Delta Connection

[Onoff]

A weird one. Not me but I watched another subbie do it.

Delta connected motor was running the wrong way. VFD driven with red, yellow and blue going to a 3 pole isolator then straight to the motor. He switched off and swapped red and yellow coming out of the VFD. Powered up and the motor continued running the wrong way. I watched it happen.

Went to the motor terminal box and swapped I guess yellow and blue. Motor ran the way he wanted.

I deal pretty much exclusively with star / double star round motors. Don't think I've ever had to reverse a delta connected motor.

Is there something special about reversing delta connected?

 

[m_c]

Nothing different about reversing between star and delta.
Sounds more like he's swapped the input wires, as there shouldn't normally be any isolator between the VFD and motor.

 

[Brad93]

A vfd should always run the motor clockwise by default (that is indicated by fwd on the vfd).

Changing line leads between the output of vfd and U1,U2,U3 connection on motor won't reverse it like it would if hooked up directly to the mains, because the vfd is probably getting feedback from the rotating field and adjusting to suit.

 

[Brad93]

I could do an experiment this week with a Mitsubishi 700 series vfd and a motor in star vs delta

 

[Onoff]

Thanks all. The chap on site is having another look today as to the "why" of it all. If I get any feedback when I seem him I'll report here.

 

[Onoff]

That was quick. @m_c was bang on. The guy had swapped 2 phases on the input wires:

Swapping 2 phases in the motor does make it reverse.

No, it wasn't me!

 

[Agroshield]

The guy had swapped 2 phases on the input wires:

Once the juice enters the VFD, the first thing it does is rectify the AC into DC so that kind of provides the reason why changing the input wires makes no difference.

I guess, in principle, you could connect a VFD to a high enough DC supply and get variable frequency AC out of it.

 

[Onoff]

So he actually changed the wiring going into the VFD?

Yes.

 

[brightspark]

That was quick. @m_c was bang on. The guy had swapped 2 phases on the input wires:

View attachment 450196
fat lot of good that would have done on 240v ac. it dont matter which way they go it wont alter the motor direction unless it was dc supply

 

[Turbo]

There should not be a contactor or isolator of any kind between the vfd & motor. Cutting off the output of the vfd from the motor suddenly is not good for the vfd & could destroy it.
There's no reason for it either, the vfd controls the motor better & will stop it quicker than simply isolating it, making it much safer.

 

[Agroshield]

There should not be a contactor or isolator of any kind between the vfd & motor.

I agree 100% that is what we are all taught or have come to believe.

However, perhaps you should enquire of the source of the document pictured above before jumping to conclusions.

Please look on page 73 here:

https://files.danfoss.com/download/Drives/doc_MG61B602.pdf

Please let us know what Danfoss say when you raise the issue with them.

 

[Onoff]

I agree 100% that is what we are all taught or have come to believe.

However, perhaps you should enquire of the source of the document pictured above before jumping to conclusions.

Please look on page 73 here:

https://files.danfoss.com/download/Drives/doc_MG61B602.pdf

Please let us know what Danfoss say when you raise the issue with them.

That image above is from the inverter manual on site. Below is an extract, heavily edited from the actual circuit schematic I've managed to get. The isolator is there to isolate as the motor is situated inside a large duct albeit an enclosed space that requires periodic access. A signal gets sent back to the bms to tel it it's off. As I say not my job or kit but I was nearby and helped out when they had an issue. I know jack about BMS stuff but it's pretty interesting tbh.

 

[morssdn]

It’s common practise to have a fourth pole on the isolator to disable the drive if isolator operated. After all you’ve got to be able to isolate the motor from any source possibly immediately, killing the feed to the drive cant guarantee this.
Turning the drive back on with the isolator means the drive will start up as normal not on full load.

 

[dobbslc]

You can't manually lock off a VFD for maintenance either.

 

[Matrixx]

Page 85 says you can have an isolator/contactor in the output, you just need to bridge the screening. if the motor is far away from the controls, then BS7671 would define it needs an isolator next to it , for safe working?