With the field off, you may be able to start it as an induction motor with the damper windings acting as the rotor bars, or you may need to use an induction motor to spin it up to near synchronous speed.
Maybe if you state your concerns and application we could help more. Are you worried about too much starting torque, too little starting torque, voltage drop, motor damage, timing of the Field Excitation etc. etc.
Generally, a synch motor will have a separate set of internal windings used to accelerate it to full speed, called the amortisseur windings, which as essentially the same as an induction motor. Typically all you need do is apply power to the stator leads, no separate switching is needed. While accelerating, the rotor needs to be connected to a field discharge resistor because it will generate high currents that need to go somewhere. When you reach the "pull-in speed" of the rotor, then discharge resistor is removed and the DC is applied to the rotor field. Issues of starting torque and voltage drop can be addressed in how you apply power to the amortissuer winding, i.e. Across-the-Line, Reactor starter or solid state soft starter.
Here is an older thread that provides some additional information. thread237-85767
Eng-Tips: Help for your job, not for your homework Read faq731-376
First one is starting motor with discharge resistor (rotor) and make the conexion of the DC field through the SPM (Synchronous protection control/ GE). Due the starting is on no load, I dont know if I can conect the DC field excitation, supresing the resistors when the slip frecuency approaches synchronous 95/98% of rated speed, without the need of install the SPM.
Second one is starting the motor increasing the frecuency (stator) from 0 HZ to rated speed through a drive of low voltage and transformer low/hight voltage. In this case, the issue is if I need to increase the DC esciter current from the begining proportionally to the increase of frecuency or just need to fix a level of current.
Hi Bedoya.
I understand that the GE SPM Controller includes a motor start-up feature. Is this a collector rings type motor or brushless?
You should have adequate contactors for the field and discharge resistor controlled by SPM to get a closed transition on the field application.
Remember that high voltage is induced in the rotor field winding when the slip between the rotor and the stator magnetic rotating field is high (accelerating condition). The field must be applied when the rotor is very close to synchronous speed and then the field discharge resistances disconnected (closed transition).
I recommend full understanding of the GE SPM instruction book before attempting any further step.
Thanks aokalde, Im going to use the discharge resistor method. Now Im looking for the way to select the dischage resitor.
I have read that the value of the discharge resitor must be around 6-10 times the field resistance. Higer the discharge resistor higer is the induced voltage on the exciter and higer the torque at the last step of the synchronizing. Although I have found in some papers this ratio (6-10 times the field resistance), nothing is said about the way to calculate it. If anyone knows the way to get it, It would be right to know the value of resistor to limit the induced field voltage during starting.
You would take field voltage divided by field current times 6 then to calculate the resistance.
I typically use 2-4 times the field resistance. I try to limit the voltage to < 1000V when using a solid-state field bridge.
As for starting, you want to get the motor up to speed and then synchronize it asap. A synchronous motor can draw 2-4 times the FLA until it is synchronized. This high current will continue even when it gets to full speed until the DC is applied. I've personally witnessed a synchronous motor still draw approximately 340% current when at synchronizing speed before the DC was applied. So, it's not really in the motors best interest to use a timer and let it just run at full speed until the timer times out. The best way to perform synchronization is using frequency detection on the field/resistor current. Synchronize as the frequency goes below your desired synchronizing speed, probably a few hz.
Now, if this is a brushless motor then you just connect line power and let it go. Use a timer to apply the DC. That's about all there is to those motors.
I would recommend keeping the reistance as low as possible, I am guessing that as it has a DC exciter we are talking about an old motor. So really the induced volts should be as low as possible. In fact many modern machines don't use a starting resistor at all, just feed into a short circuit. With a slow speed machine there is more than enough torque. Worth a try maybe
The Brushless Synchronous Motor Control is not for beginners!
Here are a few pointers, though:
In order to properly size the field discharge resistor (FDR) you need to know the induced field current from the motor manufacturer. This value used to be given at 100%, 50%, 25%, and 5% slip. As the name says, this is the current induced in the field during start. You can think of the field as a current source, and never, NEVER, ever, open this circuit when the motor is running (even if the motor itself has an internal spark gap). If you don't have that value, it's best to account for both the voltage in the field circuit and, believe it or not, the inertia of the driven train (wk**2 of the shaft, the coupling, and the load). The higher the value of the FDR, the more torque the motor delivers. If the train design is correct, the delta contribution from the FDR should only affect starting time, not the ability to accelerate. On the other hand, a lower FDR value limits the voltage in the field circuit to acceptable insulation withstand levels. Through experience, you'll find that it's best to stay around 2P.U. of the field resistance. And, in agreement with a post above, keeping the voltage low is best for older equipment.
On to the controls: if you're not familiar with this, I suggest that you download from GE the uSPM (Micro SPM) instructions book, print it, and read it over and over and pay attention to the electrical schematics, and pay extra attention to the Field Application contactor make before break design, and the required surge suppression using either selenium (sp?) type rectifiers, or high Joule, 1000V MOV's.
My personal opinion is that because synchronous motors are typically on critical loads and are very expensive to repair, they should be started with soft starters. Other choices for MV soft starters that offer synchronous motor control packages:
(copy and paste this entire URL line, the $ messes up the TGML link capability here). I believe Schneider also uses the Basler synch control unit mentioned above.
Toshiba JK Series [link ftp://ftp.tic.toshiba.com/pub/_controls/bro_jksss2005.pdf] Tosh JKSSS link [/url], who I believe also use the Basler and maybe the GE SPM.
And of course, GE Limitamp who would use the SPM you already mentioned as the synch motor control unit GE Limitamp RVSS link
Personally, I would steer clear of the Kinetics synch controller product. I have had a lot of reliability trouble with them and very poor support.
Eng-Tips: Help for your job, not for your homework Read faq731-376
Below is a snippet of a Rockwell white paper outline some of the concerns when applying any soft-starter to Synchronous motors.
" Applying soft starter technology to any synchronous machine does require a closer look at the overall impact on the motor and driven load.
The key issue, as in any reduce voltage starting method, is will there be enough starting torque generated by the motor to accelerate the load safely without motor damage with reduced voltage. Motor torque is dependant on the voltage applied and the type of starting device. The motor speed/torque and speed/current profiles will determine the level of current drawn (affecting voltage drop on the system feeding the motor). The amount of voltage drop will in turn affect motor flux and the amount of torque produced by the motor.
Synchronous motors have lower starting torque, at full voltage, compared to an equivalent rated asynchronous induction motor. A majority of synchronous machines are started uncoupled from the load because of this torque/speed concern. Every electrical motor must be matched, both electrically and mechanically, to the power supply, the equipment driven, the protective and control systems and similar constraints. In most retrofit or upgrade cases, the existing motors were usually designed for full voltage starting. This fact has to be considered if a user is considering a retrofit or upgrade to any reduced voltage starting method.
For fixed speed applications synchronous machines are normally started at reduced load, typically 30 percent of the rated nominal torque. When the near nominal speed is reached and the motor rotor field is energized, the nominal load torque is applied (after which valves are then opened or clutches are applied).
Motor starting torque is always dependant on the voltage applied to the stator and thus too the method of applying the voltage at the time of starting. The only way to validate the minimum level of reduced voltage to safely accelerate the motor and a load requires the use of the data from the original motor torque/ speed curves and load torque/speed curves.
The flexibility in parameter settings within our soft starters can provide easy field changes to accommodate custom start profiles allowing the user to modify the start profiles to safely accelerate the motor and load. There will always be the balance between how much voltage is required to safely accelerate the motor and load while still achieving the benefits of soft starting to the overall mechanical and electrical systems."
An additional item would be to investigate how the synchronous protection relay/system measures the induced current frequency across the discharge resistor as solid-start soft-starting can affect the level and frequency content. Some vendor have offered solutions that ignore this direct feedback for the use of a timer to determine field application.
PS I'm sorry Jraef feels I was trying to selling something as I don't hide the fact that I work for Rockwell.
Did you ever get your motor starting means sorted out?
Jraef;
Good list.
The Motortronics, Toshiba, GE and Siemens soft-starters all appear to use the same soft-starter control circuitry. Brochures from these companies all show the same display interface. I believe it's Motortronics components but correct me if I'm wrong.
All of the companies you listed are putting together equipment from 2 different manufacturers to control a synchronous motor. Kinetics push the "one-manufacturer builds all" but they only do the field control so they too have to be combined with another manufacturers starter to operate the motor.
Have you seen a SquareD MV soft-starter? I don't know of any in existance yet.
MVRockwell;
Does Rockwell build their own relay and DC field bridge for the field of a sychronous motor or do they rely on some 3rd party equipment? Does the unit use solid-state control or contactors? Just curious because I've never found any real info on what you guys use for the field control.
We designed and build our own synchronous control (based on PLC hardware) and DC bridges, upto 11kw (single phase) and 65kW (3 phase), with solid-state discharge resistor switch.
A couple of points
1> THE AMORTISSUER WINDING is in the rotor and its purpose is to restrain the rotor from "Hunting" rotationally. It is very similar to the squirrel cage winding in a large induction motor and acts as a squirrel cage winding. The starting is similar to starting a squirrel cage induction motor and reduced current starting methods can be used.
There is one caveat, the amortissuer winding is suitable for starting duty only and in some motors may over heat if the motor is run for too long as an induction motor. Particularly in the event of field failure.
FIELD DISCHARGE RESISTOR.
Don't forget that the inductive kick when the field is disconnected can be a much higher voltage than the induced voltages. The voltage induced when the field is De-energised will be the sum of the field discharge resistor resistance and the field resistance in Ohms multiplied by the field current in amps. (V=IR)
APPLYING THE FIELD;
Consider a field frequency relay. The rotor position relative to the rotating field is important. Damage can be done if the field is applied with the rotor poles in the wrong physical relation to the rotating field.
STARTING.
Years ago I had to start a synchronous motor every morning. The procedure was;
Close a power circuit breaker and wait.
The motor accellerated as an induction motor.
When it was close to synchronous speed, the field frequency relay applied the field at the correct time and the motor would lock in to sync.
We would look at the field current meter and adjust as required and that was it.
The basic requirements to start a synchronous motor;
A contactor
A field frequency relay,
A source of DC for the field. (Rotating or static.)
Add whatever protection you want, but don't forget "Field Failure."
Answering your original question of 'the best way'...
Based on hands-on, and having designed and commisionned units up to 9,000HP, with very severe applications such as ball mills, car shredders, MG sets for submarine testing, hammer mills, wood chippers, mainshaft drives, etc, etc... my answer is FVNR!
The amortisseur windings are designed to handle the starting of the motor, period. No matter what means you use, they will have endured the same amount of heating due to i**2t. Why? because one acceleration from stand-still to full speed requires the same amount of energy to mechanically bring the motor up to speed. You cannot abuse them though with repeated starts -just like any induction machine.
The second part is field application. Definitely no timers! you would run the risk 50-50 of slipping a pole during synchronization. Slipping a pole is not terribly terrible, but it exerts undue stress on the field and the amortisseur windings, so it's best avoided.
Basically, the optimum field application is when slip is about 5% and the induced field current is at zero crossing going from negative to positive (there is a 90 degree lag between the induced field current and the magnetic field. So at that zero crossing, the magnetic field is at maximum. If you slip a pole you would be forcing the magnetic field to change directions instantly).
Hardware is very simple: either a GE uSPM, or lot more inexpensively, GE field application relays usually called FAR and FRX (I don't work for GE, but, among many of these, not one single failure).
I believe Toshiba still makes these (I don't work for them either!)
Retraction.
swgrmfg
I stand corrected. I rechecked my old books and realised that it is on Alternators that amortisseur windings reduce hunting. On synchronous motors hunting is not an issue.
I concur with your post.
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