Using a transistor to control a motor?

Is it possible to use a N-Channel MOSFET 60V 30A to control a 36V 750W DC motor? In theory the motor should only take 20.833 Amps. (750/36=20.833) However I've heard that a motor take more power to start rotating. Would the N-Channel MOSFET 60V 30A be able to handle it?

Thanks Tyler

Be sure to use a good heat sink. Does the MOSFET have built-in over-current protection? If so it should be fine.

Probably not. Stall current about 3-5 times higher. The same time you could solder 3 transistors in parallel, MOSFET don't have temperature run-away issues

johnwasser:
Be sure to use a good heat sink. Does the MOSFET have built-in over-current protection? If so it should be fine.

What’s the best way to “heat sink” it? (I’ve never done that before.) I don’t think it has protection (I couldn’t find anything mentioning it.)

Magician:
Probably not. Stall current about 3-5 times higher. The same time you could solder 3 transistors in parallel, MOSFET don’t have temperature run-away issues

Oh ok. So putting them in parallel will reduce the current for each one? What’s “temperature run-away issues”?

Thanks!
Tyler

Oh, sorry, I used wrong word. They call it thermal runaway:

Power MOSFETs Power MOSFETs typically increase their on-resistance with temperature. Under some circumstances, power dissipated in this resistance causes more heating of the junction, which further increases the junction temperature, in a positive feedback loop. However, the increase of on-resistance with temperature helps balance current across multiple MOSFETs connected in parallel, so current hogging does not occur. If a MOSFET transistor produces more heat than the heatsink can dissipate, then thermal runaway can still destroy the transistors. This problem can be alleviated to a degree by lowering the thermal resistance between the transistor die and the heatsink. See also Thermal Design Power.

http://en.wikipedia.org/wiki/Thermal_runaway

What’s the best way to “heat sink” it?

Clamp it or bolt it to a piece of metal. The amount of surface area determines how much heat is conducted away from the device. You can buy specially shaped heat sinks with fins.
See this for why:-
http://www.thebox.myzen.co.uk/Tutorial/Power.html

Magician: Oh, sorry, I used wrong word. They call it thermal runaway:

Power MOSFETs Power MOSFETs typically increase their on-resistance with temperature. Under some circumstances, power dissipated in this resistance causes more heating of the junction, which further increases the junction temperature, in a positive feedback loop. However, the increase of on-resistance with temperature helps balance current across multiple MOSFETs connected in parallel, so current hogging does not occur. If a MOSFET transistor produces more heat than the heatsink can dissipate, then thermal runaway can still destroy the transistors. This problem can be alleviated to a degree by lowering the thermal resistance between the transistor die and the heatsink. See also Thermal Design Power.

http://en.wikipedia.org/wiki/Thermal_runaway

Ok thanks! I will take a look at that :D

Grumpy_Mike:

What's the best way to "heat sink" it?

Clamp it or bolt it to a piece of metal. The amount of surface area determines how much heat is conducted away from the device. You can buy specially shaped heat sinks with fins. See this for why:- http://www.thebox.myzen.co.uk/Tutorial/Power.html

Thanks, is that your website? It's great website! I am learning a lot from it! I was looking at the "Power Example" part, but I'm a bit confused. You calculated the power as 3.5A, but if the devices are in parallel wouldn't it only be 0.5A each? Also I looked at the spread sheet for the MOSFET and it's 62.5 C/W. If I'm running a 750W motor wouldn't it be way too hot (62.5*750=46,875)?

Thanks! Tyler

If I’m running a 750W motor wouldn’t it be way too hot (62.5*750=46,875)?

No you are confusing the power of the load and the power dissipated in the switching device.

The fact that you have a 750W motor only affects the current it will draw.
Then when you have that current it flows through the FET’s on resistance ( Ron ).
This is normally very low.
The amount of heat generated is then given by I2R
That is going to be nothing like 750W.

Two FETs in parallel will half the Ron resistance.

Ok thanks! I will have to do some more research. I have one more question. Would buying the Mosfet 60V 60A http://www.ebay.ca/itm/10-X-STP65NF06-Pb-Free-RoHS-MOSFET-N-CH-60V-60A-11-5m-/280538608048?pt=LH_DefaultDomain_0&hash=item41516771b0#ht_9522wt_986 be any better?

Thanks Tyler

Yes. The current rating of a power FET is just "big" talk, not much related to the actual current you will be able to switch. Normally thermal considerations kick in before you get any where close to the headline current capacity.

Grumpy_Mike: Yes. The current rating of a power FET is just "big" talk, not much related to the actual current you will be able to switch. Normally thermal considerations kick in before you get any where close to the headline current capacity.

Ok, after some research I think I finally get it! XD (sorry for my ignorance) So the lower "Static drain-source on resistance" the less heat it will generate?

If I use your formula "I*I*R" I get 20*20*0.0115=4.6W if I'm running the motor at full power. 4.6W*62.5°C/W=287.5°C which seems a lot more reasonable than my last answer :)

What is a safe temperature? If I connect 4 in parallel would that cut the temperature in a quarter, or would it cut the current in a quarter, there by cooling it a lot more (i.e. (5*5*0.0115)*62.5=18°C)?

Thanks! Tyler

the lower "Static drain-source on resistance" the less heat it will generate?

Yes.

What is a safe temperature?

Keep the actual chip temprature below 110C, you can work out what that is for the case.

If I connect 4 in parallel would that cut the temperature in a quarter

Not quite, it would split the heat over four devices, as each device takes one quarter current and current is squared in the formulary it is a bit more than this.

4.6W means using a moderately large heatsink or a smaller heatsink plus a fan (chipset cooling fans designed for computers are very handy for cooling I find - small and quiet.

So what about PWM and speed control? You probably want this in order to ramp up the motor to speed smoothly (a 750W motor suddenly connected to its full supply will give a hell of a kick, its not recommended - apart from anything you can chip any gears). The stall current will be whatever the supply can give, I suspect the motor can pull 100's of amps at stall if allowed. That will explode your MOSFET so make sure the supply current is limited to 30 or 40A and ramp up the drive to avoid these startup surges.

With those reasonable high voltages and currents you almost certainly need to drive the MOSFET gate with a MOSFET driver chip (otherwise drain-gate capacitance could cause the gate voltage to go out of limits and blow up the Arduino. In fact that MOSFET isn't logic level so you'll need a 10..12V supply and driver circuit anyway. I've used the DIP version of the MIC4422 driver before - its very rugged. Connect the Arduino pin to the input of the driver with a 4k7 resistor and you'll save the Arduino from damage should something go wrong with the high-power devices (this is a real risk with prototyping high-power circuits).

Putting my 2 cents in the conversation, do you know what type of transistor i need for the Baldor motor like this https://mrosupply.com/product/23247-Baldor_Electric_Motors-Motors_DC_Motors_General_Purpose is there any on the site? Thanks!

You can use a transistor to control a motor but I suggest that you check out the chips that are designed to handle the PWM aspects. Do a search for motor control chips. It is very, very important that you use a chip that has pulse by pulse current control. This is because without it the current, when the motor is not yet turning, will be very high and then when the pulses turn it off the voltage produced will blow your transistor. If you just want to turn it on or off without any speed control then a MOSFET will do the job but it needs protection from the high voltage on turn off and the inrush current when turned on. Easier and best to use a dedicated chip and use the Arduino to send a control signal to the chip.

Grumpy_Mike: Keep the actual chip temprature below 110C, you can work out what that is for the case.

Ok, I'm not sure how to work it out? I was looking at the spreadsheet and it says the thermal resistance junction(actual chip?) to case is 1.36. But I have no idea what to do with this number? Do I divide it? (110C/1.36 = 80.88 ?)

Thanks! Tyler

MarkT: 4.6W means using a moderately large heatsink or a smaller heatsink plus a fan (chipset cooling fans designed for computers are very handy for cooling I find - small and quiet.

So what about PWM and speed control? You probably want this in order to ramp up the motor to speed smoothly (a 750W motor suddenly connected to its full supply will give a hell of a kick, its not recommended - apart from anything you can chip any gears).

Yeah I'll try and use PWN. I found this article great (it talks about controlling high current with a MOSFET). http://bildr.org/2012/03/rfp30n06le-arduino/

MarkT: The stall current will be whatever the supply can give, I suspect the motor can pull 100's of amps at stall if allowed. That will explode your MOSFET so make sure the supply current is limited to 30 or 40A and ramp up the drive to avoid these startup surges.

Oh ok thanks, what's the easiest way to limit the current?

MarkT: With those reasonable high voltages and currents you almost certainly need to drive the MOSFET gate with a MOSFET driver chip (otherwise drain-gate capacitance could cause the gate voltage to go out of limits and blow up the Arduino. In fact that MOSFET isn't logic level so you'll need a 10..12V supply and driver circuit anyway. I've used the DIP version of the MIC4422 driver before - its very rugged. Connect the Arduino pin to the input of the driver with a 4k7 resistor and you'll save the Arduino from damage should something go wrong with the high-power devices (this is a real risk with prototyping high-power circuits).

Oh really? So I can't connect and control the MOSFET directly from my arduino? Or is the MOSFET driver chip just extra protection for the arduino?

Thanks! Tyler

themotorman: You can use a transistor to control a motor but I suggest that you check out the chips that are designed to handle the PWM aspects. Do a search for motor control chips. It is very, very important that you use a chip that has pulse by pulse current control. This is because without it the current, when the motor is not yet turning, will be very high and then when the pulses turn it off the voltage produced will blow your transistor. If you just want to turn it on or off without any speed control then a MOSFET will do the job but it needs protection from the high voltage on turn off and the inrush current when turned on. Easier and best to use a dedicated chip and use the Arduino to send a control signal to the chip.

Oh hmmm. Thanks! Yeah, I know what you mean (I think) :D How do I check to see if it can handle PWN?

I've been looking for a cheap motor controller but they seem really expensive, so thats why I've looked at MOSFETs.

Thanks Tyler

Ok, I'm not sure how to work it out?

Have you seen this? http://www.thebox.myzen.co.uk/Tutorial/Power.html and http://www.thebox.myzen.co.uk/Tutorial/Power_Examples.html