I'm trying to control the speed of a single phase AC motor with a variable transformer, but I'm having some problems.

Description of setup:

Motor used:
http://www.blowerwheel.com/blower-gp-3.htm

Variable transformer:
http://www.aaaim.com/u/web/aaaimc/cgi-loca...VATS.htm#VT2502

The motor has a start capacitor of 25 microfarads. There does not appear to be a centrifugal switch. (I can send photos of the disassembled motor if anyone has questions as to its type.)


Problem:

Motor speed control with the variable transformer is very touchy...a change of the transformer's voltage output of just a couple volts changes the motor speed from very slow to near maximum. It is impossible to set the motor speed at a medium speed. I can set speed at a very slow value, or near maximum, but the middle range (which is most desirable to me) is 'unstable' and very sensitive. In many cases after I set the motor speed, it will gradually ramp up in speed over the course of a minute or two.


Questions:

1. What can I do to make the middle speed range easily controlled? Is it possible to do this with the variable transformer? (Or do I need another method of speed control.)

2. If I change the value of the start capacitor...can I optimize the motor's performance and/or speed control characteristics for the desirable middle speed range?

Thanks in advance!

Hello Tom

Welcome to the forum.

Most induction motors have a low impedance rotor and are designed to operate under low slip conditions. You can not alter the speed on these motors by reducing the voltage, in fact if you operate at reduced voltage, you will destroy the motor.

There are some motors that are designed with a high impedance rotor. Thes motors tend to operate with a high slip and produce maximum torque near zero speed. These motors can be speed controlled by variable voltage. Typically, these design D motors are used as an integral part of a fan assembly with the rotor on the outside of the motor rather than the inside. There is high power loss in the rotor when it is operating with a high slip. Putiing the rotor on the outside of the rotor increases its surface area and also positions it in the air flow of the fan, providing best cooling.

Typically, the locked rotor current of a standard induction motor is in the order of 500 - 900% of the rated full load current and the locked rotor current of a type D induction motor is in the order of 200% of the rated current of the motor.

With the voltage reduced on your transformer and the motor spinning at a very slow speed, measure the voltage and the current. This will give you an indication of what the current would be at full voltage. If the full voltage current is higher than 300% current at low speed, then it is not a high slip motor and you are stuck with high speed only. (the current reduction ratio = the voltage reduction ratio)

1. Sounds to me like you have a standard induction motor with the maximum torque available at just below full speed. This can and must not be operated at reduced voltage.

2. see 1 above.

Best regards,

Destruction by overheating? Drawing too much current?



I did some measuring and find at very low speed the motor draws about 10.5 - 11 amps, at about 40-45 volts. (Startup current as the motor is spinning up is around 12-13 amps). As motor speed increases, current draw drops down to about 7 amps at near 75 volts. At full speed/full voltage (110) it draws about 5 amps.

Based on how the motor spins up initially very slowly, and spins up faster and faster as it nears max speed...it sounds like I have a standard induction motor...that I should only operate at full voltage/speed.

Am I right?

Thanks in advance.

Hello tom

Yes, you definitely have a standard induction motor. At just over one thrid voltage, you are drawing about 2.5 times the full speed current This tesl me that the ful voltage start current will be in the order of 6 - 7 times the full load current. That is a standard cage motor, you can not vary it's speed by controlling the voltage. - sorry about that!!

Best regards,

Two more questions:

1. Can I vary speed over a reasonable range by varying frequency?

2. Would this motor be a better candidate for voltage control?:

Stock #ACM8005

http://www.aaaim.com/u/web/aaaimc/cgi-loca...0CM.htm#ACM8005

1/9 HP 3000 RPM EMERSON ELECTRIC FAN TYPE MOTOR, #F33EFS-1998. 115VAC, 60Hz, 3.0 amps, single phase. Has built-in internal cooling fan and a 3-foot 3-wireAC cord and plug attached. CCW rotation. Sleeve bearing. Dimensions: 4" dia. x 4-7/8"long. Shaft: 5/16" dia. x 1" long.

Thanks in advance.

Tom

Hello Tom

Very hard to tell. The statement "fan motor" tends to imply that this may be a hish slip motor, but I would not like to claim that on the information given.

Best regards,

Mark ... from data sheet for "Pressure Blower 600 CFM 5" Inlet "
- HP - 5/8 Totally Enclosed Fan Cooled Motor
- RPM - 3450
- Volt - 115 AC
- Amps - 4.95
- Capacitor Start
Fan curve not offered in lit.

Ergo, mtr not a high slip design ... 3450 very standard rpm for 2-pole motor on 60 Hz supply.

Motor NOT designed for variable speed operation either by voltage or by frequency control or by voltage & frequency control.

For this, blower would need to be powered by a PSC (Permanent Split Capacitor) motor .... for which variable speed controllers are standardly available.....i.e. Anacon Systems http://www.anaconsystems.com/text/opti_1.html

Kind regards,

jΩ

Hi jO

Welcome to the forum, long time no see!!

Yes you are correct as usual.
I don't tend to promote the use of the single phase inverters as they are not readily in many countries and I have no experience with them. In this part of the world, the standard methode is to replace the motor with a three phase motor. The small 3 phase VSDs are now so cheap.

Have a good day,
Best regards