Never open the shunt field of a motor that is running, the loss of field magnetic strength will cause the armature to pull extremely high current and if the motor is not under load, the lack of counter emf will cause the armature rpm to run away, possibly high enough to cause damage.
What is the most important precautions with a DC shunt motor at the time of starting?
26: What is the most important precaution in any experiment with d.c. shunt motor? Answer: Before switching on d.c. supply, a sufficient resistance should be put in series with the armature of the d.c. shunt motor.
What precautions should be taken when starting DC machine?
Before starting any work, turn off the power and check that the work can be conducted safely. Shut down the operation of your product and motor when any abnormality occurred. It might cause a fire, injury or electric shock. Do not pull or pinch lead wires while the power is on.
How do you start DC shunt motor?
If armature and field of d.c. shunt motor is energized together, a large current is drawn at the start but the torque builds up gradually as the field flux increases gradually. To improve the torque per ampere of line current drawn it is advisable to energize the field first.
What is the main precautions should be taken while starting a series motor?
Speed regulation in the series motor is quite poor. With the increase in the load speed of the machine decreases. (DC shunt motor maintains almost constant speed from no load to full load). DC series motor should always require to be loaded before starting the motor.
What happens in the DC shunt motor when the notching process occurs?
What happens in the dc shunt motor when the notching process occurs? Explanation: Notching operation means cutting out the resistance. During the process of notching speed remains constant. … Explanation: The motor resistance, resistance to limit the starting current and number of resistance is calculated.
What is critical resistance of DC shunt generator?
The critical field resistance is defined as the maximum field circuit resistance (for a given speed) with which the shunt generator would just excite. The shunt generator will build up voltage only if field circuit resistance is less than critical field resistance.
What is the need of a starter in a DC motor?
Starters are used to protect DC motors from damage that can be caused by very high current and torque during startup. They do this by providing external resistance to the motor, which is connected in series to the motor’s armature winding and restricts the current to an acceptable level.
How does a motor behave during its starting operation?
Direct on-line starting
The motor starts with its own characteristics. When it is switched on, the motor behaves like a transformer with its secondary, formed by the very low resistance rotor cage, in short circuit. There is a high induced current in the rotor which results in a current peak in the mains supply.
What are the types of starter used for starting DC motor?
Detailed Solution. There are mainly 3 types of DC motor starters, such as 2 point starter, 3 point starter and 4 point starter. The basic concept behind every DC motor starter is adding external resistance to the armature winding during starting.
How is a shunt motor started why it should not be started direct on line?
Direct-on-line (DOL) starting are not used for large DC motor because the starting current will be enormously high which will cause large voltage drop in the supply mains and armature may get damaged due to excessive heating. … The armature current of DC shunt motor .
How does a DC shunt motor work?
The working principle of a DC Shunt Motor is, whenever a DC motor is turned ON, then DC flows throughout stator as well as the rotor. This current flow will generate two fields namely pole as well as the armature. … The commutator overturns the armature current flow direction at ordinary gaps.
How can the starting current of a DC motor be limited?
To prevent such an incidence from occurring several starting methods of DC motor has been adopted. The main principal of this being the addition of external electrical resistance Rext to the armature winding, so as to increase the effective resistance to Ra + Rext, thus limiting the armature current to the rated value.
What should you always do before working on an electric motor?
- You should always keep your work area clean and well lit.
- All motors include many small parts; keep very young children away from your work area.
- Motors spin very fast; always wear safety goggles.
- Super Glue, provided with most of the kits, is “instant bonding”.
What would be observed if a DC shunt motor is started with an open circuited field?
If the shunt field suddenly opens while the motor is being operated under full-load conditions, only a small voltage due to the residual magnetism of the pole pieces will be induced into the armature. Current will increase considerably. The magnitude of this current may be sufficient to burn out the motor.
Which losses can be identified from Swinburne’s test *?
3. Which losses can be identified from Swinburne’s test? Explanation: We get total rotational losses occurring in a machine, which are equal to no-load core losses (iron + copper) and windage and friction losses. We also get shunt field losses and variable loss occurring in armature resistance.