Control solutions and power saving problems when installing inverters for air compressors

I. Cases where customers should install inverters for air compressors:

The customer’s productivity of compressed air is not continuous.
The need to adjust the outlet pressure to match the needs of use.
Customers need to install to protect against pressure shock, protect mechanical details.
Energy saving (Depends on many factors).

II. Parameters to pay attention to when installing the inverter:

Install replacement inverter or install new?
– If installing new, please ask for the manufacturer’s data, engine capacity parameters.

– If replacing, still ask for the above parameters and old inverter code to check if the inverter is manufactured specifically for the machine.

Note:

– When installing new and replacing things, ask customers if they have the password of the machine?

– Do customers need more bypass mode? If the customer needs it, contact the technician to see and quote the exact price for the customer.

The inverter lines of Song Nguyen company provide for air compressors: The air compressor is a heavy-duty machine, so choose 1 level larger to ensure that when starting up, there is no overload error:
– For Yaskawa, it is recommended to use A1000, GA700, E1000.

– For Danfoss, FC 360 can still meet if the engine capacity is less than 75kW. Engine capacity greater than 75kW, choose FC102, FC202, FC 302 depending on the economic ability of the customer.

III. Analysis of air compressor control solutions in systems:

1. Difficulties of customers

Power loss, especially when the device runs with a large idling time (over 20%) of the usage time.
Direct starting causes mains voltage drop.
Faster damage to the engine and peripherals, reducing system life.
Unstable gas pressure leads to reduced product quality.

2. Solution

Using an inverter control solution to change the rotational speed of the air compressor, adjust the flow instead of using a valve, saves up to 25% of electricity.
The solution uses an inverter to control the motor running PID, the inverter with integrated PID control algorithm will automatically compare the set pressure and the feedback pressure from the pressure sensor to reduce the speed when the pressure is close enough, increase speed when under pressure and slow down or stop completely when under pressure…

3. For single machine

– PID control algorithm

The PID control method is also known as glass loop control, this algorithm will install a pressure sensor at the tank, then give the pressure feedback signal to the inverter or PLC.
Assuming that when we set the required pressure to use is 10 bar, initially the pressure in the measured tank is 0 bar, the inverter will control the motor to speed up to be enough with the set voltage and when the inverter reaches the desired level. voltage according to the setting, it will automatically reduce the frequency to save power.
For example, if the pressure is 10 bar, the motor will run at 40Hz, when starting up to 40Hz, but the pressure has not reached 10 bar because the output is heavily used by the customer, the inverter will speed up to 45Hz or can run up to 50Hz to sufficiently compensated for the set pressure. And if the customer uses little or no gas, but the pressure in the tank is 10 bar, the inverter will reduce the frequency slowly. According to the min speed that I set.

 

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4. For multiple machines

– Follow the call and fire algorithm

Here, due to the need to use gas, the current machine of the customer system is not enough, so they install more machines, for example, there are 3 machines 37kW, 45kW and 75kW.
We will use the PID algorithm and apply the call and fire algorithm. For example, a customer using an average amount of air needs a 75kW machine to run, but when the customer needs more compressed air, we will call an additional 45kW motor if it is not enough, we will order another 37kW.
We will need a pressure sensor placed in the reservoir position, and when the system works, the pressure sensor will send a signal to the PLC for processing. We will let a 75kW machine run in the background according to the set pressure. For example, the set pressure is 20 bar, then the 75kW motor runs 45Hz but during peak hours customers use a lot, then the 75kW motor runs to 50Hz but the set pressure still does not meet, we will call an additional 45kW machine. compensating operation if the 45kW motor in compensating operation still does not meet the pressure, we continue to call another 37kW motor to meet, when the demand for compressed air is low, the pressure will be redundant, we will give a 37kW machine enter Standby mode run at min speed. If after a period of time the pressure is still residual, the 37kw machine will be turned OFF. Continuing to have excess gas, we will put the 45kW machine into Standby mode. If there is still excess pressure, then OFF the 45kW machine and only run the 75kW machine.

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IV. Calculation of gas energy saving installed with inverter for air compressor:

1. Calculation formula:

We have: The relationship between power consumption and frequency is as follows: P1/P2= (f1/f2).

In there:

P is power consumption
F is the frequency of the current supplied to the motor

– Example: We consider an engine with a capacity of 55Kw and has the characteristic of running for 24 hours/day.

Information:

Electricity price is 4000 VND/kWh.
Run 24 hours a day.
Buy Yaskawa A1000 inverter with capacity of 55kW, cost 48,630,000 (VND) excluding installation service depending on the system to be surveyed.

Calculation: (Based on ideal environment and 100% lossless motor efficiency)

When the inverter is not installed, the motor always runs at rated speed f=50Hz, P=P max

P = Pmax = 24 x 55 x 4000 = 5.280,000(VND)

When installing an inverter, the motor running at 40Hz is efficient enough.

We have

P2 = (P1*F2) / F1 = (55*40) / 50 = 44kW

Ptt = (P1-P2) / P1 * 100 = 20% Save 20%

A = P + t = 44*24 = 1,056W

Electricity cost for 1 day when using inverter = 1056 * 4000 = 4,224,000 (VND).

2. Practical problems with textile factories using air compressors:

– Machine parameters

Model : KOBELION SG90A.

– Motor parameters: 90kW, 380V, 2975 rpm, 163A.

– Improved solution

Install an inverter to adjust the motor speed to stabilize the system pressure at the set value.

– Cost of improvement

The total cost of the inverter together with electrical cabinets, labor and complete installation materials is: 160,600,000 VND.

– Power test results before installing the inverter

Average LOAD time in 1 cycle: 20s
Average UNLOAD time in 1 cycle: 25s
Measurement time: 1 hour 30 minutes, from 10:15 am to 11:45 am on January 6, 2020
Sampling cycle : 1s
Total measured power consumption: 130.2 kWh

– Calculate payback time

Assuming the machine works 24/24
Average electricity price is 2000VND / 1kWh
The predicted savings after installing the inverter is 15%. Actual percentage of savings is higher due to customer machine overload
Power consumption in 1 hour is: 130.2 kWh / 1.5 = 86.8 kWh
Electricity saved in 1 hour is: 86.8 kWh * 15% = 13.02 kWh
Electricity saved in 1 day is: 13.02 kWh * 24 h = 312.5 kWh
Electricity saved in 1 month is: 312.5 kWh * 30 = 9,375 kWh
The amount saved in 1 month is: 2000VND * 9,375 kWh = 18,750,000VND
The payback period of the project is: 160,600,000 VND / 18,750,000 VND = 8.5 months

V. Summary

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