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​ = 10 AM+−En = ?T

A DC motor with k = 0.5 V·s/rad and an armature resistance of 0.4 Ω runs from 120 V and draws 10 A. Find its speed n in rpm.

V = 120 VRa = 0.4 ΩIa = 10 Ak = 0.5 V·s/rad
n = rpm
2.Easy
+−48 VRa​ = 0.2 ΩIa​ = ?M+−E = 42 VωT

A DC motor on a 48 V supply has an armature resistance of 0.2 Ω. At its present speed the back-EMF is 42 V. Find the armature current Ia.

V = 48 VE = 42 VRa = 0.2 Ω
Ia = A
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.8 N·m/A and Ra = 2 Ω is connected to 220 V. Find its stall torque Tstall, the torque at zero speed.

V = 220 Vk = 0.8 N·m/ARa = 2 Ω
Tstall = N·m
4.Medium
T (N·m)n (rpm)7.2 N·mn0​n = ?

A DC motor (k = 0.6 N·m/A, Ra = 0.4 Ω) runs from 48 V and drives a load torque of 7.2 N·m. Find its speed n in rpm.

V = 48 Vk = 0.6 N·m/ARa = 0.4 ΩT = 7.2 N·m
n = rpm
5.Hard
+−24 VRs​ = ?Ra​ = 0.4 ΩM+−En = 600 rpmT = 1.2 N·m

A DC motor (k = 0.3 V·s/rad, Ra = 0.4 Ω) on a fixed 24 V supply drives a load of 1.2 N·m. Find the series resistance Rs that slows it down to 600 rpm.

V = 24 Vk = 0.3 V·s/radRa = 0.4 ΩT = 1.2 N·mn = 600 rpm
Rs = Ω
6.Hard
+−120 VRst​ = ?Ra​ = 0.4 Ω30 AM+−E = 0n = 0T

A 120 V DC motor has an armature resistance of 0.4 Ω. Find the series starting resistance Rst that limits the starting current to 30 A.

V = 120 VRa = 0.4 ΩImax = 30 A
Rst = Ω

Answer key

  1. 1. n = 2215 rpm
  2. 2. Ia = 30 A
  3. 3. Tstall = 88 N·m
  4. 4. n = 687.5 rpm
  5. 5. Rs = 0.8876 Ω
  6. 6. Rst = 3.6 Ω

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