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 120 V supply has an armature resistance of 0.5 Ω. At its present speed the back-EMF is 110 V. Find the armature current I_a.

V = 120 VE = 110 VR_a = 0.5 Ω

I_a = A

2.Easy

A DC motor turns at 2000 rpm, and its back-EMF at that speed is 232.5 V. Find the motor constant k in V·s/rad.

E = 232.5 Vn = 2000 rpm

k = V·s/rad

3.Medium

A DC motor with k = 0.8 N·m/A and R_a = 0.25 Ω is connected to 240 V. Find its stall torque T_stall, the torque at zero speed.

V = 240 Vk = 0.8 N·m/AR_a = 0.25 Ω

T_stall = N·m

4.Medium

A DC motor with k = 0.6 V·s/rad runs from 120 V. Neglecting all losses at no load, find its no-load speed n₀ in rpm.

V = 120 Vk = 0.6 V·s/rad

n₀ = rpm

5.Hard

A separately excited DC motor (k = 0.2 V·s/rad at full field, R_a = 1 Ω, V = 24 V) drives a constant load torque of 0.4 N·m. The field flux is reduced to 70 % of its full value. Find the new speed n.

V = 24 Vk = 0.2 V·s/radR_a = 1 ΩT = 0.4 N·mΦ = 70 %

n = rpm

6.Hard

A DC chopper feeds a motor (k = 0.5 V·s/rad, R_a = 0.2 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 300 rpm with a load of 12.5 N·m.

V_dc = 220 Vk = 0.5 V·s/radR_a = 0.2 ΩT = 12.5 N·mn = 300 rpm

D = %

Answer key

1. I_a = 20 A
2. k = 1.11 V·s/rad
3. T_stall = 768 N·m
4. n₀ = 1910 rpm
5. n = 1442 rpm
6. D = 9.413 %

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