Compressor failure

Fig. 1: Typical compressor installation in factory

1. Introduction:
Compressor failure is also a common factory problem and it costs a huge loss of production if it fails suddenly. More importantly, in large factory compressors are connected in cascaded way so one single failure can affect other compressor. We can think it as an electromechanical machine/device where one electrical machine(motor) and mechanical pump are combined together. Due to poor power quality supply to the compressor, it can fail.

2. Common causes of compressor failure:
    a. Presence of VFD/DC drive nearby:
    b. High current imbalance in compressor input line
    c. High current harmonics in compressor input line

Effect of VFD/DC drive nearby

Fig. 1: Typical air compressor running with VFD/DC drive nearby

Fig. 2: Three phase voltage and typical current waveforms at VFD input

Fig. 3: Phase current input of the compressor due the presence of VFD nearby

Fig. 1 shows compressor is running with one VFD/DC drive nearby.It may happen that the electrical power input for the compressor is coming from the same panel where one or more VFD/DC drives are attached.

Fig. 2 shows typical voltage and current input at VFD input when VFD is having diode rectifier or thyristorized rectifier at input for the conversion of AC-DC. It is the current waveform which is highly distored and can carry up to 90% harmonics of different frquencies (100Hz, 150Hz, 200Hz…so on). This harmonics generated by VFD can affect other motors running paralley to same electrical panel output.

Fig. 3 shows how the impact of high harmonics generated by VFD keeps on going to the compressor attached to the same electricial panel.  Current waveform gets distorted and more importantly peak can even goes beyond fundamental peak which unneccessairily increases heating of the stator coil of the motor. In a long run insulation can get damaged and inter-turn ( a portion of the winding) can be shot circuited/burnt. If motor fails due to poor power quality whole compressor will stop working.

Effect of high voltage imbalance in compressor input line:

High voltage imbalance implies that any phase carries low voltage compared with other. Once this happens current goes higher for that particular phase and it overheats the winding. In long run, overheating can also degrade the winding insulation. In other words, high voltage also damage the winding insulation due to high (dv/dt).

Effect of high current imbalance in compressor input line:

Under voltage in the line increases the current drawn by that line when the motor is running on constant load (power mode). This additional current can create additional heating of the coil.  Voltage imbalance has the same impact as that of the under voltage.

If one of the winding gets partially damaged, its winding impedance will change and will try to create magnetic imbalance which in other turn will create current imbalance. So by measuring current imbalance we can monitor coil health.

Effect of high current harmonics in compressor input line:

Current harmonics are generated by use of SMPS based power adapter,  Led lights in factory. Improper distribution of load can also create high neutral current with high third harmonics . Basically when ever there are no of nonlinear load running in the system it can experience more harmonics.

So not only VFD/DC drive, any power converter having nonlinear rectifier ( diode based or thyristor based) for input power conversion can generate line harmonics in the system. This harmonics get conducted through line to some other motors which are connected to the same power line directly .

Solution

Isolate power source from VFD/DC drive system if they are running nearby. To do so, a transformer with proper kVA rating can be installed.

• Replace ordinary VFD/DC drive with IGBT based digital drive which has inbuilt PFC ( Power Factor Corrector) at drive input. IGBT allows bidirectional current flow and reduce harmonics significantly and thus improves power factor at input.
•  You can contact MachineSense Power Analyzer team to consult and get guidance to select better quality drive to solve your factory problem. 
• Use MachineSense Power Analyzer (PA) to measure the current imbalance and current harmonics which is passing through the compressor. MachineSense PA will monitor the abnormalities 24x7 and send you sms/email when ever anything alarming condition is detected.

If you have already measured the harmonics level and current imbalance level, please verify with MachineSense PA as it maintains IEC standard for each of quality parameter.