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 on a 110 V supply has an armature resistance of 0.8 Ω. At its present speed the back-EMF is 98 V. Find the armature current I_a.

V = 110 VE = 98 VR_a = 0.8 Ω

I_a = A

2.Easy

A permanent-magnet DC motor has the torque constant k = 0.15 N·m/A. Find the torque T it develops at an armature current of 4 A.

k = 0.15 N·m/AI_a = 4 A

T = N·m

3.Medium

A DC motor (k = 2 N·m/A, R_a = 1 Ω) runs from 220 V and drives a load torque of 40 N·m. Find its speed n in rpm.

V = 220 Vk = 2 N·m/AR_a = 1 ΩT = 40 N·m

n = rpm

4.Medium

A DC motor with k = 0.6 N·m/A and R_a = 1.2 Ω is connected to 240 V. Find its stall torque T_stall, the torque at zero speed.

V = 240 Vk = 0.6 N·m/AR_a = 1.2 Ω

T_stall = N·m

5.Hard

A separately excited DC motor on 220 V (R_a = 0.8 Ω) runs at 1500 rpm with an armature current of 15 A. The load changes and the current becomes 22.5 A, with the field unchanged. Find the new speed n₂.

V = 220 VR_a = 0.8 Ωn₁ = 1500 rpmI_a1 = 15 AI_a2 = 22.5 A

n₂ = rpm

6.Hard

A DC chopper feeds a motor (k = 0.3 V·s/rad, R_a = 0.4 Ω) from a 220 V DC bus. The average armature voltage is D·220 V. Find the duty cycle D that runs the motor at 400 rpm with a load of 3.6 N·m.

V_dc = 220 Vk = 0.3 V·s/radR_a = 0.4 ΩT = 3.6 N·mn = 400 rpm

D = %

Answer key

1. I_a = 15 A
2. T = 0.6 N·m
3. n = 954.9 rpm
4. T_stall = 120 N·m
5. n₂ = 1457 rpm
6. D = 7.894 %

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