Motor control systems

Different kinds of motor control solution are compared in the following tables.

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	Is / In	Ts / Tn	Speed control	Torque control
Direct on line	5-10	5-10	No	No
Star – Delta	2-3	1-2	No	No
Auto-transformer	2-3	1-2	No	No
Soft starter	3-5	1.5-2.5	No	Yes
Variable speed drive	1.5	1.5-2	Yes	Yes

	pros	cons
Direct on line	Reduced cost High starting torque when necessary	High in-rush current High torque so high constraints on motor, mechanics, ... not necessarily needed for the application starting
Star – Delta	Reduced in-rush current	Reduced starting torque, switching voltage & currents
Auto-transformer	Reduced in-rush current	Weight
Soft starter	Reduced in-rush current controlled start and stop Interesting solution in medium voltage as good value for money	Reduced starting torque
Variable speed drive	Controlled speed Energy saving at reduced speed Sometimes the main "modern" solution according to the driven process characteristics	Higher cost

Fig. N63a: Comparison of different motor control solution

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