Use dc injection as anti-condensation on existing motor?
Posted 27 October 2003 - 08:08 PM
Customer has aprox 50 motors with VFD existing. Motors are in high moisture area. They can't / don't want to have heaters installed in motors.
Any experience / formulas or methods greatly appreciated.
Posted 31 October 2003 - 08:21 AM
Posted 31 October 2003 - 09:59 PM
Posted 02 February 2004 - 09:02 AM
1) Wait before applying it. The VFD will probably be smart enough to not apply DC until the AC field collapses in the motor, but be safe rather than sorry. There is nothing wrong with waiting even a few minutes since the motor will be warm from running anyway.
2) Larger motors (NEMA 324 frame and over) tend to have a positive coefficient of temperature, meaning that as the temperature rises, so does the resistance. So as the motor warms up, the amount of DC current draw will decrease at a fixed DC voltage. This helps out if your VFD has a limited amount of DC current available. Smaller frames have a negative coefficient so as the temperature rises, the resistance drops. This can be troublesome for the limited resources on a VFD DC injection circuit because as it does it's job, the motor winding starts to require more current. If the VFD is limited, the motor cools off. It is therefore often more practical to add a strip heater to motors under 10HP.
3) Make sure the user and service personnel are well informed that there is power on that circuit all of the time even though the motor is mot turning. Safety is never expensive.
4) For motors from 15-125A FL, 10A of brake current is usually adequate. For motors 150 - 300A, 25ADC will work. For motors 350-600A, 50ADC and for larger, up to 900A, use 80A DC.
5) Make sure you have an overload relay in the circuit or check with the VFD mfgr to ensure that overcurrent protection is active during DC injection. Fuses on the input of the VFD may not adequately protect from a short in the motor circuit because they will be sized for the 3 phase input of the VFD, and the DC power output is only going to be on 2 windings, disproportionate to the fuse ratings.
Hope that helps.
Posted 07 February 2004 - 02:49 AM
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