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​ = 1 ΩIa​ = 4 AM+−EωT

A DC motor runs from 120 V with an armature current of 4 A. The armature resistance is 1 Ω. Find the developed (electromagnetic) power Pdev.

V = 120 VIa = 4 ARa = 1 Ω
Pdev = W
2.Easy
+−110 VRa​M+−E = 106 Vn = 2000 rpmT

A DC motor turns at 2000 rpm, and its back-EMF at that speed is 106 V. Find the motor constant k in V·s/rad.

E = 106 Vn = 2000 rpm
k = V·s/rad
3.Medium
+−VRa​M+−En = 1800 rpmT = ?

A DC motor delivers 2.2 kW of mechanical power at 1800 rpm. Find the shaft torque T.

P = 2.2 kWn = 1800 rpm
T = N·m
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.5 V·s/rad runs from 120 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 120 Vk = 0.5 V·s/rad
n0 = rpm
5.Hard
+−120 VRa​ = 0.25 ΩIa​: 20 → 8 AM+−En = ?T

A separately excited DC motor on 120 V (Ra = 0.25 Ω) runs at 900 rpm with an armature current of 20 A. The load changes and the current becomes 8 A, with the field unchanged. Find the new speed n2.

V = 120 VRa = 0.25 Ωn1 = 900 rpmIa1 = 20 AIa2 = 8 A
n2 = rpm
6.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

Answer key

  1. 1. Pdev = 464 W
  2. 2. k = 0.5061 V·s/rad
  3. 3. T = 11.67 N·m
  4. 4. n0 = 2292 rpm
  5. 5. n2 = 923.5 rpm
  6. 6. Pmax = 230.4 W

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