Electrical system of compressors: how do you start a compressor?

In order to operate, a compressor obviously needs to be powered electrically, and this aspect must also be calibrated efficiently to ensure the correct effort of the system. Let’s try to understand more aboutthe electrical system of a compressor and how it should be sized to be effective. 

First of all , it is important to know that compressors mostly use three-phase squirrel cage induction motors, low voltage for powers up to 500 kW, and high voltage for higher powers. Motors must be protected in accordance with current regulations, in particular from water and dust (since motors already protect themselves from these agents, protection lower than IP55 may be sufficient). 

The motors of the compression systems are cooled by a fan and can operate at an ambient temperature of up to 40°C at an altitude not exceeding 1000 m. Despite these limits, there are motors on the market that can withstand temperatures of up to 46°C, above which it is recommended to reduce the nominal power rating. This latter metric (the nominal value) also depends strictly on the requirements of the compressor itself and cannot deviate significantly from the required values. In fact, a motor can encounter various problems if it is either undersized or oversized. 

How is a compressor motor started? 

There are essentially three methods for starting a motor: direct, soft start, and star/delta. Let’s take a closer look at each of these. 

Direct start is the simplest method, requiring only a meter and overload protection. The disadvantage of this start-up method is the very high current (up to about 10 times the nominal current), which can damage the compressor.

The soft start system:

This is the most modern and efficient starting system. It is based on power thyristor technology. It allows for smooth and gradual starts by partially reducing the voltage using thyristors controlled by a microprocessor.

The soft start consists of two thyristors in anti-parallel per phase. During start-up, the microprocessor control unit regulates the voltage at the motor terminals so that it gradually increases to its nominal value. 

The star/delta start is based on three electrical contactors, overload protection, and a timer. The operation is very simple: the first start is given by the star connection, then the timer comes into action, switching the contactors to allow the motor to be connected to the delta configuration. The advantage of this configuration is that it reduces the starting current by about one-third compared to the previous mode (direct start). 

Positioning of compressor motors

In compressors, the motors are usually integrated into the compressors themselves, while in larger compressors they can be located separately in the control panels. This applies to direct and star starts. 

In the progressive configuration, on the other hand, the motors are located separately from the compressor (usually next to it) due to heat radiation. Alternatively, they can be integrated, but an adequate cooling system must be in place.

Compression motors: Riem Italy configurations

The Riem Italy team is able to manage the configuration of compressor starter motors in the most efficient way possible. In addition to guaranteeing top-of-the-range industrial compressors, our professionals are also able to provide analysis and valuable advice on the electrical systems to be used for starting compressors in order to maximize the efficiency of the systems as much as possible.