Machines; Synchronous Motors Flashcards
What rate does a rotor turn in a Synchronous motor?
Same speed as the rotating magnetic field of the stator (synchronous speed)
In order to rotate synchronous Motors require a magnetic field in the rotor, how can this field be supplied?
Brushed DC excitation
Brushless excitation
Permanent magnet
What is the starting current and torque like on a synchronous motor?
Low starting current per horsepower and high starting torque
What is the speed regulation like on a synchronous motor?
They operate at a constant speed from no load to full load and maintain maximum efficiency
What is a synchronous motor called when it’s operated at no load and used for power factor correction only
Synchronous condenser
What kind of synchronous motors do not require an excitation system for the rotor?
Synchronous motors with Permanent magnet Pole
What is one disadvantage of using a permanent magnet pool synchronous motor?
Cannot alter the power factor
What advantages does a synchronous motor have over a squirrel cage induction motor?
1 can be operated at a leading power factor
2 gives constant speed from no load to full load
3 operate at higher efficiency especially when operating at Unity power factor
What are some disadvantages of a synchronous motor compared to an induction motor?
1 requires DC excitation supplied from a separate source to run
2 may I followed of synchronism or stop rotating if overloaded.
3 small starting torque, synchronous motor needs assistance to get to operating speed
4 doesn’t work well in applications that require frequent starting or Direction changes
What are the rotor and stator like in a synchronous motor construction compared to AC induction motor?
Stator is similar to the stator of an AC rotating field alternator. Stator supports the Armature windings which are connected to the supply voltage.
Where are the amortisseur windings in a synchronous motor located?
Within the rotor
What sort of Supply is applied to the Armature windings of a synchronous motor?
Three-phase AC Supply to produce a rotating magnetic field
Where are the field windings in a synchronous motor?
On the rotor
How many numbers of poles and field windings make up the rotor?
As many as there are poles and Armature windings on the stator
What mechanical connection device allows DC voltage to develop the rotor magnetic fields?
Slip rings and brushes
Why is DC supply to the rotor?
To develop a constant magnetic field that interacts with the rotating magnetic field of the stator
What type of construction are most synchronous motor rotors?
Salient Pole
What does it mean to be doubly excited?
Synchronous motors have to electrical inputs the stator and the rotor
What kind of supply is used for the stator and what kind of supply is used for the rotor?
Stator= 3 phase AC Rotor= DC
When a synchronous motor is a two-pole machine how many pole pairs would you expect to see?
1 pole pair for each phase. (Ie. 6)
What stator connection is most commonly found for synchronous Motors?
Wye connected but they can also be Delta connected
What direction is each phase winding wound on the laminated steel cores?
All in the same direction
How does the magnetic field rotate through the stator?
Each phase is at a maximum 120 electrical degrees apart.
The magnetic field produced by the field coils in one particular phase depends on the magnitude and direction of current through that phase.
From one instant to the next the magnetic field smoothly shift its position and rotates as the current Rises and falls at different times in the windings
When the Wye connected point is the South Pole in a given phase that phase is that a max
** see figure 4 pg. 8 030401e**
What is the speed of the stator’s rotating magnetic field dependant on?
The frequency of the supply voltage. directly proportional
What is the relationship between rotational frequency and the number of magnetic poles produced in the stator?
Inversely proportional to each other. The greater the number of magnetic poles produced in the stator the slower the speed of the stator’s rotating magnetic field
What is the equation for synchronous speed?
N(s) = (120 × f) ÷ p
How do you control the intensity of the magnetic field produced by the rotor field windings?
Vary the field excitation
Why do you start a synchronous motor with little or no load?
So the rotor can build inertia enough to rotate. Too much counter torque wood prevent this
What are the three common methods to start a synchronous motor?
1 use a shaft mounted induction motor
2 use the load from the synchronous motor as a starting motor
3 use an armortisseur on the rotor
What is the purpose of the amortisseur windings?
The voltage induced into these findings from the stators rotating magnetic field produces current in the windings which produces a flux that interacts with the stator flux this produces torque
What’s the risky thing that happens drink start process?
Rotating stator magnetic field cuts the turns of the DC winding which induces High unwanted voltages in the DC field windings. This can damage the winding insulation slip Rings brushes and other components
How do we reduce The Unwanted High voltages across the DC windings during the start process?
Connect a field discharge resistor across the field using two normally closed contacts of a synchronizing switch
What type of switch is used in the field discharge system of a synchronous motor?
Make before break Style
What is the relationship between the supply voltage applied to the stator Armature and the counter electromotive Force induced in the Armature?
Cemf opposes the supply voltage by 180 degrees
True or false Supply phase voltage is fixed
True
True or false cemf generated voltage (E(generated)) is fixed
False it can be varied by changing the field excitation
What is induced voltage or cemf due to?
Field flux
What is the value of current in the stator windings determined by?
The resultant voltage applied to the stator winding impedance
What is the value of impedance?
The phasor some of:
Synchronous reactance
(due to inductance and armature reaction of the coils)
and resistance
(due to the resistance of the copper in the coils)
What is the torque angle?
The angle between the centre of a stator pole and the centre of a corresponding rotor Pole
What affects the torque angle?
Increased load
What does a torque angle on a phasor diagram illustrate?
That generated voltage is not 180 degrees out of phase with phase voltage. When the motor is under load it legs the 180 degree reference line by the torque angle value
What is the angle between the stator and rotor poles called?
Torque angle
What is pull in torque
The torque the rotor must develop to pull into synchronism once the DC windings have been energized. It must be greater than the counter torque of the load
What is pull out torque
The value of the load counter torque required to rip the rotor out of synchronism. The point where the load is sufficient to break the magnetic attraction between the two fields
What degree angle represents pull out torque?
90° for cylindrical Motors
60 to 70 degrees for Salient pole Motors
At what load would you find Unity power factor on the synchronous motor?
Rated load
At what load would you find a legging power factor on a synchronous motor?
Overload
At what load would you find a leading power factor on a synchronous motor?
Light load
At what load would you find no losses on a synchronous motor?
No load
What effect does changing the DC excitation have on torque angle?
Changing DC excitation is mainly to change power factor and its effect on torque angle is generally small enough to be ignored
What is the excitation of a motor when it results in a lagging power factor (Iarm lagging Vphase)?
Under excited
What is the excitation of a motor when it results in a leading power factor (Iarm leading Vphase)?
Over-excited
What is a rotary capacitor?
Another word for synchronous condenser or a synchronous motor operating add a leading power factor for power factor correction purposes only
What two values are compared to produce a v curve?
Armature current and DC field current of a synchronous motor
What loading would cause a deeper Vee curve?
No load
When referencing a v curve as field current increases what happens to the torque angle?
It becomes less lagging and more leading
Why is a full load vee curve more shallow?
Armature current is on the x axis, and more Armature current flows to produce the power to drive the load. (I(arm) values stay higher as field current values increase)
What values does a bell curve compare?
Power factor and field current
At what point on a bell curve does a synchronous motor reach normal excitation?
At the peak or Unity power factor
At what point on a bell curve does a synchronous motor reach normal excitation?
At the peak or Unity power factor
As field current is increased from Unity power factor what happens to the torque angle?
Becomes more leading
Which would take more field current to produce Unity power factor: a motor operating with a full load or with no load?
A motor operating with full load would take Moorefield current to reach Unity power factor
