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
+−48 VRa​ = 0.2 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.2 Ω is switched directly onto a 48 V supply. Its normal full-load current is 8 A. Find the armature current Istart at the moment of starting.

V = 48 VRa = 0.2 Ω
Istart = A
2.Easy
+−240 VRa​M+−E = 210 Vn = 1500 rpmT

A DC motor turns at 1500 rpm, and its back-EMF at that speed is 210 V. Find the motor constant k in V·s/rad.

E = 210 Vn = 1500 rpm
k = V·s/rad
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 1 N·m/A and Ra = 0.4 Ω is connected to 240 V. Find its stall torque Tstall, the torque at zero speed.

V = 240 Vk = 1 N·m/ARa = 0.4 Ω
Tstall = N·m
4.Medium
+−VRa​M+−En = 2900 rpmT = ?

A DC motor delivers 4 kW of mechanical power at 2900 rpm. Find the shaft torque T.

P = 4 kWn = 2900 rpm
T = N·m
5.Hard
+−24 VRa​ = 4 ΩM+−EωT

A small DC motor has an armature resistance of 4 Ω and runs from 24 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 24 VRa = 4 Ω
Pmax = W
6.Hard
+−240 VRst​ = ?Ra​ = 0.8 Ω80 AM+−E = 0n = 0T

A 240 V DC motor has an armature resistance of 0.8 Ω. Find the series starting resistance Rst that limits the starting current to 80 A.

V = 240 VRa = 0.8 ΩImax = 80 A
Rst = Ω

Answer key

  1. 1. Istart = 240 A
  2. 2. k = 1.337 V·s/rad
  3. 3. Tstall = 600 N·m
  4. 4. T = 13.17 N·m
  5. 5. Pmax = 36 W
  6. 6. Rst = 2.2 Ω

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