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​ = 2 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 2 Ω is switched directly onto a 220 V supply. Its normal full-load current is 10 A. Find the armature current Istart at the moment of starting.

V = 220 VRa = 2 Ω
Istart = A
2.Easy
+−220 VRa​M+−E = 210 Vn = 2000 rpmT

A DC motor turns at 2000 rpm, and its back-EMF at that speed is 210 V. Find the motor constant k in V·s/rad.

E = 210 Vn = 2000 rpm
k = V·s/rad
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 1.5 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 240 Vk = 1.5 V·s/rad
n0 = rpm
4.Medium
+−VRa​M+−En = 1450 rpmT = ?

A DC motor delivers 1 kW of mechanical power at 1450 rpm. Find the shaft torque T.

P = 1 kWn = 1450 rpm
T = N·m
5.Hard
+−240 VRs​ = ?Ra​ = 0.2 ΩM+−En = 800 rpmT = 30 N·m

A DC motor (k = 0.75 V·s/rad, Ra = 0.2 Ω) on a fixed 240 V supply drives a load of 30 N·m. Find the series resistance Rs that slows it down to 800 rpm.

V = 240 Vk = 0.75 V·s/radRa = 0.2 ΩT = 30 N·mn = 800 rpm
Rs = Ω
6.Hard
+−110 VRs​ = ?Ra​ = 0.4 ΩM+−En = 1000 rpmT = 15 N·m

A DC motor (k = 0.5 V·s/rad, Ra = 0.4 Ω) on a fixed 110 V supply drives a load of 15 N·m. Find the series resistance Rs that slows it down to 1000 rpm.

V = 110 Vk = 0.5 V·s/radRa = 0.4 ΩT = 15 N·mn = 1000 rpm
Rs = Ω

Answer key

  1. 1. Istart = 110 A
  2. 2. k = 1.003 V·s/rad
  3. 3. n0 = 1528 rpm
  4. 4. T = 6.586 N·m
  5. 5. Rs = 4.229 Ω
  6. 6. Rs = 1.521 Ω

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