Thermal overloads ono load side of Powerflex 525's tripping

[ASF]

I'm currently commissioning a project where we have a couple of dozen powerflex 525 drives. 6 of them run vibrating conveyors - each with one VSD and two vibrator motors. Each drive has two Schneider LRD thermal overloads on the load side to provide thermal protection to the motors.

These overloads are all constantly tripping, even when set 10-20% above the motor nameplate current. The motors are not drawing anywhere near nameplate current.

I figure there must be something about the drive that generates additional heat in the simple bimetallic strip in the TOL's, over and above what the actual current draw would normally cause. Do I have any parameters at my disposal that might mitigate this effect?

Has anyone else used TOL's downstream of a powerflex 525 (or any other drive), and did you see the same results? How did you ultimately solve it? Are there some thermal overloads on the market that work better on the load side of a VSD than others?

 

[RET]

How long do they run before they trip?
Leave the doors open to let the heat out and see if it still trips.

 

[Bit_Bucket_07]

1. Schneider LRD thermal overloads are IEC type and employ bimetallic temperature sensing. A bimetallic OLR is more sensitive to harmonics generated by modern, high frequency VFD outputs. A eutectic (solder pot) type OLR might resolve the issue.

2. What is the distance between the VFDs and the motors? Long motor lead cable runs will require output reactors or (at very long distances) an LC filter to minimize reflected high voltage pulses. This could not only cause nuisance trips with a bimetallic OLR, but could also damage motor winding insulation.

 

[Steve Bailey]

Use a motor that features built-in thermal leads and connect them to the appropriate terminals on the drive. Don't most VFD manufacturers recommend against opening the circuit to the motor under load?

 

[BITS N BYTES]

Plain motor overload will NOT work. You need a MCC device [Motor Circuit Controller]. AND if motor leads are long a Dv/Dt filter.
I have used this approach for many years,in some cases with more than 10 motors [each with its own MCC] on load side of drive.
After extensive testing of MCC's from different vendors we discovered the Sprecher & Schuh devices to be the most reliable and least affected by reflected waves.
https://www.sprecherschuh.com/products/motorcircuit/kt7/

 

[BITS N BYTES]

Use a motor that features built-in thermal leads and connect them to the appropriate terminals on the drive. Don't most VFD manufacturers recommend against opening the circuit to the motor under load?

NOT a good idea and CERTAINLY NOT to code!

 

[Bit_Bucket_07]

Use a motor that features built-in thermal leads and connect them to the appropriate terminals on the drive. Don't most VFD manufacturers recommend against opening the circuit to the motor under load?

An overload relay wouldn't open the circuit under load, although a motor starter on the load side of a VFD being tripped by a thermal overload relay would. The OLR simply opens a relay contact. I would assume that the OLR contact is wired to the drive terminal board to stop the drive on an OL trip.

 

[NetNathan]

What class are the over loads?
Maybe they are Class 10 and should be Class 20 or 30?

 

[BITS N BYTES]

I'm currently commissioning a project where we have a couple of dozen powerflex 525 drives. 6 of them run vibrating conveyors - each with one VSD and two vibrator motors. Each drive has two Schneider LRD thermal overloads on the load side to provide thermal protection to the motors.

These overloads are all constantly tripping, even when set 10-20% above the motor nameplate current. The motors are not drawing anywhere near nameplate current.

I figure there must be something about the drive that generates additional heat in the simple bimetallic strip in the TOL's, over and above what the actual current draw would normally cause. Do I have any parameters at my disposal that might mitigate this effect?

Has anyone else used TOL's downstream of a powerflex 525 (or any other drive), and did you see the same results? How did you ultimately solve it? Are there some thermal overloads on the market that work better on the load side of a VSD than others?

Changing the carrier frequency of the VFD might help but in my experience a MCC offers a far better approach for multiple motors on a single VFD.
Been using MCC's for this type of multi-motor application for 20+ years, work perfectly.
Even if motor leads are short I would still recommend a quality Dv/Dt filter between the VFD load side and the MCC.

 

[jraef]

MOST of the IEC style bimetal OL relays on the market are NOT suitable for being used on the output of a VFD. The simple trick to determining this is to look at the specs and if it says "50/60Hz", that means it is NOT suitable to be used in a variable frequency application. The reason is twofold; one being the extra harmonic current causing the bi-metal trips to nuisance trip, the other is that they will also react to the capacitive cable charging current that can take place, especially if you have multiple outputs in the same conduit (which is a no-no anyway but happens a lot more on multi-motor applications).

A few mfrs have addressed this in their product designs, either with Solid State Overload Relays, or with bi-metal OLRs that have been specifically designed to be used downstream of VFDs. You cannot guess or assume. Rockwell's new 140MT have a couple of specific versions that ARE listed for use downstream of VFDs. For applications small enough to be run from a PF525, you would use the 140MT-D9V... (insert appropriate size). The other 140MT units are NOT designed for this, you MUST use the D9V...

There are a few others, but Schneider does not appear to have one.

Here is a paper on the issue from Rockwell. It refers to a previous version of their Motor Protection Switches, but the concept is still valid.

Side note: The VFD issue was never a problem for eutectic melting alloy style OL relays, this issue only applies to b-metal (and many SSOL designs). But the OP in in Australia and outside of North America, eutectic melting alloy OL relays are no longer legal. Something to keep in mind if you are building machines here for shipment overseas.

Side side note: When you use these Motor Protection Switches, the switch will actually open the circuit downstream of the VFD, which although not immediately dangerous for some VFDs, is not a good idea. So ALWAYS use an aux. contact on the MPS that is wired to the Enable input of the VFD so that the VFD output is shut off BEFORE the main contacts open (aux contacts are always designed to open before the main contacts).

 

[ASF]

Thanks for the assistance everyone. We have used these overloads before and had no issues, although in that case the cable run to the motor was very short. In this application we've got runs of 60-90m, so perhaps that's the difference.

We are looking into filters and different MCC's, although we have been told the leadtime is 12-16 weeks at minimum.