Synchronous Motor starting sequence 3

[jhonsom]

We have a few brush type synchronous motor of different HP used in our application.
I'm thinking of bringing the motor to close to synchonous speed and then apply the DC field.
Some manuals (GE-SPM) say after it reaches between 1- 10% slip (settings) and it should be closed on positive zero crossing.

But If I wait to let it run till it reaches 3-4% slip and then close the field without looking for rotor angle, it would synchronize anyway, right? the reason I'm thinking is the slow rotor angle change and It slips pole to lock.

But if I want to synchronize at 10% slip, if I close the field at any point, wouldn't it repel and cause surge in the power system?

Do I need to consider breaker close time for higher range of slip%?

At what slip is the motor normally synchonize? we do have some high inertia motor.
Also how long can I let it run as induction motor?

Thanks!
Jhonson

 

[LionelHutz]

Are you trying to make the synchronous controls yourself? There likely are a number of issues you haven't yet considered if you are going to attempt that.

A very smart GE engineer told me to synchronize at 2.4% slip for one project. I have seen a motor require energizing at 7% slip to allow it to actually reach the synchronizing speed. Get it wrong and you can slip poles or cause damage due to the torque transients during synchronizing.

This may seem obvious, but you run it as an induction motor for as little time as possible.

 

[jhonsom]

Lionel,
Thanks for your reply!
Yes we have arrangements to do that.

I understand slip settings are different for different motors, please elaborate me more on the choice of the slip % for a given motor.

Also if we decide to pick positive zero crossing for all slip ranges, wouldn't the rotor angle change depending on breaker close time? did you consider the breaker close time for large slip? Do we need rotor angle compensation?

 

[LionelHutz]

The choice of slip is based on the motor and the load and the desire to balance the accelerating time vs possible synchronizing transients. Typically you try to synchonize at as low a slip as practically possible.

I suppose it could depend on the breaker closing time. I don't use breakers for the DC so I've never had to worry about that one.

 

[jhonsom]

Lionel,

Please let me know how you applied DC without breakers/contacts?

Also if I close DC at positive zero crossing (or)
if it had already locked before I apply the field (reluctance synchronism) 180 degree out of phase then when I apply DC at any rotor angle, I would slip a pole and run in synchronism? In this case It wouldn't see transients which you mentioned?

Reluctance synchronism occur only when lightly loaded right?


Thanks,

 

[LionelHutz]

I use SCR's.

Yes, you run a 50% chance of being 180 degrees out if you reluctance synchronize and there is a transient as the motor slips into sync, but it's usually not too bad since it doesn't require much DC to slip.

 

[jhonsom]

I have some low inetia motor, does it reluctance synchronize on no load? I'm not getting when this reluctance syc occurs...

So for reluctance synchronize: I would first check if set speed is reached and then detect positive zero crossing every cycle, if no zero crossing is detected for a fixed time say 5sec and I would close the breaker anyway, assuming it is already locked. If its 180 out of phase then it would slip a pole and run smoothy there after. Is this the right way to detect reluctance synchronize? or is there any otherway?


Should the load be applied always after field?
I see the SPM say 3sec after appling field.



Thankyou! your help is much appreciated.

 

[waross]

Google polarized field frequency relay.
Its only purpose in life is to safely and dependably connect the field of a synchronous motor at the proper time.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jhonsom]

Please don't discourage me from understanding this. thanks!

 

[waross]

I have found that a description of the operation of the polarized field frequency relay helps to understand some of the issues involved in applying the field to a synchronous motor.
Here's a picture of a polarized field frequency relay.

http://www.ebay.com/itm/EM-ELECTRIC-MACHINERY-POLARIZED-FIELD-FREQUENCY-RELAY-352C596-/300553677178

This link may help.

http://webtools.delmarlearning.com/sample_chapters/140188038X_ch07.pdf

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[gattie]

Well done Waross - Your helpful post reminded me of of this very elegant solution that I used for starting large synchronous motors in the 1960s.
Your help will certainly school jhonsom in the theory of bringing this type of motor on line perfectly.

 

[jhonsom]

"A very smart GE engineer told me to synchronize at 2.4% slip for one project. I have seen a motor require energizing at 7% slip to allow it to actually reach the synchronizing speed"

How to determine the choice of slip setting (range 0.5 to 10%), or can I set all motors high/low inertia to 3% always and wait till it reaches 3%? why would you close at 7% when motor can settle to 3% through it takes more time? what is the reason behind the choice of slip?

If I close at 3% slip and I don't have to look for the positive zero crossing as the slip is very low? and can I close at any instant? I see SPM relay looks for zero crossing but not this polarized freqency relay or Seimens SIPROTECH.

Thanks to all those who replied me !

 

[oldfieldguy]

jhonsom-

The polarized frequency relay does indeed differentiate between positive-going and negative-going crossings. Seeing one in action as the motor nears synchronous speed is both interesting and explanatory.

The polarizing coil of the relay is fed with DC, establishing the desired crossing direction. AT high slip, the operate coil is subjected to the AC wave-form across the field discharge resistor. Since the operate coil is seeing a rapid change in direction due to the AC impressed on it, the armature only hums.

As the rotor speeds up, the impressed frequency on the operate coil drops, and the hum becomes more apparent as a visible vibration. Nearing synchronous speed, the AC frequency on the operate coil drops further, and real movement is seen on the armature. At a small slip value, the AC waveform is on the order of a few hertz. As the waveform goes negative, the armature is repelled, and as it goes positive, the armature is attracted to the 'closed' position. When the slip is low enough, the attraction exists for enough time for the armature to swing in and close the contacts, applying field to the rotor.

It's quite impressive, if you're into that sort of thing. It is, however, a mechanical device, with its own set of problems.

I've replaced them with GE SPM's with good success.

old field guy

 

[LionelHutz]

7% was used because the motor stopped accelerating around 6% slip. Start the motor and watch the slip and you'll know.

The company I work for builds synchronous control so I can't be giving away all the details we've learned over the years.