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This paper presents a mechanical device with controllable inertia, in which a magnetic planetary gearbox (PG) is employed to hold two free terminals (the carrier and sun gear) and one feedforward control terminal (the ring gear). Based on the concept of inerter design and impedance control, the ring-gear motor controls the equivalent inertia of the carrier. The sealless nature of the proposed noncontact magnetic PG provides a low-inertia design ability and an over-load protection characteristic for a high-torque transmission. The control block diagram technique is utilized to represent the dynamic model of the variable-inertia device for the equivalent-inertia derivation and computer simulation, in which its equivalent inertia can be found by the transfer function from the torque to the acceleration at the carrier. Finally, the dynamic model is verified by experiments, and the controlled range of the equivalent inertia is illustrated.