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​ = 2 ΩIa​ = ?M+−E = 108 VωT

A DC motor on a 120 V supply has an armature resistance of 2 Ω. At its present speed the back-EMF is 108 V. Find the armature current Ia.

V = 120 VE = 108 VRa = 2 Ω
Ia = A
2.Easy
+−48 VRa​ = 0.25 ΩIa​ = 6 AM+−E = ?ωT

A DC motor runs from a 48 V supply and draws an armature current of 6 A. Its armature resistance is 0.25 Ω. Find the back-EMF E.

V = 48 VIa = 6 ARa = 0.25 Ω
E = V
3.Medium
T (N·m)n (rpm)1.44 N·mn0​n = ?

A DC motor (k = 0.12 N·m/A, Ra = 0.2 Ω) runs from 24 V and drives a load torque of 1.44 N·m. Find its speed n in rpm.

V = 24 Vk = 0.12 N·m/ARa = 0.2 ΩT = 1.44 N·m
n = rpm
4.Medium
+−VRa​M+−En = 1000 rpmT = ?

A DC motor delivers 1.5 kW of mechanical power at 1000 rpm. Find the shaft torque T.

P = 1.5 kWn = 1000 rpm
T = N·m
5.Hard
+−120 VRa​ = 2 ΩM+−En = ?T = 2 N·m

A separately excited DC motor (k = 0.5 V·s/rad at full field, Ra = 2 Ω, V = 120 V) drives a constant load torque of 2 N·m. The field flux is reduced to 85 % of its full value. Find the new speed n.

V = 120 Vk = 0.5 V·s/radRa = 2 ΩT = 2 N·mΦ = 85 %
n = rpm
6.Hard
+−220 VD = ?Ra​ = 0.8 ΩM+−En = 500 rpmT = 0.16 N·m

A DC chopper feeds a motor (k = 0.08 V·s/rad, Ra = 0.8 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 500 rpm with a load of 0.16 N·m.

Vdc = 220 Vk = 0.08 V·s/radRa = 0.8 ΩT = 0.16 N·mn = 500 rpm
D = %

Answer key

  1. 1. Ia = 6 A
  2. 2. E = 46.5 V
  3. 3. n = 1719 rpm
  4. 4. T = 14.32 N·m
  5. 5. n = 2485 rpm
  6. 6. D = 2.631 %

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