Hi,
I am working on a one phase motor speed control. Currently, I am using a one phase inverter to control the input frequency. While doing so, I found that my START capacitor gained some heat and even showing electrolyte's burnt and leakage.
Is it really inadvisable to control the one phase motor with inverter?
Is there any recommended way for me to control my motor speed?
Thank you
Hey!
Well, it is not advisable as the inverter will cause the capacitor to overheat and burn due to repetitive charging and discharging (as you described).
You can use a triac/diac solution with potentiometer for speed control. See the attachment: The setting of P1 will determine the phase of the trigger pulse that fires the triac. The values of the C:s and R:s will depend on your set-up and specs... you need to calculate and experiment. Good luck!
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Dear Ryan,
Please have a look on my paper (old one) with experiments:
Article Induction Motors Energy Optimized Control Strategy by Means ...
Regards,
Mohamed
If it is a starting capacitor, it should be connected only during the starting; once the motor has started, the capacitor should be out of the circuit and the motor would keep running.
Another option would be using a shaded-pole single-phase motor instead; these motors do without the starting capacitor.
By controlling the input frequency.
This is the proper and most efficient way to change the speed of a single or multiphase induction motor.
A single phase induction motor operates differently than a three phase induction motor, largely only in starting, because the direction of rotation is not established. Once spinning, the inertia of the single phase motor set the direction of motion, and carries it through complete revolutions, much like the flywheel of a single-cylinder engine.
Is is possible to control the speed of an induction motor (single or three phase) by reducing the voltage. This will reduce the magnetizing current, flux, and thereby increase the slip, by essentially reducing the rating. Not all types of motors are designed to run with significant slip (e.g. more than 5%). Most induction motors will stall, if the slip is increased significantly.
Changing the operating speed of a typical induction motor (Class A/B) with reduced voltage. A turn-down of approximately 20% is achievable, in this case with a 50% voltage reduction. Current remains high. Beyond this the motor will like stall, overheat from inadequate cooling or behave unstably.
The torque/speed curves of various induction motor classes. Notice the wide speed range of a Class D (uncommon) induction machine. Class C/D machines also have poorer efficiency than Class A/B machines.
The single-phase IM should be started first using capacitor start motor which is disconnected after starting. The speed control can be, thereafter, done through inverter which is the most efficient way or voltage reduction in some applications.
Hi Ryan,
There should be no problem controlling a single phase motor with an inverter as you suggest. You may have to derate the motor though.
If I understand matters you have a start winding which is controlled by a centrifugal switch (CS) which cuts the start winding out. When you lower the frequency the centrifugal switch will cut out and re-connect the start winding again and hence cause the heating you have noted.
You might have to bypass the CS and cut out the start winding with an external relay and this will prevent the start winding from being activated apart from starting.
Single phase motors with CS are designed to run at fixed speed.
However, single phase motors cannot start when a high frequency is applied as the motor shaft oscillates at standstill, hence the need for a start winding. However if you apply a low frequency at starting the motor will begin to turn in one direction on positive cycle and then reverse on negative cycle, so it is possible to get the motor to start without the need for a start winding, but then that will take the excitement out of your project.
A single-phase motor without any start winding will oscillate and remain stationary but if you turn the shaft in any direction it will run in that direction, it just needs a kick to get it going. You may do same with the single phase inverter if you control it correctly. Do some research there are some good examples out there which already do this.
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