In electric drives using power electronics, thyristor-based converters and choppers can be used to control the speed/torque of different direct-current (DC) motors. Identify all motor types for which thyristor control is commonly applied in practice.

Introduction / Context:Thyristor-based power converters (controlled rectifiers, choppers) are widely used for DC motor control. They provide adjustable armature voltage and, in some topologies, field control to regulate speed and torque. This question checks recognition of which DC motor types can be managed with thyristor control.Given Data / Assumptions:

• Motors considered: separately excited, shunt, and series DC motors.
• Armature voltage and/or field control achievable via thyristor converters.
• Standard industrial practices and classic drive schemes are implied.

Concept / Approach:By phase-controlling AC–DC converters or using DC–DC choppers, the average armature voltage is varied, thus changing speed. With appropriate feedback and protection, all mainstream DC motor types can be controlled. Field weakening can extend speed range for shunt and separately excited machines.Step-by-Step Solution:1) For separately excited DC motors, armature voltage control via a thyristor rectifier or chopper gives smooth speed control.2) For shunt motors, similar armature control applies; additional field control refines speed regulation.3) For series motors, thyristor control is also practical but requires attention to torque–speed characteristics and current surges.4) Therefore, all listed motor types are controllable with thyristors when using suitable topologies and protections.Verification / Alternative check:Classic DC drive textbooks and application notes show thyristor drives for all three types, with nuances in feedback and protection differing by motor type.Why Other Options Are Wrong:

• Any single motor option alone omits other valid types.
• None of the above: contradicted by extensive industrial usage.

Common Pitfalls:

• Assuming series motors cannot be thyristor-controlled; they can, with proper design.
• Confusing control limitations with impossibility; practical issues do not preclude control.

Final Answer:All of the above.