motor disconnect and drive

[jcole]

Hello guys and gals.

I have a powerflex 400 drive controlling a 100hp motor. Between the drive and motor I have a disconnect.

What would happen if this disconnect is off when the drive gets start bit?
What would happen if this disconnect is turned off while running?

Drive specialist is telling me I need aux contact on disconnect to let the drive know when off. He said it could damage drive underthese conditions.

This disconnect is required by code.

Can you explain to me why this contact is important?

 

[Jraef]

1. Condition 1; Disconnect open when drive gets Run command: When this happens and you do not have the feedback from an aux. contact, the VFD will not know the motor is not connected. In the "olden days" with strictly open-loop V/Hz PWM drives, the VFD would plod along happily, no worse for the wear because it was not necessary for the load to be connected. Newer Sensorless Vector drives look at the motor current signature to determine the performance of the motor with the load in order to optimize the output, so with no load, they typically fault. No damage, just a nuisance.
2. Condition 2; Disconnect is opened WHILE the motor is running on the VFD: Very bad for the VFD transistors. As the contacts of the disconnect separate, an arc is drawn across them and the air gap acts like a capacitor. The voltage across it rises rapidly, creating what is called a high dv/dt (delta voltage over delta time; rapid rate of rise). The damage it incurs is incremental, so although it appears you may be "getting away with it" for a while, you are causing stress to the silicon in the transistor layers, leading to eventual premature failure. There is no way to quantify that so that you can predict it or say "it's OK for X number of operations", but it is widely accepted to be a bad idea.
3. Condition 3 (you hadn't asked, but it's important); VFD is on but not connected to the motor because of Condition 1 and assuming it didn't trip right away, disconnect is closed; Again, VERY bad for the VFD. If you are running a VFD and it is already at speed, closing a switch into a motor is going to put that motor into an Across-the-Line (DOL) start condition. So the current is going to surge to roughly 600% of motor FLA and that can severely damage the transistors as well (dI/dt or delta current (I) over delta time). At low speeds the surge current will be lower, but still may be higher than what the transistors can safely deliver.

Make sure you use an "early break" aux contact, meaning one that opens the aux. circuit just prior to the main power contacts opening. Most "Handle Operated" aux. contacts will do this (as opposed to the ones operated by the switch mechanism itself). Wire that to the base Block input of the VFD; Base Block is a function that immediately turns off the transistors, i.e. no ramp down etc. As long as the transistors are off before the power is disconnected, you should be fine.

 

[augie47]

Jraef,
I just want to say "thanks" for your Forum participation. I ALWAYS learn from your posts. (now if you could just tell me how to remeber it all )

 

[Jraef]

Jraef,
I just want to say "thanks" for your Forum participation. I ALWAYS learn from your posts. (now if you could just tell me how to remeber it all )

My wife calls me a "fountain of useless information".
At least here it seems to occasionally be of some use.

 

[jcole]

Jraef. What do you mean by base block input on the vfd? Would that be the "enable" input?

Also, Ditto what augie47 said.


Thanks for replies.

 

[tish53]

have to agree with other posters. great explantation from jraef.

For all of those eloquently stated reasons, we try very hard not to put a disconnect in between the VFD and the motor. Lockable disconnects on the input power of the VFD can meet the industrial installation exception in the code and we have been able to convince the inspectors to go along.

We use disconnects with aux. contacts (as described by jraef) on across the line ( ATL) starters that require a lock-out point near the motor. This is much safer in ensuring the contactor is open until after the disconnect is closed. Start the motor with the contactor NOT the disconnect.

 

[Jraef]

Jraef. What do you mean by base block input on the vfd? Would that be the "enable" input?

Also, Ditto what augie47 said.


Thanks for replies.

Huh, I wrote a response for this, don't know what happened.

Base Block is an industry term to mean the Base circuit for the transistors is disabled so that they cannot fire. The difference is, it is done on all 6 transistors immediately so it is the closest thing you can get to an "Open Circuit" with a solid state device. There is no ramping down, there is no regeneration so no braking etc. It's just immediately off.

A-B has a tendency to not use "industry standard" terms, they want to think their way is the only way. So I have no idea what they would call this, if they even allow it on their drives. I'd have to read their manual.

 

[Jraef]

A-B has a tendency to not use "industry standard" terms, they want to think their way is the only way. So I have no idea what they would call this, if they even allow it on their drives. I'd have to read their manual.

A quick run through their manual has no mention of this kind of functionality. The have a "Coast" function like everyone else does, but nothing that totally disables the transistors. There is mention of a hardware jumper on a PC board that can be removed to disable the output, maybe that's their equivalent, but that would be virtually impossible to implement in this case. In addition, it also appears that if you use the 'Coast" function that is available to the Input programming, it will also clear any faults, which may be problematic for troubleshooting. So it appears the best you can do is to program an input to do an "Aux Fault" function, which will disable the drive (assuming you program it that way) and indicate at least that you have an external fault condition, which you could instruct users to mean the disconnect was left open.

9 ?Aux Fault? If input is enable but not active, the drive will immediately fault

.

 

[mbeatty]

My wife calls me a "fountain of useless information".
At least here it seems to occasionally be of some use.

My wife says basically the same when I tell her something is a scientific or engineering "fact".

You are an excellent source of useful information. Keep it coming.