VFD for Bench Grinder?

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icarvewood

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Jan 27, 2014
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Hi all,

First post here. I'm a woodcarver and would like to know if I can connect a VFD to a 3-phase bench grinder to control it's speed? Specifically I've been looking at a Baldor 7308 and KBAC-24D. If possible what sort of speed range could I expect? I normally grind my plane blades, chisels and gouges at 1800rpm with a Norton 3x wheel but some of my carving knives are difficult to grind at this speed without burning the tip. Thanks for your help!
 
It should work fine. Most 4-pole (~1800 RPM on 60 Hz mains) motors and VFDs can be run over the range 10 Hz to 100 Hz with no problems at all, giving around 300-3000 RPM.

Many will go a little higher.

With Sensorless Vector drives, most motors can also go lower. I think the latest KBACs are SV drives.

With a grinder, you'll obviously need to ensure you keep the top speed below the rated speed for your wheels.
 
Thanks Tim!

So I should limit my choice to 4 pole 1800rpm grinders? Do the faster two pole motors not work well with VFDs? I'm mostly interested in slowing it down. Also, any reason why I wouldn't be able to reverse the grinder with the VFD?
 
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Why not buy a variable speed bench grinder? Just out of curiosity, have you looked at the Work Sharp tool sharpener? Looks like a horizontal disk grinder, but has a glass plate. Uses psa disks and is air cooled. I've only got a few woodworking tools so I do them all by hand. Still, that WS is intriguing.
 
icarvewood said:
Thanks Tim!

So I should limit my choice to 4 pole 1800rpm grinders? Do the faster two pole motors not work well with VFDs? I'm mostly interested in slowing it down. Also, any reason why I wouldn't be able to reverse the grinder with the VFD?
Click to expand...

2-pole motors work equally well with VFDs as far as I can tell, but I don't think they can be run up to 100 Hz safely, as a rule.

Because of the way manufacturing works, a motor frame (including the bearings) and rotor is usually designed to work at a speed of 3600 RPM. It's a pretty safe bet that there is a 3600 RPM version of the Baldor you linked to.

The only thing that makes "your" one an 1800 RPM motor is the 4-pole winding, which is part of the stator assembly and does not move.

Mechanically, it will hang together to 3600 RPM.

The 2-pole, 3600 RPM motor will not have been designed to run much over 3600 RPM, so running over 60 Hz carries some risk.

The frequency range is largely down to the design and most motors are designed for optimum performance at 50-60 Hz to suit the mains supplies across the world. Electrical losses increase as the frequency moves away from this range, so the usual recommendation is to keep within the 10-100 Hz range.

Going back to basics, a VFD usually increases frequency and Voltage together when below the motor rated speed. This means that at half rated speed, there is half rated Voltage. The motor can draw its full rated current at all speeds. As Power is Current times Voltage, it means the motor will give half power at half rated speed.

Once the motor is at rated speed, it is also at rated Voltage. There is no scope for further increasing voltage safely, so the VFD just increases frequency with the Voltage remaining at the rated value. Again, power is Volts times Amps and the motor can draw its full rated current at all speeds.

This means that the VFD will run the motor at constant torque below rated speed and at constant power (reducing torque) above rated speed.

Shamelessly copying a chunk from a post on another forum:

"In many applications, what we want is not so much the power as the torque.
Power is Torque x RPM x a constant, so at their rated speeds, a 4-pole motor that makes 2 HP at 1500 RPM will produce twice the torque of a 2-pole motor that makes 2 HP at 3000 RPM.

To try to simplify things, if we compare the performance at specific speeds, the power will be directly proportional to the Torque.

Driven by a basic "V/Hz" VFD, the 2-pole motor will be able to run between 600 and 3600 RPM.

By contrast, the 4-pole will be able to run between 300 and 3000 RPM.

Taking performance at specific speeds:

At 300 RPM, the 4-pole will produce 0.4 HP at a frequency of 10 Hz.

At 600 RPM, the 2-pole will produce 0.4 HP at a frequency of 10 Hz
At 600 RPM, the 4-pole will produce 0.8 HP at a frequency of 20 Hz

At 1000 RPM, the 2-pole will produce 0.66 HP at a frequency of 16.6 Hz
At 1000 RPM, the 4-pole will produce 1.33 HP at a frequency of 33.3 Hz

At 1500 RPM, the 2-pole will produce 1.0 HP at a frequency of 25 Hz
At 1500 RPM, the 4-pole will produce 2.0 HP at a frequency of 50 Hz

At 2000 RPM, the 2-pole will produce 1.33 HP at a frequency of 33.3 Hz
At 2000 RPM, the 4-pole will produce 2.0 HP at a frequency of 66.6 Hz

At 2500 RPM, the 2-pole will produce 1.66 HP at a frequency of 41.6 Hz
At 2500 RPM, the 4-pole will produce 2.0 HP at a frequency of 83.3 Hz

At 3000 RPM, the 2-pole will produce 2.0 HP at a frequency of 50 Hz
At 3000 RPM, the 4-pole will produce 2.0 HP at a frequency of 100 Hz

at 3600 RPM, the 2-pole will produce 2.0 HP at a frequency of 60 Hz

Over the range of speeds common to both motors, the 4-pole motor is ahead on power everywhere below 3000 RPM.

At the low speeds, up to 1500 RPM, the 4-pole produces double the power of the 2-pole. Both motors are in their "constant torque" range.

Between 1500 RPM and 3000 RPM, the 4-pole is in its "constant power" range, but the 2-pole is still on "constant torque". The advantage of the 4-pole tails off progressively from double the power at 1500 RPM to equal power at 3000 RPM, but it is still ahead all the way."

Sorry it's so long-winded, but I hope it helps.
 
Thanks Tim, that is useful info!

I've used both a belt grinder and a flat disc grinder in the past and I prefer the wheel grinder. The hollow grind makes it easy to index the tool to the waterstone. Thanks everyone!
 
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