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View Full Version : VFD's and downstream switching of loads - actual behaviour and musings...



OxxAndBert
3rd Oct 2019, 05:07 PM
My understanding is that VFD's can't handle downstream switching of the load - ie you can't just hook up:
VFD ==> switch/contactor ==> motor
If you open/close the switch the VFD will throw errors.

I've also read of people having setup like this:
VFD ==> main motor (eg lathe spindle) ==> switch ==> secondary motor (eg a coolant motor).
Switching the secondary motor on/off while the main motor is running works fine and doesn't upset the VFD.

Can anyone confirm that secondary motor behaviour is correct?

Assuming that's the case, has anyone tried the following hybrid style setup for a rotary phase converter?
Single phase supply ==> VFD ==> idler motor ==> switch ==> load motor

I've been musing that if it works it might be simple alternative to using capacitors in the generation/balancing of the phantom phase of a traditional capacitor style RPC.
My theory being that the VFD will provide balanced phases to the idler to start with, and that the idler will electromagnetically "buffer" the VFD from the downstream switching.

Rocks in my head? Too much time on my hands??
Let me know what you think.

Steve

simonl
3rd Oct 2019, 05:54 PM
I can confirm that with a VFD already running a motor, you can introduce another motor without too much issue.

The caveat being that you obviously need t o be within the power rating of the VFD, cannot verify if it shortens the life of the VFD although I have been doing if with my surface grinder for several years without issue.

My surface grinder has 3 x 3ph motors; spindle, hydraulic and coolant. I can switch any combination of these on and off without issue. The circuit I designed is setup so that when you switch off the last motor running (regardless of what motor or what order) it also stops the VFD.

It's done with contractors and relays. Obviously if you have a ramp up soft start programmed into the VFD then only the first motor will see that. Same same if using a ramp down time.

I'm currently wiring a VFD with my mill. It has a spindle motor and coolant pump. I will be doing the same system.

Also, both those machines pretty much only run at 50hz so I'm not changing speeds. If you want to run different speeds you need to realise that all motors will running at a speed according the the VFD frequency. This could be an issue for some things e.g. you wouldn't want a coolant pump or hydraulic pump running at twice their normal speed!

Simon

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OxxAndBert
3rd Oct 2019, 07:50 PM
Awesome, that's what I wanted to hear - thanks Simon!!

It would just be fixed 50Hz once the idler motor was running, so no dramas there.

Think I've almost got enough components sitting around at home to set one up to for testing :)

Steve

WCD
3rd Oct 2019, 07:59 PM
My Arboga EM825 mill-drill came to me with a 3 phase 415V motor and a 240V VFD. The PO said nothing about stopping the motor before changing speeds.
The Arboga worked but the power was quite limited, so I had the 415V motor rewound to 240V 3phase.
Being a Dahlander wound motor, the two speeds are accessed by switching between star and delta winding configurations.
The changeover switch is between the VFD and the motor, and before I realised there could be a problem I used to switch between speeds without first stopping the motor via the VFD.... no problems resulted.
However after reading the VFD handbook I took to ensuring the motor was stopped before changing speeds.

It seems to me that the following aspects may assist in protecting the VFD:
1. the VFD is overrated for the motor by a factor of 2,
2. I never switched speeds when the motor was under load,
3. the inertia of moving parts on the Arboga is low, and
3. the speed switch has an 'off' (open contacts) position between the two speed settings.

Of course it may be that none of these factors is relevant, and maybe the VFD is just well built (Eurotherm).

Cheers,
Bill

jhovel
3rd Oct 2019, 09:44 PM
There are VFDs that are certified for downstream switching. I have a 415V VFD on my Arboga which is like that. But I still would only switch the motor speeds with the motor made stationary by the VFD....

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WCD
4th Oct 2019, 05:38 PM
... I still would only switch the motor speeds with the motor made stationary by the VFD....

Thanks Joe. That is what I do, and how I should have worded it.
Cheers,
Bill