Walsall - Hazardous Area ATEX Motor Starters & Isolators Zone 1 & Zone 2

Walsall - Hazardous Area ATEX Motor Starters & Isolators Zone 1 & Zone 2

Motor Starters - Hazardous Area Zone 1 & Zone 2

DOL Direct On Line & Star Delta Motor Starters - ATEX Certified

Walsall manufacture a comprehensive range of EEx d Direct on Line (DOL) motor starters and starter/isolators and Star Delta motor starters and starter/isolators. Hazardous area motor control  is available in a choice of cast iron and cast aluminium enclosures with ratings up to 30kW – the Walsall ranges of motor starters and isolators are suitable for Zones 1, 2, 21 and 22 (T6) with ATEX and GOST certification.

Direct on Line (DOL) v Star Delta Starters

Types of Motor Starter Compared..............
The Star/Delta motor starter is the most commonly used reduced voltage starter but in a large number of motor applications the performance achieved is less than ideal, and in some cases, the damage and interference is much worse than that caused by a Direct On Line motor starter.

The Star/Delta starter requires a six terminal motor that is delta connected at the supply voltage. The Star Delta starter employs three contactors to initially start the motor in a star connection, then after a period of time, to reconnect the motor to the supply in a delta connection. While in the star connection, the voltage across each winding is reduced by a factor of (1 /.'/'3) [1 divided by root three]. This results in a start-current reduction to (1 /.'/'3) [1 divided by root three] of the DOL start current and a start torque reduction to one third of the DOL start torque.

If there is insufficient torque available while connected in star, the motor can only accelerate to a partial speed compared to the full speed it would reach if connected in delta. When the timer operates (set normally from 5-10 seconds), the motor is disconnected from the supply and then reconnected in delta, resulting in full line voltage running currents and torque.

The transition from star connection to delta connection requires that the current flow through the motor is interrupted. This is termed "Open Transition Switching" and with an induction motor operating at a partial speed compared to full load speed, there is a large current and torque transient produced at the poi, unless proper protection methods are used, can cause severe damage to the supply service's infrastructire and to other connected equipment.

If there is insufficient torque produced by the motor when running in star, there is no way to accelerate the load to full speed without switching to delta and causing those severe current and torque transients. These must be allowed-for in the design of the motor and its starting system if they are to have an economic useful life.

Update : Electronic motor-control systems, which offer soft-starts in DELTA configuration, are now replacing the use of manual or semi-automatic star-delta starters.

Technical explanation:

When the windings of a 3-phase motor are connected in STAR:
* The voltage applied to each winding is reduced to only (1 /.'/'3) [1 divided by
   root three] of the voltage applied to the winding when it is connected directly
   across two incoming power service lines in DELTA.

* The current per winding is reduced to only (1 /.'/'3) [1 divided by root three] of
   the normal running current taken when it is connected in DELTA.

* Because of the Power Law V [in volts] x I [in amps] = P [in watts],

The total output power when the motor is connected in STAR is:

PS = [VL x (1/.'/'3)] x [ID x (1/.'/'3)] = PD x (1/3) [one third of the power in DELTA]

where:
VL is the line-to-line voltage of the incoming 3-phase power service
ID is the line current drawn in DELTA
PS is the total power the motor can produce when running in STAR
PD is the total power it can produce when running in DELTA.

* A further disadvantage when the motor is connected in STAR is that its total 
  output torque is only 1/3 of the total torque it can produce when running in
  DELTA.

Walsall DOL Range - Hazardous Area EEx d IIB Direct-on-Line Starters and Starter/Isolators, Group  II Cat 2, Zones 1, 2, 21, 22 - 5.5-30kW....

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