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

A DC motor on a 120 V supply has an armature resistance of 0.4 Ω. At its present speed the back-EMF is 116 V. Find the armature current I_a.

V = 120 VE = 116 VR_a = 0.4 Ω

I_a = A

2.Easy

A DC motor runs from a 240 V supply and draws an armature current of 20 A. Its armature resistance is 1 Ω. Find the back-EMF E.

V = 240 VI_a = 20 AR_a = 1 Ω

E = V

3.Medium

A DC motor (k = 1 N·m/A, R_a = 0.2 Ω) runs from 110 V and drives a load torque of 25 N·m. Find its speed n in rpm.

V = 110 Vk = 1 N·m/AR_a = 0.2 ΩT = 25 N·m

n = rpm

4.Medium

A DC motor with k = 1.2 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n₀ in rpm.

V = 240 Vk = 1.2 V·s/rad

n₀ = rpm

5.Hard

A separately excited DC motor on 120 V (R_a = 0.25 Ω) runs at 1440 rpm with an armature current of 40 A. The load changes and the current becomes 10 A, with the field unchanged. Find the new speed n₂.

V = 120 VR_a = 0.25 Ωn₁ = 1440 rpmI_a1 = 40 AI_a2 = 10 A

n₂ = rpm

6.Hard

A DC chopper feeds a motor (k = 0.8 V·s/rad, R_a = 1.5 Ω) from a 240 V DC bus. The average armature voltage is D·240 V. Find the duty cycle D that runs the motor at 400 rpm with a load of 9.6 N·m.

V_dc = 240 Vk = 0.8 V·s/radR_a = 1.5 ΩT = 9.6 N·mn = 400 rpm

D = %

Answer key

1. I_a = 10 A
2. E = 220 V
3. n = 1003 rpm
4. n₀ = 1910 rpm
5. n₂ = 1538 rpm
6. D = 21.46 %

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