super synchronous operation of an induction motor
Posted 21 October 2004 - 02:17 AM
Sunil Athavale (firstname.lastname@example.org)
Posted 21 October 2004 - 04:27 AM
There are a number of issues that must be addressed , but let me start off by stating that this is a very common and successful application of VFds, so don't let the details stop you from persuing it. Just be thorough in your approach.
1) Make sure the motor is designed to handle the higher speed mechanically. For instance if you have a 1500RPM motor, it is somewhat safe to assume that the motor manufacturer used bearings capable of operating at 3000RPM, just because he didn't want to inventory 2 different bearings. But if your motor is already 3000RPM, it is NOT safe to assume you can run it much higher than that. Best bet is to ask the motor manufacturer directly. If you have not already purchased the motor, be clear about what you will be doing when you go out for tender.
2) The same goes for all of the components connected to the motor. Cuplings, shafts, bearings, mounts all need to account for the higher speed and greater potential kinetic energy in the spinning load.
3) Safety systems. In some cases, safety systems are designed around a particular operating speed and the physics involved in that. At higher speeds, some things that were considered minor can become major concerns.
4) Understand that your motor will be operating in a constnat HP mode once you are above base frequency, so as you increase speed you will decrease torque. This may be a problem in some applications.
There are more and maybe someone else can take the time to add to this list.
Posted 21 October 2004 - 06:00 PM
I would just like to reinforce the effect of overspeeding the motor on the characteristics.
The important points to consider are:
What does the load require as it is oversped?
What can the motor deliver when it is oversped?
Loads such as pumps and fans have a rapicly increasing load with speed. A small increase in load will yield a large increase in torque/KW required. Other loads will have a small increase in load. You need to study the power/speed curve of the driven load to determine the size of motor and inverter required.
A common error is to underestimate the power required for a speed increase.
Motors operating from inverters behave as "constant power" above the rated speed of the motor. This is because the output voltage of the inverter is limited to the line voltage of the supply. As the frequency is increased, the voltage can not be increased and so the flux is reduced. This results in reducing torque as the speed is increased.
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