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	High in-rush current
Star – Delta	Reduced in-rush current	Reduced starting torque
Auto-transformer	Reduced in-rush current	High weight
Soft starter	Reduced in-rush current, controlled start and stop	Reduced starting torque
Variable speed drive	Controlled speed, Energy saving at reduced speed	Higher cost

Fig. N63a: Comparison of different motor control solutions

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