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
+−240 VRa​M+−E = 216 Vn = 2400 rpmT

A DC motor turns at 2400 rpm, and its back-EMF at that speed is 216 V. Find the motor constant k in V·s/rad.

E = 216 Vn = 2400 rpm
k = V·s/rad
2.Easy
+−48 VRa​Ia​ = 10 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.8 N·m/A. Find the torque T it develops at an armature current of 10 A.

k = 0.8 N·m/AIa = 10 A
T = N·m
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.6 V·s/rad runs from 48 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 48 Vk = 0.6 V·s/rad
n0 = rpm
4.Medium
T (N·m)n (rpm)2 N·mn0​n = ?

A DC motor (k = 0.4 N·m/A, Ra = 1.5 Ω) runs from 110 V and drives a load torque of 2 N·m. Find its speed n in rpm.

V = 110 Vk = 0.4 N·m/ARa = 1.5 ΩT = 2 N·m
n = rpm
5.Hard
T (N·m)n (rpm)5 N·m600 rpm

A DC motor with k = 0.5 V·s/rad and Ra = 0.5 Ω must drive a load of 5 N·m at 600 rpm. Find the armature voltage V it needs.

k = 0.5 V·s/radRa = 0.5 ΩT = 5 N·mn = 600 rpm
V = V
6.Hard
+−240 VD = ?Ra​ = 0.5 ΩM+−En = 500 rpmT = 1.5 N·m

A DC chopper feeds a motor (k = 0.3 V·s/rad, Ra = 0.5 Ω) from a 240 V DC bus. The average armature voltage is D·240 V. Find the duty cycle D that runs the motor at 500 rpm with a load of 1.5 N·m.

Vdc = 240 Vk = 0.3 V·s/radRa = 0.5 ΩT = 1.5 N·mn = 500 rpm
D = %

Answer key

  1. 1. k = 0.8594 V·s/rad
  2. 2. T = 8 N·m
  3. 3. n0 = 763.9 rpm
  4. 4. n = 2447 rpm
  5. 5. V = 36.42 V
  6. 6. D = 7.587 %

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