Advantages of different NEMA motor designs

[Thedroid]

I'm working on a screen for seperating different sizes of rocks. The original motor was an old GE U frame 10 horse design B. It was replaced with a Baldor NEMA Premium design A 10hp motor. Since it was replaced there has been a problem getting the motor to start. The FLA is 12.5 @ 480v. The motor pulls about 90A for 20s before it settles down to about 8A. From what I've read about design A motors this is a common problem, and is usually solved by using a soft start.

From what I've heard the old U frame motors had alot more starting torque then modern motors. I'm wondering if I should try a design C or just stick with a B. Are there disadvantages in using a C? I know that there is more slip allowed, and I will be using a Premium efficiency motor.

The A has to go. We are having to run bigger overloads in order to get it to start, and also some of the wiring from the starter to the overloads showed signs of severe heat damage, and had to be replaced.

 

[itsmoked]

High efficiency motors have higher starting currents. From your description it sounds like your overloads are actually limiting the voltage to the motor so it takes longer to start and hence draws its higher starting current for far longer. Which is heating things up.

The 20 seconds is a sure sign that you have a voltage issue supplying the motor.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[electricpete]

The design A has higher allowable starting current (but not necessarily higher starting torque). This is likely causing a higher voltage drop a Keith suggested, which is prolonging the starting time.

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[Thedroid]

What kind of things could I do to solve this problem?

Another thing I noticed was that the wires going out to the motor are aluminum. They looked like 6 or 4 ga and I would say that the run is at least 200ft maybe even a little longer.

Monday I'll check the ampacity of the wires and also the condition of the overloads. The starter is an old GE nema size 1 using overload heaters. I checked the contacts on Thursday and they looked OK.

Would hooking up a recorder at the starter show me the voltage drop at the motor, or would I have to make my measurements at the motor terminal box?

If my measurement show no voltage drop would a design C or B be a better motor for this application.

The motor drives a large eccentric which oscillates a screen to seperate different size rocks. Takes awhile to get up to full speed. Every thing turn freely at the pulley using just arm power.

 

[roydm]

Large screens take a lot of starting, I suspect the original motor was rated on the conservative side, Any chance of putting on a 15HP?
Roy

 

[Thedroid]

I agree. That was one of the plans to put on a 15hp. What would be the best design to go with in that case. I would think a design B

 

[HCBFlash]

Most people loathe aluminum wiring, and for good reason.

If you're looking at voltage drop issues AL is definitely a factor. The total circuit impedance will certainly be higher than the same size CU conductors, but the resistance of the splices and terminations will be the biggest problem for sure. To accurately determine your voltage (and impedance) characteristics you will have to measure -> at the actual motor lead at the point of connection <-.

You really need to know and consider as many factors as possible about the load presented to the motor. If the power transmission system from the motor to the presented load is shock absorbing, the startups are always on unloaded equipment, the load's mass varies widely, the electrical power supply is clean and solid, the machine has been modified, the environment is hot or dirty, etc. all have more impact on achieved efficiency than minor differences in motor designs.

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Me wrong? I'm just fine-tuning my sarcasm!

 

[DickDV]

Just change to a 10hp Nema Design C motor and most of the mentioned problems should go away. You will sacrifice a little efficiency with Design C but the high starting torque with relatively low starting current will be just what the doctor ordered!

 

[Thedroid]

Thats what was thinking also. I'm going to investigate the whole circuit on Monday and make sure everything is in order. I'll check the wire size, condition of wire, and also measure the voltage drop at the motor upon startup. I'm leaning towards the design C motor if everything checked out.

That screen ran for 50yrs without issue until the design A motor was installed.

 

[waross]

Is there enough left of the old "U" frame to rewind it? It will be worth the price.
From the Cowern papers;
"Design C is a high starting torque
motor that is usually confined to hard to start loads,
such as conveyors that are going to operate under
difficult conditions."
If you haven't discovered the Cowern papers;

http://www.baldor.com/pdf/manuals/PR2525.pdf

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

 

[Thedroid]

I'm actually in the process of replacing the bearings now. It will probably be used as a spare. The boss wants to use a spare drive we have laying around and see how that works. High efficiency is first priority.

I have to take back what I said about the aluminum wiring. It's actually copper, #8 about 250ft.

 

[LionelHutz]

An old 10hp, 284U frame motor should be equivalent in size to a new 15hp, 284T frame motor.

So, you may need a 15hp motor to get the equivalent starting characteristics. I would use a 15hp design B motor or a 10hp design C motor.

The design A motors tend to have low starting torque and high breakdown torque. The design B motors tend to have a little more starting torque and a little less breakdown torque.

 

[waross]

I agree with LionelHutz.Another option may be a design D
"D" motor but then your efficiency drops.
Efficiency is a great priority, but when you compare a 10 hp motor that won't start the plant with a 15 hp or old 10 hp "U" frame motor, neither of which is as efficient as the new 10 hp motor, the motor that actually starts the plant may contribute more to the profit column.

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

 

[Thedroid]

Agreed. Looks like for now the plan is to hook up a spare vfd to the high efficiency motor and experiment with it to see if it will do what we want.