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​Ia​ = 15 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.05 N·m/A. Find the torque T it develops at an armature current of 15 A.

k = 0.05 N·m/AIa = 15 A
T = N·m
2.Easy
+−240 VRa​M+−E = 228 Vn = 3000 rpmT

A DC motor turns at 3000 rpm, and its back-EMF at that speed is 228 V. Find the motor constant k in V·s/rad.

E = 228 Vn = 3000 rpm
k = V·s/rad
3.Medium
+−VRa​M+−En = 960 rpmT = ?

A DC motor delivers 1.5 kW of mechanical power at 960 rpm. Find the shaft torque T.

P = 1.5 kWn = 960 rpm
T = N·m
4.Medium
T (N·m)n (rpm)2.4 N·mn0​nFL​

A DC motor (k = 0.4 V·s/rad, Ra = 1 Ω) runs from 48 V. At full load it draws 6 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 48 Vk = 0.4 V·s/radRa = 1 ΩIFL = 6 A
SR = %
5.Hard
+−24 VRs​ = ?Ra​ = 1 ΩM+−En = 400 rpmT = 0.2 N·m

A DC motor (k = 0.1 V·s/rad, Ra = 1 Ω) on a fixed 24 V supply drives a load of 0.2 N·m. Find the series resistance Rs that slows it down to 400 rpm.

V = 24 Vk = 0.1 V·s/radRa = 1 ΩT = 0.2 N·mn = 400 rpm
Rs = Ω
6.Hard
+−120 VRa​ = 1.2 ΩM+−En = ?T = 3.6 N·m

A separately excited DC motor (k = 0.6 V·s/rad at full field, Ra = 1.2 Ω, V = 120 V) drives a constant load torque of 3.6 N·m. The field flux is reduced to 70 % of its full value. Find the new speed n.

V = 120 Vk = 0.6 V·s/radRa = 1.2 ΩT = 3.6 N·mΦ = 70 %
n = rpm

Answer key

  1. 1. T = 0.75 N·m
  2. 2. k = 0.7257 V·s/rad
  3. 3. T = 14.92 N·m
  4. 4. SR = 14.29 %
  5. 5. Rs = 8.906 Ω
  6. 6. n = 2495 rpm

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