Soft Motor Starter.

[pwilkinson]

I'm looking to build a soft motor starter. Currently I'm using a 40 amp relay, but upon start up I have a serious power drain issues. I've seen a couple of designs using resistors, mosfet, and capacitor. I know this only lowers the voltage the motor is receiving, which ramps up until the capacitor is charged then runs at full power. The other is using PWM which is the way I would rather go. I understand how it works for a variable speed. How to make it work so it ramps from 0-100% power over a period of about 5 seconds? Any help would be apprecieated. Also, I'm a electronics rookie.

 

[Grossel]

It depends on what kind of motor you're gonna use and the size of it

 

[pwilkinson]

The motor is a 12v DC motor. I'm using it for a radiator fan. I don't have all the specs. It draws around 30 amps continuos. Start up spike is an estimated 50-60 amps. I could scrap my current controller and spend about $130 for one that would probably work. My current set-up works really well except for start-up. I did find this schematic on the forum. https://www.electro-tech-online.com/threads/cheap-and-cheerful-dc-motor-soft-start.30314/ I believe I could adapt it to my purpose. I'm a little concerned about controlling motor speed through resistance. I would hate to burn up my motor.

 

[riccardo]

I used this before...
**broken link removed**

 

[pwilkinson]

I checked out the link. None of the ones listed are rated for amperage I will be seeing.

 

[riccardo]

They have a 100A one, but might be cheaper to just swap the transistor on the smaller units and fix on a bigger heat sink. An IRFP4468PBF should do it.

 

[pwilkinson]

At the price for the 100 amp it would be cheaper to just by a controller made for my fan. They run $100-150. I was hoping I could just add something on to soft start the motor.

 

[riccardo]

How about using varistors as inrush current limiters? Not sure how much softer the start will be, but much cheaper to try out.
**broken link removed**

 

[crutschow]

The motor is a 12v DC motor. ........ I did find this schematic on the forum. https://www.electro-tech-online.com/threads/cheap-and-cheerful-dc-motor-soft-start.30314/ I believe I could adapt it to my purpose. I'm a little concerned about controlling motor speed through resistance. I would hate to burn up my motor.

There's no problem with using a resistor in series with the motor for a short time to limit startup current. It won't hurt the motor. Just select a resistor size that gives a reasonably fast startup while still limiting the current to the desired level.

 

[pwilkinson]

There's no problem with using a resistor in series with the motor for a short time to limit startup current. It won't hurt the motor. Just select a resistor size that gives a reasonably fast startup while still limiting the current to the desired level.

I think I may go that route. I understand how this schematic works. I was also looking into using a 555 chip to make a PWM, but I don't know how to make it stop oscilating and run at full speed. I'm a novice at this electronics things. I see your from Louisiana. What part? I'm living in Baton Rouge.

 

[pwilkinson]

What size resistor do I need?

 

[crutschow]

What size resistor do I need?

To limit the peak startup current to 30A would require a resistor of about 14V/30A = 0.47Ω. Use a 0.5Ω resistor 10W resistor or so. The peak power dissipation is actually close to 450W, but since if will be on for only a very short time, a 10W resistor should be adequate.

 

[pwilkinson]

I tried a .5ohm 10W resistor the motor would not start. I did this in a hard wired configuration just to see if it would work. I did some math, but I'm not sure if I'm right. I'm thinking I need .3 ohm at 100W. I want the motor to start at 6-10 volts.

 

[ronv]

.1 ohm is a common value. 0.1 X 30 would give a 3 volt drop across the resistor. Your counting on the FET turning on to keep the resistor from frying so the 10 W would probably be ok. The varistor is made to order for your problem so take a look at that as well.

 

[pwilkinson]

I will go pick up a 2 more .5 ohm 10W resistors at lunch. I will hook them together in parallel for a total of 4 just to see if I can get the motor to come on. This should give me .125 ohms. The more I get into this simple circuit. The more I realize what I don't know. It's a good learning experience though.

 

[pwilkinson]

My next question on this. How to size a mosfet. I know My Source and Drain need to be able to see 12v at 30 amps continuous. How do I determine gate voltage?

 

[ronsimpson]

One of the MOSFETs (on the other thread) in the data sheet, there is a graph of gate voltage verses drain current. Below 4 volts the part was open, At 5 volts it was closed up to 30 amps, above 30A it opens up. That part needs 10 volts to keep it closed at high current. It should never see more than 20 volts G-S voltage.

Is there a heat sink on the part? With no heat sink you need to keep the watts below 1.

 

[pwilkinson]

I orded my mosfets, with help from an electrical engineering buddy. Now I'm thinking of alternatives. I'm thinking of using a mosfet to trigger a relay instead. That way I can probably get away with smaller heat sinks if any on the fet. I eneded up using 4 1 ohm 10 watt resistors in parallel. This seemed to start the motor relatively soft. I'm also debating building a soft motor start using a pwm based off of a 555 chip. I'm not sure if I could make it work right or not.

 

[Vimlesh Motawala]

To dissipate the Heat generated u should keep the MOSFET submerged in Transformer Oil so no problem of designing the Heat Sink.

 

[crutschow]

To dissipate the Heat generated u should keep the MOSFET submerged in Transformer Oil so no problem of designing the Heat Sink.

I have doubts that transformer oil has good enough thermal conductance to dissipate the localized high power for a MOSFET if it's dissipating more than a few watts. Even then the container holding the oil must have a least as low a thermal conductance to air as a regular heat sink would.