This part is a kind of semiconductor called MT800.

Function of this product has Thyristor Modules.

Manufacturers : TECHSEM

Image and pinout :

Some of the text within the PDF file :

MTC800 MTA800 MTK800 MTX800 MT800 Thyristor Modules Features：  Isolated mounting base 2500V~  Pressure contact technology with Incrtased power cycling capability  Space and weight savings Typical Applications：  AC/DC Motor drives  Various rectifiers  DC supply for PWM inverter IT(AV) VDRM/VRRM ITSM I2t 800A 600~1800V 22 KA 2420 103A2S SYMBOL CHARACTERISTIC TEST CONDITIONS IT(AV) IT(RMS) VDRM VRRM IDRM IRRM ITSM I2t VTO rT VTM dv/dt di/dt IGT VGT IH VGD Rth(j-c) Rth(c-h) Viso Fm Tstg Wt Outline Mean on-state current RMS on-state current 180 half sine wave 50Hz Single side cooled,Tc=85C Repetitive peak off-state voltage Repetitive peak reverse voltage VDRM&VRRM tp=10ms VDSM&VRSM= VDRM&VRRM+100V Repetitive peak current Surge on-state current I2T for fus [ ... ]

MT800 PDF Datasheet Download