# Under what circumstances would one need or not need an electronic brake for a permanent magnet motor?

My company was asked to design a custom product several years ago. The original requester never bought it, but we've sold several a year through distribution. It's been a small side-line for a while, but we'd like to understand the broader use and potential market for it.

This product sits between a motor controller and a permanent magnet motor. In the event a signal is received, perhaps indicating a light curtain is broken, this unit disconnects the motor from the controller and applies resistors across the motor leads. This stops the motor. This is clearly useful primarily for safety applications. Things I'd like to understand:

1. What alternate solutions exist?
2. Who would be most likely so specify such solutions?
3. How can I most effectively reach those people?
• So your product is just a braking resistor and a contractor of some sort? Many if not most modern motor controllers have that functionality built in. – Ethan48 Mar 11 '16 at 14:09
• @Ethan48 Three resistors and a three-pole contactor, but yes. Unfortunately I live mostly in the world of variable frequency drives, which don't have this kind of brake. Any suggestions as to where I should look for general or specific information about the kinds of controllers you're talking about? – Stephen Collings Mar 11 '16 at 14:33
• Many VFDs have points of connection for a braking resistor. I've bought many of them from SEW for example. Usually the manufacturer of the drive will also sell resistors. – Ethan48 Mar 11 '16 at 14:36
• @ethan48 oh, yes, that's our primary market. This e-stop brake for permanent magnet motors is what confuses us. I want to know why they're still sold and to whom. Distribution won't tell me. – Stephen Collings Mar 11 '16 at 14:42
• Ah, OK. Interesting. I wonder if it's mostly retrofit projects or if new systems are being developed with it. – Ethan48 Mar 11 '16 at 15:07

## 2 Answers

Your solution fits a rather narrow gap of the market.

The first part of the market is where a brake is not needed - the motor propelling devices where standard operation friction is enough to stop them when unpowered. It's a very broad domain: motor being non-critical, say, powering a fan which is harmless when allowed to spin down slowly after switching power off; high-torque (heavy industrial) machines of moderate inertia, that will stop within 1-2 turns by themselves without constant power input, machinery where duty cycle of motor operation is so long comparing to switch on/off times these become negligible (pumps operating on large reservoirs) and many others.

The other side of the market requires braking far more efficient than a couple resistors can provide. These will use mechanical clutches or external mechanical brakes that can stop the rotation within much less than several rotations, and inhibit slow free rotation (your brake does nothing against that!) - this will be safety equipment stopping machines that can be dangerous to life and health: saw blades, elevators, heavy machinery of high inertia that would take a long time to sink all the stored momentum through the resistors, vehicles and transport devices (cranes/gantries), also production lines where a fault at a point could lead to serious accident if allowed to propagate down the production line with damaged product.

You're in a narrow segment where a weak brake like yours is sufficient. It may be budget machinery which allows a delay at braking at expense of safety; it may be very heavy, high-inertia machinery where rapid mechanical braking is simply impossible due to forces involved. It may be also a comfort/efficiency feature for devices that don't require brakes, but presence of one allows faster change of operation - say, service/regulation access to a section of a machine that is locked out for duration of operation.

And even in these cases a segment of your market is stolen by controllers that have your feature integrated, and provide it as a part of control, not just extra safety.

• I would be concerned that the OP's brake is not fail-safe. If the resistor, wiring on contactor fail then there is no braking effect. – Transistor May 30 '18 at 21:00
• @Transistor: Mostly agreed. Resistors are too primitive devices to fail. But contactors have a nasty tendency to stick, and you'd need to observe the motor shutdown carefully to notice this. And of course wiring. Where there are motors, there are vibrations - and contacts break. It's an okay 'convenience' solution to reduce shutdown time, but not a thing to protect human lives. – SF. May 30 '18 at 22:58

You would need a brake when the motor is pushing a car. The reversing of the polarities in the magnets to break are currently used in Prius cars and are also to recoup the kinetic energy down hills called synergy. This has also been done to reverse engineer ceiling fans to make them into wind generators.

After talking to the author of the picture below he said this was also used in trains since the 1900 to collect kinetic energy from going downhill.

You have my permission to use or link to illustrations from my tutorial on stepping motors at -- or any mirror of that web site, on the condition that you credit those illustrations with something like:

-- Illustration from -- http://www.cs.uiowa.edu/~jones/step/, -- Control of Stepping Motors -- by Douglas W. Jones -- used with permission.

The actual wording is your choice -- make it fit gracefully into what you're doing. All I care is that the final 4 elements listed above are included in the credit. If you are crediting multiple sources, it reads better if you use a similar form of credit for all of them, but if not, just go with this:

-- Illustration from http://www.cs.uiowa.edu/~jones/step/, Control of Stepping Motors by Douglas W. Jones, used with permission.

What you may not do is imply any endorsement by me of what you are writing, since I have no idea what it is.

            Doug Jones
jones@cs.uiowa.edu
(aka douglas-w-jones@uiowa.edu)


Original image retrieved from here