View Full Version : Adding a brake to a grinder

10-14-2002, 09:49 AM
This general idea came from the Third hand page. I have a 12" x 2" old rockwell grinder in my shop, no power off spindle brake, which means it coasts for up to five minutes. The grinder must be from the 50's. I was wondering if any companies make aftermarket spindle brakes.

Forrest Addy
10-14-2002, 12:00 PM
Might be a pretty good grinder.

AC makes an induction motor run. If you apply low voltage DC to the motor it makes a stationary magnet of the motor's field. The armature with its squirrel gage of conductor will generate current as it rotates inside the magnet formed by the DC energised stator. The magnet thus creates a drag in rough proportion to the stator current.

Many very successful braking devices for induction motors have been made using a car battery and resistor (to limit current) and a suitable relay. It's called an eddy current brake.

A DC power supply can be as simple as a motocycle battery and a cheap trickle charges.

A DC brake can be quite powerful. You don't want the motor to stop so quick so the wheels spin loose on their arbors. A typical 1 HP induction motor has a DC field resistance of 1 Ohm os so. Directly applying a 12 volt battery will producing a braking effort roughly equivalent to the motor's full load torque. You need a resistor of some kind and a portion of an old wound filament dryer element with an adjustable tap will work just fine.

Add to this a manual momenraty switch or a time delay realy and suitable wiring and you have a DC brake.

Set it up so the motor has a 10 second coast down.

Bon appitit

[This message has been edited by Forrest Addy (edited 10-14-2002).]

10-16-2002, 01:24 AM
DC can fry the windings, try a large power resistor shunted across the windings when it is off. If it is high enough resistance and wattage it will brake the motor - this is what is used in VFD braking circuits.

Forrest Addy
10-16-2002, 01:56 AM

DC braking won't fry the windings of an induction motor. Look at the text of my posting where I discuss a current limiting resistor.

DC has been used to brake high inertia AC driven machinery for nearly a hundred years including rolling mill motors to kits sold by HeathKit in the '50's and '40's for home shop equipment.

The HeathKit dynamic brake is where I first became aware of the trick. The kit consisted of a transformer, a selenium rectifier (remember them and how they stunk when they blew?), a resistor and a fancy little time delay relay set up to close for several seconds on power off, a chassis, and the controls and a very complete book. HeathKit was first rate stuff. They died in the late '70's with DIY electronics.

VFD's "motor" to decellerate, that is they follow the motor to a near stop with a frequency lower than the armature sync frequency. The resistor you metnion dissipates the regenerative heat. They also have a DC braking option that can be applied in a number of ways up to full load current for any number of seconds.

[This message has been edited by Forrest Addy (edited 10-16-2002).]

Treven Baker
10-16-2002, 08:59 PM
What difference does it make how long it takes to coast to a stop? I have one that does the same thing. I think it is kind of cool. If it ain't broke don't fix it.

10-16-2002, 10:42 PM
Forrest is right on. Works great, and takes much less than full load current to stop pretty smartly.

Absolutely no danger to the windings. But I'd worry about the wheels too.

Oh, and as for why, in a small shop, remembering that the thing is still spinning and needs watching for can be a problem. you don't want to get into it by accident while you are doing something else. Thay can be kinda quiet and sneaky.

In a big shop you just walk away, of course.

[This message has been edited by Oso (edited 10-16-2002).]

10-16-2002, 10:44 PM
I would not care how long it takes, but the school safety laws do here in NH. I either put a brake on it, or sell it and buy a new one.

Personally, I think a brake on such a big wheel might cause bearing issues in the future, just like the one we are continually changing out bearings on in another school in the district - same size unit.

Have to keep the feds and state happy.....

10-17-2002, 12:12 AM
I bet a mechanical brake could be a problem.

An electrical brake won't cause any bearing problems. That's what I would do.

BUT, I guess you better sell it, because the laws doubtless won't let you modify the electrical part of it either. Forfeits your UL mark if modified..............then it doesn't meet OSHA anymore.

You probably have to have an approved brake made for that particular unit, nothing made and attached by "unqualified persons". Good luck finding such a part.

Leaves you with the faint chance of a plug-in electrical brake. It is possible, but tricky.

However, probably somewhere a school supply place has such animals. You can't be the only one with the problem.

10-17-2002, 09:09 PM
How about making a box with a on'off switch, and receptacle for the grinder to plug into.

In the box put a 6amp diode from radio shack (< a dollat at radio shack) and a heating element from a 300 watt soldering iron (about 3 amps when hot) and time relay or electroincs circuit for time delay (could use a triac and R/c Circuit). OPen the swich and the circuit across the switch will become active and send DC at low amperage to the grinder. I would be most careful about trying for rapid breaking or the nut on the grind whell may back off.

THen lock the grinder switch in the on position.

Never tried the above but have braked induction motor with diode and lamp bulb for limiting resistor. that worked well.

10-24-2002, 03:52 AM

Thank you for the correction - I was on drugs and lost my mind (no, really).

I do not agree with braking a grinder on the grounds of an abrupt stop could damage the wheel and make it dangerous. If the method you suggest is used I would adjust it for about twice the spin-up time to a dead stop and then kick out.