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.

New worksheet Practise online with hints

For teachers: free to print, copy and use in class. Every new worksheet has new values and its own answer key. A link to ExamProblems on your course page is appreciated.

Name: ____________________ Date: ____________

1.Easy

A DC motor with an armature resistance of 0.5 Ω is switched directly onto a 48 V supply. Its normal full-load current is 4 A. Find the armature current I_start at the moment of starting.

V = 48 VR_a = 0.5 Ω

I_start = A

2.Easy

A DC motor turns at 1450 rpm, and its back-EMF at that speed is 232 V. Find the motor constant k in V·s/rad.

E = 232 Vn = 1450 rpm

k = V·s/rad

3.Medium

A DC motor with k = 0.6 V·s/rad runs from 110 V. Neglecting all losses at no load, find its no-load speed n₀ in rpm.

V = 110 Vk = 0.6 V·s/rad

n₀ = rpm

4.Medium

A DC motor with k = 0.08 N·m/A and R_a = 0.25 Ω is connected to 24 V. Find its stall torque T_stall, the torque at zero speed.

V = 24 Vk = 0.08 N·m/AR_a = 0.25 Ω

T_stall = N·m

5.Hard

A separately excited DC motor on 220 V (R_a = 1.2 Ω) runs at 1800 rpm with an armature current of 20 A. The load changes and the current becomes 10 A, with the field unchanged. Find the new speed n₂.

V = 220 VR_a = 1.2 Ωn₁ = 1800 rpmI_a1 = 20 AI_a2 = 10 A

n₂ = rpm

6.Hard

A separately excited DC motor on 110 V (R_a = 1.5 Ω) runs at 1000 rpm with an armature current of 4 A. The load changes and the current becomes 8 A, with the field unchanged. Find the new speed n₂.

V = 110 VR_a = 1.5 Ωn₁ = 1000 rpmI_a1 = 4 AI_a2 = 8 A

n₂ = rpm

Answer key

1. I_start = 96 A
2. k = 1.528 V·s/rad
3. n₀ = 1751 rpm
4. T_stall = 7.68 N·m
5. n₂ = 1910 rpm
6. n₂ = 942.3 rpm

Full step-by-step solutions: practise this topic on examproblems.com.