Pumps And Fans with Drives

Hello.

I will drive 2 fans and 2 pumps with a Micromaster 440 Drive. Fans are 75kW, pumps are 132kW. Electrical cabinets are installed and last week we have driven the motors with a simple V/f control. There is no problem.

What I am curious about is MM440 have some paramaters about the application of the drive like :

P0205 Inverter application
P0500[3] Technological application
P1300[3] Control mode

In the end they all change parameter P1300, by the way I hate the parameter list of Siemens Drives very much. They must think the more complex the parameter list the better drive.

In the manual for P1300 it says centrifugal fans / pumps need parabolic V/f control becuase the demand for torque in fan/pumps varies with f2.

I want to try this fan and pump application but I have questions about what will I gain. Will it save energy? or will speed be more stable and also this is the first time I will implement fan and pump application with drives. What can you advice me? Which way should I go? Linear V/f, Parabolic V/f or Vector Control (Sensorless)

Also somewhere I have read about a critical speed for fans which should be bypassed while running. Do you have any idea about this? If there is a critical speed like this for the fan motor how can I calculate it?

The special pump/fan application won't really do much for you. I'm not familiar with the Micromaster specifically, but for most VFD manufacturers the specific application modes just hide the parameters that aren't appropriate for the particular application.

For example, it's true that a centrifugal fan or pump has torque propotional to the square of the speed. However, if you are using simple Volts/Hertz control it will work fine, since the drive will provide the current needed to maintain the set speed. I've done a lot of centrifugal blower control, for example, and haven't had any issues. The parameters that generally need adjustment are accel/decel time, motor parameters like # poles and full Load Amps, and minimum speed.

There aren't any energy savings from the parameters, since kW is dictated by the load not the VFD.

Calculating critical speeds is a failry complex project, and requires things like shaft stiffness and impeller weights that you won't have access to. If you contact the manufacturer they can tell you the critical speeds for the unit. Having said that, though, most of the time manufacturers try to have the first (lowest) critical speed at least 10% above the nominal operating speed. You should check, but this isn't a problem for most pump and fan apps.

Good on all that Tom mentions. Plus, if I understand your post correctly, you are putting more than one motor on a single drive.

If that is the case, no other control mode but V/Hz is appropriate.

If you are putting only one motor on one drive so there are four drives and four motors, you might be tempted to use sensorless vector for better speed control but it isn't worth it. V/hz is still the right choice. One of the often unmentioned negative aspects of sensorless vector control is the need to do a motor ID run and speed loop tuning (at least P and I) when commissioning the system. Also, whenever the motor is changed out or rewound, you have to do it all over again. And, the speed accuracy you gain is meaningless in centrifugal fan and pump applications.

Go with simple. It won't hurt you in this case!

Hello

Thank you both for your interest and replies.

I will go with the simple then as you recommended if the pump and fan application will not make thing any better. The basic parameters Tom mentioned are already set. Also I blocked the reverse rotation in case of any corrupted analog set value.

I think I have left some empty points, we aren't putting more than one motor on a single drive. All motors have single drives. But since I learned not to use any other control other than V/Hz if more than one more is driven by single drive, it is good you mentioned. Thank you.

For the critical speed of the fan I contacted the manufacturer and they told me they will check it and reply.

Originally posted by Tom Jenkins:

There aren't any energy savings from the parameters, since kW is dictated by the load not the VFD.

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I don't believe this is absolutely true in any motor technology, much less an asynchronous AC motor application. The above statement infers that a TENV motor running with nothing connected to the shaft will draw an amount of electrical energy equal to the heating in the rotor bearings. We all know this isn't true.

It is my understanding that a drive operating in "fan/pump mode" operates with a modified V/Hz curve that undermagnetizes the motor at low speeds. We all look at magentizing energy as effectively free since we get it back on the demagnetizing half cycle. However, that magnetizing/dmagnetizing action still requires current flow. This means you still have IR losses in the motor windings and rotor bars. Running the motor at a lower general level of magnetization will reduce the magentizing current levels and minimize IR losses. however, it also means you have less than rated torque available to you.

Having said that there is no performance issue running in constant torque configuration when running a fan or pump.

Keith

kamenges said:

I don't believe this is absolutely true in any motor technology, much less an asynchronous AC motor application. The above statement infers that a TENV motor running with nothing connected to the shaft will draw an amount of electrical energy equal to the heating in the rotor bearings. We all know this isn't true.

It is my understanding that a drive operating in "fan/pump mode" operates with a modified V/Hz curve that undermagnetizes the motor at low speeds. We all look at magentizing energy as effectively free since we get it back on the demagnetizing half cycle. However, that magnetizing/dmagnetizing action still requires current flow. This means you still have IR losses in the motor windings and rotor bars. Running the motor at a lower general level of magnetization will reduce the magentizing current levels and minimize IR losses. however, it also means you have less than rated torque available to you.

Having said that there is no performance issue running in constant torque configuration when running a fan or pump.

Keith

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You are correct that at very low speeds and loads motor current actually rises, and that this results in losses. However, the "load" I mentioned doesn't just refer to the bhp at the shaft, it also includes the electical losses. The VFD will give the motor what it needs, and parameter adjusments won't materially affect the kW that the power company will charge you for.

Without getting too pedantic, centrifugal loads (like most fans and pumps) just don't operate much at very low speeds. The discharge pressure capability falls off with the square of the speed, so in a great many applications the bottom end of useful operation is in the 50% to 75% speed range.

I can't comment on the "fan/pump mode" and it's effect on V/Hz ratio - my expertise doesn't extend that far into the VFD details. That may be the case on newer VFDS. It isn't something I've encounterd on my applications, and I think it's effect isn't going to be of much consequence in most applications.

In my experience, setting the VHz curve to "pump/fan" mode or quadratic or whatever term other manufacturers use for a squared function curve rather than linear will save at most 1-2% in the midrange (30-70% speed) and then only if the pump or fan spends the majority of its time running in that range.

And, just as kamenges said, the savings, if any, are due to undermagnetizing the motor in the midrange so its available torque approximates the load torque curve.

However, there are other parameters in a typical drive setup that can make a difference in energy consumption. For example, how about setting the minimum speed to zero or some low value way below the lowest functioning speed of the load. Leaving the motor run there simply is a waste. Or, how about forgetting to set up the sleep function in a PID loop. Again, all wasted energy at light loads.

As for the mainstream parameters, I agree with Tom.

Originally posted by DickDV:

...will save at most 1-2% ...

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1-2% of what?

Percentages are always a bit funny that way. As an example assume a vector driven motor. Magnetized and sitting still it is 0% efficient. You have the same motor running fully loaded at full speed and is running at 90% efficiency. Which is burning more energy as losses?

If you are saving 1-2% of the total energy expenditure of your system that may be a pretty significant percentage of system losses. 1-2% is about the difference in efficiency between a standard 3-phase motor and a premium efficiency motor in Marathon's catalog.

While I agree that there are many other places to find efficiency gains in a system I guess I don't understand why you wouldn't want to take on that is effectively free.

Keith