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New Electrical Math

whydontu

whydontu

I Tried, It Broke
Premium Member
Has electricity & physics changed since I went to school those many years ago?

I’m thinking of upgrading the spindle in my CNC engraver, but the one I like is way too much power ( or so I thought). The larger spindles are all advertised as 2.2kw. No way I can run 2.2kw from my single 15A circuit in my garage. But look! The actual nameplate rating is 8.4A @ 110 VAC, or in reality maybe 1kw. This I can do.

And my new shop vac is rated at 6.5 HP, not sure how it can develop 6.5 HP from 12A @ 110 VAC.

Why are advertisers allowed to rewrite physics?

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Its advertising monkey games. If you look closely at the home depot vacuum rating, it says 6.5hp "Peak". That's based on startup amps not full load. It's basically BS that shouldn't be allowed.
 
Make sure that's not 110v 3-phase. My money says it is because of to 400hz rating.

When I built my CNC router I chose 120v over 240v because I didn't want to run another 240v circuit.
So I bought a 120v kit from Aliexpress. Turns out the spindle is 120v 3-phase.
The reason why this is important is that there are no name brand VFDs that output 120v 3-phase so the only options are imports. I have an HY vfd for it but it’s a complete pain in the neck as I have to reprogram it on a regular basis.
 
Darren's in rush is the max startup current too.

I think the real problem is that there are no standards to limit the false advertizing.

But @Darren s example was important. Air compressors is a situation where higher startup hp can be important to overcome the pressure lump. Unloaders are used for the same purpose.

So maybe the vacuum and spindle motor guys are just levelling the playing field.

It's still BS in my books.
 
Typically, peak starting current is about 5 times the running current (dependent on the type of load)

I’ve always called it Marketing Horsepower.

Now if your circuit breaker can’t carry the starting current, then you could go up in breaker size and not change the cable size. But you’ll likely have to hard wire the load since now your breaker will only protect the cable on part of the operating curve.
Of course all this required approval from the authority having jurisdiction…
 
StevSmar said:
Typically, peak starting current is about 5 times the running current (dependent on the type of load)

I’ve always called it Marketing Horsepower.

Now if your circuit breaker can’t carry the starting current, then you could go up in breaker size and not change the cable size. But you’ll likely have to hard wire the load since now your breaker will only protect the cable on part of the operating curve.
Of course all this required approval from the authority having jurisdiction…
Click to expand...
A point to remember about this, is that it can only be done on motor loads, and has to be dedicated. Don't do this with a resistive load such as a space heater....
 
ok Motor heads - I have a question. Why are VFD's on motors always (or usually) 3 phase? What if you took an ordinary 120V motor and changed the power input frequency to 30Hz or 120Hz? What happens? Or the same on a 240V motor?
 
one problem is the centrifugal starting switch for the capacitor. Needs to reach rpm to switch off the start winding. Other than that, if you can get it started, a normal induction motor will react the same.
 
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Quick n dirty

3-phase motors don’t need start capacitors or start windings, so present a constant load to a VFD. Easier to predict current draw and phase effects on a 3-phase compared to single-phase motor.

Induction motor rpm is a function of frequency, current draw is a function of rpm. All other things being equal, at 120hz a standard 60hz single-phase motor will be much lower torque than at 60hz and lower current draw. Lower current = lower torque.

deltaautomation.wordpress.com

Operating Motors at Higher Speeds than their Nameplate Speed-Can it Be Done?

Occasionally we are asked the question about the ability to over-speed a motor using a variable frequency drive (VFD). This question usually comes up when there are capacity issues with a mechanica…
deltaautomation.wordpress.com deltaautomation.wordpress.com
 
Janger said:
ok Motor heads - I have a question. Why are VFD's on motors always (or usually) 3 phase? What if you took an ordinary 120V motor and changed the power input frequency to 30Hz or 120Hz? What happens? Or the same on a 240V motor?
Click to expand...

I agree with what @Darren and @whydontu have said.

I would add that even if set to 60Hz, I would full expect that most 3ph VFDs would sense that two of the output phases are not happy and shut it down, and further that the one connected phase is seriously sick (capacitors and start winding are not as expected) and also shut it down.

What are you wanting to do? Maybe we can suggest a better solution.
 
Yes I do have an application. All my 240V equipment. I generally want to have variable speed zero to the current normal operating speed. I'd like variable speed on my lathe 14x40 lathe without changing to a 3 phase motor. I have a 3x72" sander which is a pretty nice tool but it is too fast at times. If I could slow it down that would be convenient. I was thinking if I could just change the input power to the motor with a two wire single phase 240V VFD maybe that would largely solve my issues. I didn't think about the starting issues. Maybe if you just start it up to normal speed, the cap drops out, and then a simple switch disables it. Then slow it down with the VFD. ?
 
Janger said:
Yes I do have an application. All my 240V equipment. I generally want to have variable speed zero to the current normal operating speed.
Click to expand...

Zero rpm isn't gunna happen without new motors. Maybe a slight reduction.

Janger said:
I'd like variable speed on my lathe 14x40 lathe without changing to a 3 phase motor.
Click to expand...

Totally knock this out of your mind John. You cannot do this without a new motor. Lathes are not like mills. The startup power is very high and so is the power requirements at high rpm.

But I understand your interest. I have already purchased the 3ph motor and VFD for my lathe and I am currently redesigning the control box.

Janger said:
I have a 3x72" sander which is a pretty nice tool but it is too fast at times. If I could slow it down that would be convenient. I was thinking if I could just change the input power to the motor with a two wire single phase 240V VFD maybe that would largely solve my issues. I didn't think about the starting issues. Maybe if you just start it up to normal speed, the cap drops out, and then a simple switch disables it. Then slow it down with the VFD. ?
Click to expand...

The capacitor and start winding are automatically disengaged when the motor winds up. You cannot hear this happen because of other noises during startup. However you can usually hear the centrifugal switch closing again as the motor winds down after turning it off. Turn it off and then listen for the click.

Single phase VFDs exist and may do what you want but it's range of authority will be significantly reduced. Maybe a 10% lower and higher speed is possible. Any more than that will likely damage the motor. Used 3ph motors are everywhere. If you are buying a VFD, why not swap out the motor too?
 
Johnwa said:
You can get single phase output vfds. https://www.ato.com/1hp-vfd-single-phase-input-output
Click to expand...
The seller above here cautions to not slow the motor by any more than 80%. Well ok I see that is the reality - but I don't understand the fundamental reason why a 3 phase motor can be operated as a variable speed motor and a 1 phase motor is much more limited. You could solve the starting issues with more wiring and procedures. Start it up with 60 hz and full power, kick in the starting circuit coil/cap, get it running. turn off the starting circuit. Then slow the frequency. Cooling problems could be addressed with fans. Seems like that stuff is not really the problem? or is it?
 
Janger said:
The seller above here cautions to not slow the motor by any more than 80%. Well ok I see that is the reality - but I don't understand the fundamental reason why a 3 phase motor can be operated as a variable speed motor and a 1 phase motor is much more limited. You could solve the starting issues with more wiring and procedures. Start it up with 60 hz and full power, kick in the starting circuit coil/cap, get it running. turn off the starting circuit. Then slow the frequency. Cooling problems could be addressed with fans. Seems like that stuff is not really the problem? or is it?
Click to expand...
Centrifugal switch on your single-phase capacitor-start motor isn't going to like dropping RPM below its crossover point. So you could start at 60Hz, but maybe when you drop to 40Hz the switch cuts back in and puts the capacitor back in-circuit. Magic smoke appears.

A three-phase motor starts due to the phase difference between the three lines, unlike a single-phase induction motor that needs either a start winding or a capacitor to develop the phase shift needed to get the rotor spinning.

Here's a good article, from a VFD manufacturer.

www.kebamerica.com

VFDs for Single Phase Applications - KEB

Single-phase applications require special consideration when applying a VFD. The VFD must be sized accordingly and a 3 phase motor must be used at the VFD output.
www.kebamerica.com www.kebamerica.com

I suspect if I did some digging that the cost to get a 3-phase motor and matching single to three-phase VFD would be less than the cost of getting a single phase VFD to work with an existing unknown-configuration single-phase motor.

I will say that for my 12" bandsaw I just cheated - scrounged a free treadmill and adapted the treadmill DC motor and variable-speed DC controller to power the bandsaw. I only needed about 1/2 hp so it was a no-brainer.
 
I picked up my 10hp 240v 3ph motor for 50 bones, inverter rated. I bought the 2hp 3ph motor for my Doall bandsaw for 40. I see these all the time. People swap them out for single phase motors when they don't know about VFD's, RPC's etc

Also, every single 3ph VFD out there will function on single phase input.......AS LONG AS PHASE LOSS DETECTION CAN BE TURNED OFF, jumpered over, or ignored.

I spent eons on ebay looking for VFDs that could handle a 10hp motor, ignoring all the ones where they showed a photo of the dataplate and it said 3ph in/3ph out. I started looking up the actual manuals, and most had a way to make them work. Either by looping L2 to the L3 terminal. or by changing the settings. A drive tech from a local mill who takes care of hundreds of VFDs confirmed this. Even the ones where they don't specify, will sometime function on single phase, sometimes showing a fault code, sometimes tripping the breaker under higher loads. But they worked.

Point being, 3ph motors are cheap as borscht and VFD's can be had super cheap too, if you do some research.

edit: for anyone that doubts this, google reforming motor drive capacitors, or reforming vfd capacitors and see how its done with single phase through a light bulb. The drives power up and very slowly charge the depleted caps at roughly 80v 1ph ac. They have to do this when a spare drive sits on the shelf for too long, or bad things happen if they were to hit it with 600v 3ph off the batt. Some 3ph drives can even be powered by dc not ac.
 
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I have ordered this one to experiment with my mill, their claim of no need to remove capacitor has intrigued me the reviews seem good and the price is very good if it works out. I had previously done single phase in/out for my lathe with a yasksawa vfd, it took some experimenting and it can be confusing because sometimes your not sure if its a wiring problem or a parameter problem but once you have the wiring correct the parameters are easy enough to play with to improve performance.

 
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