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​ = 0.8 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.8 Ω is switched directly onto a 48 V supply. Its normal full-load current is 6 A. Find the armature current Istart at the moment of starting.

V = 48 VRa = 0.8 Ω
Istart = A
2.Easy
+−110 VRa​Ia​ = 30 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.12 N·m/A. Find the torque T it develops at an armature current of 30 A.

k = 0.12 N·m/AIa = 30 A
T = N·m
3.Medium
T (N·m)n (rpm)6 N·mn0​n = ?

A DC motor (k = 0.3 N·m/A, Ra = 0.4 Ω) runs from 110 V and drives a load torque of 6 N·m. Find its speed n in rpm.

V = 110 Vk = 0.3 N·m/ARa = 0.4 ΩT = 6 N·m
n = rpm
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.3 V·s/rad runs from 110 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 110 Vk = 0.3 V·s/rad
n0 = rpm
5.Hard
+−48 VRa​ = 4 ΩM+−EωT

A small DC motor has an armature resistance of 4 Ω and runs from 48 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 48 VRa = 4 Ω
Pmax = W
6.Hard
+−240 VRa​ = 0.8 ΩIa​: 30 → 18 AM+−En = ?T

A separately excited DC motor on 240 V (Ra = 0.8 Ω) runs at 1500 rpm with an armature current of 30 A. The load changes and the current becomes 18 A, with the field unchanged. Find the new speed n2.

V = 240 VRa = 0.8 Ωn1 = 1500 rpmIa1 = 30 AIa2 = 18 A
n2 = rpm

Answer key

  1. 1. Istart = 60 A
  2. 2. T = 3.6 N·m
  3. 3. n = 3247 rpm
  4. 4. n0 = 3501 rpm
  5. 5. Pmax = 144 W
  6. 6. n2 = 1567 rpm

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