VFD Mitsubishi F-700 22KW Drive...

[okoye]

Hi Folks...
I am considering connecting a braking resistor
without a brake unit, to the terminals P/+ and N/-- of this 22kw, F-700 Mitsubishi drive...
This particular F-700 VFD is to replace a faulty Schneider ATV-71 drive... which initially had the brake resistor wires connected to PA/+ and PB of its terminals...
Will this direct connection of the brake resistor affect the new F-700 VFD in anyway without the braking unit in between them...?

 

[jraef]

Yes, it will burn up the resistor. What the "brake unit" does is to monitor the DC bus level and fire a 7th transistor to put DC from the bus into the brake resistor. That transistor is fired as PWM as well, plus it takes care of the duty cycle (hopefully) in order to avoid damage to the resistor. With no brake unit, you are just immediately putting DC into the resistor with nothing to stop it until it burns up.

 

[okoye]

Thank you sir Jaref...

Look at this...the Brake resistor was initially connected directly to the PA/+ and PB terminals of the earlier ATV-71 Schneider VFD without a Brake Unit.... and it worked fine up until the Schneider VFD got damage over the years...

It got me thinking... because I have been wondering how come it worked with the Schneider VFD.... I even checked the Schnider ATV-71 manual... and saw in the that breaking resistor was connected directly to the Schnider VFD without the Breaking Unit....

This is my difficult puzzle...!!!
And this job must be done tomorrow... I am in a fix...

 

[okoye]

I am still making some research...

I just noticed that some VFDs have built-in Chopper transistor... and other VFDs don't have it in-built...

AC Drives like ABB, Schneider, Vectron, Lenze e.t.c have in-built chopper transistors... and can have the Brake resistor connected directly to them without the Brake Unit in-between them...

AC Drives like Mitsubishi, AutomationDirect, Yaskawa e.t.c ....do not have in-built chopper transistors...so they require Break Units must be connected to their VFDs before the Braking Resistor....

Please great day Folks...I need everyone to verify if this my finding is correct....and make necessary additions....

 

[thingstodo]

I am still making some research...

I just noticed that some VFDs have built-in Chopper transistor... and other VFDs don't have it in-built...

AC Drives like ABB, Schneider, Vectron, Lenze e.t.c have in-built chopper transistors... and can have the Brake resistor connected directly to them without the Brake Unit in-between them...

AC Drives like Mitsubishi, AutomationDirect, Yaskawa e.t.c ....do not have in-built chopper transistors...so they require Break Units must be connected to their VFDs before the Braking Resistor....

Please great day Folks...I need everyone to verify if this my finding is correct....and make necessary additions....

The Mitsubishi A700 series has built-in brake resistors below ... 50 HP or so? I don't have experience with the F700

My suggestion is to try it. The brake resistor should take full current for a few seconds. Connect the brake resistor, turn on the VFD, and watch the resistor. If it starts to glow red (or worse, white) then shut it down and you have your answer.

To use the resistor properly you will need to configure the parameters on the drive to tell it that the brake resistor is there, what max duty cycle is, etc. If those parameters are not accessible from the P7 (I think that's the keypad model) .. then it likely does not support the dynamic brake resistor without an external controller. I don't remember the mitsubishis allowing you to change what voltage that the bus is regulated to. It's about 960VDC for the A760.

 

[jraef]

The Mitsubishi A700 series has built-in brake resistors below ... 50 HP or so? I don't have experience with the F700

My suggestion is to try it. The brake resistor should take full current for a few seconds. Connect the brake resistor, turn on the VFD, and watch the resistor. If it starts to glow red (or worse, white) then shut it down and you have your answer.

To use the resistor properly you will need to configure the parameters on the drive to tell it that the brake resistor is there, what max duty cycle is, etc. If those parameters are not accessible from the P7 (I think that's the keypad model) .. then it likely does not support the dynamic brake resistor without an external controller. I don't remember the mitsubishis allowing you to change what voltage that the bus is regulated to. It's about 960VDC for the A760.

Looking at the manual, the F700 does NOT have a built-in braking transistor, it uses an external module. So DO NOT just hook it up and try it...

 

[okoye]

Nice one Guys.... you are resourceful...

 

[okoye]

Thank you Guys for your insights...

I am thinking about the possibility of introducing a contactor in-between the braking resistor and the F700 VFD....then I will program one of the logic outputs (an N-O) of the VFD to be activated the moment the DC bus voltage rises above (Input voltage × √2)....so that this contactor coil will be activated... and the excessive heat that would be generated on the DC bus.... would be conducted to the Brake resistor.... until the bus voltage falls below (√2×Line voltage).... and the contactor will open again...

How about this design...?

 

[thingstodo]

Looking at the manual, the F700 does NOT have a built-in braking transistor, it uses an external module. So DO NOT just hook it up and try it...

I should not have advised to 'try it'. That was irresponsible

Thanks for looking it up

 

[thingstodo]

Thank you Guys for your insights...

I am thinking about the possibility of introducing a contactor in-between the braking resistor and the F700 VFD....then I will program one of the logic outputs (an N-O) of the VFD to be activated the moment the DC bus voltage rises above (Input voltage × √2)....so that this contactor coil will be activated... and the excessive heat that would be generated on the DC bus.... would be conducted to the Brake resistor.... until the bus voltage falls below (√2×Line voltage).... and the contactor will open again...

How about this design...?

This solution is just not fast enough. The drive monitors it's bus voltage, performs the 'bus voltage too high' logic, uses some deadband to bring the bus down to 'nominal' and turns on the output for the contactor. That part might happen fast enough (but I doubt it). A contactor normally takes a minimum of 3 cycles (50 ms or so) to open or to close. So unless you are dumping a bunch of regenerative energy ... like slowing down a train .. to an over-sized brake resistor ... I don't think you will be happy with this solution.

Our brake resistors are sized for 20% max duty cycle, for 60 seconds ... then at least 5 minutes of cooling afterward, I think. This was a rule-of-thumb that our supplier used to size them.

The IGBT that drives the brake resistor turns on and off quite quickly ... from 0.5 Khz to 8 Khz, depending on the drive and the horsepower.

I don't remember the frequency that Mitsubishi uses, and I'm too lazy to look it up

 

[jraef]

This solution is just not fast enough. The drive monitors it's bus voltage, performs the 'bus voltage too high' logic, uses some deadband to bring the bus down to 'nominal' and turns on the output for the contactor. That part might happen fast enough (but I doubt it). A contactor normally takes a minimum of 3 cycles (50 ms or so) to open or to close. So unless you are dumping a bunch of regenerative energy ... like slowing down a train .. to an over-sized brake resistor ... I don't think you will be happy with this solution.

Our brake resistors are sized for 20% max duty cycle, for 60 seconds ... then at least 5 minutes of cooling afterward, I think. This was a rule-of-thumb that our supplier used to size them.

The IGBT that drives the brake resistor turns on and off quite quickly ... from 0.5 Khz to 8 Khz, depending on the drive and the horsepower.

I don't remember the frequency that Mitsubishi uses, and I'm too lazy to look it up

Yes, I agree. The brake transistor is not just a binary (On-Off) switch like a contactor, it pulses very very very fast in what's called "PWM" for Pulse Width Modulation, to CONTROL the amount of energy getting to the resistor. If you simple close a contactor, ALL of the DC bus energy will go there and if that doesn't damage the rectifier, it certainly will not result in a braking effect.


How dynamic braking functions is to turn the motor into an AC induction generator, then pull the kinetic energy of the spinning load as electrical energy, then burn that electrical energy off as heat in the resistor. The important functional part of that process is that the VFD must remain ACTIVE for that to happen, because it must still provide excitation energy to the motor windings. AC induction motors can only become generators by having excitation of the magnetic fields in the stator (and thus the rotor), PLUS something moving the rotor FASTER than the stator frequency. Using the VFD then, the drive reduces the stator frequency to always be slower than the relative rotor frequency and keep the motor in a continuous state of regenerating into the DC bus, while the brake transistor SLOWLY dumps that excess energy into the resistor. Using a contactor would, as I said, dump ALL of the DC bus energy into the resistor at once, possibly leaving nothing for the VFD to operate with and keep the motor in a state of regeneration.


You seem to be insisting on going out of your way to avoid buying the braking module... you cannot avoid that.

 

[okoye]

Thanks for your guidance and help.... I am backing out.

I will request they return the Mitsubishi F700 and get either ABB or Lenze or any such VFD that allows a Brake resistor to be connected directly to the VFD without the use of Brake Unit...



Thanks once again for saving me some costly troubles...

 

[okoye]

Does anyone know if MEIDEN vfd has this in-built chopper transistor.... that would allow direct connection of Braking Resistor without the use of Brake Unit for 22...?

 

[2rlp]

What is your application? Do you really need a DBR?

 

[okoye]

Yes I do need a Dynamic brake resistor...
It's a HOIST application... involving very fast stops...