variable speed ac motors

This is a simple question for you guys Im sure. I typically always use dc motors and a motor control board with potentiometer. What would I use to control the speed of an ac motor?

An Inverter or as it is known a VSD (variable speed drive) there are many to choose from, Mitsubishi, imo, allan bradley, danfoss, siemens ABB etc.

there are many different types but for simple speed control use any of them, some have pid software, braking etc,,,,

It would consist of a controller and you could still use a pot. for a speed refence just like the dc drive. But it would also give you more speed control options.

VSD/VFD Drive controllers come in a wide variety.

Some have external pots, some have a touch screen, different manufacturers have a plethura of options.

I think from the standpoint of modern engineering, A vafial Frequency Drive and AC motor is probably the better option.

Mining Trucks and Locomotives started switching to AC Drive in the early 90's. Much of the DC drive technology used in traction drive, like locomotives, is the same that it was in the 30's. Just a little bigger, with higher voltages and currents. But then, if it works, why spend more money to replace it!

regards.....casey

Generally, AC motor speed is dictated by the the line frequency, but it really depends on your AC motor.

Take for example the shaded pole motor in your dining room ceiling fan. Its speed can be easily controlled with a simple RC network connected to the gate of a triac. Commercial units can be purchased for only a few bucks at most home improvement centers. The motor torque degrades sharply at lower speed with this method and it is suitable only for lightly loaded light duty single phase motors.

You no doubt have a variable speed electric drill. This drill has what is called a wound-rotor. The poles of this rotor are attached to slip rings on the rotor shaft. Carbon brushes contact these rings and a potentiometer is used to vary the voltage deliverd to the rotor, thereby changing the amount of slip, or difference in rotation between the rotor and the rotating field created in the stator.

If we are talking about three phase induction motors, motor speed is a function of line frequency and the number of poles in the motor. To change the motor speed, you must change the line frequency. This is done with an inverter or VFD.

What kind of a motor are you needing to control?

Thru 40 to 50 hp drives, AC drives and motors cost less to purchase.

AC drives, variable FREQUENCY drives are also cheaper to operate. They work with a higher power factor at all times. DC drives and motors were the only way to change speed and get low speed torque at one time. Today with vector technology, DC drives and motors are being replaced with AC.

The control logic for AC drives is like the control logic for DC drives. You give it a RUN command and a SPEED command. The method of doing both can be drive mounted, remote control panel or touch screen via a plc to name a few.

Thanks guys. The motors I have are 1 phase. Do they make variable speed drives for 1 phase? Everything I've found is 3 phase. Im not looking for anything fancy just need to control the speed, and of course the cheaper the better.

Now you mention you have single phase motors! That is good news to those who assumed you were talking 3phase all along!

Most 1, aka single phase motors have 2 speeds ON or OFF. Some can be run at several PRESET speeds, like a fan. I know of NO drives for 1 phase varaible speed, unless you consider DC single phase.

The cost of 3 phase motors can be less than you think. Many small 3 phase variable speed drives are single phase input.

The attached PDF talks about a drive from Anacon that might do the trick for you. But I agree with Leadfoot. Going 3-phase will probably be cheaper in the long run.


Keith

ready961 said:

Thanks guys. The motors I have are 1 phase. Do they make variable speed drives for 1 phase? Everything I've found is 3 phase. Im not looking for anything fancy just need to control the speed, and of course the cheaper the better.

Click to expand...

How big is the single phase motor and what kind of a load is it attached to? This info will have a big inpact on what we can reommend for a solution.

In case you haven't figured it out yet, an AC motor's speed is determined by the frequency of the power source, not by the motor voltage. However, there are some creative things that can be done with certain limited applications. Other application will leave little choice but to use a VFD. That is why more information about the motor and the application will be helpful in recommending a speed control method.

I found a good place to start(not to plug a particular line)is
www.automationdirect.com Most of there manuals are free to download for you to get a starting point. I agree going to three phase would be helpful. I have used there GS2 drives for some conveyor automation. Well that is my 2 bits.

Jody M

I would also suggest that three phase is better.
A three phase motor tends to be simpler and more robust than a single phase, and generally you gan get these down to 0.12kW in size.
Although my colleagues disagree, I would use DC motors for anything smaller. Generally this is because small DC motors are generally cheap, throw away items and can do the job better than trying to speed control a single phase AC motor.

That is true to a point but not totally correct.

In case you haven't figured it out yet, an AC motor's speed is determined by the frequency of the power source, not by the motor voltage.

Click to expand...

A VFD is a device that controls voltage and frequency which offers more/better control and capabilities of a motor BUT a motor is nothing more than an inductor that operates by creating a magnetic (flux) field, the strength of the field is determined by the voltage, current and frequency applied. Any of these may be varied and cause a change in the field.

"SOME" single phase AC motors..ie Universal Motors, Series Wound Motors, Torque Motors, and Shaded Pole Motors may be able to use a "VARIAC" (variable AC voltage transformer). A variac changes the voltage which will change the speed when applied to an AC motor. In most cases you can use a dial (just like a pot) to adjust the voltage or the shaft can be motor driven. Most motor manufactures will have data on whether the motor can or can not be used with a variac and may recommend a specific type. www.variac.com can offer more info on variacs.

You can not use a VARIAC with split phase, cap start and some other single phase motors.

A variac changes voltages which in turn will change current values which in turn will change torque output.

Running at slow speeds may require a separate fan for cooling the motor.

I'm just trying to keep it simple Ron.

As I said earlier in this thread, speed control of some single phase motors in limited applications can be accomplished without a VFD. Speed can be controlled using a controller that consists of an RC network with a pot tied to the gate of a Triac (which is usually cheaper than a variac). But this can only be used in certain light duty appliactions because the motor torque degrades sharply when speed is controlled by voltage only. Here are some examples. I have several similar devices in my home.
http://www.grainger.com/Grainger/wwg/itemDetailsRender.shtml?xi=xi&ItemId=1611699160&ccitem=
http://www.grainger.com/Grainger/wwg/itemDetailsRender.shtml?xi=xi&ItemId=1611576608&ccitem=

In any case, none of these methods decreases the speed at which the magnetic field rotates, they allow for more slip between the field and the rotor.

This is why I asked the OP for more infomation about the application. If he is controlling the speed of a fan for instance, then he may get adequate control using a reduced voltage method or by using a wound rotor motor (like the one in a var speed hand drill), which also allows for speed control by changing the slip ratio.

If the need is to control a heavily loaded motor then such methods will not be adequate.