Everything You Need to Know about Car AC Compressor Magnetic Clutch

This article introduces the compressor magnetic clutch components and their working principle, as well as several important parameters. In addition, we also give you some experiences in dealing with the burn of air conditioner compressor clutches.

Figure 1: A car ac compressor clutch.

1. AC Compressor Magnetic Clutch Components

The car ac compressor magnetic clutch is a power transfer device between an automobile engine and air conditioner compressor. It consists of a belt disc assembly, a coil assembly, and a drive disc assembly (also known as a sucker or a pressure plate assembly).

Figure 2: The belt disc assembly.

Figure 3: The coil assembly.

Figure 4: The drive disc assembly.

2. AC Compressor Magnetic Clutch Structure

Figure 5: The ac compressor magnetic clutch structure, including a pulley, pressure plate, shim, sharp rings, pulley, and stator.

3. AC Compressor Magnetic Clutch Working Principle

When the A/C switch is powered on, the current passes through the magnetic coil of the clutch, which generates an electromagnetic field to attract the displacement of the drive assembly, so that the drive disc and the friction end face of the belt disc are tightly attached. Then the engine torque is transferred to the compressor shaft, and then it is spinning.

Figure 6: The ac compressor magnetic clutch assembly.

When the A/C switch is disconnected, there will be no current on the coil, also the coil magnetic field disappears. The drive disc assembly returns under the action of the reset spring, leading to the separation of the drive disc and the belt disc end face with friction. Therefore, the compressor stopped working, but the belt disc is still rotating.

4. AC Compressor Magnetic Clutch Parameters

Generally, the initial static friction torque is more than 32 Nm. The static friction torque after running will increase by 20%.
The minimum clutch clearance is 0.4-0.6 mm.
The minimum pull in voltage is no more than 7.5 V.
The edges and corners of the compressor belt shall not be less than 120 degrees.

Figure 7: The worker is inspecting the friction torque before delivery.

5. Case Studies of Car AC Compressor Magnetic Clutch Troubleshooting

Case 1

Within 10 days of the new car test of an OEM, several cars ran hundreds of kilometers, and there were abnormal noise and smoke in the compressor magnetic clutch, and the electrophoretic paint of the drive plate was discolored. Some cars even stopped and idled after running only ten minutes, with abnormal sound and smoke in the clutch.

Noise & smoke
The cause is initial static friction torque of the compressor magnetic clutch is lower than the standard. So when the vehicle is under a bad heat dissipation environment, the air conditioning system pressure rises, the compressor load torque increases sharply, the drive plate and the belt disc rotate with each other, producing sliding friction heat and smoke.

Lower static friction torque
This is caused by the deformation of the drive disc friction surface. Detecting the friction surface, the worker found that the surface center is slightly raised. But on a normal drive disc, the surface center is concave with raised surrounding part, which looks like a food dish. As a result, after pulling in, there was an insufficient laminating surface, causing low static friction torque.

Deformation of the friction surface
When the workers of the production and assembly line detected that the clutch clearance had been out of standard, they did not replace the drive disc but pried it to adjust the clearance illegally. This resulted in the middle deformation of the drive disc, thus causing the abnormal sound and smoke with the burn of the magnetic clutch.

Figure 8: The drive disc is deformed.

Case 2

An OEM after-sales service has received some customer complaints saying that a new car only ran thousands of kilometers, and its compressor clutch was sliding and burnt with abnormal sound and smoke.

After careful inspection, it was confirmed that the three key parameters of the magnetic clutch produced in this batch - initial static friction torque, minimum clutch clearance, and minimum pull in voltage all met the requirements. Finally, it was verified that the belt disc was pasted with a layer of non-metallic friction material to increase the static friction torque of the clutch.

To eliminate the noise, an OEM canceled the clutch tension wheel. The wrap edge angle of the clutch belt disc was reduced from 173 degrees to 88 degrees. The normal wrap edge angle should not be less than 120 degrees, otherwise, the belt and the belt disc are prone to slip. If you reduce the belt edges and corners, you need to increase the belt tension appropriately.

Figure 9: The compressor clutch is burnt.

According to the regulations, the initial tension force of the new belt is 750 Nm +/- 50 Nm, and the actual initial tension force only reaches 44%~64%. That is, before delivery, the belt disc tension force of the vehicle is far from the standard.

So during the driving, the belt loosed with the temperature rising, and the belt and the belt disc were easy to slip. This made the gear train heat instantly, leading to softening of the non-metal friction plate, reducing the static friction torque of the clutch, causing slip and ablating the clutch.

Summary

* AC Compressor Magnetic Clutch

Parameters
The three parameters of the magnetic clutch including static friction torque, minimum clutch clearance, and minimum pull in voltage are very important and must be strictly monitored. In particular, after the magnetic clutch runs, the static friction torque must be higher than the initial static friction torque.

Performance & quality
In the purchase of compressor magnetic clutch, you should not consider the price only, you must give priority to the performance and quality. It’s because the harsh working environment will directly affect its service life.

For instance, there is a well-known magnetic clutch manufacturer in China. The factory inspection indexes of products conclude 14 items of the belt disc assembly, 9 of the drive disc assembly, 11 of the coil assembly, 6 of the clutch assembly, which is a total of 40 items. And none of these compressors was found to be ablated due to clutch.

The three-point screw fastening structure
The clutch coil assembly is fixed on the compressor cylinder head by using the three-point screw fastening structure, which is reliable and firm.

If adapting a raised locating pin on the compressor cylinder head, after a long time of use under a bad working environment, the small convex hull is easy to rust and corrosion. Therefore, the shear force generated by the instantaneous rotation of the belt will cut off the convex hull, which is easy to cause a short circuit of the compressor line or even a fire accident.

Wrap edge angle
The wrap edge angle of the compressor belt should not be less than 120 degrees, if not, you should increase tension force as required.

* Piston Compressor Assembly

Figure 10: A Bristol piston compressor.

We could find that compressors are stuck, 80% of which are due to the wear of the piston and swash plate in the compressors. The black molybdenum disulfide coating on the piston and swash plate was worn away to varying degrees, exposing the metallic white underlayer.

Piston and swash plate wear
Lubrication oil deterioration. If the frozen oil poured out of the compressor is black, and there is a clear acidic smell, the frozen oil has deteriorated.

Deterioration of frozen oil
The use of low-price inferior refrigerant, or the vacuum is not clean air containing water, resulting in the deterioration of frozen oil.

If we can troubleshoot the problems of the compressor magnetic clutch, it will greatly reduce the failure rate of the car ac compressor assembly. Therefore, it can improve the service life of the air conditioner significantly, as well as the market for users.

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