单选题
一台并励直流电动机，P
N
=96kW，U
N
=440V，I
N
=255A，I
fN
=5A，n
N
=500r/min，R
a
=0.078Ω(包括电刷接触电阻)，其在额定运行时的电磁转矩为______N·m．
【正确答案】
C
【答案解析】[解析] 励磁回路电阻：R
f
=U
N
/I
fN
=440/5=88Ω，电枢额定电流：I
aN
=I
N
-I
fN
，=255-5=250A，电枢电动势E
aN
=U
N
-I
aN
R
a
=440-250×0.078=420.5V，电磁功率P
em
=E
a
I
N
=420.5×255=107227.5W=107.2275kW，电磁转矩T
em
=9550×P
em
/n
N
=2048 N/m，接近于C项答案．