DC Motor

An AC Synchronous motor holds speed from no load to full load, while under a constant voltage supply. Although they are probably not practical for your application.

I believe you are a little confused about standard DC motors. Yes, they are routinely used in applications that need to hold speed from no load to full load, however it is the controller (DC Drive) that does the speed regulating, not the motor design. This is the same for common induction AC motors as well. Both can be used for wide ranging applications, and hold speeds within a fraction of a percent, but it is a function of the drive, since the drive automatically adjusts the motor's voltage.

There are DC motors (Series bucking) that run from a fixed voltage, and hold speed (within 1-5%) from no load to full load, but typically these are more expensive than using a standard motor with speed regulating drive (AC or DC).

What's the duty cycle? (How often does it start and stop)

Does it ever start under full load?

Temperature?

What motor RPM do you need? 1800? 3600?

Shaft diameter?

footprint

Outdoor use or indoor?

explosion proof?

A DC motor is considered for variable speed application mainly and also where speed holding is required accurately like driving a generator where frequency is to maintained at rated value and within +-1%.Its quite expensive and requires a higher degree of maintenance compared to the rugged ac cage motors.If your problem is only of just adjusting the speed think of a cage motor with an ac inverter .Is the speed so critical? can u be more specific on the application or driven equipment?

Thanks for your comments.

The motor would drive a fan. This fan would suck in air from the atmosphere into a region. The mass flow of air depends on the RPM of the motor. The mass flow rate of the air being blown needs to be controlled within an accuracy of +/-0.5%. Therefore, there is a premium on being able to control the speed of the motor.

The application is such that the motor will start from rest and as the speed increases so will the load increase. We would try to calculate the load at a particular speed from apriori methods but these may not be entirely accurate. In this case, if the load is different from what we plan for, Can we make the motor provide enough power to hold the RPM.

With the above information could you suggest which type of motor would be most useful and why? Also, what kind of controller would be ideal from your experience?

Thanks.

Based on your information, I would recommend a standard induction wound 3 phase AC motor. This should be powered from a "vector" drive. You would probably require motor speed feedback (encoder or equivalent) to be wired back to drive to give the absolute best speed regulating performance.

Please note that motor speed regulation is typically stated as a function of max speed. Thus if the vendor states it is good for 0.5% they typically are stating 0.5% of 1800RPMs (or motor nameplate) not 0.5% of set point speed- thus it would only be holding 1% at 900rpm.

Your application's requirement (0.5%) seems a bit extreme for the everyday fan motor, so be advised that my advice is only intended for 99% of fan applications- yours may be falling in the extra 1%.

I recommend you contact a good supplier, capable of engineering the proper solution to your application.

Try one of these to start: http://www.ab.com/drives/motors/

As commented by fellas here, AC motor will be my 1st nomination than DC motor, with the proper AC drive, of course. AC motor have cheaper maintenance cost, the drive execution also more simple than DC, thus trouble shooting will be easier, too.

In a complex loop, the output flowrate should be feeded back to the drive to maintain set value of air you need.

Do you put a control valve in service? What I meant ; get the fixed speed of AC motors (no drive), and the flow rate will be controlled by flow controller instrument and control valve. But this will consumed a lot of electrical energy as motor speed is constant when your demand of air is lower.

Keep in mind, global warming now become the main issues of electrical power generation.

My apologies for the belated reply.This being a fan drive utmost care has to be taken while selecting a ac cage motor.The fan normally has a high Inertia.The accelerating time has to be within motor's stall withstand time.You can plot the Torque Vs speed curve of the motor and the fan with speed on X axis and Torque on Y.This can be in Per unit or actual values.The point of intersection of the fan on motor T-s Curve is the load point and projection from this to load point will give the speed..Now depending upon the fan load the motor speed will vary.Also the fan HP varies as the cube root of speed.No doubt a DC motor is IDEAL but maintenance is high particularly the commutator.The first and simple thing will be a cage motor coupled through a fluid coupling with scoop arrangement which can adjust the speed.But energy wastage will be there to an extent.Second is use a wound rotor motor with a resistance in rotor circuit .Finally cage induction motor with a inverter but proper care as explained above will be have to be exercised while selecting the motor.I will see how to post the curve on this forum.Considering the speed holding accuracy I would suggest go in for a cage motor with Inverter drive and If Maintenace is not a factor best is the DC motor from every consideration.

Three types of motors can be found

Serie

parallele

compound wiring serie and parallele motors

the serie motor is able to start any load , and parallele motor is stable while is rotating in its chosen point

combine both caracteristics choosing a dc motor with type compounded excitation

if you find motors with nominal power higher than 75 hp which is few for our times.

In the past you could find motors in this range of power, because the technology had still not brought the new drives for synchronous three phases motors.And this actualy are perfect and exist in the range of until 1000 of kw

Try also a three phases alternative asynchrone motor with wounded rotoric resistors which can start any load and , if powered with stable voltage and stabilized temperature can run at a stable speed

Happy New Year everybody.

try to visit the link below there is a lot of informations http://en.wikipedia.org/wiki/Electric_motor

Your fan should have a required HP motor needed to turn it if it was a commercially bought fan. I think your best bet would be to go with an AC motor and a VFD. You would want to get a vector rated motor and a VFD with a built in PID loop. You can then feed in a setpoint and then use the PID loop to compare to your feedback to tweak the output to give you the amount of air you need. It certainly could also be done with a DC drive and motor but remember that DC motors require more maintenance. There are a number of companies that can help you engineer the proper solution including programming the drive to get the desired output. Good luck!

Shawn

I had replied to your messgae .Some how the posting in the name of guest any how I hope the info is useful to u.I am unable to post the speed torque curve sample on this .If u can send message to my inbox i can e mail it.