Two Pole vs Four Pole Motors

Grinder Motor: 2 pole vs. 4 pole

Motor Speed

AC induction motors are designed to run at a certain speed. The number of poles or windings in the motor and the supply frequency, say 60 Hz, dictate the speed. Common induction motors have 2, 4, or 6 poles. When it comes to speed, more poles is less. The more poles a motor has the slower it runs. 

Another factor that directly influences motor speed is the frequency of the voltage you connect it to. Most motors made for North America operate at 60 Hertz (Hz) while the rest of the world operates at a mix of 50 Hz and 60 Hz.

The formula for determining the speed of a motor is :
RPM = 120 * f / number of poles

120 * 60 = 7200

7200 / 2 = 3600 RPM

This is said to the synchronous speed.

As a motor is energized it quickly gets up to running speed which is just under the synchronous speed. It is interesting to note that a motor cannot operate at synchronous speed it is always required to lag behind the supply frequency. This lag is called slip. The motor slips further as it is loaded. This is why the speed on the nameplate is 3450 RPM or 1725 RPM. These RPM ratings are when the motor is at it's full load.

So now we know the motor speed, let's look at belt speed,


Belt Speed

Belt speed is related to RPM, but takes into account the diameter of the wheel(s) that transmit power to the belt. The nice thing about belt speed is that it is an absolute reference, irrespective of all the complexities of diameters, ratios and motors.

To compute the belt speed in Surface Feet per Minute or SFM we take the RPM and diameter of the drive wheel into the following formula:

SFM = RPM * pi * wheel diameter / 12

In our example, we'll use a loaded two pole motor (3450 RPM) and a 5" diameter drive wheel.

SFM = (3450 * 3.141 * 5) / 12

SFM = 54182 / 12

SFM = 4515


To convert to SI metres per second multiply SFM by 0.00508.

m/sec = 4515 * 0.00508

m/sec = 22.94 


Jackshafts

One way to increase belt speed is to add a jackshaft or countershaft with sheaves (pulleys) and a belt that will increase the RPM of the drive wheel. If stepped sheaves are used, like in most drill presses, a variety of speeds can be realized. As with everything there are trade-offs. Jackshafts take power away from the belt and can introduce unwanted vibration if not balanced correctly.

If a speed control is required, a VFD driving a three phase motor with direct drive wheel is recommended.

Belt Speed and Abrasives

The finish the belt leaves on the steel changes with belt speeds. A faster moving belt produces a cleaner finish.  Based on data from VSM Abrasives and Klingspor, I have compiled this list of recommended belt speeds for a variety of different materials. 


Decision

When deciding on whether to use a faster two pole (3600 RPM) or a slower (1800 RPM) motor, ask yourself whether the cost of the drive wheel is going to be excessive. As wheels get larger, their prices get significantly higher.

Refer to the chart below to determine what size of drive wheel will bring your belt speed into the manufacturer's recommended belt speeds for the materials you want to grind.




Take a look at some grinder manufacturers and see where they land in the chart.

The Grizzly G1015 (http://www.grizzly.com/products/Knife-Belt-Sander-Buffer/G1015) has a four pole motor and an 8" drive wheel, putting the belt speed around 3600 SFM.

The JET J-4103 belt grinder has a belt speed pegged at 4600 SFM.  (http://www.jettools.com/us/en/p/j-4103-2-x-72-square-wheel-belt-grinder-115v-1ph/577000)

Perusing the catalog of Bader, Bee and many more you'll see a common theme when it comes to belt speed. If you get your belt moving at 4000 to 5000 SFM you will be impressed with the way your grinder performs and the finish the belts produce.

For in in-depth look at induction motors for your DIY belt grinder project please see my Belt Grinder Motor Guide.

Best wishes,

Dan

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