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View Full Version : Off topic: AC Motor drivers....


GregA
Wed 16 January 2008, 11:33
I am asking here because the people around here seem to know more about motors than anywhere else, but this is not mechmate specific.

I am designing a recirculating pump system for heated wax. It recirculates to avoid the wax freezing in the plumbing, also to avoid heat tracing the plumbing(which is also lot more complex than it sounds...). I have a very limited range of pumps and motors to work with, because of the viscous, mildly caustic, and heated requirements.

Looking around at various catalogs, I see that it is possible to get variable speed drives for 3 phase ac motors, but I don't see the same sort of options for 2 phase ac motors... Am I missing something, or do 2 phase motors spin at their rated speed/voltage without control, and if I want speed control, I just have to go with a 3 phase motor?

BernardR
Wed 16 January 2008, 12:07
Single phase ac motors require a capacitor to start them and once started their rotational speed is fixed by the number of poles.

Three phase motors do not need a capacitor to start them and once they start their rotational speed is controlled by the ac frequency being fed to them. This has led to the current situation where people who need variable speed use a 3 phase motor with the appropriate Variable Frequency Drive (VFD), in the small sizes these are driven from a single phase supply and can drive up to around 3 HP motors above that they require 3 phase input.

Like all things VFDs are excellent for what they are intended for, but can also be abused. Though they can run from fractions of a turn a second up to 360 HZ (giving 6 times nominal speed), you are limited to the torque available at the different speeds, and of course to the motor disintegrating if you drastically over speed it.

Used properly VFD are a really useful device. I have used Automationdirect.com in the past and have found their prices reasonable and service excellent.

Gerald D
Wed 16 January 2008, 12:14
Single phase ac motors require a capacitor to start them and once started their rotational speed is fixed by the number of poles.

Three phase motors do not need a capacitor to start them and once they start their rotational speed is controlled by the ac frequency being fed to them.

Both styles of motors' speed is determined by both frequency & no. of poles. Typical single phase motors are 2-pole or 4-pole. On 60 50 Hz mains they run about 5% below 3000rpm(2-pole) or 1500rpm(4-Pole). Same speeds, freq and poles applies to 3-phase motors.

Good points made on another forum (http://www.lmpforum.com/inforum/index.php?showtopic=1994).

BernardR
Wed 16 January 2008, 13:20
US ac motors run 3450 RPM (2 pole) 1725 RPM (4 pole).

Depending on the duty cycle and the amount of power needed it might be simpler and cheaper to use a dc motor and simple variable dc power supply.

Gerald D
Wed 16 January 2008, 20:41
Sorry, typed your 60Hz instead of our 50Hz. This goes to prove the relationship to frequency, and indeed why Variable Frequency Drives function as speed controls.

GregA
Wed 16 January 2008, 21:24
Well, after studying the issue today, building both systems complete(with McMaster parts prices)... Ive come to a conclusion. A recirculating system costs about the same to build as a heat traced system, only... I know the heat traced system will work. On the other hand, I assume a recirculating system will work (flow switched by solenoid) and it has at least one failure mode... That I know of...

Sooo, thanks for the education on motors. But I am gonna go with the pump/motor I have and flip it on and off to level the supply tank. Thanks for all the help though;)

Also... From reading about it, on the single phase motors, they tend to be warrantied for operation at the 50hz or 60Hz range. Although (with the motor I am using from digging around the specs) they definitely do not want you to operate it outside of that range.


Well Im off to go find my painters clothes and rubber gloves, I have smelly black goop to spread all over my pipes...