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
+−220 VRa​ = 1.5 ΩIa​ = ?M+−E = 211 VωT

A DC motor on a 220 V supply has an armature resistance of 1.5 Ω. At its present speed the back-EMF is 211 V. Find the armature current Ia.

V = 220 VE = 211 VRa = 1.5 Ω
Ia = A
2.Easy
+−120 VRa​M+−E = 104 Vn = 1800 rpmT

A DC motor turns at 1800 rpm, and its back-EMF at that speed is 104 V. Find the motor constant k in V·s/rad.

E = 104 Vn = 1800 rpm
k = V·s/rad
3.Medium
T (N·m)n (rpm)3 N·mn0​n = ?

A DC motor (k = 0.25 N·m/A, Ra = 0.2 Ω) runs from 48 V and drives a load torque of 3 N·m. Find its speed n in rpm.

V = 48 Vk = 0.25 N·m/ARa = 0.2 ΩT = 3 N·m
n = rpm
4.Medium
+−VRa​M+−En = 960 rpmT = ?

A DC motor delivers 15 kW of mechanical power at 960 rpm. Find the shaft torque T.

P = 15 kWn = 960 rpm
T = N·m
5.Hard
T (N·m)n (rpm)1.2 N·m750 rpm

A DC motor with k = 0.3 V·s/rad and Ra = 1.5 Ω must drive a load of 1.2 N·m at 750 rpm. Find the armature voltage V it needs.

k = 0.3 V·s/radRa = 1.5 ΩT = 1.2 N·mn = 750 rpm
V = V
6.Hard
+−240 VRa​ = 1.2 ΩIa​: 15 → 9 AM+−En = ?T

A separately excited DC motor on 240 V (Ra = 1.2 Ω) runs at 1200 rpm with an armature current of 15 A. The load changes and the current becomes 9 A, with the field unchanged. Find the new speed n2.

V = 240 VRa = 1.2 Ωn1 = 1200 rpmIa1 = 15 AIa2 = 9 A
n2 = rpm

Answer key

  1. 1. Ia = 6 A
  2. 2. k = 0.5517 V·s/rad
  3. 3. n = 1742 rpm
  4. 4. T = 149.2 N·m
  5. 5. V = 29.56 V
  6. 6. n2 = 1239 rpm

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