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SSRs to Run Two Motors off One VFD

Halcohead

Stainless
Joined
Apr 10, 2005
Location
Bay Area, Ca
Is there anything wrong with running two different (but identical make/spec) motors off of one VFD?

The VFD will only ever drive one motor at a time, and both motors will always be fully stopped when switching the VFD from driving one motor to the other. The motors and VFD are going on a lathe, which for various reasons would benefit from having separate motors for backgear and open-belt operation.

Assuming this is okay, would there be anything wrong with using solid-state relays (SSRs) to implement the switch between the motors? I've heard some SSRs don't liked to be switched under zero load. Would zero-crossing SSRs work for this application? Do SSRs have a problem with noise in the VFD output or regenerative braking loads?

If it matters, the motors are nominally 7.5hp 1750 rpm, with VFD output frequency from 10-125Hz using sensorless vector control.
 
Nothing wrong with the general idea of running two motors from one VFD. I'd suggest a couple things to make your proposed install more reliable:
1) VFD's do not like to be operated without a connected load. Some can fail in seconds, other only fail as they age and are operated into a missing load. To prevent this, I'd suggest an AUX contact on each load side contactor wired to the "enable" input on the VFD to assure that the output contactor is closed before the VFD operates.
2) Don't use an SSR. Use a traditional contactor. You'll need the AUX contacts for step #1 and you'll also want either a) another set of AUX contacts to lockout the other contactor for motor 2 while motor 1 is in use (and vice-versa) OR b) a walking beam interlock between the two contactors.

Having said all this.... don't do it. You said you're running sensorless vector control. Are you aware that this arrangement is "tuned" to the specific motor it controls? Maybe this matters to you, maybe it doesn't. If it matters, scratch your one VFD for two motors idea off the list and save your shekels for a second VFD.
 
+1 on the fact that SSRs will not work, and everything else he said. You would need to forego the SVC option to be able to do this, which may or may not be an issue for you. But on a lathe, I'd try to stick to a way to implement it.
 
SSRs are a bad choice. They are somewhat sensitive to the rate of change of the voltage across them, which will potentially be fast/large with the VFD. They may not be reliable as a result, turning on when not wanted, etc. AND they may allow quite a bit of feed-through to the "off" motor. They are NOT a positive disconnecting means.

I don't see why the VFD would fail with no load.... that should be the easiest situation, since there is little load current and small chance of any IGBT conduction "tails" overlapping. The only issue I can see is the potential for fast transitions on the commutating diodes, which might be an issue if they are not up to snuff. As a rule they are selected to deal with the switching speeds, and should not fail with no load.

Totally concur on the vector. You might get away with it if they are the same model/type motor. But "identical" motors do vary a bit, and it could confuse the controller, leading to possible odd results.
 
Thanks for the comments y'all. I figured SSRs wouldn't play well with a VFD, but couldn't think of any reason why.

Regarding SVC, I knew it was tuned to the motor, but (naiively?) assumed two identical part number motors from the same manufacturer would have similar enough characteristics that they could use identical SVC settings. Is this not the case? IE when you replace a motor do you have to re-tune the SVC parameters, even if the new motor is the exact same make/model as the original?

Regarding point 1: Yes, I'm aware VFD's don't like operating without a connected load. The exact details of the interlock will depend on the relays I can find (I've so far not found a source for a double-pole triple-throw contactor rated for 300VAC/24A, but that would be an ideal solution). Either way, the operator will have no way to command both relays open at once, as motor selection is controlled by a microswitch within the gearbox.

The only way to command a change in relay state while running is to shift gears, which is obviously destructive to the machine. So I'm not worried about suddenly switching motors while the VFD is enabled.

Related question: Default stop will be set as ramp to stop. But there are times when coast-to-stop is preferable. I'm looking at the Teco A510 Series VFD, which appears to allow a different ramp rate for E-stop commands. Is there anything wrong with regularly using E-stop to command a coast-to-stop as opposed to the regular ramp-to-stop?

Thanks again for your patience with the basic questions.
 
JST, for some reason I must've missed your reply. Thanks; that's an excellent explanation of why SSRs are bad for this application.

Whether or not the VFD is damaged by running while disconnected from the motor, my experience with running a Bridgeport off a VFD says it doesn't like this, and can alarm out. So that plan has always been to only operate the VFD while connected to one motor.

It sounds as if there's enough variability between "identical" induction motors that SVC still may be a problem. Would these odd results be small reductions in torque or serious issues with the VFD? Because I'm primarily concerned about torque on the open belt motor, and would be okay with lower performance on the back-gear motor.
 
It may not be an issue, it would show up in the two behaving differently for the same input command and load..... HOW differently, I have no way to predict, since I don't know the differences between the motors. It is likely to work decently. Far better than 2 different motors.

As for the E-stop, I'd think the E-stop would be the one with the fast decel, DC injection, whatever it takes to slam it to a stop. But it's your shop.
 
Reading deeper in the manual, the VFD allows one multifunction pin to switch between "motor 1" and "motor 2", which can have separate settings and parameters. Thus I think both motors can be driven with SVC, so long as that interlock circuit also notifies the VFD which motor is selected.

As for the E-stop, I'd think the E-stop would be the one with the fast decel, DC injection, whatever it takes to slam it to a stop. But it's your shop.

Fair point. The machine has a mechanical footbrake, which was going to double as the E-stop. But you're right, there should be a mushroom switch as well, and it will be ramp-to-DC injection braking, as you describe. Thank you for pointing out that oversight.

Previously I thought the E-stop digital input pin was required to command the VFD to coast to stop rather than the default ramp-stop. But on reading deeper in the manual, there's a "base block input" which commands the VFD to cut output and coast. Combined with a stop command, this achieves exactly the function needed for using a footbrake.

Thanks for the help in designing this drive. This isn't my first time setting up a VFD, but definitely the most involved system I'm taken on so far.
 
A VFD is not damaged by running disconnected. A basic V/Hz operating VFD would not even "know" the motor is disconnected, other than it might "think" the motor current is exceptionally low... The REAL problem is in reconnecting a motor to a VFD that is ALREADY in a Run state. The fact that the transistors cannot ramp the motor current subjects them to a rapid increase in current, called a "dI/dt" (delta I [change in current] over delta t [change in time]) that will damage the transistors themselves.

A drive running in SVC however WILL know immediately that the motor is disconnected and will trip off line. SVC is constantly monitoring the motor model and performance against that model, so if the feedback goes to zero, it knows something is wrong.
 
A VFD is not damaged by running disconnected. A basic V/Hz operating VFD would not even "know" the motor is disconnected, other than it might "think" the motor current is exceptionally low... The REAL problem is in reconnecting a motor to a VFD that is ALREADY in a Run state. The fact that the transistors cannot ramp the motor current subjects them to a rapid increase in current, called a "dI/dt" (delta I [change in current] over delta t [change in time]) that will damage the transistors themselves.
....

maybe.... if not well designed. A decently designed VFD ought to protect itself if you short the output wires, never mind connecting to a motor which has enough inductance to blunt the di/dt. Everything I designed has had to tolerate a dead short suddenly applied.

I have never seen a commercial one of recent make that would not shut down safely with a dead short.

The REAL issue with connecting a motor to an operating VFD is that the motor will have a surge that may (essentially always will with a matched size VFD) exceed the VFD current limit. Should shut down normally.

VFDs can be made to tolerate it. We made a small single phase output VFD that had to start a motor directly at full voltage and frequency. About 5X normal running current.
 
Agreed, but not everyone in the business is as conscientious as you with regard to designed in capabilities, and a lot of people buy on price. As we know, price is usually driven by cost, so low price means less cost, and less cost generally translates to omission of "good ideas". Caveat emptor...
 
If they aren't, they are welcome to the warranty issues and bad reputation. I figured most other than Huanyang would be essentially short-proof. It's gonna happen...... protect against it.

We had one model not protected fully. Folks in the lab blew up at least 3 of them on regular operation tests.
 
I don't think that most of the low end Teco drives I've dealt with had any output transistor protection scheme built in, the same is true for LG/LSIS and Delta, who make many of the low-end drives sold under other names (i.e. AutomationDestruct). I have seen numerous other drives from otherwise reputable suppliers with no transistor protection schemes as well, but it is typically on their "entry level" products.
 








 
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