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Old 07-03-2019, 01:54 PM   #1
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Default Trickle current through VFD for heating the motors?

Anyone has experience with using trickle current to supply DC current to the motors to avoid condensation and freeze up? These motors move one every 2-3 hours for a couple of seconds and we are trying to apply trickle current to keep them warm. I have never done this so I am not sure if VFDs support this feature. We are using Power Flex 525 VFDs. Where can I find this information since the VFD manual had no mention of this feature?
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Old 07-03-2019, 04:35 PM   #2
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Anyone has experience with using trickle current to supply DC current to the motors to avoid condensation and freeze up? These motors move one every 2-3 hours for a couple of seconds and we are trying to apply trickle current to keep them warm. I have never done this so I am not sure if VFDs support this feature. We are using Power Flex 525 VFDs. Where can I find this information since the VFD manual had no mention of this feature?
I have never heard of this being an option on any VFD. Usually you would need to have heaters installed in the motor windings and supply a second circuit to them.
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Old 07-03-2019, 07:34 PM   #3
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Default Torque control

Look and see if it has torque control mode. I think if it has torque control it will keep voltage on to hold the motor in position. Back to the pf755 drive if this is the same application
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Old 07-03-2019, 10:18 PM   #4
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Not sure about the 525s but some drives have a DC Hold feature. This will run a fairly small DC current to the motor whenever it's not running.

Note, this is different that a DC brake.
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Old 07-03-2019, 10:38 PM   #5
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We're waiting for you to respond @JRaef .....


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Old 07-03-2019, 10:42 PM   #6
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Anyone has experience with using trickle current to supply DC current to the motors to avoid condensation and freeze up? These motors move one every 2-3 hours for a couple of seconds and we are trying to apply trickle current to keep them warm. I have never done this so I am not sure if VFDs support this feature. We are using Power Flex 525 VFDs. Where can I find this information since the VFD manual had no mention of this feature?
I know some drives do have dc current to keep the motor winding warm but that kind of useage is kinda very limited due the design of the drive unit set up.

run of mill AC motors generally dont need heaters or dc injection but some appactions they used the DC injection for holding purpose ( very limited useage that it )

I am not really famuair with Power flex 525 unit.,

If really worry about keep the motor warm in super cold spots you may end up just get a heater tube and mount it on the motor to keep it warm.

very large motors is different story and different subject for a moment.,,
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Old 07-04-2019, 12:04 AM   #7
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Not sure on the AB drives but I've used this feature on Franklin Controls P drives, ABBs and Siemens. Usually call pre heat or freeze protection.

Edit: I didn't see that feature with a quick glance at the manual. I hate AB drives with a fiery passion
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Old 07-04-2019, 12:29 AM   #8
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Are you experiencing problems with condensation, and freezing? Unless they're tiny motors, or fill up with water then freeze, don't they have the torque to spin up? If they only run a few seconds, how do they develop the temp. differential to cause that much condensation? Are you "in house" maintenance? If so, I think I get it. Is there an engineering/planning department at this facility?
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Old 07-04-2019, 01:58 AM   #9
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Some VFDs do have that feature as mentioned and it’s standard on DC. It is not DC holding...that won’t work but DC fluxing as a brake has been around at least since the 80s. True holding requires an encoder since the flux vector disappears below 1-2 Hz. Although it is effectively DC it is applied at the right phase angle instead of just an arbitrary DC injection on all three outputs. This is one of the few times that encoders are a must.

On large motors aside from strip heaters the other way it is done is to use a separate DC supply (rectifier and trim resistors) and a contactor to feed it. I kind of recall a trick with blocking diodes to do it inexpensively but I don’t remember how because strip heaters are so cheap.

All arguments about ice are wrong. It depends on the insulation system how sensitive the motor is but on medium voltage motors where ground wall insulation and phase paper is more critical ieee 43 calls for a minimum of 100 megaohms temperature corrected so realistically 250+ megaohms uncorrected. On new motors I often get 1500 megaohms more or less but if it sits in a non environmentally controlled warehouse it can easily drop to close to minimum and that’s before the ravages of dirt and other contaminants in the plant have their way. When surface contamination gets too much to where voltage creep strength drops too low you get corona discharges at the surface of the fields which eats away at the insulation resulting in premature failure. The result is that in bad environments like mining 2-4 years life expectancy isn’t unusual. Using heaters doubles life or more. Switching from varnish to epoxy drastically improves moisture resistance and the vast majority of our rewinds are epoxy as are marine duty dry transformers. It helps but it’s not a cure.

That being said lots of plants don’t use heaters of any kind, even water plants. That being said I have a customer with a 2000 HP, 360 RPM (do the math on that torque!)) wound rotor motor about 50 miles inland from Savannah. Sleeve bearings. It sits under a roof but it’s pretty much open to the elements. They use heaters religiously. We just redipped it and baked it (epoxy this time). The motor has been in service for 30 years now without a rewind. No way it would still be kicking without heaters.



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Old 07-08-2019, 05:39 PM   #10
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You can do this directly with the PowerFlex 750 series drives, in the PF525 drives it's a little more indirect. You must set up one of the Digital Inputs (parameters t062 through t068) as a "Force DC" (setting 38) and then have something decide to enable that input when the drive is not running. That will put the drive into DC Brake mode and ignore the DC Brake Time setting so long as that contact is closed. You would set the DC level in A435
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