Electronics > Beginners

What this motor type?

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james_s:
The issue is more complex than failure rate, failure mode also matters. A mechanical relay usually fails safe, a triac or other solid state device almost always fails shorted. Nobody in their right mind would rely on a SSR alone to cut off the power to something that could cause injury if it powers up unexpectedly or fails to shut off when commanded. When it comes to things like momentary overloads or electrical surges, a relay is much more likely to survive an event that would blow a triac. Under normal conditions both are quite reliable, I have equipment that is decades old using relays to control loads that all still work fine.

Just one random example I've dealt with quite a lot is pinball machines, the solid state machines use transistors to control all the solenoid coils and those transistors are one of the most common failures in the machine. The old electromechanical machines use relays, I don't recall ever having to replace a relay in one despite the fact that the newest ones are now ~40 years old.

Microwave ovens, dishwashers with electronic controls, clothes washers, that sort of stuff still often uses relays. Of the machines that use triacs to control motors and solenoids the triac is usually what fails, I can't remember having to ever replace a relay in anything like that.

Zero999:
Another issue with TRIACs is they have a much higher on loss, compared to relays which lose negligible power.

Relays can also fail short circuit, but it's less common than TRIACs which nearly always fail short circuit.

As far as reliability is concerned, there are some advantages TRIACs have over relays: they're less susceptible to vibration and the number of switching cycles is unlimited. In this case vibration can be mitigated by appropriate component selection and good mechanical design and the number switching cycles is a non-issue, as the motor won't be reversed frequently enough for that to matter.

In the case of controlling both the speed and direction of a universal motor is concerned: use a TRIAC for the former and relay for the latter. Using four TRIACs to control both the direction and power makes it more difficult to control, results in double the power dissipation, compared to a TIRAC and relay and if one of them fails short circuit, the power supply will be short circuited when the other one turns on. In short, using TRIACs to control the direction of a universal motor is a bad idea. Different components have their strengths and weaknesses and good engineering is about knowing which component is best suited to a certain application.

A latching relay for directional control is probably best, since there's no chance the motor can suddenly reverse, if the power to the relay coil fails. Reversing the motor when the shaft is moving at full speed, will result in an enormous peak current, nearly double the initial starting current which will cause total destruction of the driver circuitary.

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