Question

Suppose you need to control the speed of a motor that takes 1200W
at full power. This is typical

of a big domestic vacuum-cleaner or a washing-machine motor, and is
a little more power than a

single-bar electric fire uses. First, you try the obvious method: a
variable resistor in series, and no

other complications. If you adjust the resistor to have a very high
value so little current passes,

this will make the motor run very slowly or stop. At the other
extreme, if you adjust the resistor to

have a very low or zero resistance, then the motor will run at full
power. So far, so good. But,

suppose I tell you to run the motor at one-quarter of its full
power? To do this, you adjust the

series variable resistor so its resistance is exactly the same as
the motor’s resistance. Then, the

supply voltage splits equally between the motor and resistor, and
also the current is halved

because the total resistance is double that of the motor alone.
Therefore, the power in the motor

(= current voltage) is now 1200/(2 2) = 300W as required.

(a) Under these conditions, what power is now wasted as heat in the
variable resistor? Is this a

lot? (e.g., how does it compare with the maximum power allowed in
the variable resistors

supplied in your parts kit?)

While pondering what to do about this, you remember this experiment
and PWM, and you think

about discarding the variable resistor and instead connecting an
active semiconductor device such

as a transistor in series with the motor.

(b) What current passes through the transistor when it is switched
OFF? So, since power is the

product of voltage and current, what power is wasted in the
transistor when it is switched OFF?

(c) What voltage appears across the transistor when it is switched
ON? (Careful – I didn’t ask,

this time anyway, what voltage appears across the motor! Assume for
simplicity that if the

transistor were switched ON continuously, the motor would run at
exactly full power just as it

would if connected directly to the supply.) So, since power is
voltage multiplied by current, what

power is

wasted in the transistor when it is switched ON?

(d) So, using PWM, and from your answers to parts (b) and (c) of
this question, what average

power is wasted in the transistor? How does this depend upon the
duty cycle value? Is this an

improvement over using a series variable resistor?

Answer #1

A 5-kW, 220-V shunt motor has an armature resistance of 0.15 ?
and a field-circuit resistance of 100 ?. The motor delivers rated
output power at rated voltage when its armature current is 20 A.
When the motor is operating at rated voltage, the speed is observed
to be 1000 r/min when the machine is loaded such that the armature
current is 15 A. In order to protect both the motor and the dc
supply under starting conditions, an external...

Suppose a power plant produces 811 kW of power and is to send
that power for many miles over a copper wire with a total
resistance of 12.0 Ω.
If the power is sent across the copper wires at 48.0 kV rms, how
much current flows through the wires?
If the power is sent across the copper wires at 48.0 kV rms,
what is the power dissipated due to the resistance of the wires at
this current?
What percent of...

2. If a current of 3 Adc flows in the series field winding of
your motor, what would the resultant voltage drop be?
3. If the rheostat were connected in series with the shunt field
winding and the combination placed across a 120 Vdc line, what
shunt field current variations could be obtained from your
motor?
I minimum = Adc I
maximum = Adc
4. All of the windings and even the commutator of your motor are
made of...

1.In your Ohm's law lab if you let the current (I) in your
circuit remain constant, what will happen to the voltage (V) if the
resistance (R) is tripled?
2.If you connect a 10-Volt supply voltage across a parallel
combination of two 10 Ohm resistors, what should be the ammeter
reading connected in series with the voltage source?
3.When you check a 7-Amps reading on your multimeter connected
in series with a 3 Ohm resistor, what will the voltmeter reading...

A voltmeter is a
device placed in parallel with a circuit element to measure the
voltage drop across the element. As it turns out, it is difficult
(impossible?) to measure a voltage drop directly, so a voltmeter
typically works by measuring a current and then converting to
potential. At least before the days of digital circuitry, a
voltmeter was basically an ammeter (which measures current) in
series with a very large resistance R0.
Suppose you use this
kind of voltmeter...

Prof. Grail disconnects her newly charged capacitor from the
power plant (see JiT#2), and prepares to make use of the stored
energy. When she throws the switch, the capacitor will be connected
to a rail gun (A rail gun can be thought of simply as a large
circuit with a resistor connected to a charged capacitor (for a
simplified picture, see
http://science.howstuffworks.com/rail-gun1.htm). In simplest terms,
this is an RC circuit that uses the Lorentz force to throw a
projectile (in...

Design a DC power supply that will deliver 100mA of current to a
load with a ripple of 5%. Use a full wave rectifier and a 17 Peak
Voltage Source with F= 60 Hz. Load resistance = 1k
So Basically you just need to find the capacitance of the filtering
capacitor. Please tell me how. This is what I did, but I think I'm
wrong.
The Vripple I got is (.05)*17 = .85 V.
Then according to my manual, C...

2(a). A transformer has a primary-to-secondary turns ratio of
100:1. The voltage on the primary side is 27.7 kv.a.c. at 60.0 Hz (
assume 60.0 Hz when treating electric power topics). A load is
connected across the secondary winding. Assuming that the
transformer is ideal, what voltage is developed to the load?
Briefly show your work. Sketch a diagram.
2(b). The load of part a is a 2.00-ohm load. How much current
does the load draw from the transformer’s secondary...

Question 1
1. Nonutility power producers (companies that don’t own
transmission lines) use a lot more natural gas than traditional
utilities.
Group of answer choices
True
False
Flag this Question
Question 2
2. Most of your electric bill is from the transmission and
generation of electric power, and not from its production.
Group of answer choices
True
False
Flag this Question
Question 3
3. Alternating current is used in homes, and direct current is
used in cars.
Group of answer...

Exploring Innovation in Action Power to the People – Lifeline
Energy Trevor Baylis was quite a swimmer in his youth, representing
Britain at the age of 15. So it wasn’t entirely surprising that he
ended up working for a swimming pool firm in Surrey before setting
up his own company. He continued his swimming passion – working as
a part-time TV stuntman doing underwater feats – but also followed
an interest in inventing things. One of the projects he began...

ADVERTISEMENT

Get Answers For Free

Most questions answered within 1 hours.

ADVERTISEMENT

asked 12 minutes ago

asked 16 minutes ago

asked 20 minutes ago

asked 24 minutes ago

asked 26 minutes ago

asked 27 minutes ago

asked 27 minutes ago

asked 27 minutes ago

asked 34 minutes ago

asked 34 minutes ago

asked 43 minutes ago

asked 50 minutes ago