720 RPM motor on a VFD 4

[itsmoked]

I have a customer who wants to reduce the speed of a large grinder. He wants to use a VFD to drop the speed about 20%. They're grinding ceramics so there is fairly low torque involved, usually something on the order of 10% of what the machine was designed for. In my opinion this does make this a reasonable candidate for VFD slowing.

My question is that the 25hp 60Hz 220V 50 year old motor has a plate that reads:

With these innards.

It states 720RPM which means @60Hz that we're talking about a 10 pole motor.

1) Will a typical VFD stoop to this many poles?

2) Isn't 720RPM the synchronous speed?? Does that make this a synchronous motor? Is that a problem?


Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

220 Volt motor?
115 Volt controls?
I would guess that with those ratings the motor may be over 60 years old.
Do you really want to hit those old windings with a PWM waveform?
How about a dip and rebake of the motor as well as a good filter?

Isn't 720RPM the synchronous speed?? Does that make this a synchronous motor? Is that a problem?

Yes and no;
Yes, it is a synchronous speed, but occasionally we see induction motors rated for the synchronous speed rather than the actual speed.
What would make it a synchronous motor would be slip rings and a revolving field winding.
"Is that a problem?"
It depends in part on the method of field energizaton.
As I understand it, the field energization methods look at the slip frequency rather than the applied frequency.
The original "Polarized Field Frequency Relay looked at slip frequency.
As far as I know, the modern rotating field energization circuits look at slip frequency.
I have never seen a centrifugal switch used to control field energization, but such a method, if used, may not operate at 80% speed.
I am betting that this is an induction motor.
The biggest issue that I see is hitting those old windings with PWM.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

There is no reason the expect it can't be done. With low speed motors or old motors like this, you also size the VFD to the motor FLA, not just HP, which could me an upsize in the VFD.

The picture of the motor doesn't show anything that indicates a synchronous motor.

The question of the windings withstanding the VFD waveform is probably your main concern.

 

[jraef]

That nameplate is for the entire machine (Blanchard is a famous maker of surface grinding machines), not the motor. There is likely another plate on the motor frame itself inside and it would have more pertinent information.

If you zoom out on that diagram of the motor, does it look like there is another smaller motor attached to the bottom shaft? If so, that might be an indicator that it's s synchronous motor, and that "little motor" on the end of the shaft is actually a field generator. If so, retrofitting a VFD can be problematic in that this field generator will no longer generate enough to excite the field in the synch motor, so you would need to add external means to get that voltage. Not impossible, just important.

VFDs work fine on synchronous motors, so long as you address those excitation issues and size the VFD for the motor nameplate FLA, not the "HP". Generally speaking, running older 230V motors is rarely a big problem for VFDs causing winding insulation damage, but if they are super concerned, add a dv/dt filter to the output of the VFD.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[itsmoked]

I will dig a little deeper. I'd planned to include an output reactor to round some corners.

Thank you gentlemen.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[EdStainless]

Dip and rebake, and very good filters, the smoother the waveform the better.
These old motors have a lot of iron in them but the windings are my concern.
Adding a few temperature sensors would be a good idea. And tighter current limits for sure.
Our Blanchard's had War Production tags on them, they were in use in 1941.
We have 4, and rebuild one each year (motor, bearings, ways). We can still hold better flatness and squareness than they will guarantee on a new one.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[jraef]

Old iron is good iron...


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[itsmoked]

I get the rebake an all but no way are these guys going to go that route. They are attempting to grind something that uses a new compound that demands a slower speed. It may not even work. So dropping $4k on a VFD/reactor install AND another $4k pulling that insane nightmare apart to go have a motor shop do something 'to increase the odds' just ain't gonna happen. LOL

The way these Silicon Valley places work is if the grinding works then they love it and if the motor subsequently chokes it's all about already having a sales-money-stream that now justifies the rewinding 'of that old motor'.

Besides, of the hundred VFD installs onto existing (old) motors I've seen, I can remember only a single one that the motor windings died on.

Don't get me wrong I know the VFDs are tough on insulation, just saying a lot of motors seem tolerant 'enough' to put up with it. I will admit that most of what I've seen had 20 foot or far less distances between the drive and the motor, so no long large standing waves getting after the insulation.

The one case and exception of a failed motor was a down-hole well pump 900ft down. Definitely brutal. The guy slapped on a VFD and the pump motor choked about 6 months later. He stuck a new pump/motor down-hole and it's been running for 7 years now. Not even sure why they put a VFD in that job - head scratching. But also I can't really say for sure the VFD was the old pump's demise since the replacement is still going 7 years on. I need to ask them why they used a VFD in that application.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

A growing trend here, (growing, not all) is to use VFDs to drive three phase pumps from single phase supplies.
A lot of farmers are on single phase distribution lines.
Out here a lot of water licenses specify a maximum pumping rate.
A VFD to set the pumping rate brings a lot more profit than a discharge throttle valve.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

From the CS drawing, it is clear rotor is cage rotor. I have rewound/refurbished a few GE Blanchards and they were invariably slow speed i.e. 360 to 720 RPM synch. All of them were rectangular form wound (not round wire, random wound) coils and hence should withstand VFD duty as long as they are in good physical shape.

Muthu

http://www.edison.co.in

 

[EdStainless]

If they don't fully rebuild the machine then they are taking a gamble. Grinding ceramic takes a very tight machine, all ways and bearings had better be in great shape.
As Edison said the winding pattern on these motors is usually very good, my only concern is the insulation itself.
A better VFD with smoother output waveform would be a worthwhile investment.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[waross]

A better VFD with smoother output waveform would be a worthwhile investment.

That may be a contradiction.
The sharper the transition from off to on and from on to off, the better the efficiency of the basic VFD.
The fast rise time of the pulses rather than the pulse frequency gives rise to the potpourri of high frequency components that cause the problems with the motor windings.
The orders of the frequencies may be:
Basic frequency; 10 to 400 Hertz.
The switching frequency; a few kiloHertz.
The frequencies of interest; May be measured in megaHertz and major fractions thereof, and dependent on the length of the leads.
An example:
Lead lengths of about 240 feet may tune as a one-quarter wave antenna for 1 megaHertz.
Lead lengths of about 48 feet may tune as a one-quarter wave antenna for 5 megaHertz.

The high frequencies may be mitigated by filters which are as effective outside the VFD as they are inside.
The longer the leads from the VFD to the motor, the greater the issue with motor abuse due to reflected high frequency waves.
Hence filtering is normally applied outside the VFD when needed due to long lead length or cases such as this where an old existing non inverter rated motor must be driven with a VFD.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[zlatkodo]

Rewinding to 12 poles (600 rpm) could be a cheaper option.

Motor repair and redesign

 

[itsmoked]

I need to ask them why they used a VFD in that application (900ft well)

They used a VFD to reduce pumping on the well as it's capacity dropped with the seasons.

EdStainless; These people are masters of ceramic grinding. They can hear one of their 40 machines from three rows away making an insignificant sound difference and know there's an issue. They are also masters of grinding ceramics with ancient thrice rebuilt machines that they rebuild themselves, sending out the ways for regrinding, having the lead-screws reground, laser mapping the errors, etc., etc. They prefer ancient heavy solid machines and since they're all made for metal grinding and cutting they're all far more powerful than required. They have dozens of machining centers with 480V 45A plates on them that they never exceed 3A draws on.

I'll definitely make sure there's some filtering on the VFD drive into that motor.


zlatkodo; That's not a bad idea except they grind quartz on this same machine normally. Here, they're trying to grind silicon carbide. The grinding wheel provider told them the surface speed has to be 3/4ths the Blanchard's normal speed. They tried it at the normal speed and the grinding wheel surface loaded up completely and irreparably in about 30 minutes. They're now believers. LOL





Keith Cress
kcress -

http://www.flaminsystems.com

 

[zlatkodo]

Or to use a grinding wheel of smaller diameter.

 

[edison123]

zlatkodo - These are surface grinders, not cylindrical grinders. The only way to reduce the surface speed is to reduce the speed.

Keith - Our client had similar requirement of 300 (original) and 150 RPM. I did the Dahlander rewinding and they have been using the lower speed whenever they wanted to grind harder material. It has been nearly a decade now.

Muthu

http://www.edison.co.in

 

[itsmoked]

Ohh wow. Nice trick with the Dahlander gambit!

I will run that by them. They have to partly de-skin the motor housing to get to the motor plate. Seems one of their key personnel's wives insisted she go visit her brother 'in person'. The next day her brother called to say he tested positive, two days later she's sick, tested positive, key person (husband) tested positive 3 days later. Both very sick. Sigh.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[zlatkodo]

Dahlander rewinding (10/20 poles if feasible at all) will provide just half of rpm.
Theoretically, the better option is PAM rewinding 10/12 poles, but it is feasible only for a few numbers of slots.
The next option is rewinding to two separate windings, but before anything, we need to know real motor nameplate details as well as the number of stator slots and rotor bars.
It is very likely that VFD could be the only option.
Support in Electric Motor Repair

 

[itsmoked]

[highlight #5195A4]Star date 98059.04[/highlight]

Turns out the motor was an in-house job built by Blanchard in... 1950. No plate. This should be fun.

They want the variability of a VFD so Dahlander appears to be a non-starter.

They're going to talk about the cost tomorrow.
It will need an auxiliary fan since it's currently sporting a squirrel-wheel blower bolted on the shaft (note above drawing).

We'll see what happens.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

If you want to experiment with different cutting speeds, VFD is the way to go. In all the Blanchards we rewound, we replaced the top shaft mounted weak ass fan with a 1000 RPM tubular axial fan wired to the motor contactor. The winding temps never exceeded 50 deg C.

Muthu

http://www.edison.co.in