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​ = 0.8 ΩIa​ = ?M+−E = 216 VωT

A DC motor on a 240 V supply has an armature resistance of 0.8 Ω. At its present speed the back-EMF is 216 V. Find the armature current Ia.

V = 240 VE = 216 VRa = 0.8 Ω
Ia = A
2.Easy
+−110 VRa​ = 2 ΩIa​ = ?M+−E = 102 VωT

A DC motor on a 110 V supply has an armature resistance of 2 Ω. At its present speed the back-EMF is 102 V. Find the armature current Ia.

V = 110 VE = 102 VRa = 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 = 1.5 Ω is connected to 110 V. Find its stall torque Tstall, the torque at zero speed.

V = 110 Vk = 0.5 N·m/ARa = 1.5 Ω
Tstall = N·m
4.Medium
+−240 VRa​ = 0.4 ΩIa​ = 40 AM+−EωT

A DC motor draws 40 A from a 240 V supply. Its armature resistance is 0.4 Ω and its rotational (friction, windage and iron) losses are 538 W. Find its efficiency η.

V = 240 VIa = 40 ARa = 0.4 ΩProt = 538 W
η = %
5.Hard
+−220 VD = ?Ra​ = 0.2 ΩM+−En = 600 rpmT = 0.6 N·m

A DC chopper feeds a motor (k = 0.1 V·s/rad, Ra = 0.2 Ω) from a 220 V DC bus. The average armature voltage is D·220 V. Find the duty cycle D that runs the motor at 600 rpm with a load of 0.6 N·m.

Vdc = 220 Vk = 0.1 V·s/radRa = 0.2 ΩT = 0.6 N·mn = 600 rpm
D = %
6.Hard
+−240 VRa​ = 0.5 ΩM+−En = ?T = 15 N·m

A separately excited DC motor (k = 0.75 V·s/rad at full field, Ra = 0.5 Ω, V = 240 V) drives a constant load torque of 15 N·m. The field flux is reduced to 90 % of its full value. Find the new speed n.

V = 240 Vk = 0.75 V·s/radRa = 0.5 ΩT = 15 N·mΦ = 90 %
n = rpm

Answer key

  1. 1. Ia = 30 A
  2. 2. Ia = 4 A
  3. 3. Tstall = 36.67 N·m
  4. 4. η = 87.73 %
  5. 5. D = 3.401 %
  6. 6. n = 3238 rpm

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