There are a number of conditions that need to be checked. Using the motor nameplate (NMPL) as given to be 100%.

1. motor torque at three critical stages of reaching full speed are:

a) lock rotor torque LRT per the motor frame and winding design usually about 130-160% of NMPL at zero RPM

b) minimum torque at cusp usually about 120-130 % NMPL at about 50% of rated RPM

c) maximum torque at breakdown usually about 225-300% at about 90% of rated RPM

When you start across the line or direct on line (DOL) start you must pass through all three stages. If you use a fluid coupling you only have to be sure that the pump (coupling) curve is covered by the motor torque for all speeds.

A voltage controlled soft-start can do the trick for this small motor. You can get more detail from mfgrs. such as Allen-Bradley now Rockwell International.

The point of static and dynamic is confusing. Static, I believe is refering to the steady-state speed whether empty, full or other. Dynamic, I believe is refering to acceleration and deceleration whether during drift or by forced braking. CEMA gives methods for both.

I do not understand the meaning of "dynamic resistance". Acceleration, in a simple form, deals with the acceleration of the masses supperimposed on the steady-state.

You note that you have selected a motor with a 2% margin over the acceleration value. This is puzzling in that acceleration will ocurr with any motor reserve over the needed steady-state power, its just a matter of time. Small reserve means long start time and maybe the motor overload fuse will trip. This means you could start if you had the necessary motor cooling for the prolong start time.

This simple explanation omits a lot of detail relevant to large drives. ■

Today we know that the term breakaway friction is misleading and in most cases mystical. Idlers and belt can have unusual starting characteristics that tend to draw more power to put the belt in motion. Except for the solitifaction of bearing grease and shearing of granular material in the transfer station, there is no breakaway. If fact, the other rolling resistance factors tend to become larger with increasing speed.

A 2% margin of power to accelerate a conveyor is insufficient as you experienced. A more realistic value is 20%. Care should be taken to accelerate the motor in less than 15 seconds and for some special motors 25 seconds, if you wish to start more often than one per hour.

Regarding VFD starting, this depends of the VFD and motor design. Many VFD's can provide enough starting torque to twisting the motor shaft to destruction at near zero speed. Imagine the motor curve from synchronous speed to the breakdown torque. This occurs at near full speed. As you drop the VFD frequency, but maintain the voltage, the VFD slip curve slides towards zero speed. At low speed, near 2-5 hz, the VFD can nearly pull breakdown torque. Thus, it may be possible to use the A frame motor. You need to talk to a knowledgable inverter rep. ■

more motor is one thing to help here, but do we not need to look at the head and tail pulleys to possibly UPSIZE to avoid excessive slippage or no? ■

George

When you are increasing tonnage you not only need to look at the HP required but you will also need to look at the belt speed, reducer, increase in tensions, possible counter weight increase and total carrying capacity of the belt. You will also need to look at the PIW ratings of the belt. Can the belt handle the increase in load? Does the reducer have the mechanical and thermal HP rating for the increase? etc.....

Drive slippage is a function of tension in the system and the counter weight must be designed to prevent the slippage. Other things to look at to reduce the coefficient of friction are drive pulley lagging (rubber or ceramic) and snub pulleys to increase wrap around drive pulley.

Best Regards,

Gary ■

As sugested in previus replies you can opt for VFD drive.

You can opt for fluid coupling , in case of space restriction you can think about orentation of gear box or bevel helcal type .Changing gearbox orentation and fluid coupling will be cheaper option.

In case of longer starting time your belt lifting at tripper may be reduced. you can also provide hold down pulley on tripper.

in case you send us conveyor GA drawing and technical data we can suggest proper solution of the problem

A R SINGH ■

Assuming that you are otherwise properly sized your problem is your NEMA A motor which has the very low locked rotor torque. If it is a direct drive the the locked rotor torque is the limiting torque. You can't get to the other parts of the motor curve without first passing through the low locked rotor torque. In this regard a NEMA C design is best because it has the highest locked rotor torque. On the other hand there is more slip at full running resulting in higher speed variation, though still very modest and not worth worrying about.

A fluid coupling will allow your motor to start under no load and come to full speed before incurring the conveyor load. Stalling the motor from this point will not be possible since maximum motor torque exceeds that which the fluid couipling can transmit (if it is properly set up with the correct fluid level) and there will be relative slip until the conveyor comes up to speed. Depending on the amount of inertia (in the loaded conveyor) it may take a while and both the motor and the coupling must be able to take and dissipate the heat build up. In this case a NEMA B motor is a better choice.

Though your breakaway resistance is higher than running, the difference is far less than that of locked rotor torque or maximum motor torque versus full load torque in a NEMA C motor. Unfortunately your NEMA A motor may have a locked rotor torque which is not higher than 100% FLT, possibly less, hence your problem starting when loaded.

Scrap your NEMA A in favor of a NEMA C and that problem will be solved. I hope there are not other problems afterwards.

I hope this helps,

Joseph A. Dos Santos ■

Dear Mr. clnelson

Referring to the issue of static resistance (i.e. conveyor resistance without inertial resistance) at the time of starting, it is considered practically the same as conveyor resistance during running. Static frictional resistance is always more than the dynamic resistance, if speed is just creeping. However, when you are comparing static resistance to the dynamic resistance at sufficiently large velocity, often the static resistance would be less than the dynamic resistance. The simple example is train, which will have a higher frictional resistance at 100 kmph (all types of resistance including air resistance) compared to the resistance it will have at the commencement of the motion.

The other important point is that the belt is elastic medium. So, when the conveyor is started and force is applied on to the belt, the entire belt will not start moving instantly. The forward section will acquire motional status quickly but rear sections will come in to motion after time delay. This reduces aggregate effect of higher static resistance (resistance prior to moving). This phenomenon remarkably assists the pulling of wagons by locomotive, when it starts moving.

The belt conveyor design is on the basis of various types of frictional coefficients, whether it is main resistance or skirt board resistance, and so on. This frictional resistances coefficients generally have in-built margin on safer side irrespective of whether you are using method-1 or method-2 for design. Hence, the motor corresponding to calculated kW should also be adequate. The conveyor you are referring is tripping during start, it could be due to many reasons. In case you have taken inadequate value of frictional coefficient, then it will certainly trip with 2% margin. If the electrical supply and cable system is causing voltage drop at the time of starting then also it will trip. The conveyor will also trip if it is taking longer time to accelerate compared to electrical settings. Therefore, the issue is subjective.

You have mentioned that you have tripper arrangement in this conveyor, and belt is getting lifted up during start-up. This means you need to correlate belt conveyor starting factor to the concave radius you are using in tripper. The belt lift up from tripper signifies faulty design. It is better to have fluid coupling or some means of gradual starting when there is tripper on the belt conveyor. If you provide oversize motor without proper analysis and proper type of starting, again you will have a belt lift problem, jerky movement of the take-up and reduced belt life.

I hope the above information would help you.

Regards,

Ishwar G Mulani.

Author of Book : Engineering Science and Application Design for Belt Conveyors.

Author of Book : Belt Feeder Design and Hopper Bin Silo

Advisor / Consultant for Bulk Material Handling System & Issues.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916 ■

Mr. Clnelson,

Can you please advise this Forum.

Have you replaced your NEMA-A motor with a NEMA-C?

What controls on the motor, if any, did you use?

Did this solve your problem of starting loaded?

Joseph A. Dos Santos ■