Motor/Inverter question

I ran into an interesting situation today and the resolution of it is puzzling me.

My client has a 5 HP, 460 VAC motor with an Omron 3G3JV drive. That's a basic volts/Hz (scalar) drive. We've known for a long time that the motor size is marginal for the application. The motor nameplate is 6.5 FLA and it's been drawing around 6.2 - 6.4 amps in operation. In fact, there have been times when the maintenance person has had to bump up the parameter in the drive that sets the max allowed current in order to get a complete production run. A typical production run is about 1.5 - 2 hours long and they generally only make one run per day, occasionally two. Motor speed ranges from 650 to 1000 RPM. It's constant for each production run. Different products are produced at different speeds.

Due to some changes in the product they're producing, they need to reduce the line speed. They found that as they reduce the speed down to around 20 Hz, the current remains reasonably constant, but below 20 Hz it starts to rise. The lower the speed, the higher the current. There doesn't appear to be any reason to believe that the required torque increases at lower speed.

As a result of the increased current draw, they couldn't slow down the line speed as much as they wanted without having the drive trip out on a timed overcurrent fault. The drive was set for max allowed current equal to nameplate current.

In an attempt to keep the current constant over a wider range, we decided to try to play with the slope of the Volts/Hz curve. On this drive it's done by setting two pairs of parameters. The first pair sets the high frequency and corresponding voltage. This was set at 60 Hz, 460 volts, which was the default setting. The second pair sets a low frequency and corresponding voltage. These were also at the default settings of 1.5 Hz, 24 volts.

My initial thought was to increase the voltage setting at the low frequency. My reasonong was that by increasing the voltage at any frequency, we would see a corresponding decrease in current at that frequency. It didn't work out that way. Increasing the voltage setting at 1.5 Hz resulted in higher current. Decreasing the voltage setting at 1.5 Hz resulted in lower current. We reduced the setting to 6 volts at 1.5 Hz and got a lot less variation in current all the way down to 5 Hz, which is way slower than we need to go.

Current draw before changing:

30 Hz, 6.0 A
25 Hz, 6.1 A
20 Hz, 6.3 A
15 Hz, 7.1 A
10 Hz, 8.1 A

After the change:

30 Hz, 5.6 A
25 Hz, 5.5 A
20 Hz, 5.3 A
15 Hz, 5.6 A
10 Hz, 5.4 A
5 Hz, 5.1 A

The change that worked seems counter-intuitive to me. Does anybody have an explanation?

More important, have we put the motor or drive at risk as a result of the change?

One additional change we will be making soon: The person who originally installed the machine did not pull the wires from the motor's thermal switch back to the drive cabinet. We will be doing that soon and wiring the thermal in series with the run contact so that if the motor actually overheats it will shut down.

I am impressed the motor actually turns with 6 volts at 1.5 hz. I did not think you could get enough magnetizing currents let alone enough torque producing current.

The JV drive is a basic V/F or V/HZ depending on how you like to call it. I have seen them run some off beat things with no real problems. Keep them clean and dry. I do not think you are causing a problem for the drive. You have not exceeded the max limits. I would monitor the motor physical temperature and if it does not seem to be heating at the low speeds you should be good to go.

If you want a motor that can handle low speed operation with relative ease, look at the Marathon Black Max motors.

We're not actually running down as low as 1.5 Hz. We ran it down to 5 Hz today just to see if it could do it, but in real production, we don't expect to go below 10 Hz.

The motor is a Black Max.

The heating issue is why I told the client to pull the thermal leads back to the control cabinet. If that opens, it's an unambiguous indication that we're running the motor outside of the range where it's happy.

If it's possible to add vector capability to the system, that has proved useful to help with low end speed and torque regulation for me in the past.

It looks like you basically took the low end boost out of the volts per hertz curve by reducing the low end voltage. If it doesn't stall out, I can't imagine that causing a problem.

We've known for a long time that the motor size is marginal for the application.

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If your top speed is 30Hz, is it practical to double your gear reduction and get top speed up to 60Hz? Then you may not have to upsize the motor and drive to give yourself some leeway:

Just my 2 cents.

If your top speed is 30Hz, is it practical to double your gear reduction and get top speed up to 60Hz? Then you may not have to upsize the motor and drive to give yourself some leeway

Click to expand...

That's an option, but it's not particularly easy. The motor is coupled to a gearbox with a substantial reduction. The gearbox's input torque rating is close to the motor's torque output. Going to a 7.5 HP motor or putting in an additional reducer risks turning the gearbox into an expensive shear pin. There is also an issue of space to mount any more hardware.

Speed regulation isn't a problem either. It's adequate to do the job. The only issue has been getting the torque at low speed without exceeding the current rating. If the change I made will take care of that without any unintended consequences, all is good.

Interesting,

I thought I'd bump this up in the hope that DickDV will see it.

Brian.

If you do not exceed the FLA of the motor or drive, I do not see you having a problem. Especially since you seem to be producing the necessary torque to run the load. If it becomes an issue, I would just put a 3G3MV or V7 as it is called now in place. It is an open loop vector capable drive. To get closed loop vector you would need the F7 drive.

Since you said it is a Black Max motor, I now know why it is running OK with reduced voltage.

Steve-

By increasing the voltage output above that defined by the volts/hertz value, you introduced a voltage component that wasn't matched by back EMF. So this voltage component created a current component that shows up as drive output current. Think of this at the low extreme, zero speed. If you request a voltage at the output terminals of the drive, this voltage will create a current flow limited only by the resistance of the motor windings. The higher the voltage, the higher the current.

If you need to develop some torque at very low speeds you will need to put in some voltage offset in order to generate torque. But I have always used this very sparingly. And if you don't need to run below about 10 Hz I wouldn't use any offset at all. A pretty strong case can be made that you can go down to 5 Hz pretty well with no offset. You are walking a fine line when using voltage boost. It can work pretty well to get moving but it often causes issues if you use it for normal run.

Keith

Thanks Keith. I think I'm beginning to understand. It appears that this particular drive's default settings include some low-end boost. The way they handle the volts/Hz setting is by setting the max frequency with one parameter, the corresponding voltage with another parameter, the minimum frequency with another parameter and finally the corresponding voltage at min frequency.

This drive came out of the box with max frequency 60 Hz, voltage at 60 Hz, 460. Minimum frequency was 1.5 Hz, corresponding voltage, 24. A straight line from 60 Hz, 460 volts to 0 Hz, 0 volts would put the voltage at 1.5 Hz equal to 11.5, so the default setting apparently gave it more boost than required.

My initial change (from 24 to 48 volts) made a bad situation worse. Shoulda done the arithmetic before making the change.

Lesson learned: RTFM (read the friendly manual) isn't always enough. Sometimes you have to UTFM (understand the friendly manual).

Steve, let's try this. The motor is nameplated 460V 60Hz so that is a V/Hz ratio of 7.67. If the motor was purely inductive, you would want that ratio all the way down to zero speed so the current would stay constant for a given load. Unfortunately, copper conductors have a little resistance so, a very low frequencies, the current falls off a little faster than desired due to the increasing significance of the coppre resistance which results in less torque at low speeds. Most drives including the one you were working on have the ability to artificially increase the voltage above the normal 7.67 ratio to compensate for this loss of torque.

As you discovered, while a little might be good, a lot is not necessarily better! What happens is this--remember that the motor lead current is the vector sum of the magnetizing current and the torque-producing current. The torque-producing current is the result of currents flowing in the rotor where the copper resistance is very low and the inductance is also very low. But the magnetizing current comes from the stator windings which are very inductive and somewhat resistive due to the windings. If you "overvoltage" the motor with too much boost at low frequencies, you get significantly increased field current in the stator. If you really overdo it, the magnetic circuit begins to saturate and the current goes up even faster with a corresponding decrease in torque. Did I mention that the motor tends to roast under these conditions too?!!

I am not at all clear on Kamenge's comment about back EMF. I don't find any back EMF in AC induction motors, only in DC machines. Maybe he can explain what he meant a bit further.

Hopes this helps. The above comment about there being a "sweet spot" for voltage boost is right on. Not enough and torque falls off due to copper losses, too much and the motor saturates. That's exactly why a good vector drive runs so well at slow speed. it manages all this automatically.

Thanks Dick, one more question.

As I mentioned, the out-of-the-box setting for the low end was 24 volts at 1.5 Hz. A straight line from 406/60 would have 11.5 volts at 1.5 Hz. We wound up setting it at 6 volts at 1.5 Hz. That's running a bit under the curve. Since the downside to excessive boost is inefficiency and motor heating, what's the downside to 'underboost'. Acceleration time isn't a problem. The load is primarily frictional.
I'm thinking that too little boost would manifest itself as inability to start. Any other concerns?

Many drive-motor systems have no low-end boost at all. They don't need it because the low slow-speed torque output is not a problem as with fans and pumps.

In fact, low output torque is the only result and its not a problem if the load doesn't need it. No motor noise, motor heating, drive reliability problems at all! (Not very often we can say that about anything anymore, it seems!!!)

In fact, when you have centrifugal loads like fans and pumps, it is common to alter the V/Hz curve so it follows more closely the torque curve of the load which is a function of the square of the speed. This results in a V/Hz curve that is way low in the middle and only rises steeply as you approach full speed.

When a drive is set up that way, you can usually save a couple % of energy costs since the motor is under-magnetized or field-weakened in the midrange and the motor is so starved for voltage at low speeds that the motor and fan will not even start turning until the frequency reaches 11-12Hz.

This is not a problem if the system is only passing thru this range when starting but you wouldn't want to try to operate there. Of course, fans and centrifugal pumps are normally not operated much below 40% speed anyway so it works out fine. That is why drives intended for HVAC duty come with a squared function V/Hz curve as factory default.

Steve,

Too little BOOST and the motor will not turn. All vfd's has a boost adjust at one time. Now it is automatic and usually not a problem. Vector drives do a much bettr job of controlling motor current while maintaining torque. If you can use 6 votls at 1.5 hertz and the motor turns, you really do not have a problem. The black max motors also do not have the MASS of iron in most AC motor that help develop the initial fields. Pick up a 5 hp AC motor then pick up a black max, quite a bit lighter.

You are correct about the 11.5 volts for the straight line V/hz. If you ever saw the motor "EFFECTIVE" circuit, the R, L and C circuit, you would see there is approximate 12 volts necessary for magnetizing currents. This is much like DC motor Field or the idle current in a transformer. Then you need current in the rotor. The V/HZ is actually what is happening inside the rotor not the applied energy.

Think of an AC motor as a transformer with the stator the primary and the rotor the seconday. I find this helps most folks to better understand what is happening in an AC motor. Watts in times the Efficiency = watts out.

Also think of the extra volts down low as watt loss or voltage of the I squared R losses..

Originally posted by DickDV:

I am not at all clear on Kamenge's comment about back EMF. I don't find any back EMF in AC induction motors, only in DC machines. Maybe he can explain what he meant a bit further.

Click to expand...

All motors I am aware of (with the exception of piezoelectric effect motors) develop back EMF. If they didn't all we would have to oppose applied motor voltage would be stator impedence. When you apply 460 VAC to a motor winding this would result in some really huge currents.

The DC alalogy is the correct way to think of it. In an AC machine the stator induces current in the rotor, which sets up a rotor magnetic field. This magnetic field rotates inside the stator, which induces a voltage in the stator winding counter to that of the applied motoring voltage. This is no different than in a brushless DC motor except there the rotor magnetic field comes from permanent magnet motors, not induced magnetism from the stator. It is the same thing that creates back EMF in a DC motor.

Keith

I've never seen an AC induction motor equivalent circuit with a CEMF generator in it. Can you refer me to one?

I've always thought of the motor stator as being similar to the primary winding of a transformer.