Wide speed range in VFDs

  • Thread starter Patrick Levinson
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Patrick Levinson

Hello list members.

I'm considering the replacement of the 4 motors in an industrial washing machine with a single motor/VFD combination. What concerns me is the wide speed range that the motor/VFD would need to handle, since it has to replace the current 15 hp motor at low speeds (about 30 rpm) and the current 20 hp motor at high speed (about 500 rpm). Thats a speed ratio of about 17:1 between the highest and lowest speeds. Right now the switching between motors is done through a series
of pneumatic clutches and speed reducers.

The low speed is needed for the normal washing cycle (reversing direction periodically) for about 40 minutes, and the high speed is required for the 10 minute spinning cycle. Does anybody has any comments on this wide range of speeds at this power levels? I have worked with wide speed ranges on VFDs with no problems, but maybe motor heating would be an issue at low speeds in these particular application, since it would have stay at about 6% maximum speed most of the time, and still be able to go to 100% speed for
approximately 10 minutes each cycle.

Thanks and best regards, Pat.
You can either oversize the motor accordingly or specify an inverter duty motor i.e. with electric blower.

Anthony Kerstens

You will have trouble at the lower speed. If you gear it so that 60Hz is 500RPM, then 30RPM will be 3.6Hz.
If you overspeed the motor at 90Hz geared to 500RPM, then 30RPM will be 5.4Hz.

Either way I would not recommend this. If you're
desperate to run things with one motor, maybe there's a way to mechanically engage different pulleys.

Anthony Kerstens P.Eng.

Ralph G. McDonald, P.E.

Hi Pat:

You will need to use a inverter rated, separately fan cooled motor. This will have a seperate motor on the cooling fan so it runs at 100% speed. I would also recommend a motor with RTDs embeded in the motor windings to allow you to monitor internal motor temperature. Also if you coodinate with the VFD and motor manufacturers you may be able to run the motor at greater then 100% speed. In some cases up to 200%. I hope these hints help.

You might try the newer AB vector drive. It produces same torque at 2Hz as it does 60Hz, or so they claim. I have seen it work and I believe that it does.
You may need forced air cooling for the low speeds, unless the motors are oversized, or are not under a heavy load.

Also, don't forget to consider the existing mechanical advantages of having several gear ratios. If the lowest speed uses a 10:1 gear
reduction, and the highest speed has a 1:1 reduction, then you may need to size a motor with 10x torque to handle the load. Although, you may be able to improve the situation by using some gear reduction to help the low speed/high load conditions and running the motor above base speed to get the high speed/light load conditions.

Bill Sturm
Pat's concern was motor heating so I assumed he already determined how to handle 16:1 speed range. This can be easily done with today's high
performance VFDs. 3.6 Hz can be easily handled by a vector drive with or w/o feedback, or even a good sine PWM drive. An industrial washing machine
should have high inertia which will act as a speed filter to smooth out any low Hz VFD distortion. As you mention, you can even set 500rpm to be > 60Hz to boost the low end Hz. However, the torque drop above 60Hz needs to be
factored in. Regardless, there is NO way this application needs to resort to a mechanical speed changing solution again - what he's trying to get rid of.

Anthony Kerstens

Granted, so such a vector drive would _not_ be the cheap variety that one may be tempted to install in such a machine?? Especially the AB 160's with the 4kHz carrier that produce an audible whine in the motor.

This is a washing machine, so does it oscillate
in the wash cycle? If it does, then the inertia
regularly changes directions and hence has variable load at the motor. This would be kind of like driving a low speed high inertia vibrator with a VFD. I've never tried that, so I don't know how well it would work. Does this thing have a flywheel?

Finally, barring answers to the above questions,
I would still suggest a mechanical method.
Pat said he was looking at replacing 4 motors with 1.
I tend to look for the simplest solution where I can, regardless of whether it's electrical or not.

In case you're wondering, IMOH mechanical is simpler
1. I don't think a laundry would keep spares, so a
bummed VFD would be a problem.
2. A laundry isn't likely to bring in an experienced industrial electrician, so some pulleys and solenoids would make repairs possible.
3. Not all applications require top-notch technology. I do tend to be a minimalist.

Anthony Kerstens P.Eng.
First.... Don't rule out hydraulic. (see more below)

Motor sizing...
Not a particular problem, if you select carefully. Consider that some vector drive/motor combinations hold full torque at zero speed.

You will have an external electric blower on the motor. The motor will probably be at least 60 HP, simply to reach the low speed torque requirement.

Consider an 8 or 12 pole motor. 60Hertz base speed of 800 and 600 RPM More low speed torque! This keeps the maximum motor speed lower compared to the 4 pole motor.

12 pole motor 60 Horsepower Base speed 600 RPM Top speed 2500 RPM
Results: 15 Hertz minimum = 150 RPM through 5:1 Gear Box
Max Freq requirement sounds high..... Take the motor up to 75 HP and rework, etc. (is this realistic?.....not the point. The application
can be sized!! )

The motor horsepower will be effectively sized to meet your low speed torque requirement. It would be a special motor, but certainly could be done.
Don't rule out hydraulic!!!!!!! (possible example)
25 Horsepower motor with 2 pumps in tandem on hyd. power unit
First pump large displacement operating at low pressure
Second pump small displacement operating at higher pressure
1 hydraulic motor for the washer drive with no gearbox required

When is comes to power transmission hydraulics can show some interesting advantages. This may be one of those situations.

Let me know if you have questions

David Kane [email protected]
Kane Engineering Group Inc.
Grand Rapids, Michigan
Certified Motion Control Specialist,
Association of International Motion Engineeers AIME

Sounds like there is a good reason for the two motors/drivetrains. It is likely that the wash cycle requires the low speed at a much higher
torque while the spin cycle is typically a low torque high speed application. Do you know what the actual speed/torque requirements are?
Does it need 15HP at 30 rpm (2600+ lb-ft of torque) . . . I doubt it ..

I would also assume that the drum is actually turning at the 30 rpm during wash and 500 rpm at spin while the motor(s) are probably turning
at the synchronous speed across the line. This would equate to about a 3:5 reduction for the spin cycle (1750rpm / 500rpm) and a ~60:1
reduction for the wash cycle (1750rpm / 30 rpm) - assuming a 4-pole motor running at 60Hz. It would be hard to reconcile these two operating points with a single motor / gearbox combination.

As far as heating concerns at the low speed operation, use a TENV design motor and you will be fine (even without a blower). You can also typically overspeed these motors easily by 2 - 3 x base speed or run all day at full torque zero speed without heating problems.

It is best to put an encoder on the motor (Baldor sells vector duty motors this way) and use a true Vector controlled drive - personally I like the UniDrive for this type of app because it has good open loop (encoderless) control and if you decide you want better performance, just add an encoder and change a few drive parameters and you are now
running closed loop control. We have over 400 of these running now in some very abusive applications and are pretty happy with them.

Ken Brown
Applied Motion Systems, Inc.
I agree with what you mentioned, but had one question.

Does Baldor (or other motor mfg.) rate there larger motors for TENV? Or derate larger TEFC motor?

I usually jump to the conclusion to put a blower on the back, because I don't see them in the catalog.

Dave Kane
Baldor is very conservative in the design of inverter duty motors (as is the case with most motors designed to NEMA specifications). Nearly
every motor power rating that Baldor makes for inverter / vector duty applications is available in a TENV configuration. We have used these
motor in dockside winching applications holding barges in position for loading where they run for days at a time at full torque and zero speed.
They never get much warmer than a few degrees above ambient temperature. When running with an encoder and a good FOC algorithm, the drive makes
so much more efficient use of the current to produce torque, that it is not unusual to run a motor at 125 - 150% rated current with no adverse
thermal effects. I have had motor designers from Reliance scoff at this, but the proof is in the many applications where this has been
successfully done. NEMA motors are designed for 600% in-rush and typically 1.15 service factor . . . a vector drive will never subject
the motor to this kind of abuse and it will always maintain the correct orientation between the field and stator currents so . . . . thermal
concerns are way down on the list of things to worry about when applying this technology.

Ken Brown
Applied Motion Systems, Inc.