Applications for Synchronous Motors 1

[rockman7892]

I was wondering if there were certain applications that that were best suited for use with a synchronous motor as opposed to an induction motor. In other words are there certain applications where it may be better to use a synchronous motor over an induction motor?

Most of my experience has been in cement plants and water treatment plants and most if not all of the motors I have come across in these types of plants have been induction motors. I have seen a very small amount of synchronous motors used mostly on very large pumps. Although I don't have much experience in paper mills I have seen several one-lines that show a number of synchronous motors used in these types of plants.

Is there a certain load type that is best suited for use with a synchronous motor or is it a matter of cost with the motor itself or the equipment used to supply the motor? Or perhaps a matter of experience of police personnel? Or is it simply a matter of the capabilities of an induction motor for developing the required torque and Hp for a given load type?

 

[davidbeach]

One rule of thumb I've heard is that synchronous should be used(considered) when the hp exceeds the rpm.

 

[lukin1977]

Synchronous motors are used whenever exact speed must be maintained (like DC generators) or for power factor correction. They are more expensive than other types at the lower ratings but may possibly be more economical for 100 HP and higher ratings

The main advantage is a constant speed characteristic

From: Industrial Electricity and Motor Controls - McGrawHill

 

[rockman7892]

lukin

For "exact speed" I'm assuming that you mean that the synchronous rpm stays the same throughout different Hp loading unlike induction motors that will have an rpm lower than synchronous rpm due to slip as load increased.

So "exact speed" would be for a load that needs to run at synchronous rpm and cannot afford to be run at anything less?

 

[lukin1977]

Yes.
although that characteristic if achievable with a modern closeloop VFD too

 

[electricpete]

A few powerplants use synchronous motors for main circulating water pump application for the reason davidbeach stated. It is a low speed high horsepower application and it was cheaper to use a synchronous motor with field equipment than to use the same rating of induction motor. Personally I think it is a misguided choice since it ignores the increased complexity of sync motor rotor/field circuit which will likely give higher maintenance and lower reliability. In contrast the rotor circuit of induction motor is simple and robust… the few times they degrade electrically, it is usually a slow process which can be detected long before any loss of function.

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(2B)+(2B)' ?

 

[315kVA]

I read in some papers that synchronous motors tend to have a slightly better efficiency than induction motors, and that even a difference of 1-2% can bring significant savings over time when we talk about motors rated 10 MW or more.

 

[Hoxton]

From memory, at the larger ratings, say above 2000 kW, the synchronous motor is more efficient than the equivalent induction motor. So in regions where energy is relatively expensive, then this can give a long term financial benefit.

Power factor (reactive current) can be controlled to help in improving site power factor.

The downside is relatively poor starting torque, and complexity, since there is a need for an excitation controller (what the generator guys call an AVR). The you need CT's and VT's to feed sensing supplies to the controller. You do not need all of that with an induction motor (yes you might need a starter for the induction motor).

There are technology mergers, you can have a synchronous induction motor (a wound rotor induction motor - gets up to speed like a traditional wound rotor induction motor, with a variable resistor across the sliprngs, then when up to speed put dc across the sliprngs to get it into synchronism.

In the UK we saw these driving large ventilation fans at coal mines (what's a coal mine say the children!)

So as so often it is case of the combination of application engineering and economics.

I used to have a good guide to the application of large electrical motors, but lost it years ago. Hope I remembered all the major factors.

 

[waross]

In my world, the overwhelming choice of synchronous motors was for the ability to correct power factor. This is not to rule out other reasons, but it is a very important consideration that has not been mentioned.
Power factor correction has always been important, and now possibly more important than in the past.
Historically when power factor penalties were based on the monthly average of consumption of real power versus reactive power, oversized synchronous motors were a good choice over capacitor banks.
Now, in more and more jurisdictions, power factor penalties are being based on small windows of time, in some cases 15 minute windows. Leading power factor is penalized as well as lagging power factor.
We can no longer use fixed banks that over corrected during the night and banked KVARs to somewhat offset daytime usage of KVARs.
Years ago, power factor correction may consist of a disconnect switch and a bank of capacitors.
Now it is more likely to include a power factor controller and a number of contactors to switch capacity in and out of the circuit as the plant load changes.
An oversized synchronous motor may supply an easily varied reactive current to keep the power factor right on the set point. The saving is not only in capacitor banks but also in the associated contactors.
One synchronous motor is not a good solution.
A better solution is a bank of several synchronous motors running 24/7. If some of the motors must be taken out of service the others will pick up the KVAR load rather than the PF becoming uncorrected as would be the case should only one synchronous motor be used.
I am familiar with the rule of thumb that when the HP exceeds the voltage, consider a higher voltage.
I had not encountered the similar rule for synchronous motor, but it seem reasonable. Thanks for that David.

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

 

[jraef]

One rule of thumb I've heard is that synchronous should be used(considered) when the hp exceeds the rpm.

I Like it, simple and memorable. I'll have to put that one in the bank.

From my observation, it's getting more and more to the point where the only real advantages are the higher efficiency at slow speed operation* and the PF correction capability, i.e. acting as a "Synchronous Condenser" to correct the power factor at a facility that has one or a few large slow speed motor applications and a lot of other smaller induction motors that affect the overall PF.

I did work at a steel forging facility a few years ago where they had 6 separate 600HP air compressors, all of them on 4160V synch motors running at a leading PF to correct for the hundreds of smaller induction motors running all the time. The thing was however, they were not really using all of that air any more; the system had been designed in the 1950s when their production was 10x what it was when I was there. We crunched the numbers and cut back to 3 compressors, only 2 of them running at any given moment with the 3rd in lead-lag rotation. Then we uncoupled the other 3 motors so that if necessary, we could just run them unloaded as synchronous condensers. After 3 years, they had never needed to turn them on and went ahead and sold them off as surplus.

* Efficiency is better COMPARED to a high pole count induction motor. So IF for example you need 10,500 lb.-ft. of torque at a 300RPM shaft speed, you can go with a 600HP 24 pole synchronous motor, a 600 HP 22 pole induction motor and some form of belt or gear reduction to deal with the extra few RPM (if that matters) or a 650HP 4 pole induction motor and a (roughly) 6:1 gearbox (650HP to cover the losses in the gearbox). Of those 3 options, the synch motor will be the most efficient. But if you need the shaft speed to be around what a 4 pole motor will give you, a modern 4 pole induction motor will outperform a slower synch motor and step-up gear or belt drive system from a line-to-load efficiency standpoint.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[electricpete]

Google shows a link to ABB Tech Note on selecting sync vs induction
The same type things that have been discussed in this thread

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(2B)+(2B)' ?

 

[rockman7892]

Thanks for all the good feedback.

In the case of these motors being used for Power Factor Correction is the facility power factor measured at a certain point with that information value being used in a DCS or SCADA control to adjust the synchronous motors fields for the various motors to maintain a specified power factor? I assume this would be some sort of closed loop control to control the various sync motors in order to control power factor.

 

[waross]

You can't change the power factor of a motor. You can supply the VARs from another source than the grid so that the power factor that the plant presents to the grid is improved.
Generally the PF is measured at the PCC, or point of common coupling, or the service entrance.
Similar to a capacitor based power factor controller, the power factor control adjusts the excitation so that the VARs produced by the synchronous motor raise the PF seen by the grid to the desired value.

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

 

[MatthewDB]

What do you mean by "you can't change the power factor of a motor"?

When a syncronous motor is neither producing nor consuming VARs, it has a PF of one. When excitation is increased, the PF if the synchronous motor drops in value.

 

[lukin1977]

I think he was referring to induction motors in this case

 

[cranky108]

I think the key here is that Highest PF that can be reached is 1. If you increase the excitation the PF does drop, but the vars are leading vars, vs lagging vars from induction motors.

Induction motors must intake vars to drive the magnetic field.

synchronous motors can either intake or putout vars.

Another important factor is that synchronous motors putting out magnetizing vars also support the voltage to reduce voltage dips. Capacitors can also put out magnetizing vars, but can make a voltage dip last deeper.

 

[waross]

Yes, I misread a previous post. I was referring to induction motors.

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

 

[jraef]

In the case of these motors being used for Power Factor Correction is the facility power factor measured at a certain point with that information value being used in a DCS or SCADA control to adjust the synchronous motors fields for the various motors to maintain a specified power factor? I assume this would be some sort of closed loop control to control the various sync motors in order to control power factor.

Yes, it's typically measured at the PCC, but many Synch Motor Potection and Control modules now include the PF control as a feature. So for example if you use something like a Multilin SPM unit and a synchronous motor controller with an excitation supply unit, you can have the closed loop system right there, no SCADA or DCS necessary.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington