- Joined
- May 27, 2016
- Messages
- 3,469
Ahh OK! Thanks! You now give some clues.
I think my first point is about VFDs not actually being "Variable Frequency Drives", despite the inherited name, unless they are very old types that actually did make a fake 3-Phase that would only vary the frequency that the field rotated at. They could only alter speed over a very limited range. As the frequency got lower, so did the coils inductance, and also did the mutual inductance coupling to the rotor. Induction motors make their rotors become magnets by a induced transformer type action. It fizzles out when the field rotates slowly. The back-emf disappears, and the power plummets.
A switch mode drive should start by making a rectified bus DC voltage. This would then be switched at a high carrier frequency like 12KHz or 16KHz as pulse width modulated bursts applied to the coils. I oversimplify a lot, but basically, the motor is given a demand, either speed, or sometimes torque, and the field is rotated at a nice AC frequency the motor expects. The shaft itself might be nearly stopped, or just creeping around, but the field is spinning, and the urge to move is decided by the pulse width duty cycle energy delivery.
Without the VFD, the motor surely spins at the normal 3360 rpm speed. Any belts pulleys ratios combinations had better be able to find 19.2 to 1 ratio to ever get a spindle to turn at 175 rpm. This would be before even one takes advantage of the abilities of a switched drive to go even slower.
Perhaps I need to know more about regular induction motor function when driven by switchers. My normal use was multi-pole AC brushless motor drives used to slow down the shaft to a halt, holding the load as a brake, and then even driving back and forth with the shaft rotating in the opposite direction to the field. That application was satellite dish servo tracking, but think of the load in tons! Most of these had permanent magnets, (hence not induction type), but there were motors moving stuff that were induction types. I never saw the feeble torque phenomenon, but that does not mean it couldn't happen.
I think we need some real facts and explanations about what exactly goes on when driving regular user machine induction motors with "VFDs", and maybe also untangle the differences in various common VFD kit members use in this role. Some may be way more capable than others.
[Edit - Under test, the only way I could stop a shaft was with a big spanner, and that nearly went wrong a few times! ]
I think my first point is about VFDs not actually being "Variable Frequency Drives", despite the inherited name, unless they are very old types that actually did make a fake 3-Phase that would only vary the frequency that the field rotated at. They could only alter speed over a very limited range. As the frequency got lower, so did the coils inductance, and also did the mutual inductance coupling to the rotor. Induction motors make their rotors become magnets by a induced transformer type action. It fizzles out when the field rotates slowly. The back-emf disappears, and the power plummets.
A switch mode drive should start by making a rectified bus DC voltage. This would then be switched at a high carrier frequency like 12KHz or 16KHz as pulse width modulated bursts applied to the coils. I oversimplify a lot, but basically, the motor is given a demand, either speed, or sometimes torque, and the field is rotated at a nice AC frequency the motor expects. The shaft itself might be nearly stopped, or just creeping around, but the field is spinning, and the urge to move is decided by the pulse width duty cycle energy delivery.
Without the VFD, the motor surely spins at the normal 3360 rpm speed. Any belts pulleys ratios combinations had better be able to find 19.2 to 1 ratio to ever get a spindle to turn at 175 rpm. This would be before even one takes advantage of the abilities of a switched drive to go even slower.
Perhaps I need to know more about regular induction motor function when driven by switchers. My normal use was multi-pole AC brushless motor drives used to slow down the shaft to a halt, holding the load as a brake, and then even driving back and forth with the shaft rotating in the opposite direction to the field. That application was satellite dish servo tracking, but think of the load in tons! Most of these had permanent magnets, (hence not induction type), but there were motors moving stuff that were induction types. I never saw the feeble torque phenomenon, but that does not mean it couldn't happen.
I think we need some real facts and explanations about what exactly goes on when driving regular user machine induction motors with "VFDs", and maybe also untangle the differences in various common VFD kit members use in this role. Some may be way more capable than others.
[Edit - Under test, the only way I could stop a shaft was with a big spanner, and that nearly went wrong a few times! ]
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