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
+−220 VRa​ = 0.4 ΩIa​ = 25 AM+−En = ?T

A DC motor with k = 1 V·s/rad and an armature resistance of 0.4 Ω runs from 220 V and draws 25 A. Find its speed n in rpm.

V = 220 VRa = 0.4 ΩIa = 25 Ak = 1 V·s/rad
n = rpm
2.Easy
+−110 VRa​ = 0.4 ΩIa​ = 15 AM+−En = ?T

A DC motor with k = 0.6 V·s/rad and an armature resistance of 0.4 Ω runs from 110 V and draws 15 A. Find its speed n in rpm.

V = 110 VRa = 0.4 ΩIa = 15 Ak = 0.6 V·s/rad
n = rpm
3.Medium
+−VRa​M+−En = 2000 rpmT = ?

A DC motor delivers 15 kW of mechanical power at 2000 rpm. Find the shaft torque T.

P = 15 kWn = 2000 rpm
T = N·m
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.2 N·m/A and Ra = 0.5 Ω is connected to 48 V. Find its stall torque Tstall, the torque at zero speed.

V = 48 Vk = 0.2 N·m/ARa = 0.5 Ω
Tstall = N·m
5.Hard
+−120 VRa​ = 0.5 ΩIa​: 8 → 4.8 AM+−En = ?T

A separately excited DC motor on 120 V (Ra = 0.5 Ω) runs at 1000 rpm with an armature current of 8 A. The load changes and the current becomes 4.8 A, with the field unchanged. Find the new speed n2.

V = 120 VRa = 0.5 Ωn1 = 1000 rpmIa1 = 8 AIa2 = 4.8 A
n2 = rpm
6.Hard
+−110 VRst​ = ?Ra​ = 0.2 Ω100 AM+−E = 0n = 0T

A 110 V DC motor has an armature resistance of 0.2 Ω. Find the series starting resistance Rst that limits the starting current to 100 A.

V = 110 VRa = 0.2 ΩImax = 100 A
Rst = Ω

Answer key

  1. 1. n = 2005 rpm
  2. 2. n = 1655 rpm
  3. 3. T = 71.62 N·m
  4. 4. Tstall = 19.2 N·m
  5. 5. n2 = 1014 rpm
  6. 6. Rst = 0.9 Ω

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