Answer MET Question 32
Question: Which of the following three motors has the poorest speed regulation: shunt motor, series Motor
or cumulative compound motor? Explain.
The speed regulation of DC cumulative compound motor varies depending upon the degree of compounding. It is superior to that of dc series motor and inferior to DC shunt motor. The percentage of speed regulation of DC cumulative compound motor is between 25 to 30%
DC differential compound motor
They have the best regulation among all the types of d.c motors.
The speed of this motor remains almost constant or even increasing with increase in load. Due to this reason, there is a decrease in rate at which the motor torque increase with load. The flux weakens with increase in load. There is a tendency toward speed instability and motor running away unless designed properly.
Answer: Among
Shunt motor, series motor and comulative compond motor series motor
have the poorest speed regulation. alos it can be noted that:
DC differential compound motor have the best speed regulation while
DC Series motor have the worst speed regulation.
DC Series motor have the worst speed regulation.
When the load is applied to the dc motor its speed decrease, but it is not desirable when we need constant speed.
So it is desirable that the difference between the no load to full load speed should be less.
The speed regulation is defined as the ratio of change in the speed from no load to full load to the speed corresponding to full load.
Numerically it is expressed as
$\displaystyle \small \mathrm{Percentage\ Speed\ Reg. =\frac{No\ load\ speed-Full\ load\ speed}{Full\ load\ speed}\times 100 }$
The lower the percentage of regulation the more constant the speed of dc motor.
The EMF equation of DC motor is given by
$\displaystyle \small \mathrm{E_b = \frac{NP\phi Z}{60A}}$
It is clear that back emf of DC motor is directly proportional to the speed of the DC motor.
If the load is added to the motor then the motor must produce more torque to overcome the added load and $\displaystyle \small \mathrm{T\ \alpha\ I_a}$ hence armature current also increase with the increase in the load.
To produce more torque the magnetic field of the pole must increase, and the increase in the field strength can be achieved when the armature speed decreases causing less back emf to be produced in the armature.
The decrease of the back emf allows more current to flow through the armature causing an increase in magnetic field strength.
In DC motor the speed regulation is proportional to the resistance of the armature.
The lower the armature resistance the better will be the speed regulation of the dc motor.
So it is desirable that the difference between the no load to full load speed should be less.
The speed regulation is defined as the ratio of change in the speed from no load to full load to the speed corresponding to full load.
Numerically it is expressed as
$\displaystyle \small \mathrm{Percentage\ Speed\ Reg. =\frac{No\ load\ speed-Full\ load\ speed}{Full\ load\ speed}\times 100 }$
The lower the percentage of regulation the more constant the speed of dc motor.
The EMF equation of DC motor is given by
$\displaystyle \small \mathrm{E_b = \frac{NP\phi Z}{60A}}$
It is clear that back emf of DC motor is directly proportional to the speed of the DC motor.
If the load is added to the motor then the motor must produce more torque to overcome the added load and $\displaystyle \small \mathrm{T\ \alpha\ I_a}$ hence armature current also increase with the increase in the load.
To produce more torque the magnetic field of the pole must increase, and the increase in the field strength can be achieved when the armature speed decreases causing less back emf to be produced in the armature.
The decrease of the back emf allows more current to flow through the armature causing an increase in magnetic field strength.
In DC motor the speed regulation is proportional to the resistance of the armature.
The lower the armature resistance the better will be the speed regulation of the dc motor.
Speed regulation of various motors
DC shunt motor
There is no appriciable change in speed in a shunt motor. Both back emf and ф decreses with load increase. However, backemf decreases slightly more than ф, so that o the whole there is some decrese in speed. The speed regulation of DC shunt motor is between 10 -15%.
DC Series motor
The speed regulation of dc series motor is worst among all the dc motor. It is widely variable and the speed is very high at no-load. This motor must never start and run at no-load
There is no appriciable change in speed in a shunt motor. Both back emf and ф decreses with load increase. However, backemf decreases slightly more than ф, so that o the whole there is some decrese in speed. The speed regulation of DC shunt motor is between 10 -15%.
DC Series motor
The speed regulation of dc series motor is worst among all the dc motor. It is widely variable and the speed is very high at no-load. This motor must never start and run at no-load
$\displaystyle \small \mathrm{N\alpha \frac{E_b}{\phi }}$
Chage
in back emf is neligible, but when load current hence armatur current
(Ia) falls to a small value, the speed becoms dangerously high.
DC Cumulative compound motorThe speed regulation of DC cumulative compound motor varies depending upon the degree of compounding. It is superior to that of dc series motor and inferior to DC shunt motor. The percentage of speed regulation of DC cumulative compound motor is between 25 to 30%
DC differential compound motor
They have the best regulation among all the types of d.c motors.
The speed of this motor remains almost constant or even increasing with increase in load. Due to this reason, there is a decrease in rate at which the motor torque increase with load. The flux weakens with increase in load. There is a tendency toward speed instability and motor running away unless designed properly.
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