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 240 V supply has an armature resistance of 1.2 Ω. At its present speed the back-EMF is 225.6 V. Find the armature current I_a.

V = 240 VE = 225.6 VR_a = 1.2 Ω

I_a = A

2.Easy

A DC motor with an armature resistance of 1.5 Ω is switched directly onto a 120 V supply. Its normal full-load current is 4 A. Find the armature current I_start at the moment of starting.

V = 120 VR_a = 1.5 Ω

I_start = A

3.Medium

A DC motor with k = 0.4 V·s/rad runs from 48 V. Neglecting all losses at no load, find its no-load speed n₀ in rpm.

V = 48 Vk = 0.4 V·s/rad

n₀ = rpm

4.Medium

A DC motor (k = 1.2 N·m/A, R_a = 0.5 Ω) runs from 110 V and drives a load torque of 14.4 N·m. Find its speed n in rpm.

V = 110 Vk = 1.2 N·m/AR_a = 0.5 ΩT = 14.4 N·m

n = rpm

5.Hard

A DC chopper feeds a motor (k = 1.5 V·s/rad, R_a = 0.5 Ω) from a 120 V DC bus. The average armature voltage is D·120 V. Find the duty cycle D that runs the motor at 600 rpm with a load of 12 N·m.

V_dc = 120 Vk = 1.5 V·s/radR_a = 0.5 ΩT = 12 N·mn = 600 rpm

D = %

6.Hard

A 24 V DC motor has an armature resistance of 0.4 Ω. Find the series starting resistance R_st that limits the starting current to 10 A.

V = 24 VR_a = 0.4 ΩI_max = 10 A

R_st = Ω

Answer key

1. I_a = 12 A
2. I_start = 80 A
3. n₀ = 1146 rpm
4. n = 827.6 rpm
5. D = 81.87 %
6. R_st = 2 Ω

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