Hi.
I have a wire EDM machine that must stop the 3 phase motor fast. The machine got 3 large 400 volt capacitors connected in parallel to that circuit, but i can only find a single diode connected to them. How can one diode change the caps to DC? The wire connections all look original, only the diode wire connection don't look original.

 

They are probabally connected in Star, with the Diode connected to this point terminsating all three?

 

The way to stop a 3-phase induction motor fast is to short all 3 motor terminals together as soon as possible after the power is removed. Only do this on a small motor with not too much inertia.
Back in the day I used to do this on disk cutters in a chicken processing plant - to try and save the operators fingers!
The motor was switched by a 3-pole contactor operated by a foot pedal. The contactor had auxiliary NC contacts that closed when the contactor released. Very important that these aux contacts are "make after break" - they only make after the main contacts break.
All depended on the reaction time of the operator of course. Chain mail gloves were a safer solution!

 

The best and traditional way to brake a 3ph induction motor is DC injection into the stator at switch off..
The other method sometimes seen is reversal of two phases, and zero speed detector to switch off power at zero RPM.
there is very little back emf from the rotor of a AC induction motor to just short the 3 phase leads for effective braking.

 

The best and traditional way to brake a 3ph induction motor is DC injection into the stator at switch off..
The other method sometimes seen is reversal of two phases, and zero speed detector to switch off power at zero RPM.
there is very little back emf from the rotor of a AC induction motor to just short the 3 phase leads for effective braking.

Ah! You are right. I forgot about the DC injection. I just remember bang! the motor stopped.

 

I have only ever come across one of the reversing phase methods, was on a pair of CNC rail car wheel boring machines for the chuck/unchuck process.

 

The DC method is what I have used, but it is important to be careful to not overheat the winding used for the braking. SHORTING the motor at switch-off will reduce the coasting time, but not generally produce a very quick stop. BUT some motors stop that way much faster than others. But a few seconds of full voltage DC gives a fairly quick stop.

 

But a few seconds of full voltage DC gives a fairly quick stop.

Which generally is not enough time to "over-heat" the winding, Unless of course, there is several such stops in a row!!

 

Both Malfunctions and Wiring errors have been known to occurr, and cause problems. Thus, much more attention to details is required in the application of momentary braking voltages.

 

Both Malfunctions and Wiring errors have been known to occurr, and cause problems. Thus, much more attention to details is required in the application of omentary braking voltages.

The capacitors that was in the machine was powerlytic caps. Don't know how they work as they are DC but there was no bridgerectifire in the circuit. I changed everything by installing a bridgerectifire and a large cap that i first charge with a button switch. That stop the motor in seconds when the power is off.

 

A simple one-diode half wave rectifier circuit is all that is needed to charge DC capacitors. What is complex is how to connect them to a 3-phase motor for the best DC braking. THAT portion also requires assuring that the timing avoids connecting the DC while the AC power is still connected.

 

A simple one-diode half wave rectifier circuit is all that is needed to charge DC capacitors. What is complex is how to connect them to a 3-phase motor for the best DC braking. THAT portion also requires assuring that the timing avoids connecting the DC while the AC power is still connected.

That's an important point about the timing. Preventing DC injection while AC is still connected is probably one of the most critical safety considerations in these braking circuits. It sounds like the electrical design is often less challenging than making sure the switching sequence is reliable under all operating conditions.

 

The challenge is that many 3-pole contactors do not provide a set of normlly closed contacts, to use for DC BRAKING

 

My first go-to for contactors are Telemecanique, Now Group Schnieder, They are compact DIN style and offer different kinds of optional, add-on aux contacts.

 

I discovered the Telemecanique brand of contactors one time when the AB brand priced itself out of a project's budget. And I have not looked back, except tp see how much I have saved.

 

AB has always been high (over) priced for their products, I discovered that many of their items were outsourced then sold under their banner.
I foumd their PLC to be very akward to use.