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​ = 0.4 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.4 Ω is switched directly onto a 120 V supply. Its normal full-load current is 30 A. Find the armature current Istart at the moment of starting.

V = 120 VRa = 0.4 Ω
Istart = A
2.Easy
+−120 VRa​ = 1 ΩIa​ = 10 AM+−En = ?T

A DC motor with k = 0.6 V·s/rad and an armature resistance of 1 Ω runs from 120 V and draws 10 A. Find its speed n in rpm.

V = 120 VRa = 1 ΩIa = 10 Ak = 0.6 V·s/rad
n = rpm
3.Medium
+−VRa​M+−En = 3000 rpmT = ?

A DC motor delivers 4 kW of mechanical power at 3000 rpm. Find the shaft torque T.

P = 4 kWn = 3000 rpm
T = N·m
4.Medium
T (N·m)n (rpm)3.75 N·mn0​n = ?

A DC motor (k = 0.25 N·m/A, Ra = 0.4 Ω) runs from 48 V and drives a load torque of 3.75 N·m. Find its speed n in rpm.

V = 48 Vk = 0.25 N·m/ARa = 0.4 ΩT = 3.75 N·m
n = rpm
5.Hard
+−24 VRa​ = 0.4 ΩIa​: 5 → 2.5 AM+−En = ?T

A separately excited DC motor on 24 V (Ra = 0.4 Ω) runs at 1500 rpm with an armature current of 5 A. The load changes and the current becomes 2.5 A, with the field unchanged. Find the new speed n2.

V = 24 VRa = 0.4 Ωn1 = 1500 rpmIa1 = 5 AIa2 = 2.5 A
n2 = rpm
6.Hard
+−48 VRa​ = 4 ΩM+−EωT

A small DC motor has an armature resistance of 4 Ω and runs from 48 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 48 VRa = 4 Ω
Pmax = W

Answer key

  1. 1. Istart = 300 A
  2. 2. n = 1751 rpm
  3. 3. T = 12.73 N·m
  4. 4. n = 1604 rpm
  5. 5. n2 = 1568 rpm
  6. 6. Pmax = 144 W

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