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 0.5 Ω is switched directly onto a 24 V supply. Its normal full-load current is 2 A. Find the armature current I_start at the moment of starting.

V = 24 VR_a = 0.5 Ω

I_start = A

2.Easy

A DC motor with an armature resistance of 1.2 Ω is switched directly onto a 110 V supply. Its normal full-load current is 6 A. Find the armature current I_start at the moment of starting.

V = 110 VR_a = 1.2 Ω

I_start = A

3.Medium

A DC motor (k = 0.75 N·m/A, R_a = 0.8 Ω) runs from 110 V and drives a load torque of 3.75 N·m. Find its speed n in rpm.

V = 110 Vk = 0.75 N·m/AR_a = 0.8 ΩT = 3.75 N·m

n = rpm

4.Medium

A DC motor with k = 0.8 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n₀ in rpm.

V = 240 Vk = 0.8 V·s/rad

n₀ = rpm

5.Hard

A separately excited DC motor on 240 V (R_a = 0.8 Ω) runs at 1800 rpm with an armature current of 25 A. The load changes and the current becomes 10 A, with the field unchanged. Find the new speed n₂.

V = 240 VR_a = 0.8 Ωn₁ = 1800 rpmI_a1 = 25 AI_a2 = 10 A

n₂ = rpm

6.Hard

A DC motor (k = 0.6 V·s/rad, R_a = 0.8 Ω) on a fixed 240 V supply drives a load of 9 N·m. Find the series resistance R_s that slows it down to 1500 rpm.

V = 240 Vk = 0.6 V·s/radR_a = 0.8 ΩT = 9 N·mn = 1500 rpm

R_s = Ω

Answer key

1. I_start = 48 A
2. I_start = 91.67 A
3. n = 1350 rpm
4. n₀ = 2865 rpm
5. n₂ = 1898 rpm
6. R_s = 8.917 Ω

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