What Type Of Switch?

[John Hasler]

Agreed. My only concern would be the temptation of starting more than 1 machine at a time. This way the RPG need only be large enough for 1 2/2 the HP of the biggest load.

The RPC would be happy to have all the machines running at once as long as only one is under load and they start one at a time. Each unloaded motor becomes another RPC.

 

[David VanNorman]

RPC is the way to go cheaper and easier to work with.

 

[Plas62]

Thank you all for your replies.

On the two grinders that are 1.5 hp each, one of them is a 2x72 belt grinder and the other is a disc grinder both to be used in knife making. The third grinder is a dual wheel 2x72 grinder with a 3/4 hp motor driving each of the two wheels also to be used for knife making. Variable speed is the main objective of going with a vfd in order to better control the grinding of blades.

I may have misinterpreted the information I found online about the VFD's, but I thought that if the two 3/4 hp motors running together did not exceed the 1.5 hp setup I had in the VFD that it would be OK.

I understand that I will have to power off the vfd before switching from one grinder to another to prevent damage to the vfd. I had also thought about going with the male / female plug for switching between grinders. I was just wondering if there was a switch that could be used instead of unplugging and plugging in cords. As this would be used in my home shop and I'm the only person using it I didn't have any concern about someone else trying to turn on one of the machines while another one was already in use.

What if I bought a second vfd to use on the grinder with the two 3/4 hp motors would it be more practical to use the the other vfd and switch it between the two grinders each with the same 1.5 hp motor?

As these vfd's with the NEMA 4 enclosure go for a little over $400 each I was hoping to come up with a solution that did not require a separate vfd for each setup, which results in $800 worth of electronics sitting unused two thirds of the time.

Thanks again for your replies and advice.

Jim

 

[Ulma Doctor]

If I remember correctly, an RPG will simply stop working if it is overloaded, correct?

dependent on hook up,
the supply breaker will pop on overload, or the motor overload will pop- if so equipped
after the reset, it should be ready to fire back up in either case

 

[John Hasler]

I may have misinterpreted the information I found online about the VFD's, but I thought that if the two 3/4 hp motors running together did not exceed the 1.5 hp setup I had in the VFD that it would be OK.

Depends on the type of VFD. That one has some fancy circuitry that tries to sense the phase of the motor current and adjust itself accordingly. It's likely to be confused by two motors. A simpler VFD that just blindly pulse-width modulated wouldn't care.

 

[JimDawson]

Depends on the type of VFD. That one has some fancy circuitry that tries to sense the phase of the motor current and adjust itself accordingly. It's likely to be confused by two motors. A simpler VFD that just blindly pulse-width modulated wouldn't care.

You are correct John. While it's a waste of VFD functionality, the Sensorless Vector VFDs that I have worked with have a provision for turning that function off and allow them to revert to simple V/Hz operation.

 

[John Hasler]

the Sensorless Vector VFDs that I have worked with have a provision for turning that function off and allow them to revert to simple V/Hz operation.

That would solve the problem.

[Edit] Aren't there cheaper VFDs for small motors, though? That's a pretty fancy one.

 

[fixit]

Thank you all for your replies.

On the two grinders that are 1.5 hp each, one of them is a 2x72 belt grinder and the other is a disc grinder both to be used in knife making. The third grinder is a dual wheel 2x72 grinder with a 3/4 hp motor driving each of the two wheels also to be used for knife making. Variable speed is the main objective of going with a vfd in order to better control the grinding of blades.

I may have misinterpreted the information I found online about the VFD's, but I thought that if the two 3/4 hp motors running together did not exceed the 1.5 hp setup I had in the VFD that it would be OK.

I understand that I will have to power off the vfd before switching from one grinder to another to prevent damage to the vfd. I had also thought about going with the male / female plug for switching between grinders. I was just wondering if there was a switch that could be used instead of unplugging and plugging in cords. As this would be used in my home shop and I'm the only person using it I didn't have any concern about someone else trying to turn on one of the machines while another one was already in use.

What if I bought a second vfd to use on the grinder with the two 3/4 hp motors would it be more practical to use the the other vfd and switch it between the two grinders each with the same 1.5 hp motor?

As these vfd's with the NEMA 4 enclosure go for a little over $400 each I was hoping to come up with a solution that did not require a separate vfd for each setup, which results in $800 worth of electronics sitting unused two thirds of the time.

Thanks again for your replies and advice.

Jim

In my other life before retirement I designed a machine drive for a multiple motor system with seven different motors on one VFD. The need was to keep all seven drives in sync. All motors could be started & stopped at will without hurting the VFD & all drives would ramp up & down as a single machine. Really cool some said it would not work but we showed them it would. Fact is the VFD was on line and each motor had it's own contactor with overloads for start/stop control.

fixit

 

[JimDawson]

In my other life before retirement I designed a machine drive for a multiple motor system with seven different motors on one VFD. The need was to keep all seven drives in sync. All motors could be started & stopped at will without hurting the VFD & all drives would ramp up & down as a single machine. Really cool some said it would not work but we showed them it would. Fact is the VFD was on line and each motor had it's own contactor with overloads for start/stop control.

fixit

Can't argue with success. If asked, I would have said it wouldn't work well. What was the total HP? Was it possible to start the VFD with all of the motors off, and then switch them in one at a time? This indicates that a VFD could be used in place of a RPC. Now I'm going to have to try this.

 

[John Hasler]

This indicates that a VFD could be used in place of a RPC. Now I'm going to have to try this.

There is no reason why you should not be able to do that in principle as long as you can disable all load-sensing interlocks and feedback. I think that it would be best to hook up a resistive load so that the VFD never sees an open circuit.