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
+−110 VRa​M+−E = 101 Vn = 2400 rpmT

A DC motor turns at 2400 rpm, and its back-EMF at that speed is 101 V. Find the motor constant k in V·s/rad.

E = 101 Vn = 2400 rpm
k = V·s/rad
2.Easy
+−110 VRa​ = 0.5 ΩIa​ = ?M+−E = 102.5 VωT

A DC motor on a 110 V supply has an armature resistance of 0.5 Ω. At its present speed the back-EMF is 102.5 V. Find the armature current Ia.

V = 110 VE = 102.5 VRa = 0.5 Ω
Ia = A
3.Medium
+−240 VRa​ = 2 ΩIa​ = 15 AM+−EωT

A DC motor draws 15 A from a 240 V supply. Its armature resistance is 2 Ω and its rotational (friction, windage and iron) losses are 252 W. Find its efficiency η.

V = 240 VIa = 15 ARa = 2 ΩProt = 252 W
η = %
4.Medium
+−VRa​M+−En = 1000 rpmT = ?

A DC motor delivers 2.2 kW of mechanical power at 1000 rpm. Find the shaft torque T.

P = 2.2 kWn = 1000 rpm
T = N·m
5.Hard
+−120 VRa​ = 2 ΩIa​: 2 → 1 AM+−En = ?T

A separately excited DC motor on 120 V (Ra = 2 Ω) runs at 1200 rpm with an armature current of 2 A. The load changes and the current becomes 1 A, with the field unchanged. Find the new speed n2.

V = 120 VRa = 2 Ωn1 = 1200 rpmIa1 = 2 AIa2 = 1 A
n2 = rpm
6.Hard
+−240 VRa​ = 1.2 ΩM+−En = ?T = 24 N·m

A separately excited DC motor (k = 1.2 V·s/rad at full field, Ra = 1.2 Ω, V = 240 V) drives a constant load torque of 24 N·m. The field flux is reduced to 85 % of its full value. Find the new speed n.

V = 240 Vk = 1.2 V·s/radRa = 1.2 ΩT = 24 N·mΦ = 85 %
n = rpm

Answer key

  1. 1. k = 0.4019 V·s/rad
  2. 2. Ia = 15 A
  3. 3. η = 80.5 %
  4. 4. T = 21.01 N·m
  5. 5. n2 = 1221 rpm
  6. 6. n = 1983 rpm

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