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​ = 20 AM+−En = ?T

A DC motor with k = 0.6 V·s/rad and an armature resistance of 0.8 Ω runs from 120 V and draws 20 A. Find its speed n in rpm.

V = 120 VRa = 0.8 ΩIa = 20 Ak = 0.6 V·s/rad
n = rpm
2.Easy
+−120 VRa​ = 2 ΩIa​ = 2 AM+−En = ?T

A DC motor with k = 0.5 V·s/rad and an armature resistance of 2 Ω runs from 120 V and draws 2 A. Find its speed n in rpm.

V = 120 VRa = 2 ΩIa = 2 Ak = 0.5 V·s/rad
n = rpm
3.Medium
T (N·m)n (rpm)1 N·mn0​n = ?

A DC motor (k = 0.1 N·m/A, Ra = 0.25 Ω) runs from 24 V and drives a load torque of 1 N·m. Find its speed n in rpm.

V = 24 Vk = 0.1 N·m/ARa = 0.25 ΩT = 1 N·m
n = rpm
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.75 N·m/A and Ra = 1.5 Ω is connected to 220 V. Find its stall torque Tstall, the torque at zero speed.

V = 220 Vk = 0.75 N·m/ARa = 1.5 Ω
Tstall = N·m
5.Hard
+−110 VRs​ = ?Ra​ = 1.2 ΩM+−En = 1500 rpmT = 1.6 N·m

A DC motor (k = 0.4 V·s/rad, Ra = 1.2 Ω) on a fixed 110 V supply drives a load of 1.6 N·m. Find the series resistance Rs that slows it down to 1500 rpm.

V = 110 Vk = 0.4 V·s/radRa = 1.2 ΩT = 1.6 N·mn = 1500 rpm
Rs = Ω
6.Hard
+−120 VRa​ = 2 ΩM+−EωT

A small DC motor has an armature resistance of 2 Ω and runs from 120 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 120 VRa = 2 Ω
Pmax = W

Answer key

  1. 1. n = 1655 rpm
  2. 2. n = 2215 rpm
  3. 3. n = 2053 rpm
  4. 4. Tstall = 110 N·m
  5. 5. Rs = 10.59 Ω
  6. 6. Pmax = 1800 W

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