AC motor Low RPM problem

[raf23]

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

 

[chuckey]

Single phase motors cannot be successfully controlled over any great speed range as you have found. This is because as the AC switches, the armature needs more inertia to carry it over to the next (other) field pole. The armature slows down and might not make the next pole before the field reverses again. You could put a fly wheel on the motor, this should make the low speed more controllable BUT it will accelerate even more slowly. The best option is a three phase motor, this will "cog" around at virtually zero speed with a suitable controller and go to X 10 label speed (unless the armature disintegrates first !). For a 100W motor, use a DC one ( from an electric drill?) with a proper speed sensor and controller. The only downside is that the motor will use brushes so will only run for a few thousand hours before the brushes need replacing.
Frank

 

[FvM]

1/8hp ,230v/50Hz ,6000 RPM @ No load

6000 rpm @ 50 Hz would be hardly achievable without a gear...
Maximum (synchronous) rotor speed of single pole pair motor is 3000 rpm.

 

[ArNvC]

Hi,
Its generally recommended to increase the speed gradually. While you change the electrical setting to a set speed, the mechanical system (motor) takes some time to reach the set point due to inertia. You must be using a load, right?
The best way to design a speed controller for single phase induction motor is by using PWM control techniques. Only at low speeds, PWM drives may require a voltage boost to generate required load torque.

 

[FvM]

Only at low speeds, PWM drives may require a voltage boost to generate required load torque.

Voltage boost? Voltage must be basically speed proportional in a VFD, in other words low at low speeds. Some VFD have optional current boost at low speeds. Not due to low speed conditions as such but to overcome a breakaway torque that some loads have.

As an additional point, variable frequency motor drive don't work well for single phase motors, with the exception of capacitor motors that can be rewired as three-phase. The torque generaton method of single phase motors, e.g. shadow pole or built-in capacitor is designed for a single frequency (50/60 Hz) only.

 

[ArNvC]

ok yes I was talking about current boost, thanks. I agree that one can not use variable speed controllers on most Capacitor Start motors due to the speed requirements of the centrifugal switch in the start circuit. There are variable speed drive manufacturers previously of single phase input - single phase output for small motors.I do not know if these are PWM much less variable frequency.

 

[iop95]

It's difficult to control even 3-phase induction motors below 10Hz with this method (V/f=ct) and much difficult for single-phase. Torque is propotional with V^2, so reducing speed by 6 times lead to a torque reduced by 36times...For small speed must use modified V/f control law to cover voltage lost in windings, respectivly to increase voltage for low speeds to develope same gap flux. There are control law that have good performance at low speeds (vectorial control / DTC / fuzzy).