To explain the concept of the dc motor To explain the significance of the back emf To derive the equations for the torque

developed inside the dc motor

To differentiate the types of DC Motor To explain clearly the motor efficiency and power

stages

An Electric motor is a machine which converts electric energy into mechanical energy. Its action is based on the principle that when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force.

Let Φ = flux/pole in weberZ = total number of armature conductorsP = No. of generator polesa = No. of parallel paths in armatureN = armature rotation in revolutions per minute

(r.p.m.)Eb = back emf or counter emf

Eb = ZPΦN / 60a volts if Φ is in weberEb = (ZPΦN / 60a) x 10-8 volts if Φ is in

maxwells/lines

1. A four-pole motor, having lap-wound armature winding has 70 slots, each slot containing 10 conductors. What will be the back voltage generated in the machine when driven at 1500 rpm assuming the flux per pole to be 15.0 mWb ?

2. An 8-pole d.c. motor has 800 armature conductors, and a useful flux of 0.09 Wb per pole. What will be the counter e.m.f. generated if it is wave-connected and runs at 1800 rpm ? What must be the speed at which it is to be driven produce the same e.m.f. if it is lap-wound?

3. A 4-pole d.c. motor runs at 950 r.p.m. and generates an counter e.m.f. of 240 V. The armature is lap-wound and has 792 conductors. Calculte the flux per pole.

The armature coils and the shunt field coils are connected in parallel

Note: Unless otherwise specified, the flux is assumed to be constant

The armature coils and the shunt field coils are connected in series.

Note: The flux is directly proportional to the armature current

Long Shunt Compound Dc MotorThe series field coils are connected in series with the armature coils while the shunt field coils are connected across the series combination

The series field coils are connected in series with the supply voltage while the shunt field coils and the armature are connected in parallel.

Electical Power Input = VI

Electrical power Develop in Armature = E bIa

Copper Losses

Mechanical Power output

Iron & Mechanical Losses

1. A 100V shunt motor is taking 220A. Armature resistance is 0.015Ω, the shunt field is 20Ω, calculate the back emf.

2. A 220V series motor is taking a current of 40A. Resistance of the armature is 0.5Ω and the resistance of the series is 0.25Ω. Calculate Back emf.

3. A d.c. motor connected to a 460-V supply has an armature resistance of 0.15 Ω. Calculate (a) The value of back e.m.f. when the armature current is 120 A. (b) The value of armature current when the back e.m.f. is 447.4 V.

4. A shunt motor is taking 40A at 100V. Armature resistance is 0.015Ω and the shunt field resistance is 50Ω. The stray power losses amount to 700W. What is the Hp output?

5. A 230V, long shunt machine has the following parameters: RA=2Ω, RF=460Ω, and RS=0.25Ω. When the machine is run at no-load at its normal speed and rated voltage, the armature draws 0.6A. Determine the armature current drawn if the machine delivers an output of 5Hp.

6. A shunt motor is taking 40A at 100V. Armature resistance is 0.015Ω and the shunt field resistance is 50Ω. The stray power losses amount to 700W. What is the Hp output?

1. Mechanical Efficiencyηm = (Po / Pg) x 100%

2. Electrical Efficiency ηe = (Pg / Pi) x 100%

3. Commercial or Over-all Efficiency ηo = (Po / Pi) x 100%

= ηm x ηe

1. A 200V shunt motor is taking 30A. Armature resistance 0.2Ω and the shunt field resistance is 100Ω. The stray power losses is 500watts. Calculate the Hp and the commercial efficiency?

2. A 220V, 10Hp dc shunt motor has RA=0.25Ω and RF=100Ω. If the full load efficiency is 83%, determine the value of the starting resistance in order that the starting current will not exceed 200% of the full load value.

3. A 100V shunt motor is developing 6Hp while operating at an overall efficiency of 86%. The armature and shunt field resistances are 0.06Ω and 50Ω respectively. Determine the SPL.

4. A 500 V d.c. shunt motor takes a current of 5 A on no-load. The resistances of the armature and field circuit are 0.22 ohm and 250 ohm respectively. Find the efficiency when loaded and taking a current of 100 A.

5. On no-load, a shunt motor takes 5 A at 250 V, the resistances of the field and armature circuits are 250 Ω and 0.1 Ω respectively. Calculate the output power and efficiency of the motor when the total supply current is 81 A at the same supply voltage.

6. Calculate the shaft power of a series motor having the following data; overall efficiency 83.5%, speed 550 r.p.m. when taking 65 A; motor resistance 0.2 Ω, flux per pole 25 mWb, armature winding lap with 1200 conductor.