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
+−110 VRa​ = 1 ΩIa​ = ?M+−E = 95 VωT

A DC motor on a 110 V supply has an armature resistance of 1 Ω. At its present speed the back-EMF is 95 V. Find the armature current Ia.

V = 110 VE = 95 VRa = 1 Ω
Ia = A
2.Easy
+−48 VRa​ = 1.2 ΩIa​ = 2 AM+−E = ?ωT

A DC motor runs from a 48 V supply and draws an armature current of 2 A. Its armature resistance is 1.2 Ω. Find the back-EMF E.

V = 48 VIa = 2 ARa = 1.2 Ω
E = V
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.15 N·m/A and Ra = 1.5 Ω is connected to 48 V. Find its stall torque Tstall, the torque at zero speed.

V = 48 Vk = 0.15 N·m/ARa = 1.5 Ω
Tstall = N·m
4.Medium
T (N·m)n (rpm)16 N·mn0​n = ?

A DC motor (k = 2 N·m/A, Ra = 1 Ω) runs from 220 V and drives a load torque of 16 N·m. Find its speed n in rpm.

V = 220 Vk = 2 N·m/ARa = 1 ΩT = 16 N·m
n = rpm
5.Hard
+−220 VD = ?Ra​ = 1 ΩM+−En = 500 rpmT = 3 N·m

A DC chopper feeds a motor (k = 0.6 V·s/rad, Ra = 1 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 500 rpm with a load of 3 N·m.

Vdc = 220 Vk = 0.6 V·s/radRa = 1 ΩT = 3 N·mn = 500 rpm
D = %
6.Hard
+−240 VRst​ = ?Ra​ = 0.4 Ω60 AM+−E = 0n = 0T

A 240 V DC motor has an armature resistance of 0.4 Ω. Find the series starting resistance Rst that limits the starting current to 60 A.

V = 240 VRa = 0.4 ΩImax = 60 A
Rst = Ω

Answer key

  1. 1. Ia = 15 A
  2. 2. E = 45.6 V
  3. 3. Tstall = 4.8 N·m
  4. 4. n = 1012 rpm
  5. 5. D = 16.55 %
  6. 6. Rst = 3.6 Ω

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