The Electric Motor

Electricity Compressed Air Wiki Basic Theory

In order to turn air into compressed air, you need power. This power comes in the form of electricity, generated by an electric motor. The most common electric motor is a three-phase squirrel cage induction motor. This type of motor is 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 electric motor?

The electric motor consists of two main parts, the stationary stator and the rotating rotor. The stator produces a rotating magnetic field and the rotor converts this energy into movement, i.e. mechanical energy. The stator is connected to the three-phase mains supply. The current in the stator windings give rise to a rotating magnetic force field, which induces currents in the rotor and gives rise to a magnetic field there as well. The interaction between the stator's and the rotor's magnetic fields creates turning torque, which in turn makes the rotor shaft rotate.

Rotation Speed

If the induction motor shaft rotated 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 and the speed is always approx. 1-5% below magnetic field synchronous speed (called "slip"). (Permanent magnet motors do not produce any slip at all.)

Efficiency

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 is divided into insulation classes in accordance with IEC 60085, a standard published by the International Electro-Technical Commission. A letter corresponding to the temperature, which is the upper limit for the isolation application area, designates each class. If the upper limit is exceeded by 10°C over a sustained period of time, the service life of the insulation is shortened by about half.

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.

For example, IP23 represents: (2) protection against solid 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 how the motor is cooled. 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

The installation method states, according to IEC 60034-7, how the motor should be installed. This is designated by the letters IM and four digits. For example, IM 1001 represents: 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 winding's phases are joined together to form a zero point, which 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 are joined between the different phases, which then form a delta (Δ). As a result, there will be a main voltage across the windings. The current Ih into the motor is the main current and this will be divided between the windings to give a phase current through these, Ih/√3 = If.

The same motor can be connected as a 690 V star connection or 400 V delta connection. In both cases the voltage across the windings will be 400 V. The current to the motor will be lower with a 690 V star connection than with a 400 V delta connection. The relation between the current levels is √3. On the motor plate it can, for example, state 690/400 V. This means that the star connection is intended for the higher voltage and the delta connection for the lower. The current, which can also be 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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