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
+−48 VRa​M+−E = 46 Vn = 750 rpmT

A DC motor turns at 750 rpm, and its back-EMF at that speed is 46 V. Find the motor constant k in V·s/rad.

E = 46 Vn = 750 rpm
k = V·s/rad
2.Easy
+−240 VRa​ = 1 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 1 Ω is switched directly onto a 240 V supply. Its normal full-load current is 10 A. Find the armature current Istart at the moment of starting.

V = 240 VRa = 1 Ω
Istart = A
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.8 N·m/A and Ra = 0.2 Ω is connected to 110 V. Find its stall torque Tstall, the torque at zero speed.

V = 110 Vk = 0.8 N·m/ARa = 0.2 Ω
Tstall = N·m
4.Medium
+−VRa​M+−En = 1000 rpmT = ?

A DC motor delivers 15 kW of mechanical power at 1000 rpm. Find the shaft torque T.

P = 15 kWn = 1000 rpm
T = N·m
5.Hard
+−220 VRa​ = 1 ΩM+−En = ?T = 6.4 N·m

A separately excited DC motor (k = 0.8 V·s/rad at full field, Ra = 1 Ω, V = 220 V) drives a constant load torque of 6.4 N·m. The field flux is reduced to 70 % of its full value. Find the new speed n.

V = 220 Vk = 0.8 V·s/radRa = 1 ΩT = 6.4 N·mΦ = 70 %
n = rpm
6.Hard
+−24 VRa​ = 0.5 ΩM+−EωT

A small DC motor has an armature resistance of 0.5 Ω and runs from 24 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 24 VRa = 0.5 Ω
Pmax = W

Answer key

  1. 1. k = 0.5857 V·s/rad
  2. 2. Istart = 240 A
  3. 3. Tstall = 440 N·m
  4. 4. T = 143.2 N·m
  5. 5. n = 3557 rpm
  6. 6. Pmax = 288 W

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