Electrical Installation in Compressor Systems

Selecting the right motor for a compressor is essential to ensure that the system operates at its most efficient and effective level.

This minimizes the risk of mechanical failure and prevents costly repairs and downtime. The longer the motor lasts and works, the more money is saved.

When it comes to air compressor operations, three-phase squirrel cage induction motors are commonly used. Low voltage motors are ideal for power up to 450-500 kW, while high voltage motors are better for higher power. 

The motor is usually fan-cooled and selected to work in temperatures up to 40°C and an altitude of 1000m. Some manufacturers offer standard motors with a maximum ambient temperature capability of 46°C. When dimensioning compressor installations at higher altitudes or higher temperatures, the output must be derated. 

The motor is usually flange-mounted and directly connected to the compressor. The speed is adapted to the type of compressor, but in practice, only 2-pole or 4-pole motors with respective speeds of 3,000 rpm. The rated output of the motor is also determined (at 1,500 rpm).

Using an over-dimensioned motor can result in 

• higher costs, 

• an unnecessarily high starting current, 

• larger fuses, 

• low power factor, 

• and reduced efficiency levels. 

On the other hand, using a motor that is too small for the installation can result in

• overloading
• and the risk of breakdowns.

Matching the motor's output to the compressor's requirement helps to avoid potential issues and ensures that the motor performs at its best. This is good for both the motor and compressor since it helps them last longer and work more efficiently.

The motor protection class is a measure of how well a motor can withstand dust and water. The motor protection class is regulated by standards. 

It's important to note that open motors are not ideal for use with compressors, as they do not provide adequate protection from dust and water. For example, an IP23 motor will only be able to withstand splashes of water or fine mist, but not full immersion in liquid.

The dust and water jet-resistant design (IP55) is preferred over open motors (IP23), which may require regular disassembly and cleaning.

In other cases, dust deposits in the machine will eventually cause overheating, resulting in shortened service life. Since the compressor package enclosure also protects against dust and water, a protection class below IP55 may also be used.

The starting method is also important to consider when selecting a motor. For a star/delta-start, the motor is only started with a third of its normal starting torque, so comparing the motor and compressor torque curves can be useful to ensure proper compressor starts.

The most common starting methods are direct start, star/delta–start and soft start.

• Direct start is simple, but has a high starting current and torque that can damage the motor. 

• Star/delta-start limits the starting current and consists of three contactors, overload protection, and a timer that switches the motor from star to delta connection. 

• Soft start is a gradual start method that uses semiconductor switches to limit the starting current. 

Direct start is simple and only requires a contactor and overload protection. The disadvantage it presents is the high starting current, which is 6–10 times the motor's rated current, and its high starting torque, which may, for example, damage shafts and couplings.

The star/delta–start is used to limit the starting current. The starter consists of three contactors, overload protection and a timer.

The motor is started with the star connection and after a set time (when the speed has reached 90% of the rated speed), the timer switches the contactors so that the motor is delta-connected, which is the operating mode. 

The star/delta–start reduces the starting current to approximately 1/3 as compared to the direct start. However, at the same time, the starting torque also drops to 1/3.

The relatively low starting torque means that the motor's load should be low during the starting phase so that the motor virtually reaches its rated speed before switching to the delta connection.

If the speed is too low, a current/ torque peak as large as with a direct start will be generated when switching to the delta connection.

Soft start (or gradual start), which can be an alternative start method to the star/delta–start, is a starter composed of semiconductors (IGBT-type power switches) instead of mechanical contactors. The start is gradual and the starting current is limited to approximately three times the rated current. 

The starters for direct start and star/delta–start are, in most cases, integrated in the compressor.

For a large compressor plant, the units may be placed separately in the switchgear, due to:

• space requirements,
• heat development
• and access for service.

(See more information on how to create optimal working conditions in your compressor room.)

Note that a starter for soft start is usually positioned separately, next to the compressor due to heat radiation. But it may be integrated inside the compressor package, provided the cooling system has been properly secured. High-voltage powered compressors always have their start equipment in a separate electrical cabinet.

Cables shall, according to the provisions of the standard, "be dimensioned so that during normal operations they do not experience excessive temperatures and that they shall not be damaged thermally or mechanically by an electric short circuit".

To choose the right cables for a job, you need to consider:

• the load,
• allowed voltage drop,
• routing method (on a rack, wall etc.)
• and ambient temperature.

Fuses can also be used to protect the cables from short circuits and overloads.

When using motors, you need two types of protection. Short-circuit protection, like fuses, is used to prevent dangerous electrical shorts. Overload protection, which is usually the motor protection included in the starter, trips and breaks the starter if the current exceeds a certain level. This protects the motor and its cables.

Short-circuit protection shields the starter, overload protection, and cables. To pick the right cable size, you can look at IEC 60364-5-52. 

But there's another important factor: the "tripping condition". This means that the installation should be designed to quickly and safely break if there's a short circuit. To make sure the condition is met, you need to consider the cable length, cross-section, and short-circuit protection.

Short-circuit protection is placed on one of the cables' starting points and can include fuses or a circuit breaker. Either option provides the proper level of protection, given that the solution you select is correctly matched to the system. 

Fuses work better for large short-circuit currents but don't create a fully-isolating break and have long tripping times for small faults. Circuit breakers create a quick and fully-isolating break, even for small faults, but require more planning. Dimensioning short-circuit protection depends on the expected load and the limitations of the starter unit.

For starter short-circuit protection, see the IEC (International Electrotechnical Commission) standard 60947-4-1 Type 1 & Type 2.

The selection of Type 1 or Type 2 is based on how a short-circuit will affect the starter. 

Type 1: "… under short circuit conditions, the contactor or starter shall cause no danger to persons or installation and may not be suitable for further service without repair and replacement of parts." 

Type 2: "… under short circuit conditions, the contactor or starter shall cause no danger to persons or installation and shall be suitable for further use. The risk of light welding of the contactors is recognized, in which case the manufacturer shall indicate the maintenance measures …"

Electric motors consume both active power (which turns into mechanical work) and reactive power (which magnetizes the motor). The reactive power puts a load on the cables and transformer. The power factor, cos φ, determines the relationship between the two, this is usually between 0.7 and 0.9, with smaller motors having a lower value.

You can raise the power factor to virtually 1 by generating the reactive power directly by the machine using a capacitor. This means you don't have to draw as much reactive power from the mains. This is done to avoid extra charges from the power supplier for drawing reactive power over a predetermined level. It also helps take some of the load off heavily used transformers and cables.

By taking these factors into account, you can create a properly functioning electrical system that maximizes the performance and lifespan of your compressor.

Learn more about the process of installing a compressor system below.