synchronous motor started components 1

[freetown]

Hi all,

I'd like to know few things on how large synchronous motor works. I saw one 11kV 2.8MW with components such : Power source (from MV contactor), large inductor (for starting) and motor excitation system

- From what i understand the large inductor is used as current filter, can anybody correct me and why we have to use it, i mean what will be the impact for starting motor without this large inductor?
- The excitation system is used to produce the required magnetic flux but at the same time the stator produce a magnetic field, why we need excitation when the magnetic field is already produced by stator?
- The excitation system produces AC voltage which is convert to DC by a rectifier mounted on rotor, is this correct
- We say that synchronous motor can improve power factor of the plant, how this is done?

Thanks in advance..

 

[waross]

A series impedance (In this case an inductor) may be used toreduce the starting current.
The field of a synchronous motor "locks on" to the grid frequency and the motor runs at an exact speed dependent on the frequency and the number of poles.
An induction motor depends on a "slip" between the synchronous speed and the actual speed to induce the magnetic flux in the rotor. The speed of an induction (correction) motor varies slightly as the load varies.
The power factor of a plant may be improved by adding leading VARs to the system.
This may be done by static capacitors or by a synchronous condenser.
A dedicated synchronous condenser is basically a synchronous motor with no output shaft nor mechanical load.
When a synchronous motor is over-excited it will add leading VARs to the system.
A synchronous motor may be used as a synchronous condenser or as a combination motor and synchronous condenser.
When a machine is used as a combination motor and synchronous condenser, the capacity as a motor is reduced.

 

[freetown]

A series impedance (In this case an inductor) may be used toreduce the starting current.
The field of a synchronous motor "locks on" to the grid frequency and the motor runs at an exact speed dependent on the frequency and the number of poles.
An induction motor depends on a "slip" between the synchronous speed and the actual speed to induce the magnetic flux in the rotor. The speed of a synchronous motor varies slightly as the load varies.
The power factor of a plant may be improved by adding leading VARs to the system.
This may be done by static capacitors or by a synchronous condenser.
A dedicated synchronous condenser is basically a synchronous motor with no output shaft nor mechanical load.
When a synchronous motor is over-excited it will add leading VARs to the system.
A synchronous motor may be used as a synchronous condenser or as a combination motor and synchronous condenser.
When a machine is used as a combination motor and synchronous condenser, the capacity as a motor is reduced.

I understand inductor in my case is used to limit (can we say reduce or limit?) inrush current for few seconds and also acting as an input filter? Today for synchronous motor, is there limiter other than inductor using in industry?

For the rest of my questions i didn't really understand the answer.

 

[waross]

Filter?Probably not.
Reduced current starting methods for synchronous and/or induction motors):
Series impedance. Resistance or inductance.
Shunt capacitors. (Theoretically possible but not used in practice.)
Star-delta switching.
Auto-transformer.
Soft start.
Variable Frequency drive.
A 4 pole synchronous motor will run at 1800 RPM on 60 Hz.
A 4 pole induction tor will run at less than 1800 RPM. How much less? It depends on the design and the load.
Some common full load speeds are 1760 RPM and 1740 RPM. 1800 minus the speed is called the "Slip".eg 1800 RPM minus 1760 RPM = 40 RPM slip.
The percentage of load times the slip at rated speed gives the actual slip. eg:50% load times rated slip, 40 RPM slip at full load = 20 RPM slip. So, speed at 50% load will be 1800 minus 20 RPM = 1780 RPM at 50% load.
At no load there is enough slip to support windage and bearing friction losses.
Do some independent study on Power Factor, KVA, KVARs, and kW.
Google is your friend.

 

[Gr8blu]

@freetown
1) The inductor adds impedance to the circuit. By design, it also "slows down" the rate of rise of current. Taken together, this means the inductive element acts to reduce the starting current during an on-line start attempt. This can be for several reasons, but the most common is to increase voltage stability on the bus to which the motor is connected - in this case, 11 kV.
2) A squirrel cage induction machine has one power source - current to the stator winding. This means that speed and load (current) are related in some predictable fashion - which is to say the rotor slows down as more (electrical) load is applied. For some processes, this is okay - for others, not so much. So machines with a separate power source for the rotor are used (wound rotor induction, synchronous, direct current, etc.). Having a separate source for the rotor field strength makes the speed and power not so closely tied together - allowing "fixed" speed operation independent of electrical loading.
3) The excitation system may be designed for either fixed or variable speed operation. In the fixed speed case, the usual "input" is DC current to the exciter stator which has fixed field poles (like a synchronous rotor). The exciter rotor is a wound-rotor induction design, which means its output is 3-phase AC. This gets rectified to produce a DC current and voltage for the main synchronous winding. In the variable speed case, the exciter has an AC stator winding as well as the wound-rotor construction. This means it is fed from a 3-phase AC source, not DC.
4) Non-linear loads such as induction motors, electronics, some lighting, etc. all cause the voltage and current waveforms to vary from each other. The difference has to come from somewhere - which is where synchronous machines (and to some extent static synchronous compensators) come in. By increasing the power to the synchronous rotor field, the machine becomes "over excited", which allows it to "produce" some of this compensating reactive power. Thus operating a synchronous machine at a non-unity power factor helps balance plant electrical loading, reducing the requirement to obtain "non useful" power from the utility.

 

[freetown]

@Gr8blu

Thank you for taking time to give all these explanations.

Is having an under or over excitation has an impact on how power factor is improved by synchronous motor?

 

[Gr8blu]

@freetown
"Under excited" means the synchronous machine has a relatively low rotor current and is operating like a squirrel cage induction machine - it requires more reactive power than it produces. This shows up as a "lagging" power factor when talking about motors (i.e. current waveform "lags" the voltage).
"Over excited" means the synchronous machine has a relatively high rotor current. Because of this, it produces more reactive power than it uses. This shows up as a "leading" power factor when talking about motors (i.e. current waveform "leads" the voltage).

If the machine power factor is leading, it is helping the overall facility power factor. if it is unity (PF = 1.0), it does nothing. If it is lagging, it is hurting the overall facility PF.

Generally, human-machine interfaces are programmed to show power factor as POSITIVE when leading in a motoring application.

 

[jraef]

One thing left out of the above is that the synchronous motor what’s called sn “amortisseur” winding that mainly functions as a dampener against load torque fluctuations, but ALSO allows the synch motor to act as an induction motor to start from a stop. A synch motor cannot self start with sufficient torque to accelerate, so the amortisseur winding does that until it gets to 80-90% speed and the field can be used to “pull in” the rotor to synchronous rotation with the stator. So as an induction machine, it will have the same characteristics of high starting current unless that is controlled externally. In your case, that was likely what would be called a “primary reactor starter”. It could also have been an “auto transformer” type starter arrangement.