wilans
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- Asia-Pacific
Any one has experience installing VFD electric motor as driver of reciprocating compressor without flywheel on it
can VFD electric motor drive a reciprocating compressor without flywheel on it
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ptonsparky
Tom
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- EC - retired
By the sound of your question you already know why this would be a problem.
gar
Senior Member
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- Ann Arbor, Michigan
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110130-1332 EST
wilans:
Take a simple one cylinder pump with 100 psi back pressure (pumping into a tank with a pressure of 100 psi) and measure pump shaft torque vs angular displacement. What is your result?
Mechanically what function does a rotating mass perform?
You will find a very high peak torque requirement from the pump. Thus, high peak power requirement. That power comes from the motor and inertia of the system. A rotating mass will decrease in speed as energy is removed from the mass. The kinetic energy of a mass equals M*v^2/2 where M is the mass and v is the instantaneous velocity. The larger the mass at a given speed the greater is the energy stored. If you remove a fixed amount of energy from a moving mass then its speed must change.
A motor itself is a flywheel, but not a lot of mass. To increase this mass one adds an external flywheel. The size of the total mass will determine the speed variation of the input to the pump for a given pump load.
Suppose you drive the pump directly from the motor and the speed variation is 10%, dropping from 1800 rpm to 1620 rpm, assuming none of the energy during the torque pulse was from the motor electrical input. Now increase the inertia with a flywheel by 10 times. Now the speed fluctuation is only 1%.
In reality the motor was supplying a lot of electrical energy during the torque pulse and high forces (torques) were transmitted to the motor from the pump. With an added flywheel, and since the motor is usually coupled with a belt, there will be much less peak torque transmitted to the motor shaft. Rather the motor will have a more uniform torque load vs angular rotation. The motor will add the used energy over a much larger part of the pump cycle and at a lower peak power. This is much better for the motor and the electrical system.
The size of the flywheel will be a compromise between torque variation reduction at the motor, motor startup problems, and flywheel cost.
A little pump on a large motor and maybe no additional flywheel. A large pump on a small motor and a very large flywheel may be needed.
.
wilans:
Take a simple one cylinder pump with 100 psi back pressure (pumping into a tank with a pressure of 100 psi) and measure pump shaft torque vs angular displacement. What is your result?
Mechanically what function does a rotating mass perform?
You will find a very high peak torque requirement from the pump. Thus, high peak power requirement. That power comes from the motor and inertia of the system. A rotating mass will decrease in speed as energy is removed from the mass. The kinetic energy of a mass equals M*v^2/2 where M is the mass and v is the instantaneous velocity. The larger the mass at a given speed the greater is the energy stored. If you remove a fixed amount of energy from a moving mass then its speed must change.
A motor itself is a flywheel, but not a lot of mass. To increase this mass one adds an external flywheel. The size of the total mass will determine the speed variation of the input to the pump for a given pump load.
Suppose you drive the pump directly from the motor and the speed variation is 10%, dropping from 1800 rpm to 1620 rpm, assuming none of the energy during the torque pulse was from the motor electrical input. Now increase the inertia with a flywheel by 10 times. Now the speed fluctuation is only 1%.
In reality the motor was supplying a lot of electrical energy during the torque pulse and high forces (torques) were transmitted to the motor from the pump. With an added flywheel, and since the motor is usually coupled with a belt, there will be much less peak torque transmitted to the motor shaft. Rather the motor will have a more uniform torque load vs angular rotation. The motor will add the used energy over a much larger part of the pump cycle and at a lower peak power. This is much better for the motor and the electrical system.
The size of the flywheel will be a compromise between torque variation reduction at the motor, motor startup problems, and flywheel cost.
A little pump on a large motor and maybe no additional flywheel. A large pump on a small motor and a very large flywheel may be needed.
.
weressl
Esteemed Member
Considered it. The only thing I have done is a 250HP 480V motor with a SSRV on a two cylinder WITH a flywheel. Even with a flywheel it was spikey. The controller was old AB SSRV without integral bypass, so the load was on the SCR's all the time. The SSRV was not oversized for consideration of the pulsating load and rocked like a champ for years, before the plant was shut down. The motor was a different story and the front bearing went out within 18 months. Siemens repaired it under warranty but since then I always specify if it is a BELT driven load and if it is cyclical.
The other thing to consdier that compressors are ROUTINELY driven over the motor nameplate load before the HPSW cuts it off and allows it to cool down. SO you may want to oversize - heavy duty application, if available - the ASD.
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