AC motors and Variable Frequency Drives

[skeer]

Prolly a dumb question but.. Google returns mixed results on this topic: Can you safely and efficiently control a single phase motor with a VFD? I see things suggesting both yes and no.

As an aside, what about Split-Phase?

 

[wyliesdiesels]

A VFD would be for a 3Ø motor not a single phase one.

 

[alfredeneuman]

As an aside, what about Split-Phase?

Split-Phase is just a form of single phase, so no.

 

[MBfreak]

A single phase asynchronuos motor can of course be rpm controlled by varying the supply frequency ( also the voltage in order to avoid flux saturation at lower frequencies)
The question is if the 3 phase VFD will be happy running just one phase load.
Contact VFD mfr for an answer .

Ola

 

[skeer]

So if a VFD whose output it 3 phase isn’t for single phase motors then logically a VFD with single phase out would yes?

I could have worded my original question better but I was not specifying those with only 3 phase out.

 

[N_Jay]

The issue with single phase is the starting cap will try to kick in at low speeds, and the run cap (if it has one) will not be the right spec at anything but 60hz.

 

[skeer]

I get you, but what about motors without either cap? The motor in question is a Dayton 5K977B.. putting trust in the Googles there are caps 'related' to this motor but I don't have one and she starts beautifully. I just want to safely and reliably control the rotational speed.

 

[theoldwizard1]

So if a VFD whose output it 3 phase isn’t for single phase motors then logically a VFD with single phase out would yes?

A single phase asynchronuos motor can of course be rpm controlled by varying the supply frequency ( also the voltage in order to avoid flux saturation at lower frequencies)
The question is if the 3 phase VFD will be happy running just one phase load.

I concur with MBfreak, at least in theory !

What most people don't understand is that a typical 120V/240V single phase motor is "coasting" a large percentage of each revolution. 3 phase motors motors "fill in" that coasting time with more power. This is why 3 phase motors are smaller/more "efficient" compared to a single phase.

If you start driving a single phase motor at less than 60 Hz the amount of coasting time is longer and you will lose power.

 

[skeer]

I concur with MBfreak, at least in theory !

What most people don't understand is that a typical 120V/240V single phase motor is "coasting" a large percentage of each revolution. 3 phase motors motors "fill in" that coasting time with more power. This is why 3 phase motors are smaller/more "efficient" compared to a single phase.

If you start driving a single phase motor at less than 60 Hz the amount of coasting time is longer and you will lose power.

Good information!

Sourcing a 3-phase is on the list.. however I don't use the object this motor is powering on a daily, or every other day basis. So replacing the existing motor is not high on the list unless controlling it's speed is critical to having a 3-phase motor. I'm tryin to keep it cheap but not like Scrooge McDuck cheap.

 

[theoldwizard1]

Sourcing a 3-phase is on the list.. however I don't use the object this motor is powering on a daily, or every other day basis. So replacing the existing motor is not high on the list unless controlling it's speed is critical to having a 3-phase motor. I'm trying to keep it cheap but not like Scrooge McDuck cheap.

Good luck McDuck ! 3 phase motors are expensive because they have 3 sets of windings. Likely more copper. VFDs, even small ones, cost some $.

 

[skeer]

VFDs, even small ones, cost some $.

Eh, to be fair, single in/single out, 120v ones can be had for under an Andrew Jackson.

 

[no704]

What are you trying to slow down?

 

[skeer]

Well, I was keeping the tool out of the equation in an effort to be more generic. In this case it's on the new to me drill press. But in general, slowing all AC induction motors.. the info provided in this thread is useful.

 

[American Locomotive]

In general, the only single phase motors that can somewhat reliably be controlled with a VFD are shaded pole motors. These are very basic motors - typically small fan motors. Usually around 1/40th - 1/12th HP.

- Cap Start/Run motors are out because the start switch will re-engage at low speed and cause issues.
- Split-Phase Motors (which your Dayton motor is) will not work because they also have a start switch (just no capacitor)
- PSC motors (permanent split capacitor) will not work well with a VFD because of issues with the capacitor and the secondary winding.

Eh, to be fair, single in/single out, 120v ones can be had for under an Andrew Jackson.

There is no VFD that can be purchased for $20. The "speed controls" you see on ebay and Amazon for those low prices are not VFDs. They're basically just dimmer switches.

 

[jar944]

Surplus 3ph motors are cheap, and decent vfds start about $200 assuming you are under 3hp.

 

[Innovate1]

Eh, to be fair, single in/single out, 120v ones can be had for under an Andrew Jackson.

Maybe you are talking about DC output chopper drives like drive brush motors in treadmills?

 

[skeer]

Nope I just had the wrong president.. not Jackson but Grant. And I’m sure ‘VFD’ in those cases is a strong word, but I’m sure as hell hope they aren’t glorified dimmer switches. Lol.

 

[skeer]

In general, the only single phase motors that can somewhat reliably be controlled with a VFD are shaded pole motors. These are very basic motors - typically small fan motors. Usually around 1/40th - 1/12th HP.

- Cap Start/Run motors are out because the start switch will re-engage at low speed and cause issues.
- Split-Phase Motors (which your Dayton motor is) will not work because they also have a start switch (just no capacitor)
- PSC motors (permanent split capacitor) will not work well with a VFD because of issues with the capacitor and the secondary winding.

Definitely solid motor info, thanks for this.

 

[skeer]

Sometimes Amazon can be infuriating.. anyway here’s an example of what I found https://www.amazon.com/dp/B0C7PY5FVJ/?tag=atomicindus08-20

 

[Cruzan80]

The pics when you scroll down show a 3-phase motor connected

 

[American Locomotive]

Sometimes Amazon can be infuriating.. anyway here’s an example of what I found https://www.amazon.com/dp/B0C7PY5FVJ/?tag=atomicindus08-20

That might actually be a real VFD, but that's getting into real ultra chinesium there. Plus it's only rated for 120 watts. That's a very small motor.

 

[theoldwizard1]

Surplus 3ph motors are cheap, and decent vfds start about $200 assuming you are under 3hp.

A quick scan on eBay, used 1/2 HP 3 phase motors run $150-$200 including shipping. 5 HP run $300-$400.

 

[Norcal]

Be real careful with cheap VFD's they are junk with no to very limited support, as to a 3Ø motor, industrial surplus or a scrap yard where it can be removed from decommissioned equipment, you do not want the scrap from a motor shop.

 

[skeer]

Thanks for all the info guys. Im It sure what the plan is now, but I’ll keep my eyes open for a good deal on 3 phase motor now. I still have the much larger dc motor out there not doing anything..but it’s getting close to firewood time. Might be a later upgrade project this year.

 

[jar944]

A quick scan on eBay, used 1/2 HP 3 phase motors run $150-$200 including shipping. 5 HP run $300-$400.

$49

3 Phase Motors Base Mount | AC 3 Phase Motors | Electric Motors | www.surpluscenter.com

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Or

1/2 HP 3500 RPM 230/460 VAC Reliance Motor B81V3509 | 3 Phase Motors Face Mount | AC 3 Phase Motors | Electrical | www.surpluscenter.com

1/2 HP 3500 RPM 230/460 VAC Reliance Motor B81V3509, 3 Phase Motors Face Mount, 1/2 HP 3500 RPM 230/460 VAC 3PH ELECTRIC MOTOR New, RELIANCE modelReliance,B81V3509,

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[wyliesdiesels]

What size motor do you need?