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 0.4 Ω is switched directly onto a 220 V supply. Its normal full-load current is 40 A. Find the armature current I_start at the moment of starting.

V = 220 VR_a = 0.4 Ω

I_start = A

2.Easy

A DC motor runs from 220 V with an armature current of 25 A. The armature resistance is 0.4 Ω. Find the developed (electromagnetic) power P_dev.

V = 220 VI_a = 25 AR_a = 0.4 Ω

P_dev = W

3.Medium

A DC motor with k = 0.4 N·m/A and R_a = 1.2 Ω is connected to 110 V. Find its stall torque T_stall, the torque at zero speed.

V = 110 Vk = 0.4 N·m/AR_a = 1.2 Ω

T_stall = N·m

4.Medium

A DC motor with k = 1.2 V·s/rad runs from 220 V. Neglecting all losses at no load, find its no-load speed n₀ in rpm.

V = 220 Vk = 1.2 V·s/rad

n₀ = rpm

5.Hard

A separately excited DC motor (k = 2 V·s/rad at full field, R_a = 1.2 Ω, V = 240 V) drives a constant load torque of 40 N·m. The field flux is reduced to 90 % of its full value. Find the new speed n.

V = 240 Vk = 2 V·s/radR_a = 1.2 ΩT = 40 N·mΦ = 90 %

n = rpm

6.Hard

A DC motor (k = 0.8 V·s/rad, R_a = 1.5 Ω) on a fixed 120 V supply drives a load of 3.2 N·m. Find the series resistance R_s that slows it down to 500 rpm.

V = 120 Vk = 0.8 V·s/radR_a = 1.5 ΩT = 3.2 N·mn = 500 rpm

R_s = Ω

Answer key

1. I_start = 550 A
2. P_dev = 5250 W
3. T_stall = 36.67 N·m
4. n₀ = 1751 rpm
5. n = 1132 rpm
6. R_s = 18.03 Ω

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