Insulation class

The insulation class is defined in the IEC 60085 standard and tells something about how robust the insulation system is to temperatures. The life of an insulation material is highly dependent on the temperature to which it is exposed. The various insulation materials and systems are classified into insulation classes depending on their ability to resist high temperatures.

Motor start-up

We distinguish between different ways of starting up the motor: direct-on-line starting, star/delta starting, autotransformer starting, soft starter and frequency converter starting. Each of these methods have their pros and cons, see figure 1.4.13.

Direct-on-line starting (DOL)

As the name suggests, direct-on-line starting means that the motor is started by connecting it directly to the supply at rated voltage. Direct-on-line starting is suitable for stable supplies and mechanically stiff and well-dimensioned shaft systems, for example pumps. Whenever applying the direct-on-line starting method, it is important to consult local authorities.

Star/delta starting

The objective of this starting method, which is used with three-phase induction motors, is to reduce the starting current. In one position, current supply to the stator windings is connected in star (Y) for starting. In other positions, current supply is reconnected to the windings in delta (A) once the motor has gained speed.

Autotransformer starting

As the name states, autotransformer starting makes use of an autotransformer. The autotransformer is placed in series with the motor during start and varies the voltage up to nominal voltage in two to four steps.

Soft starter

A soft starter is, as you would expect, a device which ensures a soft start of a motor. This is done by raising the voltage to a preset voltage raise time.

Frequency converter starting

Frequency converters are designed for continuous feeding of motors, but they can also be used for soft starting.

Voltage supply

The motor's rated voltage lies within a certain voltage range. Figure 1.4.14 shows typical voltage examples for 50 Hz and 60 Hz motors.

According to the international standard IEC 60038, the motor has to be able to operate with a main voltage tolerance of ±10%.

For motors that are designed according to the IEC 60034­1 standard with a wide voltage range, e.g. 380-415 V, the main voltage may have a tolerance of ± 5%.

The permissible maximum temperature for the actual insulation class is not exceeded when the motor is operated within the rated voltage range. For conditions at the extreme boundaries the temperature typically rises approx. 10 Kelvin.

Frequency converter

Frequency converters are often used for speed controlling pumps, see chapter 4. The frequency converter converts the mains voltage into a new voltage and frequency, causing the motor to run at a different speed. This way of regulating the frequency might result in some problems:

• Acoustic noise from the motor, which is sometimes transmitted to the system as disturbing noise

• High voltage peaks on the output from the frequency converter to the motor