DC motors worksheet with answers

Free printable DC motors worksheet: 6 problems from easy to exam level, with an answer key. Print it or save it as a PDF.

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1.Easy
+−240 VRa​ = 0.8 ΩIa​ = 20 AM+−En = ?T

A DC motor with k = 1 V·s/rad and an armature resistance of 0.8 Ω runs from 240 V and draws 20 A. Find its speed n in rpm.

V = 240 VRa = 0.8 ΩIa = 20 Ak = 1 V·s/rad
n = rpm
2.Easy
+−110 VRa​ = 0.2 ΩIa​ = 30 AM+−En = ?T

A DC motor with k = 0.5 V·s/rad and an armature resistance of 0.2 Ω runs from 110 V and draws 30 A. Find its speed n in rpm.

V = 110 VRa = 0.2 ΩIa = 30 Ak = 0.5 V·s/rad
n = rpm
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.6 N·m/A and Ra = 1.5 Ω is connected to 220 V. Find its stall torque Tstall, the torque at zero speed.

V = 220 Vk = 0.6 N·m/ARa = 1.5 Ω
Tstall = N·m
4.Medium
+−VRa​M+−En = 1800 rpmT = ?

A DC motor delivers 1.5 kW of mechanical power at 1800 rpm. Find the shaft torque T.

P = 1.5 kWn = 1800 rpm
T = N·m
5.Hard
+−240 VRs​ = ?Ra​ = 0.5 ΩM+−En = 750 rpmT = 37.5 N·m

A DC motor (k = 1.5 V·s/rad, Ra = 0.5 Ω) on a fixed 240 V supply drives a load of 37.5 N·m. Find the series resistance Rs that slows it down to 750 rpm.

V = 240 Vk = 1.5 V·s/radRa = 0.5 ΩT = 37.5 N·mn = 750 rpm
Rs = Ω
6.Hard
+−220 VRst​ = ?Ra​ = 0.8 Ω25 AM+−E = 0n = 0T

A 220 V DC motor has an armature resistance of 0.8 Ω. Find the series starting resistance Rst that limits the starting current to 25 A.

V = 220 VRa = 0.8 ΩImax = 25 A
Rst = Ω

Answer key

  1. 1. n = 2139 rpm
  2. 2. n = 1986 rpm
  3. 3. Tstall = 88 N·m
  4. 4. T = 7.958 N·m
  5. 5. Rs = 4.388 Ω
  6. 6. Rst = 8 Ω

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