Thread: E-stop and Teco VFD's

Firstly I should preface my comments by saying that I am well outside my area of qualification and expertise, and the statements following are as I understand the situation to be, and if I am wrong, I hope that someone who is full bottle on this topic will put me straight.
The reason why the E Stop would not be in the power in side of things is as Greg has said, the input capacitors store significant energy which would allow a motor to run on a bit, But also the VFD is capable of dynamic braking the motor, and to do that it needs power. A bit like stopping a car travelling along a road, by just knocking it into neutral, sure it will eventually stop, but nowhere near as quickly as if you applied the brakes at full pressure, but with ABS active. If the VFD still has power at the input terminals, it can activate the program which it has been set up to do, in the event of certain defined circumstances. If there is no danger of say a chuck unscrewing as Ozengines experienced, you can program it to stop very quickly, perhaps well under 1 revolution from full speed. For an emergency stop, this could be a very useful feature, if say someone was being drawn into the machine by loose clothing say, but any chuck would want to be locked to the spindle by some means and not just screwed on without any locking system.
If you have a screwed on chuck, then braking force has to be more limited, and the risk of not being able to prevent injury because of a slower stop balanced against the chance of a chuck unscrewing at speed, and chasing people around the workshop.
Depending on your VFD, you may have more than one stop mode, and therefore still be able to program the normal stop function to be a much gentler stop, maybe stopping from 1450 RPM to Zero in around 20 revolutions say, still quite a quick stop, but much easier on the drive train. Naturally this is the stop button for normal stopping of the machine, and the Emergency stop should not be used except in emergency situations. If you only have one stop mode, you have to compromise one need against the other.
Regarding the way you can use a 1000Ω resistor and a normally closed E stop switch to function as Dave has described, it sounds to me that the control is a high input impedance device, probably using a Field Effect Transistor, or Fet. Fets are voltage controlled and draw (virtually) no current, whereas normal bipolar transistors are current controlled. The 12 volts applied via the 1K resistor, and a current of 12 mA flows to ground via the E stop switch, SP1 is held at zero volts and so the device runs as per normal, but when the E stop is activated, the link to ground is cut, and the input terminal, SP1, will float at 12 volts which switches the motor off and applies the pre-programmed amount of braking on. As this resistor will be dissipating only 144 milliwatts, a half watt resistor will have a low chance of failure I would think.
That's quite enough from me, if I'm off the mark in my understanding of VFDs, please someone put me straight. For now Cheers all, and Ken I will get that book in the mail tomorrow I would hope,
Rob.