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​ = 0.5 ΩIa​ = 4 AM+−EωT

A DC motor runs from 48 V with an armature current of 4 A. The armature resistance is 0.5 Ω. Find the developed (electromagnetic) power Pdev.

V = 48 VIa = 4 ARa = 0.5 Ω
Pdev = W
2.Easy
+−220 VRa​M+−E = 212.5 Vn = 1500 rpmT

A DC motor turns at 1500 rpm, and its back-EMF at that speed is 212.5 V. Find the motor constant k in V·s/rad.

E = 212.5 Vn = 1500 rpm
k = V·s/rad
3.Medium
+−VRa​M+−En = 1200 rpmT = ?

A DC motor delivers 15 kW of mechanical power at 1200 rpm. Find the shaft torque T.

P = 15 kWn = 1200 rpm
T = N·m
4.Medium
T (N·m)n (rpm)0.4 N·mn0​nFL​

A DC motor (k = 0.2 V·s/rad, Ra = 1.2 Ω) runs from 48 V. At full load it draws 2 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 48 Vk = 0.2 V·s/radRa = 1.2 ΩIFL = 2 A
SR = %
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
+−120 VRa​ = 0.8 ΩIa​: 12 → 6 AM+−En = ?T

A separately excited DC motor on 120 V (Ra = 0.8 Ω) runs at 1500 rpm with an armature current of 12 A. The load changes and the current becomes 6 A, with the field unchanged. Find the new speed n2.

V = 120 VRa = 0.8 Ωn1 = 1500 rpmIa1 = 12 AIa2 = 6 A
n2 = rpm

Answer key

  1. 1. Pdev = 184 W
  2. 2. k = 1.353 V·s/rad
  3. 3. T = 119.4 N·m
  4. 4. SR = 5.263 %
  5. 5. Pmax = 230.4 W
  6. 6. n2 = 1565 rpm

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