Physics questions

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Randy3000

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Jun 3, 2017
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Here are two setups:

A)
3500rpm 2 pole motor direct drive turning a 4" drive wheel.

B)
1750rpm 4 pole motor belt driven with 4" driving pulley and 2" driven pulley on a shaft turning a 4" drive wheel. For simplicity and for theoretical physics discussions purposes only, suppose belt slippage does NOT occur.

Here is my understanding, please correct me if i am wrong.

The motor in A has half the torque of motor B. However, motor B is transferring half of its original torque to the driven pulley, and therefore the drive wheel. So the torque at the drive wheel is the same for both A and B. Also, the drive wheels in both A and B have the same 3500rpm. So on paper, the performance of these two are the same.

C)
1750rpm 4 pole motor belt driven with a 2"-3"-4" stepped driving pulley and a 2" driven pulley on a shaft turning a 4" drive wheel.
D)
2500rpm dc motor belt driven with a 3" driving pulley and a 2" driven pulley on a shaft turning a 4" drive wheel. Suppose there is a speed controller with a range 50%-100% of max speed, which with the pulley ratio is close to the speed range of C, let us just say it is exactly the same for simplicity.

Again, my understanding is as follows.

The speed of the drive wheel is roughly the same at corresponding speed settings of C and D (using 2" step in C corresponds to min speed on dc controller in D, using 4" step in C corresponds to max speed in D). D has constant torque at all speeds, but can only transfer 2/3 of the torque to the drive wheel. For C, with 2" step the full torque of the motor is transferred to the drive wheel, and with 4" step only half the torque is transfered to the drive wheel. So at min speed D is easier to stall, and at max speed C is easier to stall. When using the 3" step, C is just as easy to stall as D.

I would like to make a similar comparison with VFDs, but i dont really understand all the various VFD settings. Is there some interesting dynamic like with C and D above?
 
vfd is real simple. you have a read out of hertz(HZ) or percent. if the VFD is set for 60HZ and you have a 60HZ motor, you will get the rpm as listed on data plate. torque typically is not listed on data plate, but you are using direct drive so should be full torque. if you set VFD to 30hz, half rpm listed, to 120HZ, double the rpm listed. some VFDs can be programmed so torque is about the same whether 50% or 200%. you can also program the maximum current output and how long it will output at maximum. if you load down the motor, current will increase(increase rate is also programmable) to maximum then start timing. if you load it down too long, VFD will usually fault for overcurrent. all this depends on make and model of VFD.
 
Speaking to A and B, your math is correct, for all practical purposes. 2 pole motors do indeed have half the torque of a 4 pole, all other things being equal, and a 2:1 pulley ratio will reduce torque by 1/2, while doubling speed.

If you want to get into step pulley calculations, this calculator is pretty nice:
http://www.gizmology.net/pulleysbelts.htm
 
So for a vfd, as frequency increases the speed increases, with rpm = 60*Hz with a 2 pole motor and rpm = 30*Hz with a 4 pole motor. So i guess the two options are constant (full?) torque or constant (full?) horsepower.

See, what is driving all the confusion i was going through was people claiming dc motors are harder to stall at low speeds, but my calculations didnt verify this claim, as in the C&D scenario above. Then i began wondering about vfd and got way confused.

So i guess let's say

E)
3500rpm 2 pole motor direct drive turning a 4" drive wheel powered by a vfd.

Then at half max speed, E at full torque vfd setting has half the torque of C (direct comparison of motor torques), and so E is easier to stall. At half max speed, E at full horsepower vfd setting has the same torque as C (for E to maintain hp at half speed it has to double torque and so would match C). I think it is clear at max speed C and E are the same.

So a vfd only offers ease of use over a stepped pulley with ac motors, no matter the number of poles or what kind of pulley concotion you come up with. And an ac motor is superior or equal to a comparable dc motor over the speed range. Is this an accurate conclusion?
 
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a 3 phase AC motor with VFD and a DC motor with DC drive should have similar performance. both will offer either programming or internal adjustments that should prevent motor from stalling. IMHO, if you stall a belt grinder with variable speed motor, you are not using it properly. for more info, try here https://dealerselectric.com/variable-frequency-drive-motor-control.asp and here https://www.baldor.com/catalog#category=2 check the baldor site for choosing the right motor and performance curves of motors they make.
 
I am going vfd route. My dad, who is electrician, is coming over in about an hour and we are going to run a sub panel in my garage and get me some 240v going. Since the attic is over the garage, i should be able to run the wire through the attic and not have conduit everywhere.
 
Randy, you're making a wise decision going the VFD route. All this concern you hear written about torque at low speed is way over rated for grinders. At lower RPM (slower SFPM belt speeds) you're not ever going to stall a motor regardless of AC, DC, or stepped pulleys. At low belt speeds you're just not pressing all that hard with the grinding. Low speeds usually require a light touch for critical (low amount of metal removed) work. The fine grit belts (400, 600, etc) will burn your metal very quick at higher belt speeds and heavy pressure.

While it's true a 1750 rpm motor has much more torque at 1750 rpm than a 3500 rpm motor does at 1750 rpm, remember, the 3500 rpm motor is only putting out half power at 1750 rpm. With both motors running at 3500 rpm both will be doing the same amount of work, and have the same amount of torque. OR, with the 1750 motor running at 1750 and 8" drive wheel so both motors will be around the same 3600 SFPM, it will have about the same torque as the 3500 rpm motor with a 4" drive wheel.

In actual use there is going to be little difference between the two motors in real world.
 
It is all conservation of energy, a 2hp ac motor gets you 2hp no matter how you transmit that power. It was good learning though for me to understand the mechanics and have confidence i am doing the right thing.

We got the panel ran, i am pretty excited about the world of 240v before me.
 
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