Differences in Synchronous Motor Control

[rockman7892]

Recently I have come across two different types of synchronous motor control applications.

The first application is simply a case where the field is applied as soon as the breaker/contactor is closed to energize the motor. This DC field is simply adjusted with a rheostat or variable transformer in order to manually control the magnitude of the field current. Most of the time this rheostat is manually adjusted for a specific field current level and is left there.

In this first application these motors were brushless motors manufactured by Electric Machinery Company. These motors had built-in automatic static excitation and filed control system in the form of a controller located in the rotating exciter that detected when the motor was up to speed in order to apply the field to the for synchronization.

In the second application the motor used a digital excitation control system (Basler DECS-250) in order to control the motor excitation. This same motor also used a multilin SPM sync motor protection and control relay (I'm not sure why this was needed. Somebody mentioned for field application).

This second application seems like a much more advanced control method for synchronous motors. So has me questioning why one of these control methods would be used as opposed to the other? Is the first application simply a result of an older motor technology? Is this an economics issue?

With the second application you can continually vary the field and thus control the power factor of the motor where as in the first application the field is set and the power factor remains fixed in terms of the field (only dictated by load current). Are there other major advantages/disadvantages between these two control methods?

 

[waross]

The evolution of technology.
Originally the application of the field was more or less random and problematic.
It was not unusual for a motor to be damaged by the application of the field.
Then The Electric Machinery Company patented and marketed the Polarized Field Frequency Relay.
This relay detected two conditions;
First that the field frequency had dropped enough to indicate that the motor was close enough to synchronous speed that the field could be safely applied without pole slip.
Second that the polarity relationship between the field voltage and the back EMF was suitable for the safe application of the field.
The field was energized through slip rings and typically controlled by a rheostat.
Then brushless excitation was developed. The field was applied by a rotating solid state circuit and the field strength was still adjusted by a rheostat controlling the field of the brushless exciter.
Back in those days, revenue metering was typically done with electro-mechanical KiloWatt Hour Meters.
Power factor penalties were based on the monthly KWHrs/KVARHrs.
Bulk power factor correction was common.
Many plants had determined how much reactive power was needed from the synchromous motor to reduce the power factor penalties to an acceptable level and that is where the excitation strength was set and left.
I worked in a plant where the desired field current was marked on the face of the field current meter with a felt marker pen.
In the morning an electrician would start the motor and adjust the current to the felt pen mark.
As the motor warmed up, the field resistance increased and the current dropped a little. About 30 minutes later an electrician would re-adjust the current back up to the mark and there it would stay for the rest of the day.
PS: This was a slip ring motor.
As the technologies developed, electronic meters that were capable of penalizing lagging power factors in real time were deployed and matching tariffs were implemented.
Bulk correction of the power factor was no longer the most economical solution to power factor corrwction.
Power factor controllers started to be installed to correct the power factor in real time to minimize penalties.
You are looking through windows at two different time periods in the evolution of the control of synchronous motors.
EMC used to publish a control magazine.
One issue described the Field Frequency Relay.
If you can find any historical information on the EMC website please share it with us.
Thanks

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

 

[rockman7892]

Waross

Thanks for the great explanation.

I guess the thing that was odd to me was the fact that we are replacing a customers 12.47kV switchgear which feeds two 5000hp sync motors of the application 1 type above. We are replacing this switchgear with the same rheostat based excitation control. I would have figured that with replacing the switchgear and controls that these would be upgraded to more modern synchronous controls but perhaps this is a function of what resides in the existing motor and not so much the sync controls. Perhaps if you upgrade the controls you would also need to replace these motors which may be a costly venture?

I was able to get some EMC instruction manuals for the polarized field relay and some other motor components from the customer that I can share if interested.

 

[waross]

Think of several generators in parallel, or feeding a grid.
One will be a swing set and the others will be dispatched and run on fixed base load.
With more than one synchronous motor running, one may be have its excitation controlled by a power factor controller.
The other sets may be run with fixed excitation.
Two power factor controllers on one system may have a tendency to fight each other.

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

 

[roydm]

The Railway workshop where I served my apprenticeship in 1962 had a lot of WWII era machinery with big old open frame motors. One of the switch rooms used to run at a power factor of 0.3 if you can believe that.
We had a 100 HP synchronous motor running no load to improve the overall power factor, occasionally they would couple it up to a compressor by adding a flat belt.
There was hell to pay when one of the relief fitters figured there was no need to start the motor and the next bill came in.

 

[LionelHutz]

In your first case. With all the controls on the rotor, the motor really only needs DC applied. It could be possible the rotor mounted controls have field current regulation as well, but I doubt it. I would have expected you to be using some form of current regulated DC supply for the upgrade as a minimum. Even better controls might have regulated the DC to control the power factor of the motor.

In your second case. The motor has no controls built in so you have to supply the controls which apply the field at the correct time and also regulate the field current. The Basler relay is mostly marketed as a generator control and likely was missing some feature desired for motor control so the SPM was added to adapt it to motor control. The SPM only does control and is pretty dated these days. Heck, it was dated 20 years ago when I first saw it and it doesn't appear it's had any kind of significant upgrade.

It's a big door mounted variac you're installing, right?

 

[rockman7892]

LionelHutz

Yes new synchronous controller will have a variac mounted on door. Essentially a variable transformer to vary the ac voltage/current prior to conversion to DC that is sent to the exciter.

So it sounds like the Basler relay or similar is used simply to control field application as well as control the power factor? Do manufacturers still make new motors with the controls located in the rotor so that external relays are not needed, or is this more a thing of the past? So it sounds like weather or not the motor has a controller in the rotor will dictate weather or not any external sync control relay is needed?

Can motors with the controllers imbedded in the rotor be retrofitted or have the controller bypassed if it is desired to use an external relay in order to control power factor?

From what Waross mentioned it may be difficult to control the power factor for various motors at once, so is it common to see only one motor in a system have a relay to control power factor with the others having something similar to a rotor controller where you can "set it and forget it"

Also I guess with the embedded rotor controller you can set it to achieve the desired power factor in the plant if you know what that setting is however this will vary based on plant loading conditions and cannot be automatically adjusted. I guess that's where the external relay comes into play.

 

[waross]

There are two factors to consider:
1 Application of the field.
2 Control of the power factor.

1. As the motor approaches synchronous speed The frequency of the voltage induced in the field winding drops. Just low frequency is not enough to ensure that the motor will pull into sync smoothly. The polarity must also be correct. If the rotor field is energized during the wrong half cycle the resulting field will buck instead of pulling in smoothly. With the wrong polarity the rotor will be out of position by 180 electrical degrees. This will be a condition similar to pole slip and damage may result.
With a slip ring motor the Polarized Field Frequency Relay was typically mounted in the control cabinet.
With a brushless exciter, the exciter generates AC which is rectified by the rotating diodes to energize the field. Neither the frequency nor the polarity information will be reflected back past the rotating diodes.
In order to use a brushless exciter with a synchronous motor a rotating solid state circuit that performs the functions of the Polarized Field Frequency Relay must be used. This circuit must be connected between the rotating diodes and the field and so must be mounted on and rotate with the rotor.

2. The power factor may be varied by varying the voltage applied either to the slip rings or to the field of the brushless exciter.
It is not advisable to use more than one power factor controller on a system but that does not mean that that it is not possible to control more than one motor with one controller.
I am sure that there are sophisticated controllers available that are able to control more than one motor.
It may be possible to "home brew" a suitable controller by using a power factor controller designed to switch capacitors online in steps. The steps could be used to increase the excitation voltage on more than one motor.
I am not suggesting that this be done.
I am illustrating a possible method of controlling more than one motor.
It may be pointless to develop this thought any further without specific information as to the actual arrangement of equipment and knowledge of the load cycles.
I understand that Rockman's plant is working well with a controller on one motor.

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

 

[LionelHutz]

I'd say the Basler was used as a power rectifier with a 4-20mA input signal to produce the regulated DC. The SPM was likely used to provide sequencing and control the power factor.

I have never seen a brushless motor with power factor control on the rotor. I've been told they exist, but I doubt it since the rotor circuit can't have any knowledge of the motor power factor. The only way it could be done would be by wirelessly transmitting a power factor signal to the rotor controller via some outside metering controller.

You really didn't post anything about what power factor was controlled. The simplest power factor control setup would control the power factor of the motor, and the SPM can do that. You can individually control the power factor of as many individual motors as you want. It's only difficult if you try to directly control the power factor of the plant using multiple motors.

I have seen a brushless motor that used 3-phase AC fed to the exciter. The purpose was to have no brushes but still be able to supply and control the field current at any motor speed meaning you can use a VFD on the motor. We supplied a controller for a very big GE motor and drive system under development. It was for ship propulsion. I would get a call with questions about it every once in a while but nothing more ever came of it. I suspect either the project died or they eventually built their on 3-phase AC controller for the field once the motor and stator side drive development was completed.

In order to use a brushless exciter with a synchronous motor a rotating solid state circuit that performs the functions of the Polarized Field Frequency Relay must be used. This circuit must be connected between the rotating diodes and the field and so must be mounted on and rotate with the rotor.

Almost every brushless motor that I have come across didn't has any kind of field application sequencing control on the rotor. Of the dozens I have been involved with, I can only recall one motor that could have the DC applied immediately. All the rest had rather dumb exciters that were basically just a 3-phase winding feeding a rectifier connected directly to the field winding. The only electronics was a resistor (sometimes) and a SCR circuit across the field. This circuit just served to ensure the field winding was shorted during the start since an open field winding produces a very high AC voltage as the motor starts. With this exciter, it is necessary to delay the application of the DC to the exciter until after the motor had reached synchronizing speed. Then, the DC just starts to flow regardless of the state of the AC or position of the poles.

 

[crshears]

Just off the top of my head, if electronic governors can be and are used to balance the real power outputs of multiple generating units so as to achieve automatic load sharing between the units, one would think it possible to use a similar concept to control the excitation of multiple motors in the same way so as to achieve balanced reactive sharing between synchronous machines without the "duelling controllers" issue...not that I've seen it done, mind you, but provided cost/benefit analysis suggests it's warranted, such a thing should be doable, no?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

Hi CR;
I think that it is electronic load controllers that control and share loading between multiple sets. Each set may have its own electronic governor. The electronic governors will be controlled by the load controller.
But I agree. I am sure that there are power factor controllers that may control more than one motor.

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