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 2 Ω is switched directly onto a 110 V supply. Its normal full-load current is 2 A. Find the armature current I_start at the moment of starting.

V = 110 VR_a = 2 Ω

I_start = A

2.Easy

A DC motor runs from 220 V with an armature current of 50 A. The armature resistance is 0.2 Ω. Find the developed (electromagnetic) power P_dev.

V = 220 VI_a = 50 AR_a = 0.2 Ω

P_dev = W

3.Medium

A DC motor with k = 0.3 N·m/A and R_a = 0.2 Ω is connected to 110 V. Find its stall torque T_stall, the torque at zero speed.

V = 110 Vk = 0.3 N·m/AR_a = 0.2 Ω

T_stall = N·m

4.Medium

A DC motor (k = 0.8 V·s/rad, R_a = 0.2 Ω) runs from 110 V. At full load it draws 30 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 110 Vk = 0.8 V·s/radR_a = 0.2 ΩI_FL = 30 A

SR = %

5.Hard

A separately excited DC motor (k = 0.3 V·s/rad at full field, R_a = 1.5 Ω, V = 48 V) drives a constant load torque of 0.6 N·m. The field flux is reduced to 70 % of its full value. Find the new speed n.

V = 48 Vk = 0.3 V·s/radR_a = 1.5 ΩT = 0.6 N·mΦ = 70 %

n = rpm

6.Hard

A separately excited DC motor on 120 V (R_a = 0.5 Ω) runs at 1500 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 = 120 VR_a = 0.5 Ωn₁ = 1500 rpmI_a1 = 30 AI_a2 = 18 A

n₂ = rpm

Answer key

1. I_start = 55 A
2. P_dev = 10500 W
3. T_stall = 165 N·m
4. SR = 5.769 %
5. n = 1988 rpm
6. n₂ = 1586 rpm

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