Synchronous Motor controlled by a IGBT NPC Voltage Source Drive
Synchronous Motor controlled by a IGBT NPC Voltage Source Drive
(OP)
Hello everyone!
I am currently writting a technical proposal for a Project that I am trying to get Capital funding for. The proposal was reviewed in house and there was a couple of good comments that came from the review. Let's state a brief description on the project... We currently have an old Load Cummutative Inverter Drive Controlling a 1500HP Induced Draft Fan synchronous motor 2000V. This is 1980 technology and a retrofit of the controls or a drive replacement needs to be accomplished due to the fact that the Power Boiler can not run without these fans, thus the mill goes down. I have read from GE technical brochures that the LCI drive has poor PF at low motor speeds. The drive they are proposing to sell is a MV IGBT NPC Voltage Source Drive which they say has high true PF over all speeds. Here comes my questions... 1. Can you produce VARs when a synch motor is connected to a VFD? 2. Someone told me that VARs produced in a synch motor does not get pushed through XFMRs is this true and please provide some tech details concerning this. The question has been sent to GE but I wanted to hear from ENG-Tips.com as well. Thanks in advance!
I am currently writting a technical proposal for a Project that I am trying to get Capital funding for. The proposal was reviewed in house and there was a couple of good comments that came from the review. Let's state a brief description on the project... We currently have an old Load Cummutative Inverter Drive Controlling a 1500HP Induced Draft Fan synchronous motor 2000V. This is 1980 technology and a retrofit of the controls or a drive replacement needs to be accomplished due to the fact that the Power Boiler can not run without these fans, thus the mill goes down. I have read from GE technical brochures that the LCI drive has poor PF at low motor speeds. The drive they are proposing to sell is a MV IGBT NPC Voltage Source Drive which they say has high true PF over all speeds. Here comes my questions... 1. Can you produce VARs when a synch motor is connected to a VFD? 2. Someone told me that VARs produced in a synch motor does not get pushed through XFMRs is this true and please provide some tech details concerning this. The question has been sent to GE but I wanted to hear from ENG-Tips.com as well. Thanks in advance!





RE: Synchronous Motor controlled by a IGBT NPC Voltage Source Drive
Due to the fact that the DC-link is able to store energy input and output power factor are totally decoupled.
Regarding power factor at the input side it is important to distinguish between displacement power factor (cosphi) and true power factor lamda. Even with a simple diode frontend displacement powerfactor will be very close to 1, so that you can save on the utility bill if you are billed for VARs. A more sophisticated active frontend would also allow to get rid of harmonics and to get lamda close to 1. Unfortunately these kind of inverters are significantly more expensive and their efficiency is lower.
2. Of course a synchronous machine can also push VARs through transformers. Transformer impedance has to be added to synchronous reactance for evaluating such a setup. I guess sychronous machines (motor as well as generator) are connected to the grind via a transformer.
RE: Synchronous Motor controlled by a IGBT NPC Voltage Source Drive
RE: Synchronous Motor controlled by a IGBT NPC Voltage Source Drive
It is informative post. Thanks.
You wrote about True power factor Lambda. I am familiar with Cosphi but not the Lambda. Would you mind elaborating on that!
Another question is that Synchro motor can present UPF load to the inverter, is it an advantage from the inverter designer point of view?
Thanks.
RE: Synchronous Motor controlled by a IGBT NPC Voltage Source Drive
- phase shift between the fundamental of current and voltage
- harmonic currents
As long as there are no harmoncis in the current the following relation is valid (example for single phase):
P = U * I *cos(phi) with U and I beeing RMS values and phi beeing the phase shift between.
With harmonics in the current this equation is not valid any more and changes to:
P = U * I *lamda
because current distortion is beside displacement another reason for apparent power
As soon as rectifiers are involved, that are not desinged as PFC (Power Factor Correction) ar AFE (Active Frontend),
current harmomics are to be considered.