# Sizing vfd cable



## SWDweller (Dec 9, 2020)

I have never calculated at the lower speeds. 
Question will you motor cool at that low speed? I have seen many that will not.


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## SCR (Mar 24, 2019)

SWDweller said:


> I have never calculated at the lower speeds.
> Question will you motor cool at that low speed? I have seen many that will not.


It will cool because the low speeds will be at low outside temp.


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## gpop (May 14, 2018)

Apples to apples the lower speed and lower voltage will not have the same hp so i would not expect to use the FLA of the full speed as my base for calculating voltage drop. 

I hate pulling 14g as it tangles to easy so i would have gone 12g anyway. My only concern at speeds less than 16 hertz is detecting motor stalling as most vfd's will get to 8-15 hertz even with a locked rotor. Nothing that can not be program out as long as you realize it can happen at lower speeds.


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## SWDweller (Dec 9, 2020)

I have always put in a min speed/hertz and locked it. Based on my understanding of the equipment. To prevent the tweakers from playing with expensive equipment. 

Had an AC fan that typically got down to around 15% which burned up the motor. I helped out changed the pulley on the motor and kept the speed above 20% which provided enough cooling and very little noise. The people I helped were fine with changing the motor and listening to the president of the company scream when the ac did not work. Wonder why he kept them.


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## paulengr (Oct 8, 2017)

SWDweller said:


> I have always put in a min speed/hertz and locked it. Based on my understanding of the equipment. To prevent the tweakers from playing with expensive equipment.
> 
> Had an AC fan that typically got down to around 15% which burned up the motor. I helped out changed the pulley on the motor and kept the speed above 20% which provided enough cooling and very little noise. The people I helped were fine with changing the motor and listening to the president of the company scream when the ac did not work. Wonder why he kept them.


That is quite surprising.

NEMA gives guidance on low speed operation at constant torque. I’ll save you the trouble of looking: don’t go below 50% of name plate. As motor speed decreases the cooling from an integral fan decreases with the square of RPM. Granted power is decreasing too but since HP=torque x RPM / 5252 it isn’t decreasing as fast as loss of cooling. Below that point use either a blower cooled motor or an inverter duty motor with a turndown rating.

Fan affinity laws say the power draw varies with the cube of the speed. So on centrifugal fan/pump systems you can comfortably go as low as 6 Hz (10%). Ive never seen problems running this low unless someone messed with the V/Hz settings. A common one is to set torque boost to 15% instead of say 5% which would overflux the motor and burn it up. Even better is V/Hz which will vary the flux as needed. You don’t “need” it for fans but it helps with low speeds above 1 Hz.

Below 10% (6 Hz) you are approaching a condition where there is very little cooling to begin with and you are in serious danger of overfluxing the motor with open loop (V/Hz) control. At that point I’d highly recommend using vector mode even in a fan application and even then use either an inverter duty motor with a high turn down ratio (say 100:1), or a blower cooled motor (infinite turn down).

To OPs question:

First off, do at a bare minimum a static auto tune even on V/Hz mode if you can. This lets the VFD calculate VD and automatically adjust for it. If you can’t do auto tune it’s because it uses a compensation algorithm that does this automatically. Either way, it will just boost the voltage, so you are covered! On long runs though..,

Voltage drop is a function of the current, both resistive and inductive (which we traditionally ignore), and properties of the cable. As a very rough approximation we calculate VD = I x Z. More exact formulas are here:









Voltage drop calculation methods with examples explained in details | EEP


The first consideration for voltage drop is that under the steady-state conditions of normal load, the voltage at the utilization equipment must be adequate




electrical-engineering-portal.com





But it’s only dependent on current and power factor, not voltage. Now then one key thing to remember is that motors have two currents. One comes from the stator flux which creates a rotating magnetic field but it does almost no work so it is purely inductive with a power factor of zero. This current is very close to the no load current. It does not change with load (torque) or frequency but it does change with voltage which is why with a VFD we have to do torque boosting. The second current is due to the torque consumed by the load. Since it does work, it is almost entirely resistive, and power factor is 1.0. So as we change the frequency with a constant torque load like a conveyor both currents don’t change. But since Power = HP x 0.746 = V x I x PF and knowing that PF is fixed but HP is decreasing our only conclusion is that voltage must drop, So we know that V/Hz is a fixed ratio with constant torque. In a fan application though this isn’t true. Load torque and current changes with the square of the speed The flux is decreasing as voltage drops too but not as much as load current, So power factor is decreasing from somewhere around 0.9 towards zero as the speed drops but we can’t predict this without actual data.

Going back to VD then conservatively using say #14 wiring in non magnetic shielded cable we have VD of 0.44 V per A per 100 feet at 0.8 PF, decreasing to around 0.33 V per A per 100 feet at 0.6 PF, so a pretty significant decrease. Don’t assume this!! On cables under about #3 the impedance is mostly resistive and this holds true but on larger size cables the impedance is mostly inductive so a shift towards lower power factor increases the voltage drop!

Now considering currents for a fan, at 10 Hz our load torque/current has decreased by 97% just owing to fan affinity laws (torque varies with square of speed). By the time we reach that speed most of the current is just no load (flux) so don’t expect the current draw to have dropped to 3%! On a 3600 RPM fan motor expect no load current of about 10-15% of FLA. So if we had to account for VD it’s there but it’s pretty insignificant.

If you don’t have a fan there’s an obvious way to estimate it. Just do the calculation assuming FLA and taking the worst (highest) VD for the cable size in the article above. This gives you a result that works at any voltage. You can conservatively assume V/Hz with a minimum of 10% voltage. So at 480 V that’s 48 V and with NEMA recommending no more than 10% VD up the cable size if VD is more than 4.8 V, So say we have a 10 A motor 200 feet away. So we have 2000 A-feet or 20 A-100 feet. In the table worst case is 0.541 V x 20 = 10.82, too much. We can back calculate and see that we need to be under 4.8/20 = 0.24. Looking at the table that’s #10. Granted most loads are variable torque to some degree. So with no load of a 4 pole motor (1800 RPM) of 25-33% we would be at 4.8/6.667 and the standard cable size (#14) is fine.

So this long winded example shows you why few people ever bother with VD with VFDs. It rarely matters at all. And if it does just auto tune or switch to flux vector mode and you can safely ignore it…the VFD will tune it out for you.


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## JRaef (Mar 23, 2009)

Voltage drop is based on current. On a fan driven by a VFD, if the speed is low, the load on the motor is lower by the CUBE of the speed change, i.e. at 1/2 speed the load is 1/2 cubed, or 1/8 load, which means the current is going to be extremely low as well. So if you sized the cable for VD at full load / full speed, it is more than good for anything less than that.


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## SCR (Mar 24, 2019)

JRaef said:


> Voltage drop is based on current. On a fan driven by a VFD, if the speed is low, the load on the motor is lower by the CUBE of the speed change, i.e. at 1/2 speed the load is 1/2 cubed, or 1/8 load, which means the current is going to be extremely low as well. So if you sized the cable for VD at full load / full speed, it is more than good for anything less than that.


Thanks for clearing this up for me. 
On a different note, this same project will have 6 single-phase input 1KW vfds in a 4 by 4 foot steel cabinet to control fans. This vfd cabinet is right beside a 200 amp sub-panel, which is fed from a 75kva Xformer located 200 feet away. I already know I'll be installing load reactors, but do i need to worry about line side reactors being that this panel is so far away from main power source. Could I install a single line reactor to handle all 6 vfds?


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## CAUSA (Apr 3, 2013)

SCR said:


> Thanks for clearing this up for me.
> On a different note, this same project will have 6 single-phase input 1KW vfds in a 4 by 4 foot steel cabinet to control fans. This vfd cabinet is right beside a 200 amp sub-panel, which is fed from a 75kva Xformer located 200 feet away. I already know I'll be installing load reactors, but do i need to worry about line side reactors being that this panel is so far away from main power source. Could I install a single line reactor to handle all 6 vfds?


Question?

Did a Coveted stamped engineer, design this set up? Or did someone just wake up one day and said, I'm going to make your life difficult this month??


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## CAUSA (Apr 3, 2013)

SCR said:


> Thanks for clearing this up for me.
> On a different note, this same project will have 6 single-phase input 1KW vfds in a 4 by 4 foot steel cabinet to control fans. This vfd cabinet is right beside a 200 amp sub-panel, which is fed from a 75kva Xformer located 200 feet away. I already know I'll be installing load reactors, but do i need to worry about line side reactors being that this panel is so far away from main power source. Could I install a single line reactor to handle all 6 vfds?


On a more serious note:

Sooo... going to have 2-3phase motors and just use 1 KW VFD per phase? 

Or going to use 6 individual single phase motors on 6 individual single phase motors?

What is the end configuration, of this design?


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## SCR (Mar 24, 2019)

CAUSA said:


> Question?
> 
> Did a Coveted stamped engineer, design this set up? Or did someone just wake up one day and said, I'm going to make your life difficult this month??


No, I woke up one day and decided to eliminate all the coveted stamped engineers from my projects and just do things my own way, and save a pile of money in the process. There is nothing about this job that requires an engineer, though.

Question???? Not sure if i can make the question clearer. Being that this subpanel is so far from source transformer, do I actually need line reactors?


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## SCR (Mar 24, 2019)

CAUSA said:


> On a more serious note:
> 
> Sooo... going to have 2-3phase motors and just use 1 KW VFD per phase?
> 
> ...


Ok let me lay it out for you, I assumed most electricians would be able to read between the lines.
1kw vfds to be running 2 3-phase fan motors each
1.5kw vfds to be running 3 3-phase fan motors each
6 vfds total controlling 14 motors. And yes, 1 3-pole O/L per motor.


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## micromind (Aug 11, 2007)

Is the panel single phase?


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## CAUSA (Apr 3, 2013)

SCR said:


> Ok let me lay it out for you, I assumed most electricians would be able to read between the lines.
> 1kw vfds to be running 2 3-phase fan motors each
> 1.5kw vfds to be running 3 3-phase fan motors each
> 6 vfds total controlling 14 motors. And yes, 1 3-pole O/L per motor.


Thanks for answering my silly question with the unnecessary loophole of 6 single phase motors powering 6 single phase motors. Just my sense of humour.

I will send some documents your way just traveling from up North.

but then again someone might chime in a do it before I get back to home bace.


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## SCR (Mar 24, 2019)

micromind said:


> Is the panel single phase?


No the panel is 3-phase. For smaller vfds I always use single phase input vfds to save breaker space and save 50% on the breakers too. Also saves the drive from going into single-phase foldback, caused by uneven currents drawn on 3-pole input.


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## gpop (May 14, 2018)

Single phase input 3 phase output isn't a problem as long as you oversize the drive. 
Multiply motors on a vfd isn't a problem as long as you over size the drive. 

1kw drive sounds way to small.


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## paulengr (Oct 8, 2017)

gpop said:


> Single phase input 3 phase output isn't a problem as long as you oversize the drive.
> Multiply motors on a vfd isn't a problem as long as you over size the drive.
> 
> 1kw drive sounds way to small.


There are single phase specific VFDs up to 2-3 HP, no oversizing needed. You can also get voltage doubling drives. But I wouldn’t do this is 3 phase is available. It’s much more efficient (lower amps).

Multiple motors per drive can work with a couple caveats. Need an external overload per motor to be legal and you can only use V/Hz mode.


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## micromind (Aug 11, 2007)

SCR said:


> No the panel is 3-phase. For smaller vfds I always use single phase input vfds to save breaker space and save 50% on the breakers too. Also saves the drive from going into single-phase foldback, caused by uneven currents drawn on 3-pole input.


I've never heard of a VFD going into single phase foldback. I suppose if the PUCO supplies lousy power, it could be a problem though. 

For multiple small VFDs in a group, I usually use 1 - 3 pole breaker and something like a Square D 2510 3 pole switch as a line-side disconnect. Saves a ton of space in the panel plus the switches are located at each VFD.


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## SCR (Mar 24, 2019)

micromind said:


> I've never heard of a VFD going into single phase foldback. I suppose if the PUCO supplies lousy power, it could be a problem though.
> 
> For multiple small VFDs in a group, I usually use 1 - 3 pole breaker and something like a Square D 2510 3 pole switch as a line-side disconnect. Saves a ton of space in the panel plus the switches are located at each VFD.


You called it right. If the local utility supplies lousy power, the vfd draws almost no power from whichever line is most out of phase, and the drive will go into a reduced power mode, by limiting the frequency of the output. This will also a huge imbalance on panels feeding multiple vfds.


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## CAUSA (Apr 3, 2013)

SCR said:


> Thanks for clearing this up for me.
> On a different note, this same project will have 6 single-phase input 1KW vfds in a 4 by 4 foot steel cabinet to control fans. This vfd cabinet is right beside a 200 amp sub-panel, which is fed from a 75kva Xformer located 200 feet away. I already know I'll be installing load reactors, but do i need to worry about line side reactors being that this panel is so far away from main power source. Could I install a single line reactor to handle all 6 vfds?


You could do a single line reactor or individual line reactors.
The single line reactor needs to be balanced on all 3 legs. so at steady state conditions not a problem. But when a fault happens. The examination of what imbalances impose to the set up would have to be taken in to consideration.

For individual line reactors, a fault does not affect the system as much. To keep the set up simple. I would go with individual reactors.

The impendence should also be considered, For the iron losses and copper losses that will be imposed. Also the sizing need to be correct for any configuration that you decide.

I would also chose the O.L for each motor to be compatible for the VFD chosen. I would stay away from Bi-metal, known for nuisance tripping on VFD's.

I included a some documentation on the subject. Just a basic out line.


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## JRaef (Mar 23, 2009)

SCR said:


> Thanks for clearing this up for me.
> On a different note, this same project will have 6 single-phase input 1KW vfds in a 4 by 4 foot steel cabinet to control fans. This vfd cabinet is right beside a 200 amp sub-panel, which is fed from a 75kva Xformer located 200 feet away. I already know I'll be installing load reactors, but do i need to worry about line side reactors being that this panel is so far away from main power source. Could I install a single line reactor to handle all 6 vfds?


The “rule of thumb” is that for an “unbuffered” VFD, meaning one WITHOUT a DC bus choke (as most low cost Asian drives are), you should have a Line Reactor if the source kVA is 10x the drive kVA or greater. So your smallest drive is 1kW, call it 1kVA to be simple, and your source is 75kVA, so since 75>10, you should use a Line Reactor. The thing is, you could put one large LR ahead of all 6 drives and get the desired protection. No need to put one in front of each drive.


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## SCR (Mar 24, 2019)

JRaef said:


> The “rule of thumb” is that for an “unbuffered” VFD, meaning one WITHOUT a DC bus choke (as most low cost Asian drives are), you should have a Line Reactor if the source kVA is 10x the drive kVA or greater. So your smallest drive is 1kW, call it 1kVA to be simple, and your source is 75kVA, so since 75>10, you should use a Line Reactor. The thing is, you could put one large LR ahead of all 6 drives and get the desired protection. No need to put one in front of each drive.


Great! That's just what I wanted to hear. That frees up a bunch of space in my cabinet.


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