# Advice Wanted Re Vfd For 2 Speed Motor.



## richo132 (Apr 13, 2016)

Hi all,
Recently acquired a 1950 Victoria U2 mill with a 2 speed (2800/1450) motor rated at 6/4hp. Some pics:




I am arranging for the mains supply to my house and workshop to be upgraded to 3ph - will take a few weeks. In the meantime, I'd like to understand my options for fitting a 3ph VFD to the mill.

I confess to having very little knowledge of these things, although I did manage to upgrade my drill press with a VFD that runs a 1hp 3ph motor at 230V. This was achieved with a new motor, preconfigured VFD and a remote control supplied (complete with idiot sheets) by a local electric motor specialist.

However, the Victoria mill is a different challenge as I want to retain the original motor.
What I would like to achieve is variable speed control and, if possible, electric braking of the motor when an emergency stop button is pressed.

I have attached a copy of the wiring diagram in the hope that someone can offer me some advice on how to go about this. Cheers.


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## hackley69 (Apr 13, 2016)

Sorry I can't help you wth the wring but I like your mill!


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## mksj (Apr 13, 2016)

Hi,
I have worked on several VFD systems with 2 speed motors. It is doable to set up a VFD to work with the 2 speed motor and switch motor connections and VFD settings using a rotary switch. The problem has been that for some reason, the 2 speed motors do not seem to work well with a VFD. Usually this results in stuttering or rough running of the motor at anything below the base speed of the motor usually on the high speed setting. Also since the VFD gives you speed adjustment, there is really no reason to switch the motor wiring/speed setting. VFDs do work with older motors, but you are usually limit in the variable speed range that the motor will maintain constant torque and I would not run it above the base speed. You would be better off replacing the stock motor with a 5 or 7.5Hp single speed in my opinion. You could keep the stock motor if you wanted to go back to stock. It is possible to keep the stock control switch, disconnect the HV going to it and using the contacts to control the VFD functions for a speed motor. It is not recommended to switch a 2 speed motor running on a VFD while the motor is running. My last 2 speed conversion was setup to stop the motor when the motor windings where switched, and also reprogrammed the VFD to change the motor settings. This required a different rotary switch which was reconfigured for this function. Normally I would use a 3 wire control with momentary buttons and a speed pot to control the VFD, but there are ways to wire the stock controls to do the same functions. They could also power high voltage relays to control the VFD motor functions.

Configuring the machine to run with the stock switch would be a bit of a challenge. As mentioned, I think it would be easier to use a small control box for the VFD control. You could use the stock controls and place the VFD between the power in and the power to your current motor switch, you would need a switch to reconfigure the motor parameters for a 2 speed motor. You would not need the thermal overload breakers in the stock wiring. The poles and motor overload characteristics need different programing parameters. A VFD would need to have a dual motor function.

I could suggest some different wiring configurations, but would be difficult without seeing the machine and seeing how it is wired and the controls. A 5Hp or 7.5Hp inverter rated motor can usually be purchased at reasonable prices if one looks around.  I would most likely recommend going with a 1750 RPM motor, they easily can go up to 1.5-2X there rated RPM depending on the motor, or just change the pulley size on the motor, as the effective Hp at the spindle will not change.


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## richo132 (Apr 13, 2016)

mksj - thank you for taking the trouble to provide such a detailed reply.
It gives clear and helpful guidance as to my options.
In short, I now accept that it is impractical to add a VFD to the existing motor.
I had previously considered changing the motor but was discouraged by its size/weight and restricted access to the mounting bolts.
There is also the matter of finding a suitable motor pulley - currently a four groove unit on an imperial shaft. 
Better have another look into this option.


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## Wreck™Wreck (Apr 13, 2016)

I would suspect that if the motor controls are left intact after the VFD it would operate at both speeds in relationship with the frequency supplied.

A very old motor may not enjoy operating at 30 HZ however. External cooling may be required.


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## strantor (Apr 13, 2016)

That's possibly the most unconventional and unnecessarily convoluted motor control drawing I've ever set eyes on.

Do you think you can sell that old motor for more than scrap value? I don't know, but I suspect that there's not a lot of people out there looking for an ordinary 2 speed motor, winding insulation circa 1950. So I would see no reason NOT to risk burning it up. I wouldn't spend any money on a new motor until I had sufficiently ruined the original. And then I would install an inverter duty TEFC with separately powered blower fan (sometimes call vector duty).

I would try installing the VFD wired to the original motor (through all the convoluted relay circuitry). Without spending an hour deciphering that diagram, it looks like (or I assume) your motor achieves dual speeds by going from [high speed low torque - delta] to [low speed high torque - star with parallel windings (doubled poles)] by shorting x,y,z. I would keep it that way because otherwise you have to permanently wire it one way or the other. If you wire for high speed, you lose the option to have higher torque at low speeds. If you wire for low speed, you can probably still get the full range out of it by adjusting frequency higher than 60hz, maybe all the way to 120hz<--- if I had to pick one, that's my choice. Sure, that will probably decrease the life of the motor dramatically, but you're saving up for the replacement, right?


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## richo132 (Apr 13, 2016)

_strantor: 
That's possibly the most unconventional and unnecessarily convoluted motor control drawing I've ever set eyes on. _
Now I don't feel quite so dumb for struggling to understand it 

_Do you think you can sell that old motor for more than scrap value?_ 
Valid point, but it is worth something to me ATM - its in place and works.
_
it looks like (or I assume) your motor achieves dual speeds by ... shorting x,y,z. _
I will try to confirm this with a multi-meter and report back.


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## Str8jacket (Apr 13, 2016)

Gday Richo, where abouts in Oz are you. I have a 5.5kw 2900rpm motor here you can have for a carton.
As far as pulleys go that was the easiest part of my recent motor change. Lots of options and far cheaper than youd expect 
Also with changes to belt tech you probably could run that mill on 2 belts nowadays or 1 if the section is bigger. 
To have better cooling go to rs online and buy a small 240v axial fan and remove the fixed one and put it in instead. Cost $17 for my fan and works a treat. I have the vfd switch a relay when the motor is on and off.


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## richo132 (Apr 13, 2016)

Str8jacket said:


> ...As far as pulleys go that was the easiest part of my recent motor change. Lots of options and far cheaper than youd expect


I'll start looking into this - presumably a taper lock pulley is the way to go.


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## mksj (Apr 13, 2016)

strantor said:


> If you wire for high speed, you lose the option to have higher torque at low speeds. If you wire for low speed, you can probably still get the full range out of it by adjusting frequency higher than 60hz, maybe all the way to 120hz<



Actually there this all depends on the belt and gear ratios. If you change the pulley size you will get just as much torque, if you push the motor up to 120Hz, you will loose significant torque and Hp with these old motors and I doubt it will last long. Why go through all the trouble to see if you can burn up the old motor. As, already mentioned, for some odd reason 2 speed motors do not seem to work well on the high speed with a VFD. On the low speed  you will have 4 Hp at best. The 2 speed motors I have seen in these machines was a Delta on low speed, a YY on the high speed.


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## strantor (Apr 14, 2016)

mksj said:


> Actually there this all depends on the belt and gear ratios. If you change the pulley size you will get just as much torque, if you push the motor up to 120Hz, you will loose significant torque and Hp with these old motors and I doubt it will last long. Why go through all the trouble to see if you can burn up the old motor. As, already mentioned, for some odd reason 2 speed motors do not seem to work well on the high speed with a VFD. On the low speed  you will have 4 Hp at best. The 2 speed motors I have seen in these machines was a Delta on low speed, a YY on the high speed.


Rereading my post I see that I left a lot of room for misinterpretation. Sorry for being unclear.

I was not suggesting wiring for high speed or low speed. I was not suggesting going up to 120hz. I was not suggesting any wiring changes at all. Nor was I suggesting any pulley changes.

I was suggesting leaving all the wiring intact and unchanged, and simply connecting the VFD between the mains supply and the power input of the machine. That way original 2-speed functionality remains (until the poor old motor is inevitably put to rest at the hands of PWM spikes). I realize that I was a little hasty in saying so; that is a bit of an over simplification, since the drive would need to be tied in with the start/stop/fwd/rev circuit. I didn't think about it hard enough to mention that part, but I still believe in leaving it 2-speed.

I then went on to suggest that, if I was forced to pick one way or the other to permanently wire it high speed or low speed (because/if for some reason leaving it 2 speed didn't work out <--- that wasn't stated, it was implied, and vague, sorry) then I would pick low speed and I would exceed 60hz. But I agree with your suggestion to wire for low speed and change pulley size, it's a better solution.

Thank you for pointing these things out.


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## British Steel (Apr 16, 2016)

Hi, re VFD PWM spikes, you can save the motor by fitting a load reactor - just a 3-phase choke (or three separate chokes), usually sized by " percent " - 3% to 5% is usual, the inductance is calculated by taking the voltage across the choke e.g. 3% of the mains voltage at the rated current and frequency.

An example: a 4% choke for a 415v motor rated at 6A, 50Hz
4% of 415 is near as dammit 16v, the current is 6A so the reactance Z is 16/6, 2.67 Ohms (reactive)
Z = 2 * Pi * f * L 
lets rearrange that: divide both sides by 2 * Pi * f:
Z/(2 * Pi * f) = L so 2.67/( 6.3 * 50) = L
L = 2.67 / 315 = 8 milliHenries (8 mH) - this is the value of inductor you need, rated for at least the max current your motor will pull. One for each phase!

How does it work? The chopped PWM signal is approximating a sine wave, so the square edges of the current pulses are very high-frequency components (multiples of the switching frequency), the formula z = 2*Pi*f*L show that the impedance (resistance to current change) of the inductor scales with frequency, so for (e.g.) a 5 KHz switching frequency, the switching component sees  267 Ohms, 100 times the  impedance that the wanted 50Hz "average" sees - the harmonics (all odd-order with square waves, so 3f, 5f, 7f etc.) see even higher impedances, 3f sees 800 Ohms, 5f sees 1330, 7f sees nearly 1900 - this limits the current the harmonics deliver to the motor winding (and insulation!) so reduces the rate that the voltage on the winding can increase, making the voltage seen by the insulation much more like a sinewave and much safer for old insulation 
I've wound suitable inductors for a 6-amp 415v motor on salvaged cores from 24v control transformers, hefty high-temperature enamelled copper windings and a "gapped" core - I removed the interleaved E and I laminations and stacked all the E's, all the I's together and put a thin sheet of PTFE in  between to form the gap, which increases the allowable current before the core saturate (when it can't take any more magnetic field and stops acting like an inductor, basically).

You probably didn't need to know that, I do go on sometimes 

Dave H. (the other one)


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## richo132 (Apr 16, 2016)

@dave H
Thanks for taking the trouble to share your wisdom on this subject.
I now know enough about this subject to be dangerous but nowhere near enough to be useful 
Slavishly applying your formulae, I have calculated the choke inductance values for my 2 speed motor:
2800rpm FLA 9.02A -> 5.8mH
1450rpm FLA 6.2A   -> 8.5mH
As I am unable to "roll my  own", I looked for commercial products and ended up here: 
http://www.zez-silko.com/en/products/reactors-for-frequency-converters 
- where I discovered there are both Input Reactors and Output Reactors.
However, I notice that in both cases they are single value chokes.
So if I may draw further on your expertise, do you recommend an Input or Output Reactor and is it necessary to have separate reactors for each speed (current) ?


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## British Steel (Apr 16, 2016)

No problem Richo,

Output reactors are what you want - input reactors are to (somewhat) correct supply power factor, reduce switch-on current and help contain RF interference on the supply to the VFD, in your circumstances they shouldn't be necessary!

You *could* use different inductors for the two speeds but myself I'd probably pick the 5.8mH if they were available - that link shows a max. of 2.5mH, which would probably be enough to protect your motor (there'll still be the attenuation of the PWM harmonics, just not quite as much, instead of 100:1 you'd be looking at 45-ish:1 or 25-ish:1 impedance ratios for the two speeds and 5kHz switching (vs 50Hz sine), again the ratios would multiply for the harmonics.

My own application is on a 3-speed motor with 4.9, 5.2 and 6A rated currents in the three speeds, I've used a single value of about 7mH as a bit of a compromise, so far the 60 year old motor hasn't died on me, nor has the VFD!

Hope that helps,
Dave H. (the other one)

P.S. - oops. forgot something important! Put the inductors between VFD and speed switch, try NOT to switch speeds with the motor running! That requires a bit more thought, putting together a snubber circuit (to deal with voltage spikes from the motor/switching and prevent them getting to the VFD) and programming the VFD with an additional switch wafer/set of contacts/microswitch to drop its output during switching.


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