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mike9111
on FreezingBlue Flashcards.
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What factors must a motor allow for?
- -Ease of starting and stopping by operator
- -Prevention of automatic restarting after power failure, if unexpected restarting might cause injury to the operator
- -Automatic overload protection for motors exceeding 370W rating
- -Phase failure or reversal of phase rotation protection, if this is liable to cause injury or damage
- -Provision for the remote starting and stoppping of a motor, should this be required
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State two ways speed control can be obtained for motors that are single phase
- -Tapped field winding
- -Continuously rated variable resistance
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State two ways speed control can be obtained for motors that are three phase
- -Frequency
- -Pairs of poles
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State the typical current levels when a motor is started DOL
A DOL starter will draw around 600 per cent or 6 times its normal full load current
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State the relationship between voltage and torque
Torque produced by an induction motor at a given speed is proportional to its supply voltage squared
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In the protection of motors there are two types of overcurrent ralays used to protect a motor against overload
State them:
- -Magnetic relay
- -Thermal relay
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5 ways of reducing the high current starting a motor DOL
- -Star Delta starter
- -An autotransformer
- -Primary resistance starter
- -Secondary resistance starter
- -Electronic starter
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With respect to other varying starters whta is the advantage of using an Autotransformer starter
It has various tappings by which an appropriate starting voltage can be chosen to provide a starting torque and current to suit a specific application. They are also efficiant as no energy is wasted, as it is in other type starters
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Under what circumstances can two autotransformers be used for starting a 3 phase induction motor
This method can be used when the required starting torque is greater than can be obtained by by star-delta starting
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Why does the primary resistance starter have a lower efficiancy than other starters?
Because energy is dissapated as heat in the starting resistors
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When positioning a starter or controller as well as protection and accessibility state THREE other considerations that should be taken into account
- -Protection
- -Accessibility
- -Protective devides
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Over-temperature protection for motors, in general, is deemed a requirement. However, there are situations where that type of protection is not required. Give an example where over-temperature protection can be omitted
Unattended shaded pole motor 480VA
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Why is it important to consider thermal effects when positioning a starter or controler?
Locating a controller outside iont he sun could cause the thermal overloads to become pre-heated and likely to trip whilst the motor is operating normally
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