**Maria Martinez \*, David Reigosa, Daniel Fernandez and Fernando Briz**

Electrical, Electronic, Computers and Systems Engineering, University of Oviedo, 33204 Oviedo, Spain; diazdavid@uniovi.es (D.R.); fernandezalodaniel@uniovi.es (D.F.); fbriz@uniovi.es (F.B.)

**\*** Correspondence: martinezgmaria@uniovi.es; Tel.: +34-985-103000 (ext. 6436)

Received: 31 December 2019; Accepted: 20 January 2020; Published: 28 January 2020

**Abstract:** Torque estimation in permanent magnet synchronous machines and synchronous reluctance machines is required in many applications. Torque produced by a permanent magnet synchronous machine depends on the permanent magnets' flux and *dq*-axes inductances, whereas torque in synchronous reluctance machines depends on the *dq*-axes inductances. Consequently, precise knowledge of these parameters is required for proper torque estimation. The use of high frequency signal both for permanent magnets' flux and *dq*-axes inductances estimation has been recently shown to be a viable option. This paper reviews the physical principles, implementation and performance of high-frequency signal injection based torque estimation for permanent magnet synchronous machines and synchronous reluctance machines.

**Keywords:** torque estimation; online parameters estimation; permanent magnet synchronous machines; synchronous reluctance machines; high frequency signal injection
