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​M+−E = 202 Vn = 900 rpmT

A DC motor turns at 900 rpm, and its back-EMF at that speed is 202 V. Find the motor constant k in V·s/rad.

E = 202 Vn = 900 rpm
k = V·s/rad
2.Easy
+−240 VRa​M+−E = 210 Vn = 2000 rpmT

A DC motor turns at 2000 rpm, and its back-EMF at that speed is 210 V. Find the motor constant k in V·s/rad.

E = 210 Vn = 2000 rpm
k = V·s/rad
3.Medium
+−VRa​M+−En = 1200 rpmT = ?

A DC motor delivers 7.5 kW of mechanical power at 1200 rpm. Find the shaft torque T.

P = 7.5 kWn = 1200 rpm
T = N·m
4.Medium
+−VRa​M+−En = 2000 rpmT = ?

A DC motor delivers 5.5 kW of mechanical power at 2000 rpm. Find the shaft torque T.

P = 5.5 kWn = 2000 rpm
T = N·m
5.Hard
+−240 VRa​ = 0.5 ΩM+−En = ?T = 22.5 N·m

A separately excited DC motor (k = 0.75 V·s/rad at full field, Ra = 0.5 Ω, V = 240 V) drives a constant load torque of 22.5 N·m. The field flux is reduced to 90 % of its full value. Find the new speed n.

V = 240 Vk = 0.75 V·s/radRa = 0.5 ΩT = 22.5 N·mΦ = 90 %
n = rpm
6.Hard
+−120 VRst​ = ?Ra​ = 0.4 Ω50 AM+−E = 0n = 0T

A 120 V DC motor has an armature resistance of 0.4 Ω. Find the series starting resistance Rst that limits the starting current to 50 A.

V = 120 VRa = 0.4 ΩImax = 50 A
Rst = Ω

Answer key

  1. 1. k = 2.143 V·s/rad
  2. 2. k = 1.003 V·s/rad
  3. 3. T = 59.68 N·m
  4. 4. T = 26.26 N·m
  5. 5. n = 3160 rpm
  6. 6. Rst = 2 Ω

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