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 with an armature resistance of 1 Ω is switched directly onto a 240 V supply. Its normal full-load current is 30 A. Find the armature current I_start at the moment of starting.

V = 240 VR_a = 1 Ω

I_start = A

2.Easy

A DC motor turns at 1000 rpm, and its back-EMF at that speed is 212.5 V. Find the motor constant k in V·s/rad.

E = 212.5 Vn = 1000 rpm

k = V·s/rad

3.Medium

A DC motor with k = 0.8 N·m/A and R_a = 1.5 Ω is connected to 120 V. Find its stall torque T_stall, the torque at zero speed.

V = 120 Vk = 0.8 N·m/AR_a = 1.5 Ω

T_stall = N·m

4.Medium

A DC motor (k = 1 V·s/rad, R_a = 0.2 Ω) runs from 220 V. At full load it draws 40 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 220 Vk = 1 V·s/radR_a = 0.2 ΩI_FL = 40 A

SR = %

5.Hard

A small DC motor has an armature resistance of 0.8 Ω and runs from 24 V. Neglecting rotational losses, find the largest mechanical power P_max it can develop at any speed.

V = 24 VR_a = 0.8 Ω

P_max = W

6.Hard

A separately excited DC motor (k = 0.3 V·s/rad at full field, R_a = 0.25 Ω, V = 120 V) drives a constant load torque of 9 N·m. The field flux is reduced to 80 % of its full value. Find the new speed n.

V = 120 Vk = 0.3 V·s/radR_a = 0.25 ΩT = 9 N·mΦ = 80 %

n = rpm

Answer key

1. I_start = 240 A
2. k = 2.029 V·s/rad
3. T_stall = 64 N·m
4. SR = 3.774 %
5. P_max = 180 W
6. n = 4402 rpm

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