Use dc injection as anti-condensation on existing motor?

[grobert]

Has anyone had experience using dc injection from a VFD to motor as a anti-condensation method?

 

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.

[Raka]

I've seen this in synchronous generators. What is the rating power for those motors?

[grobert]

There are aprox 50 motors ranging from 5HP to 25HP.

[Raka]

Which is the problem for the heating elements?

[grobert]

I'm not sure I understand your question. It is a general application for existing motors driven by existing VFDs. The customer is having moisture problems on some motors due to their locations. They do not want down time that would be required to have internal heaters installed in the motors to correct this problem. I know people have successfully used the DC injection available from most drives to power up slightly, and use as heaters, the motor windings. I am looking for past experiences and / or guidlines to follow.

[jraef]

It is done all the time, but the trick may be in getting your VFDs to cooperate. Some allow "forcing" of the DC injection braking, some only do it automatically at the end of a decel cycle when programmed to do so. If you have VFDs that allow you to apply the DC injection on-command, here are a few tips.

 

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.

[grobert]

Jraef, thanks for the info. It is very helpfull. We have toshiba G7 drives which allow for dc injection (for other than stopping a moving motor). It is typically used to hold a motor shaft still when the motor is not used.