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
+−240 VRa​ = 1.5 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 1.5 Ω is switched directly onto a 240 V supply. Its normal full-load current is 6 A. Find the armature current Istart at the moment of starting.

V = 240 VRa = 1.5 Ω
Istart = A
2.Easy
+−220 VRa​M+−E = 208 Vn = 750 rpmT

A DC motor turns at 750 rpm, and its back-EMF at that speed is 208 V. Find the motor constant k in V·s/rad.

E = 208 Vn = 750 rpm
k = V·s/rad
3.Medium
T (N·m)n (rpm)12 N·mn0​n = ?

A DC motor (k = 1.2 N·m/A, Ra = 2 Ω) runs from 240 V and drives a load torque of 12 N·m. Find its speed n in rpm.

V = 240 Vk = 1.2 N·m/ARa = 2 ΩT = 12 N·m
n = rpm
4.Medium
+−VRa​M+−En = 2900 rpmT = ?

A DC motor delivers 0.75 kW of mechanical power at 2900 rpm. Find the shaft torque T.

P = 0.75 kWn = 2900 rpm
T = N·m
5.Hard
+−120 VRa​ = 0.5 ΩM+−En = ?T = 6 N·m

A separately excited DC motor (k = 0.6 V·s/rad at full field, Ra = 0.5 Ω, V = 120 V) drives a constant load torque of 6 N·m. The field flux is reduced to 75 % of its full value. Find the new speed n.

V = 120 Vk = 0.6 V·s/radRa = 0.5 ΩT = 6 N·mΦ = 75 %
n = rpm
6.Hard
+−24 VRs​ = ?Ra​ = 0.5 ΩM+−En = 750 rpmT = 0.75 N·m

A DC motor (k = 0.15 V·s/rad, Ra = 0.5 Ω) on a fixed 24 V supply drives a load of 0.75 N·m. Find the series resistance Rs that slows it down to 750 rpm.

V = 24 Vk = 0.15 V·s/radRa = 0.5 ΩT = 0.75 N·mn = 750 rpm
Rs = Ω

Answer key

  1. 1. Istart = 160 A
  2. 2. k = 2.648 V·s/rad
  3. 3. n = 1751 rpm
  4. 4. T = 2.47 N·m
  5. 5. n = 2405 rpm
  6. 6. Rs = 1.944 Ω

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