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​ = 0.8 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.8 Ω is switched directly onto a 120 V supply. Its normal full-load current is 5 A. Find the armature current Istart at the moment of starting.

V = 120 VRa = 0.8 Ω
Istart = A
2.Easy
+−110 VRa​ = 1.2 ΩIa​ = ?M+−E = 98 VωT

A DC motor on a 110 V supply has an armature resistance of 1.2 Ω. At its present speed the back-EMF is 98 V. Find the armature current Ia.

V = 110 VE = 98 VRa = 1.2 Ω
Ia = A
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.5 N·m/A and Ra = 0.8 Ω is connected to 48 V. Find its stall torque Tstall, the torque at zero speed.

V = 48 Vk = 0.5 N·m/ARa = 0.8 Ω
Tstall = N·m
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 1 V·s/rad runs from 220 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 220 Vk = 1 V·s/rad
n0 = rpm
5.Hard
+−120 VRs​ = ?Ra​ = 0.5 ΩM+−En = 300 rpmT = 25 N·m

A DC motor (k = 1 V·s/rad, Ra = 0.5 Ω) on a fixed 120 V supply drives a load of 25 N·m. Find the series resistance Rs that slows it down to 300 rpm.

V = 120 Vk = 1 V·s/radRa = 0.5 ΩT = 25 N·mn = 300 rpm
Rs = Ω
6.Hard
+−240 VRa​ = 0.4 ΩM+−En = ?T = 18 N·m

A separately excited DC motor (k = 0.6 V·s/rad at full field, Ra = 0.4 Ω, V = 240 V) drives a constant load torque of 18 N·m. The field flux is reduced to 80 % of its full value. Find the new speed n.

V = 240 Vk = 0.6 V·s/radRa = 0.4 ΩT = 18 N·mΦ = 80 %
n = rpm

Answer key

  1. 1. Istart = 150 A
  2. 2. Ia = 10 A
  3. 3. Tstall = 30 N·m
  4. 4. n0 = 2101 rpm
  5. 5. Rs = 3.043 Ω
  6. 6. n = 4476 rpm

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