4 Pole Motor Instead of 6 Poles - the Torque Issues

Torque is provided by the load. The motor is there to overcome it.

PWSlack is correct, the required torque is determined by the load. To determine the maximum available torque from the motor, look at the motor power (5KW). Power equals speed times torque. If both motors are rated at the same power, equally efficient, and geared to the same output rpm, then the maximum torque available to the load should be the same.

...and calculable.

No torque problems. However, check the revised sprocket sizes to be sure there is enough space for the larger one, and that the smaller one has enough teeth to meet the minimum recommendation.

why not ditch the chain and gear and use belts and adjustable pulleys and dial in your choice of paramters?

I can't see any problem doing that. Probably simplest to reduce the sprocket dia on the motor (if it's not already the smallest that fits on the shaft) and keep original dia on the driven. That way the chain tension is unchanged and the guard still fits. Alternatively you could increase dia of the driven side and in principle use a lighter chain (higher speed, lower tension).

Theoretically,the same torque would be available.However,the output gearing ratio must change by approximately 50%.

The faster motor will be physically lighter and less expensive(less copper),but the gearing will be larger and more expensive.

From what you say, gear availabilty is no problem,and 6 pole motors are a problem, so looking at possible downtime in case of a failure,I would choose the faster motor.

However, if the output speed is critical,it may be difficult to find the exact ratio needed.A variable speed control may be needed at some point,either mechanical, as previously mentioned via a variable speed pulley,or electronic with a VFD.

good luck.

If you are going to use 4 pole, you will need 7.7Kw : next size up 10KW motor.

I don't agree with that. The power is unchanged.

You are correct that power is unchanged but 5kw with more speed is going to have less torque, If you want to maintain the torque that the machine was designed for and more speed, naturally you will need more power.

I don't agree with that, either. Subsequent speed reduction regains the same torque, final speed, and power as before.

Frankly I don't agree that the torque has to be anything at any speed but low slip. I thought the NEMA torque curves for different rotor designs would be common knowledge with people who specified induction motors. With more 4 pole motors in the market I'm certain there will be more variation in the torque curves available with identical power specifications.

If they choose a different design (A/B/C/D), then of course all bets are off.

• "...we are required a 3phase 5KW 950rpm motor to provide 600rpm at driven side. A question raises here that what would happen if we select a 5KW 1450rpm motor (instead of 5KW 950rpm) and reduce the teeth of chain sprockets to get 600rpm at driven side...."

Somebody needs to do the math, so here it is.

Tq (in N-m) = kW x 9550 / RPM

Stated motor size of 5kW 6 pole, 950RPM. So motor Tq. = 5 x 9550 / 950 = 50.26 N-m

Stated shaft speed requirement is 600RPM, so that means there must be an existing sprocket reduction ratio of 950/600 = 1.58:1

Work shaft torque is then 50.26 x 1.58 = 79.58 N-m

New motor is 5kW at 1450RPM, so motor shaft torque is 32.93 N-m.

New reduction ratio to get to 600RPM from a 1450RPM motor becomes 2.42:1.

New work shaft torque becomes 32.93 x 2.42 = 79.58 N-m

Bingo. It will work, as long as your revised speed reduction ratio is exactly 2.42:1 with the new sprockets and you have not increased losses somewhere along the way.

Redfred's other issues notwithstanding of course. I am just addressing the simple question asked about torque.

I don't think we need any math(s) to come to that conclusion. OP hasn't told us what torque is required by the load at 600 rpm, so we can't check that a 5kW motor is OK. We can only assume it is, and if so, same power but 4-pole and changed chain drive ratio is also OK.

Only time there might be a problem is if the design is right on the knuckle and load ends up going slightly above 600 rpm due to limitation on available number of sprocket teeth, but this is unlikely.

BTW 5kW is not a standard motor, nearest is 5.5kW and that gives extra margin. Ref eltech in #9, 10kW is not a standard motor either, nearest is 11kW.

Ref Redfred's other issues, we can't address them without more infomation on the load - constant or squared torque, high inertia etc. But I'd be surprised if the characteristics of 6- and 4-pole motors differ enough for scheme A to work but not scheme B.

In a folding operation it is critical to have the proper speed to ensure that the fold is in the right position, not too slow or too fast.

It may be impossible to source the exact proper pulley size unless custom made($$$$).There should be feedback from the input speed to the speed controller to synch the output to the input, and there should be an accumulator section to smooth out minor variations in speed. Servos are best suited for this type control, especially if different folds are frequently required, and the accuracy is critical. During the upgrade, explore the possibility of servos.

Servo motors? Who uses them anymore? Today, precision motion is achieved using a micro-stepping motor with computer/PLC control.

The main advantage of steppers over servos is the human factor. Someone who does not understand control theory and tweaks the control settings of a stepper will only fail the motor in a stall. A servo motor's misadjusted servo loop can fail in oscillations or other truly spectacular ways.