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
+−110 VRa​ = 0.25 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.25 Ω is switched directly onto a 110 V supply. Its normal full-load current is 20 A. Find the armature current Istart at the moment of starting.

V = 110 VRa = 0.25 Ω
Istart = A
2.Easy
+−110 VRa​ = 0.25 ΩIa​ = 20 AM+−E = ?ωT

A DC motor runs from a 110 V supply and draws an armature current of 20 A. Its armature resistance is 0.25 Ω. Find the back-EMF E.

V = 110 VIa = 20 ARa = 0.25 Ω
E = V
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.75 N·m/A and Ra = 0.5 Ω is connected to 240 V. Find its stall torque Tstall, the torque at zero speed.

V = 240 Vk = 0.75 N·m/ARa = 0.5 Ω
Tstall = N·m
4.Medium
+−VRa​M+−En = 1450 rpmT = ?

A DC motor delivers 2.2 kW of mechanical power at 1450 rpm. Find the shaft torque T.

P = 2.2 kWn = 1450 rpm
T = N·m
5.Hard
+−240 VD = ?Ra​ = 0.4 ΩM+−En = 1500 rpmT = 40 N·m

A DC chopper feeds a motor (k = 1 V·s/rad, Ra = 0.4 Ω) from a 240 V DC bus. The average armature voltage is D·240 V. Find the duty cycle D that runs the motor at 1500 rpm with a load of 40 N·m.

Vdc = 240 Vk = 1 V·s/radRa = 0.4 ΩT = 40 N·mn = 1500 rpm
D = %
6.Hard
+−48 VRst​ = ?Ra​ = 0.2 Ω20 AM+−E = 0n = 0T

A 48 V DC motor has an armature resistance of 0.2 Ω. Find the series starting resistance Rst that limits the starting current to 20 A.

V = 48 VRa = 0.2 ΩImax = 20 A
Rst = Ω

Answer key

  1. 1. Istart = 440 A
  2. 2. E = 105 V
  3. 3. Tstall = 360 N·m
  4. 4. T = 14.49 N·m
  5. 5. D = 72.12 %
  6. 6. Rst = 2.2 Ω

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