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Author Topic: Constant Torque vs. Varaible Torque VFD  (Read 832 times)

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Constant Torque vs. Varaible Torque VFD
« on: June 01, 2019, 09:45:07 AM »
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?

Offline Davek0974

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Re: Constant Torque vs. Varaible Torque VFD
« Reply #1 on: June 01, 2019, 10:33:10 AM »
I have a 3Hp on my mill, use a sensorless vector drive - i used this one - Bosch Rexroth EFC5610, i'm running the motor up to 4000rpm with it.

It works really well, i am using 240v though no converter.
Re: Constant Torque vs. Varaible Torque VFD
« Reply #2 on: June 01, 2019, 10:47:33 AM »
Thank you for the response. Is Bosch Rexroth EFC5610 available in the US? I didn't find much info on US sellers for this drive. I've been researching VFDs online.

Here's a good introduction to drives. https://precisionautomation.net/2019/04/10/adjustable-speed-drives/

Offline Davek0974

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Re: Constant Torque vs. Varaible Torque VFD
« Reply #3 on: June 01, 2019, 10:57:51 AM »
No idea about US availability but i should think any good sensorless-vector drive will be good, as long as the power/voltage matches what you have. ;)
Re: Constant Torque vs. Varaible Torque VFD
« Reply #4 on: June 01, 2019, 04:36:12 PM »
Hitachi VFD's are available in the US and have a good reputation.

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.

I had a cheap Indian made 3 phase 1 hp motor given to me and I used it as a motor for a grinder with a VFD. It didn't
last, after about three months the insulation let go. I am now using a GEC (US made) 3 phase motor and its been running for
five years. I can only guess that the insulation in the GEC motor is that much better.

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.

My GEC is a 'two pole pair' motor, and its synchronous speed in New Zealand with 50Hz AC supply is 1500 rpm. The same
motor in the US with 60Hz supply would have a synchronous speed of 1800 rpm. Naturally being induction motors there must
be some slip so the motor will never achieve synchronous speed but rather 1425 rpm or 1725 rpm respectively.

I guessed that GEC would have used the same rotor in their 'one pole pair' design stator, that is to say that the rotor
could handle rotational speeds of 3000 rpm and 3600 rpm depending on the frequency of the supply. I guessed that I could
with a VFD spin them a little faster WITHOUT the rotor flying apart. I have taken the GEC to 4500 rpm but got nervous and backed
it off to my now self imposed limit of 4000 rpm.

Note that with my Delta VFD which is capable of 400 Hz output I could with the 'two pole pair' motor run the motor up to
12000 rpm. I would expect either the bearings or the rotor to explode long before I got to that speed though.
Just because its possible to get a lot more speed from a motor, and incidentally more power, does not mean that it a good
idea.....you could compromise reliability.

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