speed of the motor through the DSP, so that the actual pressure P is kept close to preset reference SV. Alarming function is set. When the pressure P exceeds the upper limit, alarm AL1 will be given, and when P is lower than the lower limit, alarm AL2 will be sent out.

 
The coordination of frequency converter, pressure sensor and PID self-tuning control function forms the closed loop automatic control of the gas compressing system. It automatically adjusts the output pressure to make each compressor works in balance and improves stability and reliability in the whole system and the gas pipe network. It is very convenient for control and maintenance the system.

The original system powered by grid electricity is reserved for use in case of any problem or maintenance in VFD.

The work can be started by the inverter, and the soft start is realized, which avoids the start of the inrush current and the mechanical shock caused by starting the air compressor.

 

iii Economical benefits after retrofit

1) Saving energy

Compared to the old system, the most practical outcome of the retrofit is energy saving efficiency. The gas is supplied according to actual demand and make the air compressors work at most economical state.

2) Lower operating cost

The operating cost of the system consists of purchase cost, maintenance cost, and energy cost. Among them the energy cost accounts for about 77% of the operating cost. By reducing energy costs by 44.3%, along with reduced impact on equipment with variable frequency inverter soft starting, and lower maintenance and repair workload, the operating cost is greatly reduced.

3) Improve pressure control accuracy

The motor speed controlled by VFD has very high speed accuracy. It enables the gas pipe network pressure tolerance keeps within 3pisg, i.e. 0.2bar.

4) Extend the service life of the compressor

The inverter starts the compressor from 0Hz, and its acceleration time can be adjusted, thereby reducing the impact on the electrical components and mechanical components of the compressor during starting, enhancing the reliability of the system and prolonging the service life of the compressor. In addition, VFD can reduce the starting current fluctuation which greatly reduce the impact to the grid. The lower frequency limit is set to 40Hz. We use the infrared thermometer to monitor the temperature rise of the air compressor motor and the oil temperature of the pipeline for a long time and found that motor temperature rise is only 2-4°C, oil temperature almost does not change, and venting air temperature drops by 5 °C. So the lower frequency limit 40Hz offer very safe running of the system.

5) Reduce the noise of the air compressor

After the retrofit with VFD, the compressor motor speed is obviously lower and so efficiently lower the running noise. After detecting, the noise has lower 3-7 decibels

IV Case study and parameter setting

One mold design/manufacturing factory has very large demand of gas supply and has installed 2 sets of Altlas 110kW air compressors. One set air compressor could not satisfy the gas supply demand while 2 sets compressors running together will waste lots of energy and cause unnecessary wear to the compressor motor. The customer use AD300 series AC drives to realize the energy saving retrofit.

Air compressor working parameters: The gas required by production is 0.76Mpa, one remote pressure sensor is installed. The sensor will send 4-20mA current signal to inverter and make the whole system become closed-loop via inverter’s PID function. When actual pressure arrives the set pressure upper limit, air compressor starts unloading and the inverter gives command to make air compressor runs at frequency lower limit. When the actual pressure is lower than the preset value, the air compressor starts loading and speed up to run at frequency upper limit. In this way, the production gas demand is satisfied, energy saving is optimized, and air compressor service life is extended.

Inverter parameter setting:

F0.02 = 1 (terminal control)

F0.03 = 7 ( PID control setting)

F0.13 = 25 (frequency lower limit)

F0.19 = Specific setting according to site conditions (acceleration time)

F0.20 = Specific setting according to site conditions (deceleration time)

F1.08 = 1 (Coast to stop)

F2.12 = 1 (reverse rotation prohibited)

F6.01 = 1 (forward running)

F8.00 = 0 (PID digital setting)

F8.01 =1 (AI2 terminal)

F8.02 = specific setting according to site conditions (analog PID digital setting)

 

Air compressor AC drives retrofit case study

Inverter model: AD300-T4022GB/030PB

Motor: 3 phase Asynchronous motor 22kW

Air compressor original control method:

Power on the air compressor. The air compressor star contactor pulls in and the motor is firstly started in star connection mode, after a few seconds, it is converted into a delta starting, the air compressor is loaded and pumped. When the air compressor detects the pressure to be 8 kg, the air compressor starts to unload. While the motor is still running at high speed without load until the pressure is lowered to 6 kg, the air compressor is loaded again. The air compressor is wasting a lot of energy when the air consumption volume is not very high.

Retrofit plan with AD300 series variable frequency drive

Change the motor starting mode from star-delta to fixed delta starting mode. The inverter output terminals are connected to the motor delta type connection point. Use the contactor auxiliary contact point as the inverter starting signal. Frequency inverter is set to PID control, when the pressure is close to 8 kg, the inverter drives the motor to have decelerated running and drives the motor to stop running and enter sleep mode when the air consumption is stop for a period of time. By this way, constant pressure control is realized and the impact to load and electricity grid is greatly reduced. Disconnect a signal cable on pressure sensor of original system and connect it to inverter AI2; and using a cable to connect GND back to air compressor pressure sensor signal detection point where its cable was disconnected. The detected pressure signal is fed back to inverter as the PID feedback signal (Signal is current type)

A frequency lower limit 25Hz should be set to avoid the air compressor running at low frequency for a long time. The added variable frequency drive offers constant pressure gas supply, sleep mode, and thus realizing energy saving.

Inverter parameter setting