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 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 I_start at the moment of starting.

V = 240 VR_a = 1.5 Ω

I_start = A

2.Easy

A DC motor on a 48 V supply has an armature resistance of 0.4 Ω. At its present speed the back-EMF is 46.4 V. Find the armature current I_a.

V = 48 VE = 46.4 VR_a = 0.4 Ω

I_a = A

3.Medium

A DC motor (k = 1 V·s/rad, R_a = 0.5 Ω) runs from 220 V. At full load it draws 20 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 220 Vk = 1 V·s/radR_a = 0.5 ΩI_FL = 20 A

SR = %

4.Medium

A DC motor (k = 1.2 N·m/A, R_a = 1 Ω) runs from 220 V and drives a load torque of 24 N·m. Find its speed n in rpm.

V = 220 Vk = 1.2 N·m/AR_a = 1 ΩT = 24 N·m

n = rpm

5.Hard

A 240 V DC motor has an armature resistance of 1.2 Ω. Find the series starting resistance R_st that limits the starting current to 80 A.

V = 240 VR_a = 1.2 ΩI_max = 80 A

R_st = Ω

6.Hard

A separately excited DC motor on 110 V (R_a = 0.4 Ω) runs at 900 rpm with an armature current of 30 A. The load changes and the current becomes 18 A, with the field unchanged. Find the new speed n₂.

V = 110 VR_a = 0.4 Ωn₁ = 900 rpmI_a1 = 30 AI_a2 = 18 A

n₂ = rpm

Answer key

1. I_start = 160 A
2. I_a = 4 A
3. SR = 4.762 %
4. n = 1592 rpm
5. R_st = 1.8 Ω
6. n₂ = 944.1 rpm

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