VFD reversing

[thekman]

I cannot get an answer from the mfr, nor does a search produce an answer....but I have a VFD controlling a 3HP 208v motor. I need to occasionally reverse the direction somewhat immediately. I've hooked it up and have been playing with it and it seems to work, but without actually telling it to STOP between direction changing, am I overworking the VFD (and/or motor) by switching directions on the fly? I realize the frequency will ramp up/down based on my VFD settings, I just want to be sure that I'm not going to do any harm by not stopping it between changes.

 

[winflex]

You can't reverse speed instantaneously. The VFD would draw an infinite amount of current. Practically speaking, you will blow a fuse in the VFD or the circuit supplying electricity to it.

What you can do is decrease the ramp time and keep an eye on the resulting current draw. Keep decreasing ramp time until your current draw approaches the limits of the supply circuit.

Good luck!

 

[RajeevCell]

Stopping is not needed. You are using a two-wire configuration, I guess. One wire signals clockwise, the other one counter-clockwise rotation. If this is the case, then you can switch your "command" instantaneously, without "stopping".

The drive will not, however, change direction instantaneously. It will follow the deceleration ramp to zero speed, then acceleration ramp upto commanded speed in the opposite direction.

 

[thekman]

This is correct. Each direction has it's own momentary input on the VFD. When I want it to switch directions, it will do so "instantaneously" in the sense of ramping down, and ramping back up at a rate determined by the ramp times I've got programmed in. I know it will not defy physics and magically begin moving the other way, I was just wondering if by not stopping between would hurt the VFD. I currently have the motor connected to a transmission, and it seems to work, however there's no load on the transmission, and it will be a substantial one. I will be adjusting the ramp times accordingly.

 

[itsmoked]

No problem. However your ramp down time is limited to the internal chopper and resistor in the drive. It will only support the deceleration of a certain amount of inertia at a certain speed.

If you want to increase the slow-down speed, or do it very often, you need to install an external resistor rated for the energy dissipation.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Actually you can damage the VFD if you work at it. Most VFDs will allow you to program the stopping method, either as deceleration, braking or coast to stop. If you select coast to stop, the output is effectively disconnected upon removal of a Run command. So when you have a reversing situation, you are removing the Fwd Run command and engaging the Rev Run command, with no deceleration or stop in between. Most VFDs also have a feature called "speed search" or "flying restart" or some such name, that has the VFD look at the motor field effects to determine what is going on with the motor before energizing the transistors. that feature would help to prevent damage, but it too can often be disabled in programming. So the combination of disabling the speed search function and using a coast to stop method in a rapid reversing situation can cause the VFD to trip (hopefully) or be damaged if the trip isn't quick enough.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[krsh]

Best solution for you is to install an external brake resistor, this will allow the VFD to dissipate the energy created by the ramp down function without damage to the drive. This solution will protect the VFD when you will connect the transmmision to the load(particularly recommended when you have a high inertia load). Also you will have to adapt the ramp down func. to the load not to burn out the resistor.

 

[ozmosis]

if you are going from FWD to REV and under a controlled deceleration/acceleration, then the motor will pass through a zero point at some stage so this is infact stopping.
If you are using a controlled ramp time, then I see no reason why this should cause problems with the motor if the ramp times are correct for the load inertia. It is only the same effect if you were stopping/starting in the same direction.
If the ramp down time is too short then the drive will typically trip on overvoltage due to the regenerative energy. If the ramp up time is too short then it will typically hit current limit.
Either way, if the drive is constantly tripping and you ignore all these trips then, as jraef points out, it has the potential to damage the drive (eventually) but before that time happens, I'm quite sure you would be so tired of resetting trips that you would have increased the ramp times.

 

[thekman]

I reconfigured my control program to 'stop' it before giving a new command in the opposite direction. Thanks for the good info.....

Another question I've got, and I'm kind of surprised how little info I can get on it...there's NO mention of it in the installation and operation manual (Leeson 174279)...there are B- and B+ terminals on the unit. My guess would be that's where I would install a resistor if necessary. After reading up on it several months ago, I seem to remember that I would most likely not need to worry about it at the time.

 

[ozmosis]

B- & B+ will be a direct connection to the DC Bus I would imagine. Unless it actually stipulates in the manual that this is the connection for a dynamic brake resistor, I wouldn't put one on there. It could just be a measurement point for the DC bus voltage.

Leeson are usually quite good with tech support questions, have you asked them?