Exercise 01
The nameplate of a separately excited DC motor indicates:1.12 kW 1200 rpm
Armature: 230 V 5,7 A
Excitation: 230 V 0,30 A
57 kg
1. Calculate the rated useful torque (in N.m).
2. Calculate the motor efficiency
Exercise 02
A DC motor has separate and constant excitation. We neglect its armature magnetic reaction. It has a resistance R = 0.20. It is supplied with a constant voltage U = 38 V.
1. At rated load, the armature carries a current I = 5 A and it rotates at a rotor speed of 1000 rpm.
1.1 Calculate the electromotive force e.m.f of the armature.
1.2 Calculate the moment of the electromagnetic torque Tem.
1.3 Show that we can express E as a function of the rotor speed n following the expression: E = k.n
2. Following a load variation, the current across the armature becomes I' = 3.8A. Calculate:
2.1 The new moment of the electromagnetic torque T'em.
2.2 The new rotor speed n'.
Exercise 03
A DC motor with separate and constant excitation has the following characteristics:
- Armature supply voltage: U = 160 V
- Armature resistance: R = 0,2 Ω
1. The e.m.f. of the motor is E = 150 V when its rotor speed is n = 1500 rpm. Deduce the relationship between E and n
2. Determine the expression of I as a function of E.
3. Determine the expression for Tem (electromagnetic torque in N.m) as a function of I.
4. Deduce that: Tem = 764 - 0,477.n
5. We neglect the collective losses of the DC motor. Justify then that: Tu (useful torque) = Tem
6. Calculate the rotor speed of the motor at no-load operating mode.
7. The motor drives a load whose resistive torque varies proportionally with the rotation speed according to the following relationship: Tr = 0,02.n
7.1 Calculate the rotational speed of the motor under rated load.
7.2 Deduce the armature current and the useful power of the motor.