Constant Torque vs. Varaible Torque VFD

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Forum topic by LukeBarber posted 07-21-2019 08:14 PM 257 views 0 times favorited 4 replies Add to Favorites Watch
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3 posts in 1034 days

07-21-2019 08:14 PM

I have a vertical knee mill with a 3hp 3-phase motor that I would like to power with a VFD, using mach 3 for speed control. It is a 4 pole, 1725 RPM, 8.9 amp unit.

Would I need a constant torque VFD, or a variable torque VFD for this application? What is the difference?

Would the motor lose any power by using a VFD vs. using a rotary phase converter?

4 replies so far

View GrantA's profile (online now)


1983 posts in 1970 days

#1 posted 07-21-2019 08:24 PM

You will pay a good bit more for a sensorless vector drive (what you’re calling constant torque). If you know you’ll be needing to run it at low frequency and need max torque then by all means get it. I have v/hz drives on my lathe and mill and have no issues, just like lots of guys.
If you can get a sensorless vector for not much more go for it but I haven’t seen that to be the case.

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1983 posts in 1970 days

#2 posted 07-21-2019 08:26 PM

Here’s some good reading, check out the article I posted a link to

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1 post in 141 days

#3 posted 07-21-2019 11:27 PM

There are lots of Chinese brands, many of them questionable but Delta and Teco (actually Taiwanese) are both
very good, both easily the match of any Japanese/USA/European brands.

It is a questionable practice to use ordinary 3 phase motors with VFD’s. VFD’s by necessity generate, or rather synthesize the required variable frequency output by switching (in a PWM manner) a high DC voltage very rapidly, usually 9-20kHz.

The high speed switching causes voltage spikes within the motor. As a consequence the insulation between windings and between windings and the metal part of the stator come under stress.

A motor intended for such service is called ‘Inverter Ready’, it is in most respects just an ordinary motor but with beefed up insulation.

If your motor is good quality it should be fine but if its a cheap one then using a VFD might be a bit chancy.

Here’s a good introduction to drives.

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7531 posts in 2761 days

#4 posted 07-22-2019 12:23 AM

Your application requires constant torque, which all VFD driven motors are, up until they reach their rated speed (typically 60hz). Also, just about every VFD I’ve seen has the ability to alter the torque curve if needed, which is usually for applications such as centrifugal pumps.

As for the difference between V/Hz and sensorless vector drives… the latter can be considered a ‘constant speed’ controller (not torque). It will sense the increase/decrease of RPM’s and adjust accordingly to maintain the same speed across different loads. They used to be way more expensive than their simpler V/Hz cousins, but with the advances in electronics over the last 10 years or so, they are now just slightly more expensive. As an example, the V/Hz Westinghouse TECO FM50-203 (3 hp) drive can be had for ~$185, while the sensorless vector version (L510-203 3hp) can be had for $190.


-- Brad in FL - In Dog I trust... everything else is questionable

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