Question

2. Why it is dangerous to supply power to an unloaded series connected DC motor? Puerto Rico Polytechnic University Department of Electrical Engineering Electromechanical Energy Conversion Systems I Laboratory 89 3. How can you reverse the direction of rotation of a shunt DC motor? 4. Name and explain two methods to control the speed of a DC motor.

Answer 1

Ans 2 Starting Current and starting torque of series DC motor is very high. Due to high starting torque, is load is not connected to DC motor its shaft and ar mature may be thrown out " It could also bum the DC motor etely due to high Starting current (forush). so we require load during starting of DC Series motor And 3: The direction of rotation of Dc Shunt motor can be a reversed by changing the polarity of Either the Armature call or field coil. Simbly reversing the Aronature leads can change the rotation of DC. Shunt motor toirection

Pago ns.4: the spect Methods to control of De motor are ② flux control method The magnetic flux produced by field windings is varied in this method Since & fluor) defends upon the current flowing through field winding so is current is valued en field windenge speed can also be varied- A vacable resistance is attached in series with the field winding to vary the field Current, Hence the then(o) • When resistance is Increased keeping the supply voltage constant, field current 'In decreases and a decrease this reflects the increase in Speed of moto above rated Speed Nat

Armature Control Method In this method, speed is controlled by varying and controlling the Armateive Resistance Sof BC Moter. A variable Resistance is attached to series with Armature Winding. This helps in Controlling the voltage drop across Armatúre. When Variable Resistance value is increased the potential difference or Voltage across Armature d'écreased Since (Na VA) Speed also decreases with decrease in voltage This method helps in achieving the Speed below ito normal Range