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 runs from 110 V with an armature current of 12 A. The armature resistance is 1.2 Ω. Find the developed (electromagnetic) power P_dev.

V = 110 VI_a = 12 AR_a = 1.2 Ω

P_dev = W

2.Easy

A DC motor with k = 0.8 V·s/rad and an armature resistance of 0.25 Ω runs from 240 V and draws 30 A. Find its speed n in rpm.

V = 240 VR_a = 0.25 ΩI_a = 30 Ak = 0.8 V·s/rad

n = rpm

3.Medium

A DC motor (k = 0.75 N·m/A, R_a = 0.5 Ω) runs from 220 V and drives a load torque of 18.75 N·m. Find its speed n in rpm.

V = 220 Vk = 0.75 N·m/AR_a = 0.5 ΩT = 18.75 N·m

n = rpm

4.Medium

A DC motor with k = 0.1 N·m/A and R_a = 0.5 Ω is connected to 24 V. Find its stall torque T_stall, the torque at zero speed.

V = 24 Vk = 0.1 N·m/AR_a = 0.5 Ω

T_stall = N·m

5.Hard

A DC chopper feeds a motor (k = 0.8 V·s/rad, R_a = 1.5 Ω) from a 240 V DC bus. The average armature voltage is D·240 V. Find the duty cycle D that runs the motor at 1000 rpm with a load of 4.8 N·m.

V_dc = 240 Vk = 0.8 V·s/radR_a = 1.5 ΩT = 4.8 N·mn = 1000 rpm

D = %

6.Hard

A 110 V DC motor has an armature resistance of 0.8 Ω. Find the series starting resistance R_st that limits the starting current to 40 A.

V = 110 VR_a = 0.8 ΩI_max = 40 A

R_st = Ω

Answer key

1. P_dev = 1147 W
2. n = 2775 rpm
3. n = 2642 rpm
4. T_stall = 4.8 N·m
5. D = 38.66 %
6. R_st = 1.95 Ω

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