Synchronous vs Induction on a centrifugal compressor

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Sep. 02, 2024

Synchronous vs Induction on a centrifugal compressor

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Synchronous vs Induction on a centrifugal compressor

Lektrikman

(Electrical)

(OP)

20 Jan 06 13:59

I have a Hp, RPM, synchronous ODP motor driving a centrifugal compressor through a 1:2 speed increaser. This motor because of its location needs to be removed and cleaned approximately every two to three years. The speed increaser has recently developed some strange vibration spectrums on our PDM analyzer and it appears the gearbox is going to need a rebuild. Due to the combined costs of having to rebuild the gearbox as well as the periodic cleaning costs I am contemplating changing the motor to a 2 pole TEFC design and driving it direct to eliminate the speed increaser as well as the need for periodic cleaning.
My question is: How do I calculate any energy savings, if there are any, between my current set up with the synchronous motor and speed increaser and a straight induction motor? There must be some losses in the speed increaser and I'm just not sure whether the synchronous motor is more expensive to operate than an induction motor, especially when you consider the exciter costs. I'd appreciate any thoughts you guys might have, Thanks, Lektrikman

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RE: Synchronous vs Induction on a centrifugal compressor

davidbeach

(Electrical)

20 Jan 06 14:05

A rule of thumb that has been posted several times is that motors with horsepowers greater than speed in rpm are good candidates for synchronous while motors with horsepower less than speed in rpm will likely be induction. So, a hp, rpm motor would most likely be an induction motor. On the other hand, it won't actually be a rpm motor, more like rpm. If you really need rpm, you would need a drive to run the motor at something over 60 Hz or a synchronous motor.

RE: Synchronous vs Induction on a centrifugal compressor

aolalde

(Electrical)

20 Jan 06 15:08

Since the power factor (PF) of synchronous motors can be adjusted to 1.0, the operating efficiency (EFF) can be very high as compared to that of an induction motor. So the daily energy consumption for the induction motor will be higher.
The benefits of an induction motor directly coupled are the initial investment savings and service expenses for the gear box. As Davidbeach said check if your compressor accepts the speed reduction due to the slip in an induction motor.I assume this is a centrifugal compressor since reciprocant type could require an additional flywheel.
May be the overall expense after several (10) years will be similar for both options.

RE: Synchronous vs Induction on a centrifugal compressor

waross

(Electrical)

20 Jan 06 15:36

If you're using a synchronous motor now and change to an induction motor, you may have power factor issues. You may have to include a cpacitor bank with the induction motor. If the synchronous motor is over-excited to improve plant power factor you will need even more capacitors.
Re the speed loss with an induction motor, I believe that the Horspower speed curve for a centrifugal water pump is a third power function. A speed reduction of 2% on a water pump may cause a 6% drop in work done. There are other factors such as change in dynamic head with a change in speed which tends to increase loading etc.
Can any-one out there tell us what the Speed-Horsepower relation-ship is for a centrifugal compressor?

RE: Synchronous vs Induction on a centrifugal compressor

jraef

(Electrical)

20 Jan 06 18:22

So should you switch to induction it may not be just a matter of adding a PFC capacitor bank to this motor in order to maintain efficientcy, you may be required to add capacitors to ALL of the motors in the plant!

The power factor issue is an important one to investigate fully before making a change. Many facilities put synchronous motors on large loads that are required to run continuously, because they use them as a synchronous condenser by runnig it over unity to improve the power factor for the entire factility. In many facilities that I have been in where a lot of compressed air is used, that synchronous condenser is the air compressor.So should you switch to induction it may not be just a matter of adding a PFC capacitor bank to this motor in order to maintain efficientcy, you may be required to add capacitors to ALL of the motors in the plant!

Eng-Tips: Help for your job, not for your homework Read FAQ731-376

RE: Synchronous vs Induction on a centrifugal compressor

motorspert

(Electrical)

31 Jan 06 08:40

A 2 pole synchronous motor at this output will be expensive. I'd seriously consider a 2 pole motor and a VFD. This may give you overall cost savings as you can control the pressure/flow with speed rather than with dampers. The removal of the gearbox will save energy. Modern gearboxes can be as high as 99% efficicient, the old rule of thumb for a gearbox was to use 98.5% as its efficiency. The removal of the gearbox reduces maintenance costs and has less power consumed by oil lubrication system

The VFD is expensive, but can incorporate power factor correction - if that is an issue, there may be other synchronous motor that can take up the burden.

As the correspondents state above, to make a decision you need to determine:
Is the load constant?
Is power factor an issue?

Waross
the speed HP relationship is a cube law for a centrif compressor

RE: Synchronous vs Induction on a centrifugal compressor

oftenlost

(Industrial)

31 Jan 06 19:14

Four pole synchronous motors of recent design are wound on star punchings and tend to cost more to rewind the rotors as the poles are not removable and need to be rewound on an elevated fixture or with a shaft pit. The centrifugal forces are high and there are problems with copper extrusion and blocking. The two pole units are Vvvveeeerry expensive to rewind as they are normally wound on forgings and the manhours are high.
Two pole induction motors are problematic as ball bearings fail frequently, rotors are long with respect to diameter and tend to flex.
All things being equal and ignoring down time, I would go for induction at this HP and speed.

RE: Synchronous vs Induction on a centrifugal compressor

waross

(Electrical)

31 Jan 06 23:25

Hello motorspert
Thanks for the information.
Lektrikman
Comparing a RPM synchronous motor with a RPM induction motor, the slip is 2.8%. When this is cubed, your loss of capacity will be over 8% {1-(^3/^3)}
If you have lots of capacity then there is no problem, but be aware that a small drop in speed is significant.
If you can withstand the capacity loss, you can make up the power factor with capactors. If you don't add capacitors, everything will work fine but you will probably get an expensive penalty on your next power bill.

RE: Synchronous vs Induction on a centrifugal compressor

electricpete

(Electrical)

31 Jan 06 23:33

In the typical induction vs syncronous debate, I like to throw in my favorite overlooked aspect: sync motors are much more complex, more equipment to maintain, more equipment to possibly fail. In contrast the rotor of a squirrel cage motor does it's job reliably forever with no external support (excepting rare rotor bar problems).

My vote on that basis alone is to go induction. Also if you have a chance to get rid of a gearbox by putting in a higher speed motor, another great opportunity to reduce maintenance requirements and improve relaibility.

I'm not sure I fully understood the question and I didn't read the responses.In the typical induction vs syncronous debate, I like to throw in my favorite overlooked aspect: sync motors are much more complex, more equipment to maintain, more equipment to possibly fail. In contrast the rotor of a squirrel cage motor does it's job reliably forever with no external support (excepting rare rotor bar problems).My vote on that basis alone is to go induction. Also if you have a chance to get rid of a gearbox by putting in a higher speed motor, another great opportunity to reduce maintenance requirements and improve relaibility.

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RE: Synchronous vs Induction on a centrifugal compressor

electricpete

(Electrical)

31 Jan 06 23:35

Another aspect of sync motors,they have torque oscillations during startup at a frequency twice slip. That frequency starts very high and ends very low. If there is a torsional resonance anywhere in that range, it will be excited during startup. Even with a brief time, this can be damaging to powertrain components.

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RE: Synchronous vs Induction on a centrifugal compressor

motorspert

(Electrical)

2 Feb 06 07:35

Oftenlost
Because of the high inertia the "star punching" type design would probably not have enough thermal capacity to dissipate the heat during start up and so solid pole rotors are often offered for compressor duties.

electricpete - i agree that the ocilllating torques seen during start-up need addressing - but the existing drive is synchronous, so the mechanical system would be able to withstand the oscillating torques.

There are many pole induction machines in service with sleeve bearings which have no problems with shaft flexing, so this should not be too much of a concern

RE: Synchronous vs Induction on a centrifugal compressor

electricpete

(Electrical)

2 Feb 06 09:13

I am not saying by any means saying we have enough info to draw any conclusions or to say torsional resonances are at fault.. but conversely, we also can't use the existing installation to prove the system is working properly.

With the existing drive, they had to rebuild the gearbox due to "strange vibrations".I am not saying by any means saying we have enough info to draw any conclusions or to say torsional resonances are at fault.. but conversely, we also can't use the existing installation to prove the system is working properly.

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RE: Synchronous vs Induction on a centrifugal compressor

motorspert

(Electrical)

3 Feb 06 07:26

Electricpete - well spotted, if the number of starts is quite high, that could be a reason for the box problems.

RE: Synchronous vs Induction on a centrifugal compressor

mc5w

(Electrical)

4 Feb 06 00:12

If you do decide to go with a variable frequency drive and an induction motor do not forget to attach a universal harmonic filter to the branch circuit for the motor.

With a VFD you could run a 4-pole motor at 122 or 124 Hertz - a 4-pole or 6-pole induction motor usually has optimal design and usually has the best efficiency and mechanical ruggedness. However, a 2-pole motor started across the line has simplicity going for it and if your compressor can tolerated slightly less than 3,600 RPM that would be the best way.

RE: Synchronous vs Induction on a centrifugal compressor

snox135

(Industrial)

6 Feb 06 20:42

Wouldn't removing the 1:2 speed increaser decrease the load on the motor? If this is the case couldn't he use a smaller hp motor? Thus increasing his savings.

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News

Induction vs. Permanent Magnet Motor Efficiency

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Cost
Efficiency -- torque, core losses, frequency & motor speed control
Material opportunities
Application

Permanent Magnet Motor vs. Induction Motor Efficiency

The figure below shows the general layout of both the Permanent Magnet motor (on the left) and the induction motor (on the right). In the permanent magnet design, the rotor contains a series of magnets either internal or external to the OD of the rotor. The stator is wound with copper wire creating a magnetic field that interacts with the rotors permanent magnets resulting in rotation and torque. Compare this to the induction motor where rotor and stator are traditionally stamped lamination steel with the motor windings only on the stator which induces an opposing magnetic field in the rotor. This interaction results in rotational torque.

(Comparison of AC induction motor design vs. permanent magnet motor)

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Permanent Magnet Motor Efficiency

The inherent efficiency of a permanent magnet motor is higher than an induction motor &#; eliminating the intrinsic lag of the applied and induced field. Permanent magnet motor run synchronously with the applied frequency - allowing the motor to operate at a speed set by the frequency drive. As you increase the frequency,&#;total losses in induction motors will be far greater than in permanent magnet motors &#; having efficiencies up to 97.5%.

A 50 kW (about 70 HP) permanent motor typically&#;weighs less than 30 lbs. At any given frequency, the rotational speed of the permanent magnet motor is always greater than that of its induction counterpart due to the inherent slippage necessary in the induction design. The synchronous speed can be represented by the following equation:

Ns = 120 * frequency / pole count

(Ns is synchronous speed. Pole count is the total pole count per phase, including both the north and south poles)

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Induction Motors:

As noted earlier, an induction motor operates by the stator winding inducing an opposing current in the rotor (thus creating a magnetic field). That opposing field results in rotor rotation. The lag between the applied stator current and resultant rotor opposing field results in slippage between the applied field and rotation. The maximum speed of an induction motor is represented by the same equation as for the permanent magnet motor. However, inherent with induction is the requirement for slippage (asynchronous operation). As shown in the figure below, when the amount of slip in an induction motor approaches zero, the torque generated also goes to zero. Thus, it is impossible to operate an induction motor synchronously. For example, a two pole AC induction motor operating at 60 Hz will have a synchronous speed of RPM but there is typically a 5% loss in speed due to the slippage; thus, the maximum motor speed will be about / RPM. This intrinsic design characteristic limits the maximum efficiency of the induction motor to about 90- 93%.

The maximum efficiency of an induction motor is 90 / 93% whereas that of a permanent magnet motor is at 97% plus. Although a 4 to 7% improvement doesn&#;t seem like a lot - imagine the cost of operation over a 10 year or more life span and that relatively small improvement in efficiency results in a considerable energy savings with reduced greenhouse gas emissions.

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Cost Vs. Performance

One major consideration in permanent magnet motors is&#;the cost of the magnets. If you&#;ve used high-energy magnets (such as iron neodymium boron), you&#;ve felt the pain in your budget (or your boss has). The potential waste of stamping the lamination material only compounds the problem.&#;

Opportunities for powder metallurgy are abundant in these types of motors. The rotors of a permanent magnet motor can be made via&#;sintered powder metal, regardless of whether you&#;re taking the internal or external design route. The stator can also be produced via soft magnetic composites. At the&#;high switching frequencies&#;expected, the losses in SMCs are lower than that of laminated 3% silicon iron,&#;further improving the efficiency of this design. Simply put, soft magnetic composites are custom-built for high frequencies.

There&#;s an opportunity for powdered metal to provide additional efficiency to a permanent magnet motor vs. an induction motor. The 3D shape-making capabilities of powder metallurgy&#;allow you to form the stator to totally encase all the wire in soft magnetic composite to eliminate end turn losses.

These are some of the many advantages that powder metal -- both sintered soft magnetic materials and SMCs -- offers.

Induction Vs. Permanent Magnet Motor Efficiency: The Winner Is...

The clear winner here is the permanent magnet motor. Now, couple the permanent magnet motor, with a unique topology enabled by Soft Magnetic Composite (SMC) technology and your motor will be lighter and more efficient, with a higher torque density and lower bill of material cost &#; all while reducing supply chain complications and using a sustainable manufacturing process.

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(Editor's note: This article was originally published in April and was updated on November 29, and July 27, )

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