ELEC 1213 3 Phase test

What is the 3 phase frequency formula?

# of poles x N (in rpm) / 120

How does a 3 phase AC induction motor works (operation steps).

1) A. C. voltage is applied to the stator creating the affect of a rotating magnetic field. 2) Rotating magnetic field cuts the rotor, inducing a voltage into the rotor. 3) Rotor voltage causes rotor current. 4) Rotor current causes rotor field. 5) Rotor field interacts with stator field resulting in motor action.

What are the 5 steps of operation for an induction motor?

1) A. C. voltage is applied to the stator creating the affect of a rotating magnetic field. 2) Rotating magnetic field cuts the rotor, inducing a voltage into the rotor. 3) Rotor voltage causes rotor current. 4) Rotor current causes rotor field. 5) Rotor field interacts with stator field resulting in motor action.

What are the requirements to generate a voltage?

1. A magnetic field 2. A conductor 3. Relative motion between the conductor and magnetic field

What are wound rotor motors 5 operation steps?

1. Apply 3 phase AC to the stator creating a rotating magnetic field. 2. Voltage is induced into the wound rotor. 3. Because the rotor windings are connected through the resistor bank, current will flow. 4. Rotor current causes rotor flux. 5. Interaction of the two fields cause rotor rotation.

What are the operating steps of a synchronous motor

1. At start, no DC is applied to the rotor field. 2. Three phase AC is applied to the stator creating a rotating magnetic field. 3. The magnetic field cuts the amortisseur winding (small squirrel winding) inducing a voltage. 4. Rotor voltage causes rotor current which causes a rotor field• The amortisseur winding will accelerate the rotor to about 95% of rated speed. 5. As the rotor approaches rated speed, DC is applied to the rotor winding and the rotor field will snap into synchronous speed with the rotating stator field. 6. Armortisseur winding is not designed to carry a load because it is too small and will overheat. 7. When the rotor is at synchronous speed, no current flows through the armortisseur winding, because there is no slip.

What are the advantages of a three phase system over a single phase system?

1. Lighter 2. Smaller 3. More efficient 4. Require less maintenance

Wound rotor motor construction

1. Rotor windings are similar to a 3 phase stator winding 2. Rotor windings are connected wye 3. Rotor windings are connected to sliprings 4. Slip-rings are connected to a wye connected resistor bank 5. Resistors are either wire wound, edge wound enamel or cast grid.

What are 6 prime mover examples?

1. Steam Turbine 2. Diesel or gas engine 3. Electric motor 4. Human power - Gilligan bike 5. Hydro-electric 6. Air or wind turbine

What are the factors that affect the magnitude of an induced voltage?

1. The amount of flux 2. The number of turns linked by the flux 3. Angle of the cutting of the flux 4. Rate of the relative motion

what do synchronous speed motors do?

3 phase AC motor which operates a constant speed(synchronous) from NL to FL

What are the advantages of a three-phase motor over a single-phase motor?

3-phase motors are smaller, lighter, more efficient, and require less maintenance than single phase motors. Also: Single phase power is pulsing. At unity, power goes through zero twice every cycle. For a lagging pF, power goes through zero four times a cycle. In a three phase system, the total power delivered at any one time is the same. Operating characteristics of three phase equipment is superior. A balanced three phase, three wire circuit uses only 75% the copper required for a single phase system of the same capacity.

What is a closed transition starter?

A closed transition is when additional resistors and contacts are added. Resistors are added to continue current flow to the coils

Single Phasing definition:

A condition that occurs in a three phase system when one of the phases fails. There are only two phases, which causes the motor to act as though it was in a single phase operation.

Delta connection definition:

A connection pattern for three phase motors and generators. (drawing on page 118 in textbook)

Soft start controller (reduced torque starter) definition:

A controller that is used to reduce the line current delivered to a motor in order to reduce the starting torque and create a smooth acceleration up to full speed.

Wiring diagram definition

A diagram that is laid out to show the physical relationship of components in a circuit and how the wire is run from one to another.

Wiring diagram definition:

A diagram that is laid out to show the physical relationship of components in a circuit and how the wire is run from one to another.

What is dynamic braking?

A form of electrical braking that uses the spinning rotor as a generator and temporarily applies an electrical load to the generator output

Motor definition:

A motor is a device that converts electrical energy to mechanical work through a rotary action, and is rated in horsepower.

What is an open transition starter?

A power disconnection from the motor, then switching lead connections and re-applying power is a open transition.

Prime mover definition:

A prime mover is a mechanical energy source that drives or turns a motor.

Describe the principle for DC braking

A rotor will follow the stator field. Adding DC to a stator field after AC power is removed the stator field will become stationary.

Wye connection definition:

A three phase motor that has its internal windings in a wye pattern. (drawing on page 118 in textbook)

What 3 Phase motors is required to be marked with its secondary voltage according to NEC?

A wound rotor motor NEC 430.7 (A) (10) (remind me to check with steve).

How is a rotating magnetic field developed on the stator of a 3-phase AC motor?

A. C. voltage is applied to the stator creating the affect of a rotating magnetic field.

Apparent power formula for three phase systems:

AP= 1.73 x VL x IL or 3 x Vphase x Iphase

What is the definition of three phase power

Alternating current that has 3 windings or phases that are 120 electrical degrees apart.

What are synchronous motors steps of operation?

At start, no DC is applied to the rotor field. Three phase AC is applied to the stator creating a rotating magnetic field. The magnetic field cuts the amortisseur winding (small squirrel winding) inducing a voltage. Rotor voltage causes rotor current which causes a rotor field The amortisseur winding will accelerate the rotor to 95% of rated speed. As the rotor approaches rated speed, DC is applied to the rotor winding and the rotor field will snap into synchronous speed with the rotating stator field. Armortisseur winding is not designed to carry a load because it is too small and will overheat. When the rotor is at synchronous speed, no current flows through the armortisseur winding, because there is no slip.

Why is excessive starting current bad?

Brownouts Electrical stress (heating) on power delivery equipment transformers, contacts, circuit breakers, fuses, etc...resulting in premature failure

How is power factor controlled in a synchronous motor?

By adding DC power to the rotor.

how do multiple speed motors change speed?

By changing the number of poles they have

How is the direction of rotation of an AC single phase motor changed?

By switching the two start winding terminals (t5 and 58)

What is a torque vs speed curve?

Check page 120 of the textbook

How is DC power supplied to the rotor in synchronous speed motors?

DC is supplied to the rotor in two ways: Externally by a DC source through 2 slip rings and brushes. Electronically via rectifiers mounted on the rotor shaft.

Duel voltage Wye and Delta drawings

Day 16 powerpoint page 3 for Wye drawing, and day 16 powerpoint page 4 for delta drawing

What do type A-D design letters indicate?

Design A motors Relatively constant speed or low % SR High starting torque Rapid acceleration High breakdown Design B motors General purpose design motors - most common type Low starting torque Medium starting current Medium breakdown torque Design C motors Dual squirrel cage rotor High starting torque Low starting current Very low breakdown torque Reacts quickly to load changes - good running torque Design D motors High resistance rotor High starting torque High starting current High slip• 5- 13 % slip Suited for high inertia loads such as flywheels

What does design letter E indicate?

Design E motors Torque vs speed curve is similar to Design B motors for sizes 3HP and greater Starting torque is less than a Design B motor Design E values match the IEC standards Starting current is higher than a Design B motor (55% more) {6 times running current times 1.55} Energy Policy Act of 1992 Variations in lamination steel, slot shape, air gap, ventilation, etc.... Easy way to increase % eff is to reduce I2R losses - reduce the effective turns in the winding This results in higher starting current and reduced service • Factor Design E - SF of 1.0 and don't need to meet no load power sound requirements A, B, C, - SF of 1.15 and must meet sound requirements (We better not need to know this bullshit)

What do Design Letters do?

Design letters indicate the operating characteristics of the motor.

What are the two types of dual phase motors?

Dual voltage wye connected motor Dual voltage delta connected motor

How is the direction of rotation of an 3-phase AC motor changed?

For three phase it is switched by changing any two line leads.

What is generation of electricity based on in 3 phase?

Generation of Electricity is based on a simple principle if you force a conductor through a magnetic field, cutting lines of flux this implies you will induce a voltage into the moving conductor if you hook up a load to the conductor ends, electrons will flow from the negative terminal to the positive terminal.

Advantages of wound rotor motors:

High starting torque with moderate running torque Smooth acceleration under heavy load No excessive heating during starting Good running characteristics Adjustable speed

What are the basic delta current and voltage formulas?

IL= 1.73 x I phase VL= V phase I phase= IL / 1.73

What are the basic Wye current and voltage formulas?

IL= I phase VL= V phase x 1.73 V phase= VL / 1.73

How do you determine which 3 phase motor you are wiring if all the lead markings and the nameplate are gone?

If delta wound, wires 1,2, and 3 will have continuity with two other leads. If a wye motor configuration, wires 1,2, and 3 will have continuity with one other wire lead. The last three should have continuity with each other. (someone remind me to double check this with steve).

How does a power factor become lagging for synchronous motors?

If you decrease DC field current below this, the stator field will have to provide more of the magnetizing current and the PF will become more lagging.

What happens if you decrease the DC field too much on a synchronous motor?

If you decrease the DC field current too much, the rotor field will no longer be able to latch onto the stator field ..... this is called slipping a pole.

How does a synchronous speed power factor become leading?

If you increase DC field current beyond unity the rotor will actually pick up some of the reactive load up off the stator and the power factor will become leading.

What are the three 3 phase motor types?

Induction motor Wound rotor motor Synchronous motor

What are the types of manual motor control for three phase?

Integral or Fractional HP manual motor starters Drum switches

What type of motors use dynamic braking?

It can be used by AC motors with rotor lead connections (example: wound rotor motors) and it can be used by DC motors.

What does a prime mover do?

It is a mechanical energy source that drives or turns the rotor

What is plugging?

It is a method of stopping a motor a motor by momentarily reversing the direction of rotation of the rotating magnetic field.

What is anti-plugging protection?

It is a way the plugging control is applied, like a device stopping the motor torque from being reversed until the motor speed has slowed to an acceptable rate.

Why does a synchronous motor need to be brought up to speed before applying the field to the rotor? How can you do this with something different than a amortisseur winding?

It needs to disconnect the rotor and amortisseur winding before dc power will take over keeping the rotor turning at a constant speed. If the amortisseur winding was still connected to AC power it would burn out after a bit. You can do it with a field frequency relay instead of an amortisseur winding.

What is the starting sequence of a synchronous motor that is started with an amortisseur winding?

It starts the same as an induction motor but a small winding, the amortisseur winding, wound on the rotor gets voltage induced into it and helps the motor get to about 95% of its synchronous speed.

Magnetic control rules (idk what to call it)

Ladder diagrams always start at L1 Stop button is wired to L1 (left rail) Loads (MS,PR,CR) are wired to L2 (right rail) Loads are wired in parallel Ladder rungs are numbered from top to bottom on the left Numbers in ( ) on the right indicate that the coil on that rungs controls contacts in the ladder rung that is numbered in ( ). If the number is underlined, that contact is normally closed. Contacts are always shown with the circuit de-energized Control circuits are????? --------------->single phase

What are synchronous motors used for?

Large loads that need to run at a constant speed. Used to correct system power factor (synchronous condenser).

What are wye connections (also called star connections) voltage and current relationships?

Line current equals coil or phase current Line voltage equals coil or phase voltage times 1.73

What are delta connections voltage relationships?

Line current equals coil or phase current times 1.73 Line voltage equals coil or phase voltage

Alternate poles definition

Motor is built with one winding for each speed. Each winding is wound for a different # of poles. This allows a closer variation in speeds.

How do synchronous speed motors differ from induction motors?

Requires a separate DC source of power for the rotor field. Needs special starting components. The rotor is a salient pole rotor with winding on the poles that have DC applied The rotor has an auxiliary squirrel cage (armortisseur) winding to start the motor.

Simple LVP control circuits

Reversing circuit Electrical interlocks Mechanical interlocks

As load is added to the motor what happens to it?

Rotor speed goes down CEMF goes down and Rotor bars get cut by the stator field at a faster rate Stator flux goes up because CEMF went down Rotor current goes up because Rotor voltage went up Td goes up until it equals Tr

What is general info about a wound rotor motor?

Rotor windings are similar to a 3 phase stator winding Rotor windings are connected wye Rotor windings are connected to sliprings

How do you remove the resistance to speed up the motor, either manually or automatically?

Rsec goes down causing Irotor to go up causing Td to go up causing rotor to speed up

What is the three phase induction motor stator construction?

Similar to single phase: Slotted laminated steel core. Stator windings are symmetrically placed into the slots to establish 3 windings placed 120 electrical degrees apart. Windings can be connected either Wye or Delta

What are the advantages of a three phase system?

Single phase power is pulsing. At unity, power goes through zero twice every cycle. For a lagging pF, power goes through zero four times a cycle. In a three phase system, the total power delivered at any one time is the same. Operating characteristics of three phase equipment is superior. A balanced three phase, three wire circuit uses only 75% the copper required for a single phase system of the same capacity.

Generator definition:

Something that converts mechanical energy into electrical energy

Wound Rotor Motor definitions

Special code issues Big and bulky Requires special controllers Replaced by the variable speed solid state drives

How is speed control manual or automatic

Speed control can be either automatic - with the use of magnetic relays or manual - with the use of a drum controller or a faceplate controller or a rheostat.

What is the rotor construction of three phase induction motors?

Squirrel cage rotor Bars of copper or aluminum Rotor bars are connected at each end by shorting rings Bars carry a large amount of current.

What are the six prime movers?

Steam turbine Diesel or gas engine Electric motor Human power - Gilligan bike Hydro-electric Air or wind turbine

How do you reverse a three phase motors running direction (clockwise to counterclockwise or counter clockwise to clockwise)

Switch any two line leads

When are synchronous speed motors used?

Synchronous motors are used where they can start unloaded or with light mechanical load

True power formula for three phase systems:

TP= 1.73 x VL x IL x PF or 3 x Vphase x I phase x PF

Starting torque definition:

The amount of torque developed by a motor as it starts from a stationary, or stopped, condition. It is higher than running torque because of the inrush current when a motor first turns on.

What do code letters do?

The code letter indicates the type of rotor bars the squirrel cage rotor will have. We have learned that the code letter indicates the amount of locked rotor KVA/hp for a motor. It is also an indication of the ratio of the starting current to the full load running current.

Break-down torque definition:

The maximum torque that will develop with the rated voltage applied at the rated frequency without an abrupt drop in speed.

How do you control the speed of a wound rotor?

The more resistance you remove from the secondary circuit the faster it goes. Adding resistance also slows down the motor.

Consequent poles definition

The stator windings are rewired to produce different # of poles. Can only be half.

Speed Control Definition

The torque developed by a motor is proportional to the strength of the magnetic field and the current in the rotor.

What is torque strength proportional to?

The torque developed by a motor is proportional to the strength of the magnetic field and the current in the rotor.

What is the relationship of each winding in a 3-phase motor?

They are 120 degrees apart from each other electrically

what does wound rotor motors being at full resistance applied to the rotor mean

This means that rotor current and starting current is limited.

What are the different code letters and their meaning?

Type A Highest starting torque Lowest starting current Type B - E Medium starting torque Low starting current Type F - V Medium starting torque High starting current

What is the purpose of a Reduced voltage/torque starter?

Used to bring the motor up speed without drawing excessive amounts of line current. Also called acceleration circuits.

How do synchronous motors reach 100% power factor (also known as unity)

When just enough DC is supplied to the Rotor field to achieve synchronism, the PF of the system is 100% or unity.

Can synchronous motors control the Power Factor of a system?

Yes

Do mechanical brakes use electricity? and if so, what do they use electricity for?

Yes they use electricity. When there is no power a spring holds the mechanical brake shut. It opens when electricity is applied but closes once again when there is no power.

How do you increase or decrease power factor in a synchronous motor?

You can increase a PF by adding DC power to a motor, or decrease the power factor by taking away DC power from it.

Schematic diagram definition

represents the electrical relationships of the circuit laid out in a logical sequence.

Schematic diagram definition:

represents the electrical relationships of the circuit laid out in a logical sequence.

Definition of starting torque

the % of full load torque a motor will develop at start.

Conductor definition

the armature or the part of the electrical device that has voltage induced into it. (most generators have a rotating field and stationary armature)

One line diagram definition

the control circuit of a schematic diagram.

Definition of breakdown torque

the maximum torque a motor will develop without a rapid decrease in speed.

Torque definition

turning or twisting force which when applied about an axis will produce rotation.