# Three-phase motor VFD frequency / current draw

I am planning to convert a 1920 Drummond B type lathe to a 3-phase motor / VFD combo and am trying to understand the math involved with VFDs, mainly relating to frequency and current draw. How do they achieve constant torque? Here's the nameplate from a 3-phase motor I'm thinking of buying:

This question has been heavily modified, please bear this in mind when viewing comments.

So what I think I can deduce from this nameplate:

• Speed is 2860 RPM.
• So torque is 2.75 ft-lb / 3.72 Nm
• Can be wired either STAR or DELTA.
• If speed = 120 x frequency / #poles = 3000. So after losses this must be a 2 pole motor.

My target is 50 - 2500 RPM. I'm thinking out loud now, say I run the motor to max 10,000 RPM (not sure if mechanical limit will be met) by setting the VFD to 166.66 Hz. If I use a 4:1 pulley I will be back to down to 2500 RPM but with four times the torque. What's going on with the amperage?

The motor is cheap and just for kicks here's the Chinesey VFD I plan to use with it. Capable of 8 amps, motor is capable of 4.19 amps @ 230 V.

Thank you.

• Where is the data sheet for the VFD? Do you think that an ebay link is good enough to make this judgement call? Do you think that buying something like this from ebay is sensible? What would an electronics/electrical design engineer do? Hint: find the data sheet, find the original manufacturer, check their quality, check their reliability and speak with them. Commented Jun 5, 2021 at 10:12
• Well lets assume the quality is top notch Chinese, on paper does this appear that it would work, thats all Im asking. At £60 Ill take a punt. Cheers Commented Jun 5, 2021 at 10:15
• Your motor is 230 volt rated - what is the VFD rated maximum output voltage. Caveat emptor!!! Buy some tissues also. Commented Jun 5, 2021 at 10:20
• It sounds like your searching for someone to pat you on the back and say "it'll be fine". If you find someone to do this, you ought to ask them on what basis they make their judgement and how sound is their engineering expertise or qualifications. Commented Jun 5, 2021 at 10:33
• I know what it sounds like but your wrong. Ill edit the question in about 20 mins after Iv taken the dog out. Will have more time to word it better. Commented Jun 5, 2021 at 10:45

1. The motor datasheet says 3.4 Nm @2850 RP/M 50Hz. The torque will decrease with speed.

2. With no load you can spin it as fast as the VFD can generate. I did this with a cheapy motor recently and found out about the problems of motor imbalance. The no load speed is of little interest as if there is not enough torque, then it is useless to you.

• I understand VFDs produce either constant torque or constant speed. How does this relate to amperage ? The VFD supplies upto 8a but the motor can handle upto 4.19a @ 230v (what I have). Is it not possible to calculate the potential torque without load speed data? (also Iv updated question and will do so until its clearer and more helpful) Commented Jun 5, 2021 at 11:20
• also am I right in that this is a 2 pole motor? I understand torque / speed / HP equation but how do I calculate the amps? When you say motor imbalance you mean varying voltage of each phase ? Commented Jun 5, 2021 at 11:53

How do they achieve constant torque?

A VFD provides the motor with the capability of producing rated torque at reduced speed by providing the optimum ratio of voltage to frequency. That provides the magnetic field for which the motor is designed without excessive current. There are a number of techniques for doing that. Many VFDs do that quite well without using shaft speed/position feedback. The technique is called "sensorless vector," "direct torque control," and perhaps other terms that are unique to the various manufacturers. The term "V/F open loop" used in the eBay listing generally describes a VFD that is not designed for constant torque but best suited to drive fans and centrifugal pumps.

My target is 50 - 2500 RPM. I'm thinking out loud now, say I run the motor to max 10,000 RPM (not sure if this is a mechanical limit) by setting the VFD to 166.66 Hz. If I use a 4:1 pulley bringing I will be back to down to 2500 RPM but with four times the torque. What's going on with the amperage?

There is a mechanical limit. The motor rotor is not likely to be well enough balanced to run at 10,000 RPM without excessive vibration. I believe that an IP65 motor is totally enclosed with an external fan attached to the shaft, At 10,000 RPM, that fan will require more than 30 times the power that it uses at 2880 RPM if vibration does not break it apart. The motor bearings may not be rated for that speed even if the rotor is properly balanced for 10,000 RPM.

The motor current will be pretty much proportional to torque if the V/Hz is optimized for constant torque. However with no load, the current only drops to something like 30 to 50 percent of rated current.

If speed is increased above the rated motor frequency with constant V/Hz (increasing the voltage above the nameplate voltage) the current mains constant for constant torque and the motor gives a "free" increase in horsepower. If the speed is increased by increasing only the frequency. The torque capability decreases roughly following a constant power curve. The current remains relatively constant. At some point, generally about 1.5 X rated frequency, the motor will be operating at peak torque (breakdown or pull-out torque). At that point the motor torque capability is below the constant power curve.

• That make much more sense, thanks Charles. So about 75 Hz will produce peak torque with constant roll off after that. Sounds like I need a bigger motor, may comprise on a 2HP. Also I should be looking for a VFD that states V/Hz or 'constant torque. Closed cage - fixed fan definately. With a bang-for-buck approach what spec motor would get me as close to my target, 2 pole offer fastest speed, I guess 6 pole is higher torque, since it will drive a pulley the latter is probably better, but I need to weigh this up against the cost of a suitable VFD ? What makes a VFD more expensive / desirable ? Commented Jun 5, 2021 at 16:06
• Either way its clear this combo won't work with just one reducer pulley. I think a 2 or 3 quick change V pulley system would better compliment any motor / VFD setup. Plus its got to be alot quieter than all those gears, treadles etc! ) Commented Jun 5, 2021 at 16:18
• You need a VFD that states "constant torque," "sensorless vector," "direct torque control," "heavy duty," or something else that I haven't thought of. Some terms manufacturer's use are proprietary and some have been adopted by several manufacturers. Unfortunately, there isn't a descriptive term that you can depend on. The better products have detailed specification sheets, good instruction material and perhaps application notes. The well documented and supported products are more expensive than the poorly documented products, but there is a range of prices among the better ones.
– user80875
Commented Jun 5, 2021 at 17:00
• If you stick with the reasonably documented products, the documentation will tell you which products are good for constant torque applications and which are intended for fans and centrifugal pumps. You may need to learn to interpret and understand the specs, but you can get help by posting questions here.
– user80875
Commented Jun 5, 2021 at 17:05
• Will do thanks, hopefully no interpretation needed ) Commented Jun 5, 2021 at 17:20