Two speed motor or VFD?

[Barry1961]

A customer has an application for a mixer that will run at 120 rpm for most of the time but needs to run at 480 rpm for 5 min once or twice a day. The high speed is a cleaning cycle.
The required torque will be around 100 in-lb for both the low and high speeds. At low speed it will be mixing soft ice cream and at high speed water.
The speeds are not critical and could vary +/- 20% if needed. The torque is a guess base on previous known drives.
Would it be better to do this with a 2 speed motor or a VFD? Why?
Can the 2 speed motor produce higher than rated torque at high speed for 5 min?
Total package cost, motor, gearbox and controls is a major factor of course.
My gut tells me a VFD, but I can be swayed.

Barry1961

 

[weh3]

How big is the motor? What is your voltage? Single- or three-phase?

Is there such a thing as a motor that can switch pairs of poles in and out? E.g., an eight pole motor, nameplate speed 120: two poles for the 480 rpm and eight for the 120.

It seems as though I saw a thread about pole switching during the last couple of days. Is that what pole switching is about?

William

 

[Barry1961]

The low speed horsepower would be around .2hp and the high speed around .8hp.

The voltage is single phase 230-208 volt.

Barry1961

 

[Barry1961]

The customer says he is looking at a 2 speed, center tap, AC motor. I have heard of center tap on steppers and DC but am not familar with the AC center tap. Is this something new?

Barry1961

 

[weh3]

I would assume that two-speed means a starting speed and a running speed, with the start reactance switched out by a centrifugal switch. Center tap sounds like 120/240.

William

 

[Skogsgurra]

No. Center tap windings - aka Dahlander winding - is old hat. But can only produce 1:2 speed ratios. So I'm afraid that you cannot use that technique.

What about using two motors? One for mixing and one for cleaning. If you select speeds carefully you may be able to leave the low speed motor coupled while the high speed motor runs. But check for overspeed!

Two identical motors may be possible. Use different pulleys to get low and high speed. I do not think that the rinsing cycle needs as much power as the mixing cycle. And even if it does, I do not think that oversizing the mixing motor will cost very much. So the motors could probably be the same size. Reduces inventory and makes life simpler.

 

[itsmoked]

Why not a VFD? Gear it so at um say 100% it's the rinse cycle and at 25% it does the product? It could be small with its low HP requirements. Can use a microdrive. Gets to be a 3ph motor so it's more reliable.. I like it!

 

[sreid]

I've been looking at something similar. Reliance is selling (for example) a 2 HP sensorless vector drive package for about 1200 dollars. It is available with single phase 220 VAC Input.

It is a 1800 RPM motor so gear it down 3:1 with a belt drive.

VFD's give constant torque below base speed. They also allow programmable speeds and accel/decel ramps (easy on the transmission and load as compaired to off the line starts).

 

[itsmoked]

I would think this could be done for about 500 bucks. If that!

 

[Barry1961]

I was thinking of a 5:1 reduction and running overspeed during rinse cycle, was hoping the 5 min would not be long enough to damage motor. The customer is leaning toward a two speed motor 2/8 pole. I am looking for some reasons to go VFD. I don't like 2 speed motors because I have had problems getting them quick on breakdowns in the past.

sreid
You should be able to buy a 2hp sensorless vector drive package with 1000:1 turn down vector duty motor for about $700 if your a user, and about $600 if your a OEM. When your Reliance distributor takes you out to lunch order the steak and lobster.

Barry1961

 

[sreid]

Well, I was quoting the list price. It's been my experience that substantial price reductions for a single unit (I'm assuming this is the case, it wasn't stated otherwise)don't happen but I could be wrong.

My recolection from other posts is that the maximum NEMA overspeed allowed is 50% (?).

 

[itsmoked]

Here's the first drive I found at Automation Directs web site:

http://web5.automationdirect.com/static/specs/gs1drives.pdf

Amazing! The 1-hp version is $165!

Looks to me barry, like you spec the design to provide .8hp at say 140% speed and the .2hp will be there for the 1/4 speed.

 

[Barry1961]

Automation direct is a great place if your buying just one or two. I checked the 2hp sensorless vector price

http://web4.automationdirect.com/ad...-_460_VAC_Sensorless_Vector_Control)/GS3-22P0

it was $349 which is a fair price for a user. I have never heard of the Durapulse before though. They used to carry Hitachi which has worked well for me in past.

Back to the main topic. Why do I not want to use a 2 speed motor?

Barry1961

 

[edison123]

High inrush current. Low starting torque.

 

[DickDV]

Seems to me that the choice comes down to two: two motors on the same shaft or a VFD.

For two motors pick a 3600 rpm for high speed and 900rpm for low speed. Both should be reasonably available in single phase and reasonably cheap. The 2 pole motor will require 4 times the hp of the 8 pole motor. Make sure you pick a single phase type that gives you the starting and accel torque you need. Connect the 8 pole motor to the machine thru an over-running (one-way, or sprag) clutch. That way, when the faster motor is energized, the slower motor automatically looses the load and doesn't even need to be stopped.

I'd prefer the VFD system because I'm more familiar with them and the extra complexity doesn't bother me. It may be the deciding factor for you. For a VFD system, choose a two pole three phase motor with adaquate hp with a matching drive and run this motor up to 4800rpm (80 hz). You will not hurt the motor. For the slow speed, drop the frequency to 20 hz (1200rpm). You can expect at least the same level of torque as at high speed, probably a little more without any concern for motor heating, assuming a TEFC motor. Be sure the motor is Insulation Class F and has an MG1 Part 31 rating. Any "inverter ready" motor should have these specs.

The big benefit with the VFD system, in my view, is that, if in the future, you decide your first speed choices aren't quite right, its easy to adjust. Also, if you should decide for whatever reason, you need a third speed, again, no problem. The two-motor system wouldn't do either one very easily or cheaply or quickly.

Do not choose a two speed motor. With four to one ratio, the motor would be special and expensive. Also, expensive to repair or replace. Either of the above two suggested systems would cost less.

 

[mc5w]

To do that much speed change in a multiple spped motor you would need to use a 4-speed motor. These are no longer made. Almost all consequent pole motors that are made today are 4-pole by 8-pole. You could have a 2-pole by 8-pole motor made but it would have to be 2 winding. The overload relay that is for the higher current winding would need to be in the line all of the time such as connected ahead of the speed changing contactors. However, it would probably cost more than a bariable frequency drive.

If I were using a VFD in this application I would make the high speed say 80 or 120 Hertz and the low speed 20 or 30 Hertz respectively. Induction motors tend to have performance problems below 15 Hertz related to low induced voltage in the rotor. The only motor design possible below 20 Hertz is design D-ish because of the low induced voltage. However, in your application a design D-ish torque characteristic could be an advantage because of clumps so I would try different gear reductions and different frequency pair while monitoring motor current. The best ammeter for the job is a repulsion moving iron meter that has a D.C. to 133 Hertz frequency range in addition to a true RMS that can also pick up the high frequency current from the carrier. The repulsion moving iron meter will pick up the wanted frequency current that is driving torque and the true RMS will pick up all of the components that heat the motor.

Another altenative would be a 2-speed pneumatically shifted gearbox like on heavy trucks. A 16-speed transmission is in actuality 3 transmissions in series. The first one is a 4-speed gearbox with reverse and it it shifted with a level. The next two are 2 speed and are shifted with vacuum or compressed air.

Mike Cole, mc5w at earthlink dot net

 

[DickDV]

Ammeters of any design are subject to large error when placed on VFD driven motor leads.

Far better to use the 0-10V or 0-20ma analog output on the drive that represents current for your metering.