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 k = 0.8 V·s/rad and an armature resistance of 1.2 Ω runs from 240 V and draws 15 A. Find its speed n in rpm.

V = 240 VR_a = 1.2 ΩI_a = 15 Ak = 0.8 V·s/rad

n = rpm

2.Easy

A DC motor with an armature resistance of 0.4 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 = 0.4 Ω

I_start = A

3.Medium

A DC motor with k = 1 N·m/A and R_a = 0.5 Ω is connected to 240 V. Find its stall torque T_stall, the torque at zero speed.

V = 240 Vk = 1 N·m/AR_a = 0.5 Ω

T_stall = N·m

4.Medium

A DC motor delivers 1.5 kW of mechanical power at 750 rpm. Find the shaft torque T.

P = 1.5 kWn = 750 rpm

T = N·m

5.Hard

A separately excited DC motor on 220 V (R_a = 0.4 Ω) runs at 1500 rpm with an armature current of 30 A. The load changes and the current becomes 45 A, with the field unchanged. Find the new speed n₂.

V = 220 VR_a = 0.4 Ωn₁ = 1500 rpmI_a1 = 30 AI_a2 = 45 A

n₂ = rpm

6.Hard

A separately excited DC motor on 110 V (R_a = 1.5 Ω) runs at 1440 rpm with an armature current of 5 A. The load changes and the current becomes 7.5 A, with the field unchanged. Find the new speed n₂.

V = 110 VR_a = 1.5 Ωn₁ = 1440 rpmI_a1 = 5 AI_a2 = 7.5 A

n₂ = rpm

Answer key

1. n = 2650 rpm
2. I_start = 60 A
3. T_stall = 480 N·m
4. T = 19.1 N·m
5. n₂ = 1457 rpm
6. n₂ = 1387 rpm

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