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mooimooi (Electrical) (OP)
10 Nov 03 1:33
Hi,

Just curious to know why most motor brake and clutch coils are mostly designed or rated in DC voltage and not in AC. In the case of ordinary relays used for control purposes, they can either be constructed in AC or DC.

Thanks
Chandran.
dspDad (Electrical)
10 Nov 03 1:49
Motor braking occurs when the motor is used as a generator and the power it develops is dissipated as heat.  The dc magnet is the fixed field of the generator.

Sometimes for small motors, the input to the motor is simply connected to a dc source, usually a full or half-wave rectified input of the mains.

In a three phase motor, reversing the phases momentarily can also be used as a brake.  The currents are fairly high during this procedure.  For a single phase motor, 'reversing the polarity' will simply cause it to spin up again.

For a mechanical clutch, the use of ac or dc is irrelevant except that the solenoid or actuator is designed for either ac or dc.

An experiment striking enough to take it home and show your kids is to get a NdFeBa magnet about 0.5 x 0.5 in, and drop it down a 2 or 3 foot length of extruded aluminum tubing or square pipe.  The magnet pushes itself away from the walls of the pipe, and takes about 15-30 seconds to fall through.  In square pipe, the sides act like a kaleidoscope which adds to the visual effect.  This is an example of magnetic braking.

DspDad
mooimooi (Electrical) (OP)
11 Nov 03 23:38
Hi DSPdad,
Thank you for the explanation, that was very useful.
Joy to you.
jraef (Electrical)
18 Nov 03 22:56
Actually, I believe he was referring to a motor brake COIL, not a DC injection brake. They are 2 different things. A motor brake coil operates a mechanical brake that is attached to the motor shaft. It is held on with springs so that it is fail-safe. The coil must pull the brake OFF of the motor shaft when the motor is energized, so often the brake coil is simply connected to the motor leads. The reason that they often use DC coils is that an AC coil would require a lot more power to maintain it energized for long periods of time, and more power = more heat = earlier failure. A DC coil with an optimizer circuit will pull in and stay in with very little "sealed" current so it stays cooler and lasts longer.

Quando Omni Flunkus Moritati

Helpful Member!  gsimson (Electrical)
26 Nov 03 6:18
DC is preferred to Ac in brake coil / contactor coil/ clutch coil, to avoid chattering. Also if the air gap increases due to wear & tear of brake lining, ac coil will burn as the current required for the same flux in the increased airgap is more. Only in DC we have to add economy resistor during holding.
jraef (Electrical)
2 Dec 03 23:01
Thanks gsimon.
I hadn't heard the chattering issue and the flux changes as the linigs wear makes sense. I have added to my knowledge base today.

Quando Omni Flunkus Moritati

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