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List two reasons why a shaded pole motor is used to power the fan in a portable heater?
- any two of:
- it can be stalled
- its quiet operation
- low torque
- cost effective
Why is a three phase induction motor more efficient than the single phase equvilant size induction motor?
- Three phase induction motors have a balanced rotating magnetic field
- single phase have a pulsating one
Give an application where a three phase motor would require phase reversal protection?
- Where it may injure people or cause damage to property and equipment
- eg, lathe or conveyor
State two reaons for using reduced voltage starting for a three phase induction motor?
- Limit mechanical shock
- Reduce high starting currents
What is the purpose of the maintaining contacts in a direct online reversing starter?
- To maintain supply to the contactor coil allowing user to release start button
- To prevent auto restarting when supply is interupted
What is the cheapest type of reduced voltage starter for three phase motors, and what special requirement, if any, does this method of starting impose on the internal connections of the motor winding?
- Needs a six terminal connection
State One important reason for inserting external resistance into the rotor circuit of a slip ring motor at start up
- Any one of:
- to reduce starting current
- To produce a higher starting torque than that availiable from a squirrel cage motor started DOL
- To provide a more gentle startup for high inertia loads
- To provide a ramped acceleration for high inertia loads
- To provide a higher start torque at low speeds starting through improved power factor
What is the main advantage of a single phase synchronous motor over other single phase motors?
It rotates at a fixed speed independant of load
Sketch a Torque Speed graph for a Three phase sqirrel cage induction motor
on graph show:
Pull out torque
Full load torque
Full load speed
A primary resistance starter reduces the motor terminal voltage during starting by inserting resistance into each supply line.
The motor terminal voltage ramps smoothly up during accelaration as a function of the decreasing line current and voltage drop across the line resistances.
Explain, in principle, how an electronic soft starter achieves the same thing?
What type of three phase ac motor is most likely to have a wound rotor and an external resistor bank?
Slip ring or wound rotor induction motor
Give four reasons why universal motors are more suitable than single phase induction motors for use in portable power tools?
- High power to weight ratio
- Maximum torque at zero speed
- Can be easily speed controlled
- High speeds availiable
Give a reason why reduced voltage starting of a three phase induction motormay be required?
- Any one of:
- To reduce high starting current
- Permit installation of smaller cable
- Smaller transformer size
- Mechanical shock
What effect does a drop in supply voltage have on the supply current to a fully loaded, three phase, squirrel cage induction motor?
The current will increase
List two advantages a cap-start, cap-run motor has over a resistance start, split phase induction motor?
- Any two of:
- Better power factor
- Higher power factor
- more efficient
- Lower starting current
- Higher starting torque
- More starts per hour
List two types of protection afforded by a DOL starter and overload supplying a three phase motor?
- Any two of:
- No voltage / Low voltage
- Single phasing
List three types of reduced voltage starters (excluding electronics) that can be used with a three phase squirrel cage induction motor?
- Auto transformer
- Primary Resistance
Give Two reasons other than cost why a universal motor is used for portable tools?
- High power to weight ratio
- Maximum torque produced at zero revs
- easy to control speed
The thermal protection device in a three phase motor starter is primarily designed to disconnect the motor on the occurance of what type of fault?
Small Overload of long duration
Sketch a typical current and torque curve for a DOL starter?
Sketch the Current/Torque curve for a Star/Delta connected motor?
When using a AC speed drive Harmonics are being generated, Why must it be kept at a minimum?
- Can cause problems in distribution network if high and unchecked
- particularly with respect to resonance and elevated harmonic currents and heating effects
When AC motors run at lower than rated speeds what precautions must be taken and why?
- Fan acts as an insulant cooler
- motor running slower means fan runs slower
- which can damage motor
- can be overcome by having a seperately controlled fan supplying a fixed volume of air
A Three phase induction motor is started by a Star/Delta starter
(i) State the percentage of the DOL starting torque in the start (star) position?
(ii) State two forms of protection incorperated in the starters circuitry?
(iii) Briefly explain the purpose of interlocks?
A three phase induction motor is started by an auto transformer starter
(i) What would be the main advantage of this starter over a star delta starter?
(ii) State which auto transformer tapping should be chosen first, and give the reason?
(iii) If the motor fails to start satisfactorily on the initial choose of tapping, what action should be taken and why?
List Three reasons for using reduced voltage starters with squirrel cage induction motors when DOL may accelerate the load up to speed to quickly?
- Initial starting current to high for supply main- ie, voltage drop limitations, or switchboard limitations - with respect to other connected apparatus.
- Initial starting current too high for the power supply authority,- ie, other consumers effected by voltage variations.
- Too much stress on mechanical components, ie- smoother starting.
Sketch a circuit diagram of a single phase, cap start induction run motor.
a) Sketch the power circuit diagram of a resistance starter connected to the wound rotor windings of a slip ring induction motor ( control circuit not required )
b) Explain what special requirements are needed when slip ring motor controllers are used for speed control?
c) Give two applications of a slip ring motor with starter, and reasons for their use?
A Three phase motor has windings rated at 230V and is to be connected in Star to a 400V Three phase supply. How many terminals must their be on the terminal block?
- any one of:
- Six terminals
- Three terminals
How is the direction of rotation reversed in a delta connected, three phase squirrel cage motor?
By reversing any two phase connections
How is the direction of rotation reversed in a single phase, split phase induction motor?
By reversing the connections to the start winding or run winding,
A three phase, 8kw induction motor connected to a 400V, 50hz three phase supply has a power factor of 0.81 and an effiency of 84% when fully loaded.
(i) The input power
(ii) The input kVA
(iii) The line current
- P=root3 VL IL pf
- IL=P/root3 VL pf
- IL=9500/root3 x 400 x 0.84
State one reason why reduced voltage starting of a large three phase induction motor may be required?
- Reduce starting current
- permits the installation of a smaller cable
- smaller transformer size can be used
- reduce mechanical shock
- reduce the effect of voltage drop on the system
State three technical advantages that electronic starters have over electro-mechanical starters (for example, an auto transformer starter).
- Smoother starting
- easier to adjust for overcurrent and other perimeters
- Visual feedback through display panel
- physical size smaller than auto transformer
- more economical to maintain
- reduces mechanical shock to equipment
- more even current flow
- more even torque
- torque is very constant at start
- cheaper in the larger sizes
- built in protection and monitoring
A three phase induction motor is started by a Star/Delta starter, State two reasons why interlocks are used in the starter.
- To prevent the starter from switching motor windings to delta before run up in star
- to prevent phase to phase short circuit
- to prevent both star and delta contactors becoming energised at the same time
A three phase induction motor is started by an auto transformer
What would be the main advantage of this starter over a Star/Delta starter?
- Because their is higher starting torque per amp of line current
- their is a closer match between the tapping and load at start
- smoother start up
The auto transformer has tappings of 40%, 60% and 80%, state the reason why the 40% tapping should be used first?
- Any one of:
- To keep starting current at a minimum
- to ensure their is less voltage drop on the system
An auto transformer starter fails to start satisfactorily on 40% tapping, what action should be taken and why?
The next higher tapping should be tried, this will increase starting voltage current and torque
Draw and label a circuit diagram of a 230V control circuit for a three phase DOL motor starter,
only the items below are to be listed;
fuse protecting entier circuit
a stop button that controls the circuit
a start button that controls the circuit
a hold in (maintaining) contact
a thermal overload relay contact
a 230V coil
State one other type of protective device that could be used in place of a thermal overload
Describe the method by which a single phase supply may be used to simulate a rotating magnetic field effect for a split phase single phase induction motor
- The two stator windings are spaced 90 electrical degrees apart and connected in parallel to a single phase supply
- The start winding has a higher resistance and lower reactance than the run winding
- the current in the start winding is closer in phase with the applied voltage
- therfore the current through the start winding has a phase displacement to the current in the run winding
- this produces the simulated rotating magnetic field
If an external starting relay is to be used for a single phase, split phase induction motor, which winding will it switch?
The start or auxillary winding
In a single phase cap start induction motor,
what would be the effect on the performance of the motor if the run winding was open circuited
(i) and the motor was switched on whilst on load
(ii) while the motor was running whilst on load
- (i) the motor would not start
- (ii) the motor would slow down until centrifugal switch closed
- and for light loads - the motor will speed up again as the start winding is energised, - the centrifugal switch will open around 75% full load speed so the motor will cycle as the switch opens and closes
- or for heavy loads - the motor will stall and eventually burn out
You have been requested to wire a DOL forward reverse starter that controls a three phase delta connected motor.
Draw and label the power circuit for the DOL starter. the drawing must show the contactors and conductors, that enables the motor to be operated in the forward and reverse directions
Draw and label the 230V control circuit for the DOL forward reverse starter
You have been requested to wire an automatic star/delta starter for a 15kW motor in a workshop, the motor starts in star and runs in delta
Draw and label the power circuit showing all components to make it work correctly and safely