3P/VFD or DC motor

[Bamban]

Putting the cost differential discussion aside, HP on either unit being the same, in this case 2 HP, which would the better choice to run a lathe? Why?

 

[den-den]

I would choose a VFD and 3 ph motor due to a flatter torque curve.

 

[Ulma Doctor]

i suppose it would depend on how the lathe was going to be used.

a DC motor can have incredible amounts of torque available at low speeds,
very helpful for threading to shoulders or short length threads. little need for step pullies and belt position changes.
Downfall- increased maintenance, dc motors most times have carbon brushes that need to be replaced-a commutator to be serviced, and bearings to replace as necessary. an excellent choice- small motors can produce a lot of ponies to the spindle- a DC drive can easily be constructed from recycled/repurposed parts and a controller

3P motors can offer some pretty nice surface finishes.
when fully equipped with inverter rated motors and precaution taken to minimize overheat conditions , 3P would perform the same function in respect to speed control, but the DC motor's performance would surpass the 3p's output given the same [lowered] rpm.
generally DC motors have broader operating RPM ranges than a similar watt AC motor.
3P motors generally require little maintenance and can operate in harsh conditions. service is generally simple bearing replacement and or lubrication of rotor bearings.
3P is also a good choice- AC drives are generally reliable , 3P motors are extremely reliable .
a little more costly but well worth the added expense if your business depends on the machine.
IMHO
mike)

 

[Bamban]

i suppose it would depend on how the lathe was going to be used.

a DC motor can have incredible amounts of torque available at low speeds,
very helpful for threading to shoulders or short length threads. little need for step pullies and belt position changes.
Downfall- increased maintenance, dc motors most times have carbon brushes that need to be replaced-a commutator to be serviced, and bearings to replace as necessary. an excellent choice- small motors can produce a lot of ponies to the spindle- a DC drive can easily be constructed from recycled/repurposed parts and a controller

3P motors can offer some pretty nice surface finishes.
when fully equipped with inverter rated motors and precaution taken to minimize overheat conditions , 3P would perform the same function in respect to speed control, but the DC motor's performance would surpass the 3p's output given the same [lowered] rpm.
generally DC motors have broader operating RPM ranges than a similar watt AC motor.
3P motors generally require little maintenance and can operate in harsh conditions. service is generally simple bearing replacement and or lubrication of rotor bearings.
3P is also a good choice- AC drives are generally reliable , 3P motors are extremely reliable .
a little more costly but well worth the added expense if your business depends on the machine.
IMHO
mike)

Mike,

Thank you for articulating the merits of both systems.

I am not in the machining business, just a retired trigger monkey who wants to hot rod an older ACER ATrump 1236 lathe. I just bought and received a fairly inexpensive VFD, but I am not opposed to changing direction, if a BrushLess DC motor for a reasonable price were to become available, I will go with it.

 

[Karl_T]

I'll just add one more little fact to UD's discussion.

Controllers for 3 phase motors, or VFDs , are quite sophisticated with tons of high end options.

Without going to a major expense, the same things just aren't around for DC motors.

Karl

 

[arvidj]

Are there low speed cooling issues that would need to be addressed in the DC, the 3ph\VFD or both cases?

I have no experience with DC but the 3 phase motors I have are the 'enclosed fan' type. At speeds below their "normal RPM" I feel the enclosed fan does not get the job done and have added external fans that are permanently on and cooling the motor no mater what the speed.

This isn't meant as a pro or con of the 3 phase\VFD solution, just a fact of life that may or may not be applicable to the DC solution.

 

[mksj]

I would concur on staying with a 3 phase VFD AC motor and a decent VFD (tunable, vectorless). If you compare an industrial type DC motor (AO Smith 2 HP, 180 VDC Volts, 143TC/145TC Frame, 1750 RPM) to an inverter rated 3 phase motor (like the 1750 RPM Marathon Max series or similar), the later has a much broader operating window (torque and HP). The constant torque (CT) for the AO smith motor is 20:1 (most DC motors are worse) where the Marathon's (CT 1000:1) essentially has full torque at 0 RPM with a VFD. When you get into inverter rated motors, many are TENV types, where cooling is not dependent on speed. Even general 3 phase VFD rated motors have a minimum 10 fold speed range for constant torque, so like 200-1800 RPM which you will never use on a lathe with more than 1 speed range. The issues of cooling is a concern if you run the motor under full load at slow speeds, I can't even get my lathe (non-VFD rated 3 phase) 2 HP motor to get anywhere above luke warm running at 25% of its rated speed. The AO smith motor is 3X the price of the Max motor. Treadmill motors are fine for smaller lathes, but have a different operating envelope and would require a different belt ratio to optimize their higher operating RPM range.

The usable RPM range on most larger 2-5HP lathes with AC VFDs is around ~10 fold, the 12-16 x 40 types with variable speed ranges use 3 phase motors with a VFD. If you are retrofitting a geared or belted mult-speed lathe, you are at most using a 4-5 fold motor speed range. Just about any 3 phase AC motor would work, especially with a decent VFD running vectorless CT. Decent 3 phase 2 HP inverter rated motors are plentiful, and probably a much easier retrofit then a treadmill DC motor in this size range.

 

[rbertalotto]

Unfortunately, it does come down to cost.

Do you have 230v / 1ph where the lathe is going?

I converted my 12 X 36 to DC for $260

$179 for a 3/4HP Brand new DC motor and $81 for a brand new SCR controller. Very easy to install and wire. Runs on 120v...Forward / Reverse / Slow Start / infinite variable speed / tachometer speed control /

I've never been able to stall it, no mater what lame brain thing I do!

www.rvbprecision.com

 

[Bamban]

Unfortunately, it does come down to cost.

Do you have 230v / 1ph where the lathe is going?

I converted my 12 X 36 to DC for $260

$179 for a 3/4HP Brand new DC motor and $81 for a brand new SCR controller. Very easy to install and wire. Runs on 120v...Forward / Reverse / Slow Start / infinite variable speed / tachometer speed control /

I've never been able to stall it, no mater what lame brain thing I do!

www.rvbprecision.com

I do have 220 phase to phase available by the lathe. The lathe has 1 1/2 HP motor from the factory, the reason I am looking to use a 2 HP if I were to replace the original motor.

Is conceivable to downsize the motor? I am on the train of thought to upsize. The lower HP rated motors are definitely cheaper.

 

[rbertalotto]

I went from 2.5hp 230V / 1ph to 3/4hp 90V DC and the lathe performs much better power wise. I'm no engineer, but I did some "Glove on hand" testing. With a 3" pulley on the 2.5hp motor, I could stall it. Same pulley on the DC motor and I couldn't stop it. The control circuit just kept adding current and the motor kept going.

The 2.5hp motor was about 10 years old at the time so I have no idea if it was up to spec. But I tried the test with a brand new 1hp 120v / 1ph motor I had. With the same pulley I could EASILY stall it. Took much less effort to stall the 1hp than it did the 2.5hp.

Just thought I'd throw this out there.