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 220 V supply has an armature resistance of 1 Ω. At its present speed the back-EMF is 190 V. Find the armature current I_a.

V = 220 VE = 190 VR_a = 1 Ω

I_a = A

2.Easy

A DC motor turns at 750 rpm, and its back-EMF at that speed is 224 V. Find the motor constant k in V·s/rad.

E = 224 Vn = 750 rpm

k = V·s/rad

3.Medium

A DC motor with k = 0.6 N·m/A and R_a = 0.5 Ω is connected to 220 V. Find its stall torque T_stall, the torque at zero speed.

V = 220 Vk = 0.6 N·m/AR_a = 0.5 Ω

T_stall = N·m

4.Medium

A DC motor with k = 0.5 V·s/rad runs from 120 V. Neglecting all losses at no load, find its no-load speed n₀ in rpm.

V = 120 Vk = 0.5 V·s/rad

n₀ = rpm

5.Hard

A separately excited DC motor (k = 0.6 V·s/rad at full field, R_a = 1.2 Ω, V = 120 V) drives a constant load torque of 7.2 N·m. The field flux is reduced to 75 % of its full value. Find the new speed n.

V = 120 Vk = 0.6 V·s/radR_a = 1.2 ΩT = 7.2 N·mΦ = 75 %

n = rpm

6.Hard

A separately excited DC motor (k = 0.6 V·s/rad at full field, R_a = 1.2 Ω, V = 120 V) drives a constant load torque of 4.8 N·m. The field flux is reduced to 75 % of its full value. Find the new speed n.

V = 120 Vk = 0.6 V·s/radR_a = 1.2 ΩT = 4.8 N·mΦ = 75 %

n = rpm

Answer key

1. I_a = 30 A
2. k = 2.852 V·s/rad
3. T_stall = 264 N·m
4. n₀ = 2292 rpm
5. n = 2139 rpm
6. n = 2275 rpm

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