Motor Failure On Batch System

[fornhamspark]

A customer came in with a burnt out motor on a batch system. The motor was replaced without sucess, any help appreciated.
Layout of system dol starter to motor, motor via belt and pulley to gear box 20/1 Ratio, to electric clutch and direct drive to paddle wheel to meaaure out fixed amounts of grain. The final motor will pull 1.9A clutch disengaged and 3.2A when clutch engaged. After 2 minutes of cycles motor gets hot and trips. Motor No.3 was a frame size larger to give a little nore inertia.

Starter checked gearbox refurbed by Fenners and batch wheel bearings replaced.

I must be missing something about the old motor ?????????

Original Motor Brook crompton
Frame D71B 0.37kw, 1360 rpm, 3 phase, 1.3A, Class F, RiseB, Rate MCR, IP54.

Replacement No.1 Motor Electro Motor
Frame TMA-71-G4, 0.37kw, 1400rpm, 3 phase, 1.04A, Class F, IMB3.

Replacement No.2 F.F.D
Frame 71x 4C 0.55kw, 1500rpm, 3 phase, 1.9A

 

[davidbeach]

What was the torque/speed curve of the first motor vs. the torque/speed curve of the subsequent motors? Sounds like the original motor might have been a high slip motor while the replacements were more along the lines of an "energy efficient" or low slip motor. Low slip is great for some loads, but those high slip motors exist for a reason.

 

[waross]

It sounds as if your motor is drawing full load amps with no load. That could be a bad gearbox, but I accept that you have had the gearbox and bearings checked.
Have you checked the electric clutch for dragging?
The next most obvious concern is the supply voltage. It sounds like unbalanced voltages.
This can be a bad connection somewhere that is affecting only this motor. It may be a plant wide condition that this small motor is more suseptible to than the other motors.
And don't overlook davidbeache's advice on motor slip. The original motor is definitely high slip (9%).
The third motor is rated at synchronous speed rather than actual speed so the slip is unknown, but it will probably be low.
respectfully

 

[fornhamspark]

Thanks for advice chaps. The supply balanced load etc has been checked as has the bearings etc.

With regard to the slip, could i tell if the motor was high slip from the name plate??, or only from the manufacturer

i know the slip is a function of lagging the supply frequency, i think.

But could you please try and explain how this relates if any to the torque or how this relates to the problem.

What are high slip motors used for.

Thanks again

 

[davidbeach]

High slip and low slip motors have very different speed-torque curves. In the NEMA world, a type D would be a high slip motor while a type B would be a low slip motor. So for two motors of the same hp, a low slip motor will run closer to synchronous speed than a high slip motor. In general, the lower the slip the higher the efficiency. But certain loads, such as those where load is frequently connected and disconnected via a clutch, will have better operating characteristics using a high slip motor than using a low slip motor. Applications where the motor drives a fly wheel and chunks of energy are extracted from the fly wheel, such as punches, are also better served from high slip motors than from low slip motors. On the other hand a fan or pump running a fairly stable load will work with a high slip motor but will be much more efficient if a low slip motor is applied.

The torque transients experienced by your motors as the load is added produce much higher rotor current transients (and therefore also much higher stator current transients) than would be seen by a high slip motor.

 

[waross]

Hi fornhamspark and davidbeach;
This looks like IEC land and our NEMA letters may nnot apply, but your post is right on.
fornhamspark, The synchronous speed of all the motors that you have used is 1500 RPM. The difference between the synchronous speed and the actual speed is the slip. Slip is usually expressed in percentage, sometimes in RPM.
david and I are used to 1800 RPM motors running in the range of 1740 to 1760 RPM. Ballpark 50 RPM slip, or 2.5% to 3% slip.
Your 1500 motor actually running at 1360 RPM has 140 RPM slip, or 9%+slip.
The next motor at 1400 RPM had 100 rpm slip or 6.7% slip.
The last motor, rated at 1500 RPM probably had 2.5% to 3% slip. I can't support this figure, it's an educated guess.
The slip of a motor depends on the shape, resistance and placing of the squirrel cage bars in the rotor.
When checking voltage balance it is well to check at the incoming panel, at the motor starter with the motor running, and at the motor terminals with the motor running.
Not withstanding the motor slip, your motor is rated at 1.9 amps full load current and is drawing 1.9 amps with the clutch disengaged. Take your pick, mechanical drag or unbalanced supply voltage.
Is the clutch actually a clutch brake and is the brake dragging. Is the belt too tight and causing drag.
respectfully

 

[jraef]

Excellent responses.
To relate this back to your problem fornhamspark, slip is indicative of the torque response in the motor. High slip means rapid torque recovery when the load changes suddenly, referred to as step change in loading. Without it (i.e. a lower slip "regular" motor), a motor will take slightly longer to recover from a step change in loading, such as your clutch engaging, so it will pull higher current for a longer period of time to regain speed. It may only be a few seconds longer, but in repetitive cycles like that it adds up to a lot of extra thermal stress on the motor, especially the rotor. The trade off as mentioned above is that high slip motors are less efficient. But if the more efficient motor burns up, the entire machine becomes "100% inefficient"!

So even though you replaced the original motor, the replacements were likely not suited for the application. In fact, the original motor may have even been marginal to begin with. Next time make sure ALL of the nameplate data matches and you may even want to consider increasing the motor size if possible on this application.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

davidbeach and jraef;
What do you think of the posted currents. Apart from the issues of slip, it appears to me that the motor will fail at those currents even if it is running steadily, let alone cycling. There are applications where a low slip motor on a forgiving duty cycle may do the job even though a high slip motor would be a much better choice.
Here though the motor appears to be rated at 1.9 amps and is drawing 1.9amps unloaded.
That's a 100% load before it starts to work.
Full load current appears to be 3.2 amps or 168% of rated current. I don't think that motor characteristics are the only or the most important problem here.
Respectfully

 

[jraef]

I suppose you're probably right, but I interpreted it as a short time transient peak reading as the clutch engages and the paddle passes through the grain. Technically though he didn't say. Not only that, but now that you got me reading his post more thoroughly, if the newest "one frame size up" motor is drawing FLA at 1.9A uncoupled, that original motor must have been overloaded even then!

fornhamspark,
Pursuant to what waross brought up, what is the cycle timing on this? How long on (clutch engaged), how long off (disengaged), how many times per minute or hour? Is the motor turned off in between or left running continuously as the clutch comes on and off?

Additional issues; how did you (or someone) determine that this was the correct motor size for this application? Also, I don'tr mean to insult you, but this is a really common problem beleive it or not. Are you absolutely sure there is oil in the gear box? If it was brand new, most mfrs ship them without oil now because of hazardous material transport issues and if you haven't bought one in a while that may catch you by suprise. Like I said, it happens a lot.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[fornhamspark]

waross
Thanks so much for your description of slip, now i understand!!.

jraef
To answer you questions, clutch in on 1sec off 1sec, motor left running. Motor was replaced by engineer who replaced like for like, or what he thought was right. The gear box was refurbished by Fenner professional company, on site engineer installed. We have checked oil belts clutch etc.
Electrician checked suply etc, how well that is food for thought.

The original motor, we did not know the running current.

Any way you have all given me food for thought and a greater understanding, i will pass the knowledge on.

Then we can all learn from our mistakes.

Cheers

 

[jraef]

If your current is peaking at 168% for 1 second when the clutch engages I wouldn't find that terribly unusual, but as waross pointed out, running at FLA without the clutch engaged points to an undersized motor to start off with. Not being high slip would exacerbate the problem.

Someone who posts in here a lot (can't remember who at the moment) has a signature line that says (sic)
"If we learn from our mistakes, I'm getting one heck of an education!"

 

[electricpete]

I'm still not sure I understand the duty cycle. Aside from the issue of selecting new motor equivalent to the original (which may be the problem), I think you need to consider very carefully the possibility that the main problem causing your motors to fail is within your complex mechanical system. As waross pointed out 3.2A indicates a likely mechanical overload.

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[electricpete]

Should have been "..causing your motors to fail or trip..."

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