Best practice for motor overload protection using a vfd

[fgen]

Hello.
I have am trying to set up a vfd to power/control a 5 horse three phase 220 v motor on a lathe in a single phase shop.

I'm trying to determine the best place to have overload protection (say within VFD vs conventional thermal relay) and
to thus determine what features I have to have in a suitable VFD.

The lathe has separate contactors for direction and these go into a thermal relay to drop input current if an overload occurs.
VFD use will alter at least the directional control part of this.

Some VFDs say not to put any kind of contactor after the VFD.
Given this I assume overload protection is to be done via
(a) detting a limit in VFD configuration so it knows how big a motor it is running.
(b) trusting the VFD to limit or zero the output current appropriately.

Alternatively one could rig the overload condition to interrupt the power into the VFD (I suppose).

Some VFDs don't seem to let you set a max current and presumably infer that from the HP rating of the VFD.
I'm guessing those should be avoided.

I'm trying to decide whether VFD based overload protection is adequate and if not what is the best way to
achieve it.

Thanks,
f

 

[JST]

You SHOULD be able to set motor voltage, current, max speed, etc by parameter, and may be able to set overload time/current curves, at least to some extent. You get the values off the motor plate, and add a "cushion" to prevent unwanted stoppage.

There are many VFDs now officially rated as "motor controls" so there really should not be an issue.

Switches after the VFD are an issue. Not that they should be expected to do damage (maybe back in the old days), but that if you open a switch and stop the motor, you cannot close it and re-start the motor. So there is little point.

Best to just eliminate them and connect direct, unless you drastically over-size the VFD to handle start current.

 

[Jraef]

There are many VFDs now officially rated as "motor controls" so there really should not be an issue.

To make sure that you get this, only look at VFDs that are UL listed. Many of the cheap junk VFDs being sold for ridiculously low prices on Amazon, eBay and the like are un-listed and not only prone to premature failure, but often have minimal to no motor protection. UL began requiring VFDs to provide both Overload and Short Circuit protection for VFDs way back in 1990.

Side note: ALL VFDs are capable of reversing the motor, do NOT use the contactors for that any more. It’s true that opening a contactor under load on the output of a VFD runs a risk of damaging the VFD. There is no reason for it any longer. It’s also not a good idea to use a contactor ahead of a VFD if it can be avoided. It shortens the life of a VFD to be powered up and down a lot.

 

[JST]

........................... It’s also not a good idea to use a contactor ahead of a VFD if it can be avoided. It shortens the life of a VFD to be powered up and down a lot.

Generally, the reason for that is the "inrush" protection of the VFD. While the actual protector is rated for a large number of operations, and should not be an issue, there are details.

Without inrush protection, the bus capacitors of the VFD would experience a large surge of current when the unit is switched on. That can damage the capacitors, which do have current limits.

There are two common types of protector. One type, common on smaller VFDs, is a thermistor. These change in resistance as they heat up. They are typically sized to carry the maximum inrush current when cold, and to decrease in resistance when hot, so that they pass the load current. They only limit the surge when cold, so the assumption is that turn-on only occurs after the unit has been off for some time.

The problem is that they take some time to cool down and increase in resistance. So if the VFD power is shut off, and turned on again quickly, there is much less limiting, and the capacitors get stressed more than the designers allowed for.

The other type uses a fixed resistor to limit, and a relay to bypass the resistor after the surge is past. These work better, but if the VFD is turned off and back on again quickly, the resistor may still be hot, and it may fail open due to overheating, which causes the VFD to not turn on. The resistor is usually sized to withstand a limited number of turn-on events per hour.

Even if the inrush protector works perfectly, there still is a significant current surge, which does slowly degrade the bus capacitors, but that is taken account of in the design, using an assumed amount of turn-on events over the life of the VFD. Having a lot of additional off-on cycles may use up that design life earlier.

 

[rons]

I'm trying to decide whether VFD based overload protection is adequate and if not what is the best way to
achieve it.

The best way to achieve this is have a little faith in your VFD.

A VFD can simulate a heater by storing samples and counting seconds.

 

[jim rozen]

Nobody's mentioned overcurrent protection for the input power to the VFD yet.

 

[JST]

Nobody's mentioned overcurrent protection for the input power to the VFD yet.

That is likely because it is "mentioned" in the manual for every credible VFD I am aware of.

Certainly every VFD that carries a UL recognition. There is a specific test which assumes a catastrophic failure in the VFD, and requires the VFD to withstand the resulting fault current safely. That in turn requires the incoming power limit to be a specific type or category of fuse etc, in order for the VFD to be recognized in your application.

If you have properly installed the VFD, you will already have the input side protection which is necessary to preserve UL recognition.

 

[markz528]

The answer is neither one does a great job of protecting the motor from overload. Just way too many variables and even high end ASD's (industry and most standards are switching to Adjustable Speed Drives from VFD so I try to always call them ASDs now) can't model them well. The best motor protection includes embedded winding temperature detectors, but that is not something you will probably have. Some ASD rated motors may have embedded thermal switches. I will add them to mine if my motors ever fail.

The issues are that the heating of the windings and rotor is dependent on speed if TEFC (airflow over the motor greatly decreases with speed), internal temperature differences, motor design, etc. I have seen worst case 35 degree C difference inside a motor amongst 6 embedded winding temperature RTDs running at 25% speed at full load on a dyno. Impossible to get an algorithm to protect against that. That was maybe a 100 hp motor.

Another 400 hp or so motor was running excessively hot (at rated temperature) when load was around 70% and speed was probably 80 - 85% of rated. Found that someone had set the carrier frequency to 1.5 khz. Changed it to 3 khz and motor temperature dropped 30 degrees C. Windings are rated for 155 degrees C.

With that said, I would probably just set the drive overload to a conservative number and run with it. If the motor has a thermal switch, wire it into an estop input or the run circuit if necessary.

 

[jim rozen]

"If you have properly installed the VFD...

Well there is that *specific* question being thrown about in this thread, right?

Better a well-sized fuse in the incoming line than a motor starter between the VFD and motor.

 

[JST]

"If you have properly installed the VFD...

Well there is that *specific* question being thrown about in this thread, right?

Better a well-sized fuse in the incoming line than a motor starter between the VFD and motor.

Refer to manual, which will tell you exactly what is needed.

 

[fgen]

Hello,
Very much appreciated info in the responses above. I thought I'd update on the path taken.

The lathe has a late 80's vintage heater based protection relay for each of the spindle motor and the coolant pump. It is a
three phase system, and has many very nice looking contactors, a 110 volt single phase control bus (contactor activation voltage),
and is very well integrated with the forward/reverse controls, the brake/stop system, and the panic button. All this relay/control
switch stuff prevents for example a power fail from spinning the thing back up when power returns. Given that I like the ergonomics and
am impressed with the arrangement of all the power stuff I hated to throw it away for inferior VFD controls with sadly absent
documentation from either the purveyor of the device or the manual. Therefore I undertook the following:

- Contactors for Forward and for Reverse each now control only the 5 volt VFD control bus wires. They both assert STOP on deactivate.

- Three phase from the VFD goes into the Thermal Relay preserving any motor protection features of that system, which include moving the
VFD control inputs to Stop and interrupting the connection of the VFD from the motor. This is a potential exposure to VFD damage due to
having a contactor in the output path of the VFD but the should be an emergency/rare event and at this point I have no love for this VFD at
all. From previous information kindly provided in this thread I'm thinking that may not be a big risk anyway.

- Main power disconnect provides input power to the VFD as well as the control circuit transformer.

- In normal operation the VFD gets one power up event at the start of each session with a 220 V on switch (the Main disconnect on/off switch) and
for all operation just gets for/stop/rev singling on the 5V control buss. The output from the VFD is never interrupted by a contactor unless a meltdown
is in progress. The foot brake stops the spindle but via the VFD control bus.

- All of this is achieved with the existing 'tronic hardware on the lathe and no cutting of wires (just removal of some runs).

Bottom line - I'm relying on the original safeties to protect the lathe. I suspect (because I can't verify due to it being some kind of secret) that the
VFD just output trips if it gets over 10 Horse + delta. I'm not comfortable trusting that approach on my nice vintage (kinda) 5 hp motor.
The happy part is I get ALL my ergonomics as they were meant to be.

Thanks again for all the info.
f

 

[JST]

Hello,
Very much appreciated info in the responses above. I thought I'd update on the path taken.
................................................................

Bottom line - I'm relying on the original safeties to protect the lathe. I suspect (because I can't verify due to it being some kind of secret) that the
VFD just output trips if it gets over 10 Horse + delta. I'm not comfortable trusting that approach on my nice vintage (kinda) 5 hp motor.
The happy part is I get ALL my ergonomics as they were meant to be.

Thanks again for all the info.
f

The VFD should be settable for motor current etc. That will relatively automatically set the overload points. Sometimes you can set those as well, but most won't have the information needed to do any detailed setting beyond the motor current at full load.

So, you should be able to set up a 10 HP VFD for the motor current needed for your 5HP motor. The VFD will then trip on overcurrents beyond that setting, with variations depending on the way it was designed.

When setting up any standard VFD, it is normally required to set up motor rated voltage, motor rated current, and motor rated frequency as part of the minimum set of parameters. Also acceleration and deceleration, and others, with some depending on the VFD. You really should to do that, if the VFD controls that motor only.

If the VFD just supplies 3 phase to a bunch of motors in the shop, then you still have to rely on motor controllers to handle overload. Quite a few folks use a larger VFD to power a shop that way, similar to using a rotary phase converter.

 

[fgen]

The VFD should be settable for motor current etc. That will relatively automatically set the overload points. Sometimes you can set those as well, but most won't have the information needed to do any detailed setting beyond the motor current at full load.

So, you should be able to set up a 10 HP VFD for the motor current needed for your 5HP motor. The VFD will then trip on overcurrents beyond that setting, with variations depending on the way it was designed.

When setting up any standard VFD, it is normally required to set up motor rated voltage, motor rated current, and motor rated frequency as part of the minimum set of parameters. Also acceleration and deceleration, and others, with some depending on the VFD. You really should to do that, if the VFD controls that motor only.

If the VFD just supplies 3 phase to a bunch of motors in the shop, then you still have to rely on motor controllers to handle overload. Quite a few folks use a larger VFD to power a shop that way, similar to using a rotary phase converter.

-Hello,
On the point about setting motor current, I completely agree. This one has no settings in the manual at all for motor size, full load current etc. Sellers
similarly silent. ( To explain further, there are even worse things than a certain proscribed VFD make, and that is a make that is a knock off of a
proscribed VFD make that is advertised as the Proscribed make but is not even that. I'm not trying to discuss it mind you, just noting that
I seem to have read the meta posts in this forum in the too late order.)

On the subject of a single VFD being a 3 phase shop source - I thought the VFD was critically dependent on having the motor windings as an integral
part of the output capacitor circuit hence you had to view them as per motor. I was under the impression you had to physically (like reconfigure with
a set of disconnects) to choose which motor to use as load at any given time, and you'd then have to reset parameters (like the ones I don't get to)
for each different machine you used. I thought this was to allow the chopper logic a chance at keeping each phase coherent for a single motor/load/rpm
condition. I'd be happy to be wrong on this as I'm currently (so to speak) going to get another VFD for the coolant motor.
I'm not planning on using VFD as a speed controller, so that makes the mini-powerplant model more feasible...

Regards,
f

 

[JST]

If you use a VFD as a 3 phase source, you just leave it at 60 Hz (50) and do not change speed. All it does is provide the voltage, just as if you use it normally, but do not change speed.

The well-known "Phase-Perfect" 3 phase converter is basically a VFD with some extra stuff applied to it. It is designed to operate a number of machines, with single phase as its input. You hook up the machines as if you had powerco 3 phase.

To do that with a normal VFD, you just have to oversize it enough that it can supply the loads plus the starting current for the largest motor. Many folks do that, although it is less common than other methods.

All the fancy extra motor parameter stuff is generally associated with the more sophisticated modes, such as what is commonly called "vector" mode. For standard "volts per Hertz" mode, only the basic rated voltage, current, etc is needed.

It sounds like you have one of the versions that is special purpose, with a different program in it, made for some particular use with a particular motor and machine. There have been a lot of them sold with no mention (possibly ignorantly) of the fact that they are non-standard.

You are correct that your best move is to leave the motor protector in place. It probably will not damage anything, but if it did damage the VFD, you could replace it with a decent TECO or Hitachi.

 

[mksj]

You do not mention the VFD you are using, but most VFD's you can set the motor parameters, and this would be different for each individual motor if they are not the same. Some VFD's allow you to tune the parameters to the individual motor, and can be used to switch between motors and use it's specific programming parameter file. I have only seen a few bare bone VFD's that one cannot set the motor parameters/overload current. Probably a waste to add another VFD for the coolant pump, as there are single phase versions for not much, otherwise KB electronics has some non-programmable basic VFD's (KBVF) that can be used.

Most circuit overload devices are not specifically designed to work with VFD's and those that are usually have operating restriction so that they will function correctly. If the plan is to use it a a fixed frequency source to drive multiple motors and retaining the stock controls, then you would probably be better off with an RPC. The following describes some challenges for overload protection devices at the output of VFDs:
For electronic overload relays, their current sensor technology may not be able to measure the load current and harmonics correctly when operating at frequencies outside their nominal sensing range.
For standard motor protection circuit breakers (MPCBs), the challenges are related to reflection of voltage pulse that causes high dielectric stress on the MPCB magnetic trip coils, which results in accelerated aging.

 

[JST]

He seems to have one that is a special variant (possibly of a disallowed type).

I've seen them before in threads, they either have no settable parameters, due to being custom-made for a specific motor and usage, or perhaps have a (now not available) code that must be entered to access the parameters.