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
+−120 VRa​ = 1.2 ΩIa​ = ?M+−E = 114 VωT

A DC motor on a 120 V supply has an armature resistance of 1.2 Ω. At its present speed the back-EMF is 114 V. Find the armature current Ia.

V = 120 VE = 114 VRa = 1.2 Ω
Ia = A
2.Easy
+−220 VRa​ = 1.5 ΩIa​ = 15 AM+−En = ?T

A DC motor with k = 1.2 V·s/rad and an armature resistance of 1.5 Ω runs from 220 V and draws 15 A. Find its speed n in rpm.

V = 220 VRa = 1.5 ΩIa = 15 Ak = 1.2 V·s/rad
n = rpm
3.Medium
+−VRa​M+−En = 750 rpmT = ?

A DC motor delivers 15 kW of mechanical power at 750 rpm. Find the shaft torque T.

P = 15 kWn = 750 rpm
T = N·m
4.Medium
T (N·m)n (rpm)7.5 N·mn0​n = ?

A DC motor (k = 0.5 N·m/A, Ra = 0.25 Ω) runs from 110 V and drives a load torque of 7.5 N·m. Find its speed n in rpm.

V = 110 Vk = 0.5 N·m/ARa = 0.25 ΩT = 7.5 N·m
n = rpm
5.Hard
+−220 VD = ?Ra​ = 1.5 ΩM+−En = 1000 rpmT = 18 N·m

A DC chopper feeds a motor (k = 1.5 V·s/rad, Ra = 1.5 Ω) from a 220 V DC bus. The average armature voltage is D·220 V. Find the duty cycle D that runs the motor at 1000 rpm with a load of 18 N·m.

Vdc = 220 Vk = 1.5 V·s/radRa = 1.5 ΩT = 18 N·mn = 1000 rpm
D = %
6.Hard
+−24 VRst​ = ?Ra​ = 1 Ω10 AM+−E = 0n = 0T

A 24 V DC motor has an armature resistance of 1 Ω. Find the series starting resistance Rst that limits the starting current to 10 A.

V = 24 VRa = 1 ΩImax = 10 A
Rst = Ω

Answer key

  1. 1. Ia = 5 A
  2. 2. n = 1572 rpm
  3. 3. T = 191 N·m
  4. 4. n = 2029 rpm
  5. 5. D = 79.58 %
  6. 6. Rst = 1.4 Ω

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