Styx
Active Member
As you have realised you need PWM and !PWM
A high duty (>50%) will give you positve speed
a low duty (<50%) will give you negative speed
50% should give you an average of zero speed but as has been stated this is more of a hover around zero speed (ie a bit forward, a bit backward).
There are three options, what you choose depends on what you want by zero speed
1) have a dedicated speed loop with some form of speed-feedback (via a taco or something). This will enable better zero-speed control.
However zero-speed is a special case for motor-drives and even at zero speed it will hover around it due to the nature of having a PWM. One of our drive's is full speed and current control ranging from +12500 to -12500rpm. To us 50rpm is zero-speed
2) Do as I suggested, ie take the two PWM's. Use two AND gates.
Take PWM and pass it into one AND gate (say input A) and into the other input pass a control single. Into the other AND gate pass the other PWM (!PWM) and the same control signal.
If oyu want to rotate, have that single go HIGH, you want to stop, that singal goes LOW.
That will setup a zero-volt loop and it will stop the rotor dead!!. Limitations is that it will still rotate If something forces it, but the electronics will not be able to drive it round. Its holding toruqe will be low
3) a Mechanical brake (this is prolly the best option) only real way to ensure no rotation of the rotor with alot of toque handling.
One point since you have a bi-directional control here you have not mentioned any form of regeneration protection?
Any for of reversing rotation (ie from clockwise to anti-clockwise or vice-verce) will result in the machine behaving like a generator and this will boost your DC-link up uncontrolled. We use a brake cct (ie short a 3R power resistor across the DC-link) if the DC-link (nominal 270V) ecceeds 450V. Without this the links does exceed 600V - we forgot to attach the DC-link sense lead and the brake cct did not kick in and it took out another bit of the cct whos rating was 500V
A high duty (>50%) will give you positve speed
a low duty (<50%) will give you negative speed
50% should give you an average of zero speed but as has been stated this is more of a hover around zero speed (ie a bit forward, a bit backward).
There are three options, what you choose depends on what you want by zero speed
1) have a dedicated speed loop with some form of speed-feedback (via a taco or something). This will enable better zero-speed control.
However zero-speed is a special case for motor-drives and even at zero speed it will hover around it due to the nature of having a PWM. One of our drive's is full speed and current control ranging from +12500 to -12500rpm. To us 50rpm is zero-speed
2) Do as I suggested, ie take the two PWM's. Use two AND gates.
Take PWM and pass it into one AND gate (say input A) and into the other input pass a control single. Into the other AND gate pass the other PWM (!PWM) and the same control signal.
If oyu want to rotate, have that single go HIGH, you want to stop, that singal goes LOW.
That will setup a zero-volt loop and it will stop the rotor dead!!. Limitations is that it will still rotate If something forces it, but the electronics will not be able to drive it round. Its holding toruqe will be low
3) a Mechanical brake (this is prolly the best option) only real way to ensure no rotation of the rotor with alot of toque handling.
One point since you have a bi-directional control here you have not mentioned any form of regeneration protection?
Any for of reversing rotation (ie from clockwise to anti-clockwise or vice-verce) will result in the machine behaving like a generator and this will boost your DC-link up uncontrolled. We use a brake cct (ie short a 3R power resistor across the DC-link) if the DC-link (nominal 270V) ecceeds 450V. Without this the links does exceed 600V - we forgot to attach the DC-link sense lead and the brake cct did not kick in and it took out another bit of the cct whos rating was 500V
