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Electronic motor starters and drives
Basics of drives engineering

DOL starting
In the simplest case, and especially at low rated output (up to about 2.2 kW), the three-phase motor is connected directly to mains voltage. In most applications, the connection is made with an electromechanical contactor.

In this control mode, – on the mains at fixed voltage and frequency – the asynchronous motor’s speed is only slightly below the synchronous speed [ns ~ f].
Due to rotor slippage, The operating speed [n] deviates from this value in relation to the rotating field [n = ns × (1 – s)], slippage being
[s = (ns – n)/ns].
On starting (s = 1), a high starting current occurs, reaching up to ten times the rated current Ie.

For low- and medium-power three-phase motors
Three connection lines (circuit layout: star or delta)
High starting torque
Very high mechanical load
High current peaks
Voltage dips
Simple switching devices

If an application demands frequent and/or silent switching, or if adverse environmental conditions prevent the effective use of electromechanical switching elements, electronic semiconductor contactors are required. In addition to short-circuit and overload protection, the semiconductor contactor must be protected with a super fast fuse. According to IEC/EN 60947, type “2” coordination requires the use of a super fast semiconductor fuse. For type “1” coordination, – the majority of cases – a super fast semiconductor fuse is not necessary. Here are a few examples:

Building services management:

Reversing drive for lift doors
Starting heat-exchanger units
Starting conveyor belts

In critical atmospheres:

Controlling filling station petrol pump motors
Controlling pumps in paint processing plants.

Other applications: Non-motor-driven loads, such as

Heater elements in extruders
Heater elements in kilns
Controlling lighting systems.