AC motor Low RPM problem

Question

I'm working on a controller for ac induction motor and it's spec is given below. I could control the speed without any trouble at higher speeds like 2000 rpm and above. I'm using TRIAC phase angle control topology in the controller. But at lower RPM like below 1000 RPM but motor takes so much time to reach the set rpm which is not desired as it will add error to the set time period (a timer is there to run the motor @ set RPM for a set period). Also is there any problem if I don't increase the speed gradually.

Motor Spec

1/8hp ,230v/50Hz ,6000 RPM @ No load, I'm using a load of around 1KG

Answer 1

Assumption: Some mechanism exists to sense RPM, either through an encoder attached to the motor shaft, or via back-emf sensing.

An approach to achieve better results that what the question describes, for low RPM operation of a motor, is to use a PID controller algorithm thus:

• Motor is provided maximum rated power at start-up, as specified in the motor datasheet
• As the RPM sensed approaches the desired set-point, the power is systematically reduced my modifying the triac trigger phase
• Once the RPM set-point is achieved, the PID controller continues to sustain that RPM by increasing or decreasing power to compensate for loading effects
• If load drives the motor to stall, or beyond acceptable power ratings, the controller initiates a controlled fail-safe spin-down of the motor, and also triggers an alarm indication.

From the Wikipedia article linked above, this graph might help explain this process visually:

Depending on the acceptability of overshoots (or not) and desired system behavior, the Proportional (P), Integral (I) and Derivative (D) values of the PID algorithm need to be tuned. The diagram above specifically covers tuning the I value: For absolutely no overshoot of RPM, but with greater time to reach set-point, the red line on the graph shows the preferred behavior, as achieved with a Ki = 0.5.

On the other hand, the black trace, with Ki = 2, achieves (and overshoots) the set-point fastest, and then over/undershoots the set-point in diminishing cycles till it settles down.

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There exist excellent motor controller ICs which incorporate both back-EMF sensing (if applicable to your type of motor) and PID controlling, in one package.

Also, assuming the OP has limited experience in designing such systems, off-the-shelf PID controllers for motors are available for a variety of power ratings. These allow a set-point to be interactively set, along with tuning parameters or constraints.

There are also several projects by hobbyists out there, designing and implementing PID AC motor control using microcontrollers or microcontroller boards. For instance, see this YouTube video, for one such Arduino-based PID controller for AC motors.

Links to details are provided in the description text of that video.

Answer 2

As other people have mentioned, typically you change the speed of an AC induction motor by varying the frequency. However, what you are attempting to do is change the speed by varying the voltage. This is an acceptable method in some applications and with some types of induction motors. See the picture below (taken from Handbook of Electric Motors by Toliyat and Kilman, p. 734).

On the graph you can see the speed/torque curves for two different motors run at two different voltages. Motor A is a standard slip motor. As you can see, when you apply 70% of the voltage to this motor (while keeping the frequency the same), the speed will not change very much (point A' on the graph). With a higher slip motor, however, a change in voltage will cause a significant change in speed (compare points B, B', and B''; these represent nominal voltage, 70% voltage, and 40% voltage, respectively).

However, you can see that at 40% voltage on the high slip motor that the speed torque curve is starting to flatten out and that your operating point is near the bottom of the curve. By changing the voltage you are essentially changing the slip of your motor. One of the problems with this method of speed control is that at high slip, you will have high losses and thus your motor will get hotter. Because of this, this method of control isn't used very often and when it is used, care has to be taken to make sure the motor doesn't overheat.

It is not surprising that you are having issues at lower RPM's. 1000 RPM's is probably around where the peak torque for your motor and once you are operating below that point, the motor will probably be difficult to control, due to the way the load characteristic and the motor torque curve become almost parallel to each other. Whether this is what is happening for sure in your situation, I don't know since it depends on what your load characteristic and your motor speed/torque curves look like.

Answer 3

There are several ways to control a Induction Machine, the most common and "simple" way is to use \$V/F\$ curve. This manual of National Instruments is a good guide for implement this control mode.

Take a look at the \$V/F\$ curve: (from the NI manual)

And there's something wrong with your motor caracteristics:

\$Ns = 120*f/P\$

\$P = 120*f/Ns\$

\$P = 120*50/6000 = 1\;pole\$