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 a 220 V supply and draws an armature current of 10 A. Its armature resistance is 2 Ω. Find the back-EMF E.

V = 220 VI_a = 10 AR_a = 2 Ω

E = V

2.Easy

A DC motor with an armature resistance of 0.2 Ω is switched directly onto a 110 V supply. Its normal full-load current is 25 A. Find the armature current I_start at the moment of starting.

V = 110 VR_a = 0.2 Ω

I_start = A

3.Medium

A DC motor with k = 0.25 N·m/A and R_a = 0.2 Ω is connected to 24 V. Find its stall torque T_stall, the torque at zero speed.

V = 24 Vk = 0.25 N·m/AR_a = 0.2 Ω

T_stall = N·m

4.Medium

A DC motor with k = 0.25 N·m/A and R_a = 1.5 Ω is connected to 48 V. Find its stall torque T_stall, the torque at zero speed.

V = 48 Vk = 0.25 N·m/AR_a = 1.5 Ω

T_stall = N·m

5.Hard

A DC motor (k = 0.75 V·s/rad, R_a = 0.5 Ω) on a fixed 110 V supply drives a load of 18.75 N·m. Find the series resistance R_s that slows it down to 800 rpm.

V = 110 Vk = 0.75 V·s/radR_a = 0.5 ΩT = 18.75 N·mn = 800 rpm

R_s = Ω

6.Hard

A DC motor (k = 1 V·s/rad, R_a = 1.5 Ω) on a fixed 240 V supply drives a load of 8 N·m. Find the series resistance R_s that slows it down to 300 rpm.

V = 240 Vk = 1 V·s/radR_a = 1.5 ΩT = 8 N·mn = 300 rpm

R_s = Ω

Answer key

1. E = 200 V
2. I_start = 550 A
3. T_stall = 30 N·m
4. T_stall = 8 N·m
5. R_s = 1.387 Ω
6. R_s = 24.57 Ω

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