Can a Design D motor be replaced with a design B? 4

[Handasee]

We have a number of chip spinners that use a Design D motor due to the initial high torque required to start.
An outside contractor was asked to refurbish one of these spinners and returned the spinner with a Design B motor that utilized a soft start. When we tried the unit, it failed to start. Has anyone experience in replacing
design "D" motors with design B controlled by VFD or softstarts?
Specifications of the chip spinner are: 7.5Hp 600V 1800RPM
3ph. 60HZ. Weight of brass chips that are to be spun is 500lbs.
I say that the Design B motor failed to start, because we measured a start-up current draw of 5 times the motor nameplate rated current for a period of 60 seconds and the spinner was not up to half the required speed. The Hp of the design B motor was 10 HP.
The Design D motors can usually start this load in 20 to 40 seconds.
From previous experiences, I have tried a 15Hp Design B motor to replace a 7.5Hp Design D motor without success.

 

[sreid]

The basic torque curves at this web site probably show why the NEMA design B motor is not working.

http://www.engin.umich.edu/labs/csdl/ME350/motors/ac/induction/3-phase/

 

[mc5w]

Method 1:

You would a 30 HP design B motor that is hooked up to a 30 HP variable frequency drive. The drive would be one that can do reduced voltage running after starting, say 60% of normal voltage Which would give 36% running torque.

Method 2:

Install a 10 HP or 15 HP motor that drives a 2 stage hydraulic pump, such as a log splitter pump. Route through a 3-way solenoid valve that allows motor to start unloaded.
the 3-way valve then drives a hydraulic motor. This is NOT necessarily more efficient than a design D motor.

Method 3: couple a 20 HP and a 10 HP design B motors together. Start both motors using a soft start or just simply drop across the line. When up to full speed disconnect the 20 HP motor. One way to do this is with a belt for the 10 HP motor. Another way to do this is to remove the fan from the 20 HP motor and couple the 10 HP motor in place of the fan. You would need to build a plenum with separate blower to cool the 20 HP motor.

Variation of method 3 is to use a torque converter and belt to couple the 20 HP motor to the 10 HP motor. The 10 HP motor direct drives the load without the torque converter.

 

[DickDV]

Assuming the soft starter is programmed properly, if you are seeing 5 times nameplate for 60 seconds and the load is accellerated only to half-speed, you have a load that is not only hard starting but is difficult to accelerate up to speed as well.

If you have no need for variable speed in your process and inrush currents are manageable, I would agree that the NEMA B motor is a poor choice and replacing it with a NEMA D is appropriate.

If, on the other hand, you could use variable speed to improve your process, then I would save the NEMA B motor and power it with a sensorless-vector drive that has output ampacity of not less than 250% of motor nameplate.

A further improvement if possible would be to change the power train between the load and the motor so, at maximum load speed, the motor is turning at 150% (90Hz) of its nameplate speed. This is easy to do if the motor is belt or chain coupled to the load, not so easy if direct coupled or thru a gearbox.

Properly programmed, this drive and motor package should be able to accelerate the load as fast and maybe a bit faster than the old NEMA D motor and with lower inrush currents as well.

You will need someone who is skilled in drive programming and motor torque optimization to get this done properly but the results will be most satisfactory.

The machine rebuilder is trying to get you away from a motor that, in the future, will be very hard to replace. They just forgot to figure the accel torque properly. I think the situation can be saved with a little ingenuity.

Understand that I am prejudiced (I'm a rep for ABB) but, if I was charged with solving this problem, I would use an ABB model ACS800. It has performance capabilities that match this application perfectly. Just be sure any drive you choose has output ampacity of 250% of motor nameplate for one minute--not a bit less.

 

[aolalde]

Your application is a high inertia load that requires high torque to accelerate at full speed in 20 to 40 seconds (say 30 sec). The mathematic expression of the process is:

t = Drpm * WK^2 /(308 * Tave)

t = acceleration time (sec)
Drpm = Speed change in revolutions per minute (RPM)
WK^2 = inertia constant of the accelerated system. (LBf*Ft^2)
Tave = average accelerating torque ( Lb-Ft, Motor torque minus load torque)

The NEMA design B motor has lower torque and higher starting current, and then it is a disastrous selection for this application.

An electronic soft starter will chop the applied voltage to control the starting current which will reduce the torque more and will not work.

Larger capacity design B motors ( 15, 20, 30HP as somebody proposed) are not adequate solutions since will draw higher current and the efficiency will be very low at full speed operation.

The only option I see is the design B, 7.5 HP motor with a Vector Variable Frequency Driver which softly will start your load at low frequency controlling the current to your line capacity. The difference will be a kind of synchronous starting with more effective motor torque and reduced current. The time to accelerate your inertia could increase a little bit but I do not think will exceed the 40 seconds.

 

[jraef]

I agree with others that the standard design B motor and soft starter was a poor choice for replacement. It goes to show that (unfortunately) many people do not understand the nature and proper application of soft starters, thinking of them as "poor man's drives without the speed change" as in this application. Soft starters are best at REDUCING output torque from the motor if starting torque is excessive, but cannot reduce the REQUIRED acceleration torque as aolalde has shown in his formula.

While I have successfully applied soft starters to centrifuges and separators (similar applications), they all had specially designed motors that could be held in current limit for an extended time, and the soft starters were sized to provide 300% FLA virtually continuously. Standard motors would not work with standard soft starters. That said, here are my opinions on options:

1) If you are going to keep the design B motor, go with a Vector VFD with the highest output torque you can get as per DickDV's post, he is right on (other than the brand recommendation, but we work for competitors so take that with a grain of salt from all of us!). The reason is that with that extra torque capability in the VFD, you will be able to accelerate it faster than without it. You can probably get by with a lower overload rating, but it will require a longer accel time and a lower current limit setting to avoid overheating the motor. If you try that, get a separate motor cooling blower to avoid problems of overheating at low speeds in the beginning.

2) Go back to a design D motor and across-the-line start. You know that it worked. Using a soft starter on a design D motor is an oxymoron.

3) If Across-the-Line starting is a problem for your power system and a VFD is too expensive, you could possibly get help from the equipment manufacturer to find a suitable centrifuge motor design that will work for it, and use a soft starter, oversized 3x to allow for continuous current limit with enough torque to accelerate the load. Honestly though, it will probably be less expensive to go with the VFD option above.



"Venditori de oleum-vipera non vigere excordis populi"

 

[DickDV]

Hey, jraef! Thanks for being so gentle with my brand recommendation. How about telling me your brand affiliation so I can be sensitive back. (Interpret that as me needing all the friends I can get!)

 

[macmckim]

Not many Design D motor manufactures still around in North America. A good mfg for 1800RPM Design D motors is company called Imperial Electric in Ohio. IE mfg's mostly hoist and elevator, but can offer specials for centrifuge applications
Web site is "

http://www.imperialelectric.com

For what is worth, might help you find a supplier for replacement purposes.

 

[jraef]

Hello DickDV,
I respect your opinions and helpfulness, but if it's all the same to you I prefer not to post my affiliation. Some members here know it, but I believe in the sanctity of the non-commercial nature of this forum. Other forums that I visit which are not as strict about commercial messages are bogged down in aggregious advertising, making them into places where nobody gets useful info. While I don't consider your post inappropriate, I prefer to practice what I preach as religiously as possible.

If you visit the Motor Control Forum at

http://www.lmphotonics.com

I use the same member name there, and that forum includes a private messaging system called U2U. If you are not signed up on it, it is a good one too, I highly recommend it.

"Venditori de oleum-vipera non vigere excordis populi"

 

[mc5w]

Baldor also still makes design D motors but the National Energy Policy act restricts their use to specialized aopplications such as yours. This is because they have lower efficiency than design B or design C.

That said, design D motors need an oversize motor frame to get rid of the heat from the high resistance rotor. Any kind of shaft, gearbox, belt, or drive chain needs to be rated for the starting torque. Horsepower ratings based on the more typical design B or C motors can lead to lubrication failure during starting of short belt life.

 

[Handasee]

I would like to thank everyone for their input to my problem. In summary, the most practical solution would be to purchase a design D motor.
I will look into Vector Variable Frequency Drivers.
Reading the manual on correctly programming a vector Frequency drive for this application will be a good learning project. At this point, I do not know what the cost of these drives are, my sales reps will be able to supply me with the viability of replacing design D motors with VVFD.

 

[mc5w]

Another possibility is to parallel a design D motor and a design B motor. You would start using the design D motor, energize the design B motor, then deenergize the design D motor.

The extra money may not be worth the extra expense but is in the middle as far as expense is concerned.

 

[sreid]

And just as we thought design D motors are hard tog;

http://www.baldor.com/news/new_products.asp?id=336

 

[Farkel]

Tatung Electric makes both ODP, and TEFC design D machines in both slips of 5-8%, and 8-13%. They are special order, but can be had. U.S. Drives can supply a drive to power them. Another thought would be go to a design C machine, depending on the lock rotor torque required for the application.