Hi All,
I've been meaning to revamp my lathe's control cabinet for some time now. There were a few things I did on the original build some years ago which made me less than comfortable with the safety of the wiring. Since then, I have taken a job designing industrial control panels and I am much more aware of good practices and code requirements. I figured while I was correcting the panel, I would also experiment with a method of quickly stopping the motor using its own magnetics.
My lathe is an import ENCO 12x36 manufactured back in '94. It uses a common single phase 4-wire reversible motor with two capacitors. The motor can be started in either direction from a stop by reversing the polarity to the start winding. For those of you unfamiliar, a single phase AC motor is unable to produce any torque at a dead stop. A couple of tricks are used to start these motors, most common of which is phase shifting the AC line using a capacitor. This creates pseudo 2-phase power at the motor and gives it the torque necessary to start. A 3 phase motor does not have this issue. Once the motor gets part of the way to its operating speed, a centrifugal switch inside the motor opens and disconnects the capacitor and the so called "start" winding.
Many of these motors run in one direction, but some can be provided with separate connections for the start and run windings. By reversing the polarity of the AC line on the "Run" winding relative to the "Start", you can control the motor's direction. Again, the centrifugal switch disconnects the "Start" winding once the motor is at speed. This is typically done with a pair of contactors which are interlocked to prevent them from coming on at the same time.
The experiment came in play for when I want to stop the motor. Normally both contactors open, power is removed from the motor, and it spins down under friction. Instead, I added an additional pair of interlocked contactors labeled "Run" and "Brake". The Run contactor is pulled in whenever the motor is running forward or reverse. When the motor shuts off, the Run contactor turns off and the Brake turns on. This disconnects the motor from the AC line and connects a 24VDC 10A power supply across the Run windings of the motor. The effect is a rapid deceleration of the motor to a stop.
Here is the panel I built to do this,
From the bottom up: the power enters and is protected by 10A Class CC fuses. Terminal blocks allow easy-ish hookup of all the switches and buttons on the lathe. The right pair of contactors in the middle is the FWD/REV controls. The left pair is the RUN/BRAKE control. The big DC supply up at the top is for the DC injection braking, and in the top left is a PLC to control the timing. This could be easily accomplished with a timing relay, but I had the PLC on hand so I went for it.
The whole idea of this braking goes back to a concept known as slip. In an AC induction motor, AC voltage is applied at the stator and generates a rotating magnetic field. The rotor experiences this magnetic field and a voltage is induced in its windings. As long as there is a difference in the angular velocity of the stator's magnetic field and the rotor's mechanical angular velocity, there will be voltage induced on the rotor and torque generated. This leads to the realization that an AC motor can never match the electromagnetic field speed since there would be no difference in speed between stator and rotor, no induced voltage and no torque. The more load that is placed on the motor, the slower it rotates to generate more torque. This speed difference is known as slip.
The reverse can be used to brake the motor. When power is removed, the stator's magnetic field disappears, but the rotor remains spinning. At this point, we apply DC voltage to the stator which creates a stationary magnetic field. Now again, there is a speed mismatch between stator and rotor and torque is generated - but this time in the opposite direction, stopping the motor. All the mechanical energy is converted to heat in the rotor, so this has to be used carefully. For my lathe, I am not concerned.
Still finishing the program to time the brake, but it looks like it is working pretty well. I'm interested to see how much the stopping time is reduced.
I've been meaning to revamp my lathe's control cabinet for some time now. There were a few things I did on the original build some years ago which made me less than comfortable with the safety of the wiring. Since then, I have taken a job designing industrial control panels and I am much more aware of good practices and code requirements. I figured while I was correcting the panel, I would also experiment with a method of quickly stopping the motor using its own magnetics.
My lathe is an import ENCO 12x36 manufactured back in '94. It uses a common single phase 4-wire reversible motor with two capacitors. The motor can be started in either direction from a stop by reversing the polarity to the start winding. For those of you unfamiliar, a single phase AC motor is unable to produce any torque at a dead stop. A couple of tricks are used to start these motors, most common of which is phase shifting the AC line using a capacitor. This creates pseudo 2-phase power at the motor and gives it the torque necessary to start. A 3 phase motor does not have this issue. Once the motor gets part of the way to its operating speed, a centrifugal switch inside the motor opens and disconnects the capacitor and the so called "start" winding.
Many of these motors run in one direction, but some can be provided with separate connections for the start and run windings. By reversing the polarity of the AC line on the "Run" winding relative to the "Start", you can control the motor's direction. Again, the centrifugal switch disconnects the "Start" winding once the motor is at speed. This is typically done with a pair of contactors which are interlocked to prevent them from coming on at the same time.
The experiment came in play for when I want to stop the motor. Normally both contactors open, power is removed from the motor, and it spins down under friction. Instead, I added an additional pair of interlocked contactors labeled "Run" and "Brake". The Run contactor is pulled in whenever the motor is running forward or reverse. When the motor shuts off, the Run contactor turns off and the Brake turns on. This disconnects the motor from the AC line and connects a 24VDC 10A power supply across the Run windings of the motor. The effect is a rapid deceleration of the motor to a stop.
Here is the panel I built to do this,
From the bottom up: the power enters and is protected by 10A Class CC fuses. Terminal blocks allow easy-ish hookup of all the switches and buttons on the lathe. The right pair of contactors in the middle is the FWD/REV controls. The left pair is the RUN/BRAKE control. The big DC supply up at the top is for the DC injection braking, and in the top left is a PLC to control the timing. This could be easily accomplished with a timing relay, but I had the PLC on hand so I went for it.
The whole idea of this braking goes back to a concept known as slip. In an AC induction motor, AC voltage is applied at the stator and generates a rotating magnetic field. The rotor experiences this magnetic field and a voltage is induced in its windings. As long as there is a difference in the angular velocity of the stator's magnetic field and the rotor's mechanical angular velocity, there will be voltage induced on the rotor and torque generated. This leads to the realization that an AC motor can never match the electromagnetic field speed since there would be no difference in speed between stator and rotor, no induced voltage and no torque. The more load that is placed on the motor, the slower it rotates to generate more torque. This speed difference is known as slip.
The reverse can be used to brake the motor. When power is removed, the stator's magnetic field disappears, but the rotor remains spinning. At this point, we apply DC voltage to the stator which creates a stationary magnetic field. Now again, there is a speed mismatch between stator and rotor and torque is generated - but this time in the opposite direction, stopping the motor. All the mechanical energy is converted to heat in the rotor, so this has to be used carefully. For my lathe, I am not concerned.
Still finishing the program to time the brake, but it looks like it is working pretty well. I'm interested to see how much the stopping time is reduced.
I must say quite ingenious. Innovative and imaginative. Kudos to you sir.
