How slow can VFD go?

[S'mise]

I have an old Allen Bradley 160 ba03nps1 frequency drive feeding a conveyor belt.

Speed is adjust by a potentiometer except it doesn't go slow enough for the conveyor.

Motor is a 3/4 hp induction that goes into a gearbox (No, I don't know the ratio.)

Minimum Speed set to 0hz and all other parameters look normal to me.

Now I'm wondering how slow can a simple drive like this go?

 

[ptonsparky]

It's not the drive you need to worry about, it's the motor. Typically the motor is fan cooled. Slow the motor and you loose cooling. Newer motors will be VFD rated and have a ratio indicating lower safe speeds. Look at the nameplate. Change the gear ratio or if sheaves on it, change them.

 

[S'mise]

Motor health aside, Why does VFD make motor spin significantly even with the speed pot turned all the way down?
The parameter for min speed is set to 0hz.

I can't make the motor go any slower.

 

[winnie]

If the VFD is actually outputting 0Hz, then it should DC brake the motor.

My guess is that there is some configuration which you are missing with a higher minimum frequency, or the 'speed pot' has an analog minimum value.

-Jon

 

[GoldDigger]

Motor health aside, Why does VFD make motor spin significantly even with the speed pot turned all the way down?
The parameter for min speed is set to 0hz.

I can't make the motor go any slower.

First thing I would look at is the circuit of the speed control pot.
Is it a potentiometer fed from a reference voltage and ground? Is there a series resistor that might be offsetting the zero voltage?
Just what type of signal is the speed control input of the VFD expecting? Voltage? 20ma loop? Resistance?
The slower speed/lower frequency will draw higher current per volt through the motor inductance, with the reverse EMF playing a part.
Depending on how your current limits are set and whether you are using open loop, speed transducer, or sensorless vector control, the VFD may be deciding that it cannot run the motor any slower and still stay within operating limits.
(The unit you describe may not have any of the fancier features, but it may still contain some protective operating limits.)

 

[Jraef]

I don’t understand your question.

The parameter for min speed is set to 0hz.

I can't make the motor go any slower.

You want it to go slower than zero?

I believe maybe you left out something in your description of what’s happening.

But generally, small cheap V/Hz type drives like this are only good for about a 4:1 turn down ratio. What that means is that below about 15Hz, the ability of the drive to create stable torque in the motor is compromised by the amount of current it takes just to energize the windings. Better drives use “vector control” of sone sort to correct for that issue. This drive existed prior to Vector Control being a commonly available (as it is now).

Another aspect is that given the fact that Allen Bradley stopped selling those drives well over a decade ago, there is likely a 100% chance that it has been used previously. So you will have no issues what oddball programming someone may have left behind. It’s always a good practice when suing old uses drives to do a factory reset to default values in order to wipe out any surprises left behind by the previous owner.

 

[GoldDigger]

I don’t understand your question.


You want it to go slower than zero?

With the pot set to zero the motor is rotating too fast for the application. And the cause is not an obvious parameter of the drive.

 

[S'mise]

Thank you all for your response.
The min speed is set at 0hz but the VFD outputs aprox 6hz (..I think) with speed pot at 0.

I recall the drive outputs 10v which goes to pot with reference to gnd. Then the middle wiper of pot is the reference voltage back to the drive.
I confirmed the reference is 0v with speed knob turned all the way down.

I think your probably right Goldigger. I'm not sure what the setting is offhand, but there's no external current feedback. Probably Sensorless vector control not letting is go any slower.

I'll post the parameter list when I get back to the shop.

Sounds like i might have to change out the motor to one with a electric fan.
Perhaps also a fancier drive.

 

[GoldDigger]

Sounds like i might have to change out the motor to one with a electric fan.
Perhaps also a fancier drive.

You do not see replacing the gear drive as an option? What about a motor with internal or external reduction gearing? Or multiple poles for a lower rotation speed?
Do you ever need to go close to the maximum speed of the motor? What fraction of maximum?

 

[S'mise]

Changing the gearbox is an option although harder to acquire and (sadly) even harder to convince mechanics to change it out.

Max speed set to 60hz which application never requires. Speed pot range is smooth between slow and fast.
I just need it to go slightly slower.

@Jraef, Sorry I overlooked your comment that this doesn't use vector control.

I recall the current setting was 130% nameplate.

I'm hoping there is a setting I can tweak to slightly slow it down. I understand this comes with the expense of it running hotter and higher current.

I do have a powerflex4 onhand that I could use if it has better range.

Thanks, I appreciate your help.

 

[ptonsparky]

Changing the gearbox is an option although harder to acquire and (sadly) even harder to convince mechanics to change it out.

Max speed set to 60hz which application never requires. Speed pot range is smooth between slow and fast.
I just need it to go slightly slower.

@Jraef, Sorry I overlooked your comment that this doesn't use vector control.

I recall the current setting was 130% nameplate.

I'm hoping there is a setting I can tweak to slightly slow it down. I understand this comes with the expense of it running hotter and higher current.

I do have a powerflex4 onhand that I could use if it has better range.

Thanks, I appreciate your help.

Use it. They are easy. Reset the parameters and start fresh.
Will it help? Good question.

 

[Jraef]

Probably Sensorless vector control not letting is go any slower.

The 160 was not a Sensorless Vector Control (SVC) drive, it was a low cost “shaft turner” V/Hz drive. So is the PF4 by the way. If you want SVC (and you do), you need at least a PF40, which just went obsolete too, or now the PF523 or 525.

 

[ptonsparky]

Chain or belt on the output of the gear reduction?

 

[GeorgeB]

6 pole ("1200 rpm") motor?

 

[ptonsparky]

VFD output is 6hz.
SEWAG the motor is running @ 180 rpm, then thru a gear reduction plus the belt or chain output reduction , , if any, and it's still too fast ? And a new VFD will cure everything?

 

[drcampbell]

First thing I would look at ...

First thing I would ask is whether this system ever worked correctly. Does it need to be redesigned or merely repaired?
(this is a common problem among engineers: instinctively leaping to redesigning something that only needs repair)

Next thing I would look at is the conveyor itself and whatever process it's feeding. What are the maximum & minimum speeds required? Have they changed since the day it was first installed & commissioned? What motor speeds and reduction ratios are required to make that happen?

Only then would I look at today's minimum frequency output of the VFD, to decide whether it's a VFD problem or a system problem. You can't assume that the VFD isn't working up to spec if you have no idea what the spec actually is.

Yes, that's going to require forming a consensus among the mechanics & mechanical engineers, the electricians & electrical engineers. But that's what it's going to take to get it right.

 

[ptonsparky]

First thing I would ask is whether this system ever worked correctly. Does it need to be redesigned or merely repaired?
(this is a common problem among engineers: instinctively leaping to redesigning something that only needs repair)

Next thing I would look at is the conveyor itself and whatever process it's feeding. What are the maximum & minimum speeds required? Have they changed since the day it was first installed & commissioned? What motor speeds and reduction ratios are required to make that happen?

Only then would I look at today's minimum frequency output of the VFD, to decide whether it's a VFD problem or a system problem. You can't assume that the VFD isn't working up to spec if you have no idea what the spec actually is.

Yes, that's going to require forming a consensus among the mechanics & mechanical engineers, the electricians & electrical engineers. But that's what it's going to take to get it right.

IDK how many times I've been asked to do fix something when they actually needed to start at beginning with exactly what the goals were. Generally what I ended up doing. Got paid so, what the heck.

 

[S'mise]

I don't believe conveyor ever worked correctly.
Perhaps the process or the workers have slowed down. Or, half century ago it used different motor and controls?

What workers routinely do is just hit the conveyor stop button when they need to catch up.

No, I don't think it's broken, just not designed well for the application.

I'm going to see if the sprockets can be changed.
To change the motor and VFD doesn't seem feasible.

Thanks

 

[Macbeth]

The drive has analog input scaling..
Look at
P60 Analog input Offset
P75 Analog input Min
P76 Analog input Max

 

[drcampbell]

IDK how many times I've been asked to do fix something when they actually needed to start at beginning with exactly what the goals were. Generally what I ended up doing. Got paid so, what the heck.

Yeah, I'm an engineer but as often as not end up in the trenches changing sprockets or whatnot.