Select Ion For Ver Y Sm A Ll Ca Pa Cit Y Mot - Hitachi H20-T Technical Notes

F ig. 25 Motor t em per at u r e-r ise r at io a t
ph a se-fa ilur e
Wh en a str ict ph ase-fa ilu r e pr ot ect ion is necessa r y t h at in clu des a lso su ch a specia l case as over-cu rr en t flows only t o
l wir e at t h e seconda r y side due t o ph ase-fa ilu r e at t he pr im a r y side of t h e t ra n sform er , 3-elemen t t h erm a l over loa d
r ela y or 2E t h erm a l over load r ela y shou ld be select ed.
3-3-3 Select ion for Ver y Sm a ll Ca pa cit y Mot or
Th e ra t ed cu rr en t of a ver y sm a ll ca pacit y m otor lik e th e h ydr a u lic pum p m otor is ext r em ely sm a ll, so th e h ea t ers
of t herm a l over loa d rela y cor respondin g to t h is r a t ed cur r en t h a ve a ver y h igh r esist a n ce va lu e. Th er efor e, a volt a ge
dr op ca used by th e t h er ma l over loa d r ela y becomes la r ge, so t h a t fu ll at t ent ion m ust be pa id t o insu fficien t st ar t in g
t or que a nd volt a ge u n bala n ce wh en usin g 2-elemen t t ype th er ma l over loa d r ela y.
Wh er e t h e loa d is ligh t , u n ba la n ce of cu rr en t du e t o volt a ge u n ba la n ce t en ds t o becom e la r ge. It is r ecomm en ded
t h a t 3-elem en t t ype t h er ma l over loa d r ela y be select ed.
Wh er e on ly a sm a ll-sized 2-elem ent t h er m a l over load rela y ca n be inst a lled du e to limit ed spa ce, its r at ed cur r en t
sh ou ld be abou t twice t h e r at ed cu r ren t of th e m ot or exclu sively for prot ect ion of r estr ict ion.
F ig. 27
Refer ence 1. In sufficient st ar t in g t or qu e
E xa m ple of RC0.1A
E (Hea t er volt a ge dr op) = Is (Motor st a r tin g cu r ren t) × R (Hea t er r esist a nce valu e)
Vm (Mot or inpu t volt a ge) ≒200 － (√3 × 78) ≒ 65 (V)
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Fig. 26 Motor t em per a tu r e-r ise r at io a t
ph ase-fa ilu r e oper at ion
・・・・・・ Motor: 200V, 0.lA (r at in g)
＝6 × 0.1 × 130 = 78 (V)
in pu t volt a ge ・・・・ 38%
st ar t in g t or que ・・・ 15%