AC to DC motor thyristors speed and direction control

[Didawdidaw]

So i have been given this question:

You have also been asked to produce a circuit design that can control the speed and direction of a conveyor with a AC to DC motor thyristor system. What are the the operational features (how it works with speed and direction circuit diagram/s)

I am really confused how a AC to DC motor thyristors controls the direction of the motor. I originally thought that on a 1 phase system this would look like the diagram provided. Thyristor T1 and T3 are fired together when the supply is A is positive and then T2 and T4 are fired together when B is positive creating a rotation. To rotate the motor the opposite direction you will have to fire the thyristors when both terminals are negative thus giving you your stop, start and direction control. But I'm unsure if this is now the case as I'm finding it really hard to find information on the direction control property's of this system.

Firstly I'm wondering if this circuit I've attached can be used to control the direction of the motor and secondly if my explanation isn't right how do you control the direction of the motor?

Any help will be appreciated, thanks in advance.

 

[Lucien Nunes]

That circuit is not for a reversible drive. The parts labelled T1-T4 are not thyristors but ordinary diodes, forming an uncontrolled bridge rectifier that feeds the rails top (+) and bottom (-) with pulsating DC (two pulses per supply cycle) at a fixed voltage. Thyristor T turns on for a controllable fraction of each pulse, by which means the average armature voltage (hence speed) is controlled, while the shunt field remains at the full average DC voltage. The polarity of armature and field are not changeable by the circuit itself therefore neither is the direction; however it would be possible to implement a separate direction selector switch using a double-pole changeover to reverse the armature polarity relative to the field.

To provide full 4-quadrant control of the motor, i.e. where the speed control circuit itself can accelerate or brake in either direction, the armature needs to be within a full bridge of four controlled devices. These allow a voltage of either polarity to be impressed on the armature cycle by cycle. If you search for something like 'single-phase full bridge controlled converter' you will get a deluge of information on that configuration.

 

[Didawdidaw]

That circuit is not for a reversible drive. The parts labelled T1-T4 are not thyristors but ordinary diodes, forming an uncontrolled bridge rectifier that feeds the rails top (+) and bottom (-) with pulsating DC (two pulses per supply cycle) at a fixed voltage. Thyristor T turns on for a controllable fraction of each pulse, by which means the average armature voltage (hence speed) is controlled, while the shunt field remains at the full average DC voltage. The polarity of armature and field are not changeable by the circuit itself therefore neither is the direction; however it would be possible to implement a separate direction selector switch using a double-pole changeover to reverse the armature polarity relative to the field.

To provide full 4-quadrant control of the motor, i.e. where the speed control circuit itself can accelerate or brake in either direction, the armature needs to be within a full bridge of four controlled devices. These allow a voltage of either polarity to be impressed on the armature cycle by cycle. If you search for something like 'single-phase full bridge controlled converter' you will get a deluge of information on that configuration.

Thank you! I have found a direction control circuit diagram and now see how that works. its surprisingly hard to find if you don't know what to search. However I can't find a diagram that both controls speed and also the direction, so to answer this question I'm going to have to use 2 different diagrams to explain this. Would the image that I've provided work for the speed control of a thryristor system? I've just changed the 4 diodes to thyristors and hoping that it can work like that.
[automerge]1586253233[/automerge]

That circuit is not for a reversible drive. The parts labelled T1-T4 are not thyristors but ordinary diodes, forming an uncontrolled bridge rectifier that feeds the rails top (+) and bottom (-) with pulsating DC (two pulses per supply cycle) at a fixed voltage. Thyristor T turns on for a controllable fraction of each pulse, by which means the average armature voltage (hence speed) is controlled, while the shunt field remains at the full average DC voltage. The polarity of armature and field are not changeable by the circuit itself therefore neither is the direction; however it would be possible to implement a separate direction selector switch using a double-pole changeover to reverse the armature polarity relative to the field.

To provide full 4-quadrant control of the motor, i.e. where the speed control circuit itself can accelerate or brake in either direction, the armature needs to be within a full bridge of four controlled devices. These allow a voltage of either polarity to be impressed on the armature cycle by cycle. If you search for something like 'single-phase full bridge controlled converter' you will get a deluge of information on that configuration.

Ignore my last post I'm confusing my self. So this circuit can be used to change the direction by selecting which converter is used but how would speed control be achieved hear? would it be based on the angle the thyristors are fired? or would I need something added to the circuit do give it that property?

Thanks for your help

 

[Lucien Nunes]

Yes, speed control is achieved by adjusting the conduction angle of the controlled bridge(s), just as it was with original simple circuit you posted. Obviously with a greater number of thyristors to fire the circuit is more complex, but the principle is identical.

Because thyristors only conduct unidirectionally, as you have noted it is necessary to use two full bridges to achieve 4-quadrant control. Another topology is to full-wave rectify the supply with an uncontrolled bridge again as per your original circuit, but then drive the motor with a fully controlled H-bridge (like an inverter). The power has to be handled twice hence greater losses, although only one set of thyristors is needed.

Don't forget that the controlled bridge can't power the motor's shunt field, only the armature circuit (including any series field if compound). The shunt field usually needs to remain at full voltage and must not reverse with the armature, so a separate low-current rectifier is usually provided.

 

[Didawdidaw]

Yes, speed control is achieved by adjusting the conduction angle of the controlled bridge(s), just as it was with original simple circuit you posted. Obviously with a greater number of thyristors to fire the circuit is more complex, but the principle is identical.

Because thyristors only conduct unidirectionally, as you have noted it is necessary to use two full bridges to achieve 4-quadrant control. Another topology is to full-wave rectify the supply with an uncontrolled bridge again as per your original circuit, but then drive the motor with a fully controlled H-bridge (like an inverter). The power has to be handled twice hence greater losses, although only one set of thyristors is needed.

Don't forget that the controlled bridge can't power the motor's shunt field, only the armature circuit (including any series field if compound). The shunt field usually needs to remain at full voltage and must not reverse with the armature, so a separate low-current rectifier is usually provided.

Would the supplied image be the correct way to show this? I take it that it wouldn't be required of me to show where the gates of the thyristors were connected up to as long as i put a explanation about this? I'm not able to use a H- bridge system as it is part of this as it is part of my next question. I really appreciate your reply's, its surprisingly hard to find out the information when your not to sure what to search so thank you.

 

[Lucien Nunes]

Yes, something like that. Obviously I'm looking at it from a practical engineering point of view without knowledge of your course. Conveyors can serve all kinds of duties so don't forget to account for how the nature of a load dictates the configuration of the control. E.g. does it need to reverse without a stationary pause? Does it need any dynamic braking at all or will the mechanical brake suffice. If so, is torque needed at standstill? How critical is the speed control; would errors in P, I or D just slightly reduce the utilisation of a hopper or would it affect process efficiency and perhaps product quality?

PS if you like the help, hit the like button!

 

[Didawdidaw]

Yes, something like that. Obviously I'm looking at it from a practical engineering point of view without knowledge of your course. Conveyors can serve all kinds of duties so don't forget to account for how the nature of a load dictates the configuration of the control. E.g. does it need to reverse without a stationary pause? Does it need any dynamic braking at all or will the mechanical brake suffice. If so, is torque needed at standstill? How critical is the speed control; would errors in P, I or D just slightly reduce the utilisation of a hopper or would it affect process efficiency and perhaps product quality?

PS if you like the help, hit the like button!

Thanks!
my course is quite basic so it's just touching on all the mechanical and electrical applications and devices so i wont need to go in to that amount of detail; for the time being any way. You have a vast amount of knowledge on this subject and I now understand this system to a level that is required for my course. Thanks for your help!

Sorry i didn't see the like button so i have liked all the comments that have helped!