Using a VFD as Starter for RPC

I am wondering if a VFD could be used as a soft starter for a RPC. My thought would be to get the idler to speed then use a bypass contactor to engage the RPC. Probably overly complicated. I have multiple machines and a VFD for each does not make sense to me. Both space and power are at a premium and do not really have space for a pony motor.

Graham O

Either diy a Fitch Williams, or buy a ready made panel and supply your own motor, or just buy a complete unit, all 3 of these options would have an idler motor and not require a pony motor.

I already have a Panel and idler[ actually 2, a 10hp and a 15hp[that I can't use]] am trying to reduce inrush current. The vfd would be acting as a softstarter and supply 3 phase to initialy start the motor.

Graham O

graham o said:

I am wondering if a VFD could be used as a soft starter for a RPC. My thought would be to get the idler to speed then use a bypass contactor to engage the RPC. Probably overly complicated. I have multiple machines and a VFD for each does not make sense to me. Both space and power are at a premium and do not really have space for a pony motor.

Graham O

Click to expand...

How many machines? What are their horse power sizes?
I would rethink that "VFD for each" thought, as they are low cost commodity items. In my shop, I've removed single phase motors and replaced them with 3phase for all the advantages that a VFD gives, the biggest is variable speed. With this I never change the belt settings on drill presses, milling machine, or lathe with belts. I set them to their middle range reduction, and program the drive to run the motor to 3600RPM, as all my motors are 4 pole, so I can safely run them at 120Hz.
I started my shop with a home built RPC, a major noise generator, as well you have to start it before using the current machine of interest. It ran out of power for the 7.5HP lathe I acquired.
But we don't know what size of motors you're trying to operate.
I have two machines that have 7.5HP motors, a large lathe and a 14" table saw. I use 10HP drives on these, as I'm never going to use the max HP of either of these machines. And if you program in the HP of the actual motor, the drive will protect itself from damage. I've not see the input diodes fail. The typical derating once your larger then 3HP VFDs is 2x the motor HP rating when using single phase input.
The soft start is another benefit.

As far as machinery fairly modest a horizontal mill with a 5hp main motor and a .5hp motor that drives the rapids, several automatic drills for 2hp each, a 5hp compresor when the wood shop works a couple of 5hp shapers a 5hp dust collectorand 3hp table saw. I also dabble with repairing motors [antique to relatively modern] mostly 10hp or less.

Graham O

graham o said:

I already have a Panel and idler[ actually 2, a 10hp and a 15hp[that I can't use]] am trying to reduce inrush current. The vfd would be acting as a softstarter and supply 3 phase to initialy start the motor.

Graham O

Click to expand...

That is the kind of info we needed in the first post. I guess the question now is which is cheaper, a vfd for every machine, or bringing in more power? I guess you could experiment with driving an idler to speed with a vfd, never heard or seen that, I would pick smarter brains than mine for that info. Have you tried a pony motor to see if concept of bringing it up to speed before powering will solve your problem?

I'm gonna second Ignator's suggestion, Graham... if you're gonna VFD, just go straight to VFD on each machine. Keep your RPC in operation, but VFD the machines in stages... start with the machine you use the most, get it converted, then move to the next.

As Ign mentions, having intimate speed control means you have soft start, by virtue of just ramping-it-up-slowly. You can get some amazing performance out of smaller motors using a little drive reduction combined with overspeed...

But one of the biggest bonuses... especially with woodworking machines... is dynamic braking. When you hit the STOP button, you can bring a howling blade to dead-stop right quick... and that's a feature that is REALLY hard to beat.

As Ign alluded, running a VFD off single-phase requires a little investigation and thinking. Many VFDs are designed to run on single phase, and while not 'advertised, will identify it accordingly in their ratings. Some WILL run on, but are not designed or 'advertised' as single-phase capable... and some machines simply won't run on single at all. In the case of any VFD not rated for, it is always best to use a larger VFD than needed, so that the VFD doesn't expire due to having it's input power focused on just two of the three legs of the input rectifier. As an example, if you've got a VFD that will run on, but is not 'rated for' single phase input, and you're running a 5hp motor, use a 7.5hp VFD.

If you'd still rather run the RPC, you COULD use the 'self starting' arrangement in the Fitch Williams design (starting capacitors), but you WILL still have high inrush current.

One way you COULD do it... is to liberate a treadmill of it's permanent magnet DC motor... they're usually 90-140v range. Couple it to your idler, and use a light dimmer feeding a bridge rectifier, feeding the motor. With the current dialed down about halfway on the dimmer, it'll bring the idler up really slow.

The 'gotcha', is that if your electrical service doesn't tolerate the inrush of starting an RPC idler, it's probably not gonna be happy pulling the RPC's current demand once you throw the switch on a big power tool...