- The speed of a cumulatively compound DC motor can be controlled by varying
The speed control of a cumulatively compound motor can obtained by varying any of one following -
1. Armature voltage, 2. Armature resistance, 3. Shunt field resistnace.
- The function of field regular in a DC compound motor is to
In DC compound motor, the field regulators inserted in series with shunt field is to control field current..
- In Ward-Leonard method of speed control, the directional of rotation of motor is reversed usually reversing the connection of the
The direction of rotation of motor armature can be reversed by reversing the direction of exciting current of the generator. This is done with help of reversing switch.
- Ward-Leonard system for controlling the speed of a DC motor is used where
Ward-Leonard system is used for speed controlling of DC motor where speed in both direction, accuracy in speed control and very fine speed control the wide range are required. Such the system is required in steel rolling mills, paper machine, elevators, cranes, mine hoist etc.
- The shunted armature control method of speed control for DC shunt motor is
In DC shunt motor, speed variation depends not only upon the controlling resistance but on the load current also. This double dependance makes it impossible to keep the speed sensibly constant on rapidly changing load.
- The field flux method of speed control of DC shunt motor is generally used which one of following?
Since the flux cannot usually be increased beyond its normal value because of saturation of iron, so speed control by flux is limited too weakening which gives an increase in speed. So it is applicable over only a limited range, because if the field is weakened too much there is a loss of stability. Hence field flux method is generally used constant hp drive above the base speed.
- The speed of DC shunt motor can be raised above its rated value by
The flux produced by field current. By inserting a resistance cause to reduce field current and hence the flux is also reduced. The reduction in flux will result in an increase in speed. Because of saturation of iron, the flux cannot increased beyond its normal value. So the motor runs at a speed higher than normal speed.
- A DC shunt motor is supplying a load at its rated speed. The speed can be reduced by
In armature resistance control, the field is directly connected across the supply. Hence the voltage drop across the armature drops as the current passes through the series resistance and the remaining voltage applied to the armature is lower than the line voltage. Thus the speed is reduced in direct proportion to this voltage drop at the armature terminals.
- The resistance of the field regulator of a DC shunt motor is of order of
We know the flux is produced by field current. In shunt motor, field regulator is connected in series with the shunt field winding. The connection of field regulator reduces the field current and hence the flux φ is also reduced. The reduction flux will result in an increase in the speed. If we want to reduce shunt field current then the resistance of field regulator would be higher value.
- A DC series motor is running with a divertor connected across its field winding. If the divertor resistance is reduced then the speed of the motor will
The divertor of series motor is connected in parallel with series field winding to reduce the current flowing through the field winding. This reduce the flux and increase the speed. If the resistnace of divertor is reduced then the current flowing through the shunt field is increased and flux also be increased and thereby the speed will be decreased.
- The current drawn by a DC motor at starting is
When the motor is at rest the speed of DC motor is zero, therefore back emf is zero nad if a DC motor is connected directly to the supply mains a heavy current (about 10-12.5 times of rated current) will be drawn because from emf equation of armature circuit (Eb = V - IaRa, armature current is given as (V - Eb)/Ra and armature resistance Ra very small.
- In DC shunt motors, at start, the field excitation is kept at maximum value in order to
A large field current, therefore, a higher value of flux will result in low operating speed and in higher motor torque for a particular value of starting current because motor torque is proportional to the product of flux per pole and armature current. Thus for given load torque, the motor will accelerate quikly and take less time to reach the lower operating speed from the starting instant. This will result in less heating of armature during starting.
- High inrush of current during a starting of a DC motor can efficiency be restricted to a safer value by increasing the _________________ resistance.
The DC motor draws high current at the starting instant due to absence of back emf. In order to prevent this high inrush of current at start, a resistance is connected is series with the armature.
- If starting is not used with large DC motor for starting, it will draw a starting current which
Larg motor without starter will draw high inrush of current at starting instant may cause - (i) Heavy sparking at the commutatot and even flash over, (ii) damage to armature winding either by heat developed in the windings or by mechanical forces set up by electromagnetic action, (iii) large voltage drop in the supply voltage.
- Direct-on-line (DOL) starters are not suitable for starting large DC motors, because
Direct-on-line (DOL) starting are not used for large DC motor because the starting current will be enourmously high which will cause large voltage drop in the supply mains. It is used for starting of motor of ratings up to 3.75 KW.
- A 4-point starter is used to start and control the speed of a
The armature current of DC shunt motor Ia = (V - Eb)/Ra. At the time of starting Eb = 0. If the full supply voltage is applie to the motor it will draw a large current due to low armature resistance. A variable resistance should be connected in series with the armature resistance to limit the starting current. Hence a 4-point starter is used to start and control speed of DC shunt motor.
- DC motors are stopped by opening line switches and not by forcing the handle of the motor starter back to OFF position. This is due to occurance of
If the motor is to be stopped the main line switch should be opened. To stop the motor, the starter handle should never be pulled back at this would result in burning the starter contacts.
- In a DC shunt motor, three point starter
Three point starter is to limit the starting current and to provide volt protection by "NO VOLT RELEASE" coil and overload protection by "HOLD-ON" coil.
- If the no-voltage release in a DC motor starter fails to operate on failure of supply, on resumption of supply the motor will
When the supply voltage is switched on, no back emf will acting in the circuit, the armature being directly across the supply mains and resistance of armature being low, the armature will draw excessive current and will get damage.
- If the handle of a three point starter goes to back to OFF position, it may due to
The starter handle goes to 'OFF' position because
1. The supply voltage is either zero or lower than rated voltage.
2. There are high field circuit resistance.
3. The motor may be over loaded
- The DC motor starter handle should be removed
The DC motor starter handle should be moved smoothy in steps otherwise heavy sparking occur the stud of atarting resistance steps.
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