Soft start for a 2 phase inductive load/Transformer

[eee291]

I have seen some slow start or soft start devices for 3 phase electric motors, but I need something for a two phase AC 400v Transformer that has a motor connected to it.
The output is about 90v, which is connected to a rectifier that is then connected to a DC motor to drive a cart.
The issue is that if it switches at the wrong part of the waveform or perhaps with too much load it trips the breakers.
I guess the easiest solution would be to get a slower tripping breaker.

At one point I was thinking to just run it with a DC brushed controller and a battery, but the issue is that it needs to be inspected every year or so since it is used commercially (I don't know how it passes/passed since the wiring is not... Good).

Anyone have any tips or suggestions?

 

[amberwolf]

It's not totally clear, but it sounds like you have an AC transformer powering an AC motor that somehow creates a 90v (AC? DC? ??) output that thru a rectifier then runs an unspecified DC motor, with no controller, just full on or off.

If that's correct: most AC motors (especially high voltage ones) have a start/run capacitor, and wouldn't run off DC, so you would have to use the brushed controller on the secondary motor...but changing the start/run cap on the AC motor might fix the problem on it's own. (replacing with a new one of the same rating first).

If that's not correct, and the transformer does not actually have a motor connected to it, but just the rectifier, when then has the motor connected to *it*....then if it's a standard brushed DC motor:

--you could add sufficient capacitors to the rectifier to fliter the rippled DC to run a standard brushed controller of whatevery type you want, that can handle the worst-case current the motor will ever pull. (you'll need to know how much that is, by testing, before implementing any limiting solution, anyway).

--you could try using one of the various AC-side limiters or soft starts:
----stuff like these https://www.amazon.com/230V-20A-electric-Electric-applicable/dp/B0BZSJZCZV (dunno which one you'd actually need) whcih ramp up the output on their own,
----using an induction-tolerant lighting dimmer / AC motor speed control (like these: KB Electronics KBWC-13K (H9030), 2.5 Amps @ 120 Vac, Wall Mount, AC Fan Motor Control and changing the triac to suit the current the motor must pull (if it doesn't already). Then you can set the limit lower than max, and turn it up once it's started.

 

[eee291]

It is outputting 80v ac and rectifying to whatever DC voltage. The issue with the soft start is the 400V phase to phase I can't use a 230v tool soft start. Other 400v soft start devices have 3 phase input and I don't know if that will work with a 2 phase load.

 

[dgc]

May be no use but VersiStart II LDS [3 - 15kW]

 

[fechter]

I've seen some equipment that just used a big NTC thermistor in series with the AC line. Without it, the breaker would trip sometimes. With it, the breaker never tripped. The trick is to choose the right size for the application.

https://www.digikey.com/en/products/filter/inrush-current-limiters-icl/151?s=N4IgTCBcDaIJIDsBOBXAzgCwAQGEVKQFMEAXLAGQEsBbSkwpNEAXQF8g

 

[eee291]

Thanks for the suggestions.

I initially considered using NTC resistors but was too afraid of them burning them up or adding too much resistance.
It's on a timer, so every time a worker pushes the button it runs for 15 seconds.
Sometimes it's run right after (after about 10 seconds) more often though it gets about a minute break before it's run again. How fast does the NTC resistor cool to for this not to be an issue?

 

[fechter]

I'm not sure how long they take to cool off. It might be more than 10 seconds. If the power is cycled too quickly, the breaker might trip.

When the motor starts, it's going to have a pretty big current inrush until it spins up.

They make time delay circuit breakers designed to handle motor starts. This would replace the one in the electrical panel.

 

[amberwolf]

It is outputting 80v ac and rectifying to whatever DC voltage. The issue with the soft start is the 400V phase to phase I can't use a 230v tool soft start. Other 400v soft start devices have 3 phase input and I don't know if that will work with a 2 phase load.

If it's outputting 80vac, that's a lot less than the 230v, so it would work fine, as long as it supports the current draw of the motor at peak load.

You would need to measure that current in operation as it is now to find out how much whatever device you use has to supply.

 

[fechter]

With the transformer issue I was referring to before, it was the transformer itself and not really the load that was causing breaker trips. They make solid state relays that only switch during the zero crossing. If you could turn on the transformer with a zero crossing relay instead of a switch, it might solve the problem.