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
+−120 VRa​ = 0.5 ΩIa​ = ?M+−E = 105 VωT

A DC motor on a 120 V supply has an armature resistance of 0.5 Ω. At its present speed the back-EMF is 105 V. Find the armature current Ia.

V = 120 VE = 105 VRa = 0.5 Ω
Ia = A
2.Easy
+−24 VRa​M+−E = 22.75 Vn = 1800 rpmT

A DC motor turns at 1800 rpm, and its back-EMF at that speed is 22.75 V. Find the motor constant k in V·s/rad.

E = 22.75 Vn = 1800 rpm
k = V·s/rad
3.Medium
+−VRa​M+−En = 1000 rpmT = ?

A DC motor delivers 3 kW of mechanical power at 1000 rpm. Find the shaft torque T.

P = 3 kWn = 1000 rpm
T = N·m
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 1.2 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 240 Vk = 1.2 V·s/rad
n0 = rpm
5.Hard
+−48 VRa​ = 2.5 ΩM+−EωT

A small DC motor has an armature resistance of 2.5 Ω and runs from 48 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 48 VRa = 2.5 Ω
Pmax = W
6.Hard
+−240 VRa​ = 1 ΩM+−En = ?T = 24 N·m

A separately excited DC motor (k = 2 V·s/rad at full field, Ra = 1 Ω, V = 240 V) drives a constant load torque of 24 N·m. The field flux is reduced to 90 % of its full value. Find the new speed n.

V = 240 Vk = 2 V·s/radRa = 1 ΩT = 24 N·mΦ = 90 %
n = rpm

Answer key

  1. 1. Ia = 30 A
  2. 2. k = 0.1207 V·s/rad
  3. 3. T = 28.65 N·m
  4. 4. n0 = 1910 rpm
  5. 5. Pmax = 230.4 W
  6. 6. n = 1203 rpm

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