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51 Cards in this Set
- Front
- Back
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Two most common categories of three-phase motors are |
Synchronous motors Induction motors |
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Two common types of induction motors are |
Squirrel-cage induction motor Wound-rotor induction motor |
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The average range of slip for a squirrel-cage induction motor is between.... |
2% - 6% |
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The construction of the rotor of a squirrel cage induction motor consists of |
Heavy copper bars that connect together at each end by end rings usually made of copper or brass. This forms a carge around an iron core |
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Motor slip formula |
Slip = (Ss➖Sr) ➗ Ss ((Synchronous Speed of the Stator ➖ Actual Speed of the Rotor)➗ Synchronous Speed of the Stator Synchronous speed of the Stator = (Frequency in hertz✖️120)➗Number of Field poles |
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State the advantages of a three phase induction motor |
High Efficiency ( 85%-90%) Low initial cost High power factor (80%-90%) Good starting torque (100-275% of full-load torque) Excellent speed regulation (higher slip poorer speed regulation) |
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Name disadvantages of three phase motors |
Three phase power must be available to use a three phase power motor, not every location has three phase power Readily available. Three phase motors are not available below a certain size usually 1/8 hp or .010kW |
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List three applications of three phase induction motors |
Large conveyer systems Large Pumps because of good starting torque and long life Machine tools in machine shops or factories (mills,lathes and drill presses) because of efficiency and easy speed control through gear or belt drive systems |
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What is the purpose of laminations in the cores of both the rotor and stator |
To reduce eddy currents and hysteresis core losses |
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What is the purpose of laminations in the cores of both the rotor and stator |
To reduce eddy currents and hysteresis core losses |
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Why are the bars of the squirrel cage slightly skewed so they are not parallel with the slots in the stator? |
To make the motor run smoothly and quietly. |
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What is the purpose of laminations in the cores of both the rotor and stator |
To reduce eddy currents and hysteresis core losses |
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Why are the bars of the squirrel cage slightly skewed so they are not parallel with the slots in the stator? |
To make the motor run smoothly and quietly. |
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What is the Main purpose of the end bells on a motor? |
To house the bearings that support the rotor shaft and keep it aligned with the stator |
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Name two ways motor enclosures are classified? |
Way they’re protected from environment. The way they are cooled |
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What name is given to the machine with an enclosure that does not prevent the free exchange of air between the inside and the outside of the machine |
Open Machine |
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What name is given to the machine with enclosure that prevents the free exchange of air between the inside and the outside of the machine |
Totally enclosed machine |
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The winding of a three phase wine thing is made up of three single phase windings placed how many electrical degrees apart from each other |
120 electrical degrees |
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What are the two purposes of the stator core |
To provide a low reluctance path for the magnetic circuit Support stator windings |
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What does the service factor of the motor tell about the motor |
Service factor multiplied by horsepower gives the maximum continuous power capable of being supplied at rated voltage and frequency without damaging the motor |
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Insulation system destignation ratings. |
Class A - 55⁰C over 40⁰c +-10⁰ 105C conductors thermal image Class B 80⁰C over 40⁰C +-10⁰ 130⁰C conductor thermal image Class F - 105⁰C over 40⁰C +-10⁰= 155⁰C conductor thermal image Class H - 125⁰C over 40⁰c +-15⁰= 180⁰C conductor thermal image |
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What relationship is expressed by design letter on a motor nameplate |
Relationship between motor torque and motor speed |
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Define the term full load current |
Full load current is the current the motor will draw when it operated rated voltage and supplies rated horse power |
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The magnetic flux created by any winding will be at maximum when the current through that winding is at |
Maximum |
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What effect does increasing the frequency of the supply have on the speed of the rotating magnetic field |
Increasing frequency of the supply increases the speed of the rotating magnetic field |
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What effect does increasing the number of poles have on the speed of the rotating magnetic field? |
Increased poles, decreased speed of the magnetic field |
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Define synchronous speed |
Synchronous speed is the speed of the rotating magnetic field |
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Since there’s no electrical connection between the stator and the rotor how is current made to flow in the rotor of a squirrel cage induction motor |
Electromagnetic induction |
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Define slip speed |
Slip speed is the difference between synchronous speed and rotor speed |
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Define the term break down torque |
The maximum torque the motor will produce referred to as the stalling torque because if it increases anymore the motor will stall |
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Defined the term locked rotor torque |
The maximum torque The motor will develop at rest for all angular positions of the rotor with rated voltage and frequency applied to the stator |
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At what rotor speed with the stator current be the highest |
When the rotor is at a standstill slip is equal to 100% |
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What type of loads is A NEMA designed class c motor suitable for? |
Loads with starting torques equal to or greater than their break down torque example flywheels crusher drums compressors |
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Which part of the linear induction motor actually moves |
The stator |
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Relative to the direction of the moving magnetic field in what direction does the linear induction motor move |
Stator travels in the direction opposite of the moving magnetic field |
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Describe design class B |
Design B General purpose motor High reactance rotor that limits the starting current to about 5-6 times the full load value. Low starting torque compared to design C, and D. NOT used to start high-inertia loads
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Describe design class A and B |
Class A Was the original class of SCIM Normal starting torque up to 150% FLT Relatively high inrush currents between 6 and 10 times FLA Not used to start high-inertia loads |
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Describe design Class C Motors |
Design class C Combines high starting torque with low starting current More suitable for high Inertia starts A double cage rotor would has this type of torque vs speed characteristics used to supply loads such as large centrifugal blower’s, fly wheels, crusher drums, loaded compressors, loaded piston pumps, and loaded conveyors |
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Describe design class D motors |
Describe design class D High Resistance rotor that provides starting torques close to 300% of normal. Lower starting current inrush Higher slip under load Low efficiency |
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What happens to the Power Factor in a synchronous motor when you increase load? |
Power factor becomes more lagging |
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What happens to the Power Factor in a synchronous motor when you increase load? |
Power factor becomes more lagging |
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What happens to the stator current and Power Factor in a synchronous motor when you increase field excitation? |
Current in the stator increases to supply lagging Vars reactive power Power factor will become more leading |
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What happens to the Power Factor in a synchronous motor when you decrease load? |
Power factor will become more leading. Current will decrease |
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What happens to the Power Factor in a synchronous motor when you decrease Or reduce field excitation? |
Power factor will become more lagging |
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Describe power factor in an Under excited rotor field in a synchronous motor |
Power factor is lagging when rotor field is under excited |
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Describe power factor in a synchronous motor when rotor field is over excited. |
Power factor is leading when over excited |
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Define the term pull in torqe |
Pull IN torque Torque required to pull the rotor in to synchronous speed with the rotating stator field once the DC excitation has been applied |
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Define the term pull out torque and what happens if it’s exceeded |
The maximum sustained torque that the motor can develop at synchronous speed, If exceeded he motor will pull out of synchronization and slow down |
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What is the term used to describe the angle between the centre of the stator poles and the center of the rotor poles? |
Torque angle |
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What controls the magnitude of the current flowing in the stator windings? |
The resultant of the applied voltage to the stator and the voltage induced in the stator by the rotor field |
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How does the torque angle of the rotor affect the stator currents? |
When the torque angle changes the angle of the induced voltage in the stator also changes. This results in a new resultant EMF |
