233Mhz

So I’m not trying to untangle this mess again a year from now — here’s what l want to remember about vacuum ratings.

A bar, one that is, is like a thousand millibar. That is one atmosphere, that is 14.7psi. That is a bad vacuum.

Let’s stick with no vacuum for a minute: that’s also 760 Torr (Torr is the same as mmHg — millimeters of mercury). When we talk about mm or inches in pressure readings, we’re talking about how far the atmosphere pushes a column of mercury or water up a tube. If Torr is the same as mmHg, then 760/25.4 = aprox 30 inHg, as in barometric pressure. For what it’s worth, that’s about 400 in H2O.

Think of a U-shaped tube, with the atmosphere applying pressure on one end and the liquid being pushed up the other.
Now, if we start evacuating the atmosphere, we will gradually pull-down the column of liquid.

If I’m talking millibar, the lower I take the number, the closer I’m getting to a perfect vacuum e.g., 10mbar is more vacuum than 20mbar; same for Torr, same for……

….now, here’s where it’s easy to get confused: are we talking vacuum or atmosphere, is right on the gauge more vacuum or left? Hopefully someone has been nice enough to put an A or a V after the reading, or a negative sign to differentiate; but, sometimes you just have to look at the context.

For example: a typical pump for a small pick and place application might have the following on its nameplate:

VAC -.8 bar
CAP 12 CFM (I’ve already converted from m3h)

So, what we have here is an 800 mbar (or 600 Torr, or 6.3 inHg) pump — not much, eh? That’s because it’s not an 800 mbar pump, its a pump capable of pulling atmosphere down 800mbar to 200mbar. See? Even the guys who work with vacuums have to look at stuff a couple of times. So, be careful, or you can make some really stupid assumptions.

For fun, let’s look at a shop-vac to get an idea of where we’re at — typically about 150 Torr (200 mbar) at around 100 CFM. It would appear that it could serve the purpose of our industrial three-phase application above, and maybe it could for awhile — I think the difference might be in the way they describe the horsepower — whereas the industrial pump may have a 1 hp motor on it, the shop vac is rated at 1 hp peak.

Also, it would be interesting to compare curves. I have been told, as a rule of thumb, to look at the 200 Torr — about 22 in Hg — points to get a reasonable idea of what a pump can do.

Here are curves for some bigger industrial pumps

The top curve is for a pump that is rated as:
MAX VAC 28.5 in Hg
MAX FLOW 180 SCFM

That’s a lot. Could I use it to temporarily replace a pump with a smaller vacuum rating? Depending on the application — yes (if I’m not going to distort something). I have been told though that you should not run an oil-filled pump at a high vacuum (that is, at a high flow rate with high vacuum, say — only 400 Torr). This is due to the increased air-flow causing trouble with the oil-air separating function of the filters –too much air flow to allow the oil to separate and fall back sufficiently.

As far as carbon vane pumps, they apparently don’t care. Though I was told in the past that you should never dead-head one of these, I recently spoke with a rep that said you can (due to the fact that most will have a relief) but it is still best not to due to over-heating.

Links to conversion charts and pages here — under “Vacuum”.

So, here’s a way some OEMs work around the torque limitations we discussed in Part 1 — they lie to the inverter. They tell the inverter that the motor is wired for 480V, but actually wire it for the lower voltage, 277V.

The reason the motor doesn’t burn-up is that they also tell the inverter that it is rated for 104Hz (that’s the ratio of 480/277 or 1.73 times 60Hz).

Here’s why. As we previously saw with an 8 to 1 V/Hz ratio, we ran out of voltage at 60Hz. Further increases in Hz increases speed, but likewise increases the CEMF that the motor generates, thus reducing the effective voltage across the stator windings.

But by doing the creative wiring and programming we spoke of above, here’s what happens:

As you can see, now with the 4.26 to 1 ratio of 480V/104Hz, our curve looks a lot different, specifically look at what the inverter will put out at 60Hz —- 277V, and that’s just what we have our motor wired for.

Now, start increasing the command frequency; the motor speed and the CEMF will go up — trying to force our current (torque) back down; but now we’ve got 203 more volts to go on our curve — basically allowing constant torque well into the 100Hz range. And, we did it with the same motor.

The only trade-off is that you need to have that extra capacity available in your inverter — you can’t do this trick with a 1hp inverter and a 1hp motor, you need the extra headroom of a slightly larger inverter.

Has this ever happened to you? You’re troubleshooting a problem with a motor that is being driven by an inverter; you know the supply voltage is 480V, and the parameters in the inverter appear to be for a 480V motor, but upon looking at the wiring for the motor, you find it is wired for a lower voltage.

Before you call your friends over to celebrate your swiftness in discovering the problem, consider the following discussion.

I’ll begin with a quick review of voltages and currents as they apply here.

The simplest formula for finding out how much current will flow in a given circuit is Ohm’s law, Current = Voltage/Resistance or I = E/R. Though this is actually the formula for current in a DC circuit, we can use it as an approximation of what goes on in an AC circuit, or for the purpose of this discussion, an AC motor.

Anyone who has used a multimeter to measure resistance in an AC 3-phase motor knows that you will usually read anywhere from 0 to 10 ohms depending on how big the motor is (bigger windings = less resistance). If we plug these values into our formula for a 480V motor (say 480V/1 ohm) we see that our current should be about 500 amps and the motor should explode in our faces.

Part of the reason it doesn’t, is that there are some other factors involved — reactance in the windings (affects AC circuits) and counter electromotive force or CEMF. CEMF is key to understanding how an inverter controls a motor.

As our motor begins to turn, the magnetic field of the rotor starts to induce a voltage back into the stator (to which we are applying our operating voltage), this is the CEMF, and is approximately directly proportional to rotor speed. This acts to actually lower the effective voltage on the stator and therefore, lower the current. The faster the motor turns, the more it generates this back voltage. So once we have a motor up to speed, say at 60Hz, the current is a lot lower than when we started.

To tie the inverter into this discussion, consider what kind of current we would be drawing if we applied the total 480V across the stator at a reduced Hz. You would be pulling much higher amps due to the smaller amount of CEMF that would be offsetting the applied voltage. This is why an inverter doesn’t do that. It actually scales the voltage back to maintain the correct Volts per Hertz ratio for the motor.

So, if we have a motor that is rated at 1hp at 60Hz at 480V, that’s an 8 to 1 ratio (480/60). If we want to run the motor slower by reducing the Hz, say to 30, then the inverter will scale back the voltage accordingly. The following chart shows this.

At 30Hz, the applied voltage is only around 240V (30 x 8 = 240V — this maintains the motor’s original ratio). Once you reach the rated Hz, 60, the full voltage, 480, is applied. Up to that point, we have met our obligation to maintain the correct V/Hz ratio. After we reach this point though, we are no longer doing so, and as we keep increasing the speed of the rotor, our friend CEMF will begin robbing us of torque — as we have no more voltage to apply to offset him.

Next, we’ll see how this limitation can be overcome with some creative wiring.

480V Panel - work on energized parts, including voltage testing.
(More than 10KA short circuit current available)
= Hazard/Risk Category 2* per Table 130.7(C)(9)(a) NFPA 70E

per Table 130.7(C)(10) NFPA 70E
-V rated tools
-V rated gloves w/leather protectors
-double layer switching hood
-hearing protection
-FR long-sleeve shirt and pants or FR coveralls
(min arc rating of 4) over non-melting or untreated natural fiber pants and T-shirt
-leather work shoes

208V Panel - work on energized parts, including voltage testing.
(More than 10KA short circuit current available)
= Hazard/Risk Category 1 per Table 130.7(C)(9)(a) NFPA 70E

per Table 130.7(C)(10) NFPA 70E
-V rated tools
-V rated gloves
-FR long-sleeve shirt and FR pants or untreated natural fiber pants
-hard hat
-safety glasses

same scenario @ <10ka = Hazard/Risk Category 0
-non-melting long sleeve shirt and pants
-safety glasses

See NFPA 70E for actual requirements and specifics

Measure the diameter of the inner hole in inches (5). Measure the outer diameter of the roll (8.5). Count the total number of turns (31).

Add the two measurements together and multiply by the number of turns:

(5 + 8.5) * 31 =  418.5

Multiply above by .13 (.13 * 418.5= 54).

Answer is in feet. 54 ft