Air compressor electric motor key principles

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To generate compressed air, an air compressor electric motor uses energy to produce power. The most common type is a three-phase squirrel cage induction motor, used in all types of industries. It is silent and reliable, and is therefore a part of most systems, including compressors.

What are the main parts of an air compressor electric motor?

electrical motor

An air compressor electric motor consists of two main parts, the stationary stator and rotating rotor. The stator, connected to the three-phase mains supply, produces a rotating magnetic field. Energy becomes converted into movement, i.e. mechanical energy with the rotor.

The current in the stator windings creates a rotating magnetic force field, which induces currents in the rotor. This results in a magnetic field there as well. The interaction between the stator's and the rotor's magnetic fields creates turning torque, making the rotor shaft rotate.

Rotation Speed

formula synchronous rotation speed in rev/min, motor supply frequency and number of poles per phase

If the induction motor shaft would rotate at the same speed as the magnetic field, the induced current in the rotor would be zero. However, due to various losses in, for example, the bearings, this is impossible. Therefore, the speed is always approx. 1-5% below magnetic field synchronous speed (called "slip"). (Permanent magnet motors do not produce any slip at all.)


formula for efficiency in energy conversion, stated power and shaft power in W, applied electric power in Watt

Energy conversion in a motor does not take place without losses. These losses are the result, among other things, of resistive losses, ventilation losses, magnetization losses and friction losses.

Insulation class

The insulation material in the motor's windings gets divided into insulation classes in accordance with International Electrotechnical Commission (IEC) 60085 standards. A letter corresponding to the temperature, which is the upper limit for the isolation application area, designates each class. If the upper limit exceeds 10 °C over a period of time, the insulation's service life becomes cut by nearly one half.

Insulation class




Max. winding temp. °C




Ambient temperature °C




Temperature increase °C




Thermal margin °C




Protection classes

Protection classes, according to IEC 60034-5, specify how the motor is protected against contact and water. These are stated with the letters IP and two digits. The first digit states the protection against contact and penetration by a solid object. The second digit states the protection against water. See below for what each class represents.

IP 23: (2) protection against objects greater than 12 mm. (3) protection against direct sprays of water up to 60° from the vertical.

IP 54: (5) protection against dust. (4) protection against water sprayed from all directions.

IP 55: (5) protection against dust. (5) protection against low-pressure jets of water from all directions.

Cooling methods

Cooling methods according to IEC 60034-6 specify cooling methods for the motor. This is designated with the letters IC followed by a series of digits representing the cooling type (non-ventilated, self-ventilated, forced cooling) and the cooling mode of operation (internal cooling, surface cooling, closed-circuit cooling, liquid cooling, etc.).

Installation method

installation method

The installation method, represented by the letters IM and four digits, states how the motor is installed according to IEC 60034-7. Below are two examples on what this means.

IM 1001: two bearings, a shaft with a free journal end, and a stator body with feet.

IM 3001: two bearings, a shaft with a free journal end, a stator body without feet, and a large flange with plain securing holes.

What are star and delta connections?

A three-phase electric motor can be connected in two ways: star (Y) or delta (Δ). The winding phases in a three-phase motor are marked U, V and W (U1-U2; V1-V2; W1-W2). Standards in the United States make reference to T1, T2, T3, T4, T5, T6. With the star (Y) connection, the "ends" of motor windings phases become joined together, forming a zero point. Visually, it looks like a star (Y).

A phase voltage (phase voltage = main voltage/√3; for example 400V = 690/√3) will lie across the windings. The current Ih in towards the zero point becomes a phase current and accordingly a phase current will flow If = Ih through the windings. With the delta (Δ) connection, the beginning and ends become joined between the different phases, which then form a delta (Δ). As a result, there is a main voltage across the windings.

The current Ih into the motor is the main current. This becomes divided between the windings to give a phase current, Ih/√3 = If. The same motor can be connected as a 690 V star connection or 400 V delta connection. In both cases, voltage across the windings is 400 V.

A 690 V star connection has a lower current to the motor than a 400 V delta connection. The relation between the current levels is √3. With this, the motor plate might state 690/400 V (as an example). The star connection is for the higher voltage. As implied, the delta connection is for the lower. The current, also stated on the plate, shows the lower value for the star-connected motor and the higher for the delta-connected motor.

What is torque?

An electric motor's turning torque is an expression of the rotor turning capacity. Each motor has a maximum torque. A load above this torque means that the motor does not have the capability to rotate. With a normal load the motor works significantly below its maximum torque, however, the start sequence will involve an extra load. The characteristics of the motor are usually presented in a torque curve.

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