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Electrical Engineering
Electrical engineering is a field of engineering that deals with the study and application of electricity, electronics and electromagnetism.
23,056 Questions
Does the RPM of a motor vary if a load is added to the motor?
The speed of an electric motor is directly proportional to the frequency of supply. The rpm written on the name plate is the maximum operating speed for the motor design. Typically, for a squirrel cage induction motors, the speed is constant by design and this type of motor cannot operate for a long time at speeds below the rated value.
It is true that loading affects motor speed to some extend but the electric motor will accelerate to the rated speed. If the loading is within the design parameters of the motor, the electric motor speed will not drop.
What typically happens is that if the loading increases, the speed goes down, and the current increases. Because voltage is constant, this result in a high I2R loss in the windings and the motor circuit protection trips on thermal and electrical overload.
The formula for electric motor speed is SRPM=(120f)/P.
The above is for AC motors. If you are referring to a DC motor, what you have stated is correct. The motor will slow down as load is added. That is why you are not supposed to run many DC motors under no load conditions - they will overspeed.
What is the main function of an electric circuit?
The purpose of an electrical panel board or electrical distribution panel is to supply individual circuits from the main distribution board. As the board is supplied with one circuit of high capacity , it needs to be broken down into smaller branch circuits. Breakers accomplish this obligation by plugging into the panels main bus-bars. The load side of the breaker then goes to the specific load. The size of the breaker is dependant upon the size of the load of that circuit. The conductor that feeds the load is dependant upon the size of the breaker feeding that load.
Why there is a phase shift of 30degree in Dyn11 3 phase transformer?
The phase shift is caused by inductance in the transformer. Any inductance from magnetic flux that fails to link both windings is called leakage flux, and the resulting inductance is called leakage inductance.
How do you counter the induced voltage spark during the generator?
By creating a rotating electric field. The generator has two main sets of windings - the field windings (the spinning ones on the rotor usually) and the stator windings(the stationary part connected to the grid). The field windings are electrified to create an electromagnet out of the rotor. Spinning the rotor creates a rotating electric field, which induces a voltage on the stator windings.
How can you reverse a split phase electric motor?
Just swapping the hot and neutral connections on a plain and simple single phase ac motor (one which was not designed to be reversible) is usually a waste of time as the voltage is alternating and the motor sees this as the same either way and it will still run in the same direction.
For a DC motor which uses a permanent magnet to provide its magnetic field, simply switch the two power leads to make them run in the reverse direction.
For a DC motor which has a wound field winding instead of a permanent magnet you have to reverse the connections either to the field winding or to the armature. (If you reverse the connections to both the field winding and the armature the direction won't change!)
For a three phase motor simply switch any two of the three power leads to make it reverse.
Another opinion
To change the direction in a single phase ac electric motor you must find a way to reverse the connections to the field magnet (outer windings) or to the armature or rotor (which is the center part that spins). Some motors are reversible and the run direction can be changed simply by swapping the 2 plugs sticking out of them that are connected to the field windings.
All single phase ac motors can be reversed by physically reversing the field magnet or rotor ( front of motor is now the back) although this can be more difficult in motors with brushes because of the brush holder which is mounted at the front and usually has to stay in the front.
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Changing motor rotation
A split phase induction motor has two sets of coils and a centrifugal start switch. The start winding is in series with the start switch. The start winding provides a rotating magnetic field in one direction enabling the motor to start. The motor can be reversed by reversing the connections of either the start winding or the run winding but not both.
What is the technical difference when it comes to the implementation of AM and FM?
In a analog transmitter the RF carrier have to be modulated with the analog signal either by AM or FM where in digital transmitter the carrier is simply switched on and off a off = a zero and on = a one
How much percentage of current takes motor runs at no load?
The FLA of a motor is the amperage that the motor operates at. So to answer the question, 100%.
How much starting current required for starter of 1hp motor?
For 1 hp motor, a single phase starter is used. Because a single phase starter gives economy consumption,easy trouble shooting and easy installation when compared to three phase starters and supply....
i need help with this one as well
How can you measure the ampere or watt for power supply?
You can measure the current and power of a 'power supply', using an ammeter and a wattmeter.
With the power supply connected to its load, the ammeter must be connected in series with the power supply's input. The wattmeter's current coil must also be connected in series with the power supply's input, and its voltage coil must be connected in parallel with the supply, taking the instrument's polarity markings into account.
How do you calculate resistance in series and parallel circuits?
when in series you can use this: R1+R2+R3+enz=you total resistance.
and when in parallel you can use this: Req = 1 / (1/R1+1/R2+1/R3+1/R4+enz).
If there are 2 resistors in parallel, use Product over sum: Req = R1*R2/(R1+R2).
For 3 resistors, a little more tricky: Req = R1*R2*R3/(R1*R2 + R1*R3 + R2*R3). For more than 3, use the general formula above for a quantity of resistors in parallel.
The total current in a circuit consisting of six operating 100 watt lamps connected in parallel to a 120 volt source is 5 amperes.
Since power is volts times amps, take 600 watts (100 times 6) and divide by 120 volts to get 5 amps.
What do line voltage thermostats have that low voltage thermostats don't?
Line voltage sometimes varies according to how much load there is from neighbours and other users. When the voltage is high, the water heats more quickly so that the thermostat cuts out sooner, so the amount of energy used ia about the same.
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It is the difference in construction of the two thermostats. A line thermostat uses a bi-metal sensor to open and close the line voltage. The room temperature fluctuation has to be greater between the stat turning on to off, as the differential setpoint can not be regulated as fine as a low voltage stat.
A low voltage thermostat uses a mercury switch that is operated from a spring that senses the room temperature. Low voltage thermostats do not control the line voltage to the heater but use a remote contactor to open and close the voltage supply to the heater. There is a compensator in the low voltage stat that can be set for the contactor's coil current. This way the temperature generated by the coil of the remote contactor is nulled and the thermostat only reads the room temperature. The differential setpoint on a low voltage thermostat has a finer setting than that of a line thermostat, so a more accurate temperature is maintained.
How you calculate no load current of 30 hp motors?
You need to specify the rating , voltage and phases of the motor to answer this question.
What is the temperature coefficient of BJT?
No. Semiconductor has negative temp coefficient, because increase in temp causes the increase in the k.e of the electrons bu t not in the no of electrons . these highly energised electronsel increase current, & in terms conductivity.
What is the difference between dry and wet contact?
A dry contact is a contact that does not provide voltage. For instance, the push-to-talk switch of a microphone, which just closes a circuit without providing voltage.
A wet contact is a contact that will provide voltage when closed, like the switch on the wall that activates the 110 VAC outlet to turn a lamp on in a room.
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+------o o---------(o)
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+---------------------(o)
Dry contact
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+------o o---------(o)
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--+--
--- Wet contact
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-+-
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+---------------------(o)
What prevents transformer laminationsfrom shorting to the adjacent laminations?
You need to prevent transformer laminations from shorting to each other because, otherwise, there would be eddy currents that affect the Q of the transformer, effectively impacting its efficiency. They would also cause the transformer to run hot.
If the laminations were not insulated from each other, they would constitute a one-turn shorted winding, with high internal current. Usually, the laminations, in the shape of an E, are inserted into the windings alternately from each end, enhancing magnetic coupling while not causing the one-turn short.
This is the same issue involving routing a conductor through a hole in a panel - the hole must be relieved, or slotted, to prevent eddy current, or (better) both conductors must be routed through the same hole, balancing the current, resulting in no eddy current.
Why frequency is constant in a single phase transformer?
a transformer or inductor or capacitor does not change frequency
frequency is controlled at the generating station with the speed of the motor or turbine
the number of phases will not make a difference
an inductor or capacitor can shift phase up to 90 degrees
you can make 3 phase power from single phase power with inductors capacitors and transformers
A resistance of 10 ohms is placed across a 9 volt battery what current flows through the battery?
No current flows through the battery.
There is a current through the external circuit. I = E/R = 9/10 = 0.9 amperes.
When does a motor draw current more than its rated current?
This is a good indication that the motor is being overloaded. If the motor load is belt driven, remove the belt and then check the motor amps. If it goes back to normal FLA then there is a problem with the driven load. If the motor amperage stays high check the motor bearings for the problem.
Why RC circuit is preferred over RL circuit in waveshaping?
There is no true advantage of RC circuits over RL circuits, as they perform different functions. RC circuits contain resistors and capacitors, while RL circuits contain resistors and inductors.
What would happen if ammeter were connected directly across a voltage source?
An ammeter measures current by either being inserted in series with the load being measured or using a clamp-on device that induces a proportional current that the meter measures. Nothing should happen if you connect an ammeter across a voltage source unless it was very high voltage compared to the rating of the meter.
What is the voltage rating of a resistor having power of 1500W?
There are zero volts in 1500 watts. Watts are the product of amps times volts. Without a voltage or amperage stated, an answer can not be given. W = Amps x Volts.
