Controlling BIG DC motors using "H" bridge & MOSFETs
John Bond
Posts: 369
I want to switch a “REAL” motor – a 24 volt cordless drill motor. I intend using an “H” bridge and PWM. ·Attached is an example of a powerful (yet surprisingly small) DeWalt cordless drill motor. Note the 190 Amp stall current.
http://www.robotcombat.com/products/BP389010-00.html
1. ·Can I use an ULN 2803A to switch the MOSFETs? Are they fast enough for PWM?
2. ·What are suitable MOSFETs? Exotic components are hard to find in Africa so can I put several smaller MOSFETs in parallel on each leg?
3. How do I read the power consumed by the motor when using PWM? I intended using a resistor in the ground leg but even a 0.01 Ohm resistor will produce a lot of heat!
4. How do I do regenerative braking using an “H” bridge? This isn’t too important but I am interested in learning.
4. Is there a simple way to count revolutions without using a rotary encoder? I have encoders but they are as big as the drill gearbox and are difficult to mount.
Kind regards from Darkest Africa
John Bond
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http://www.robotcombat.com/products/BP389010-00.html
1. ·Can I use an ULN 2803A to switch the MOSFETs? Are they fast enough for PWM?
2. ·What are suitable MOSFETs? Exotic components are hard to find in Africa so can I put several smaller MOSFETs in parallel on each leg?
3. How do I read the power consumed by the motor when using PWM? I intended using a resistor in the ground leg but even a 0.01 Ohm resistor will produce a lot of heat!
4. How do I do regenerative braking using an “H” bridge? This isn’t too important but I am interested in learning.
4. Is there a simple way to count revolutions without using a rotary encoder? I have encoders but they are as big as the drill gearbox and are difficult to mount.
Kind regards from Darkest Africa
John Bond
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Comments
The motor choice is quite interesting because it is really geared for heavy torque. I don't see why you couldn't use UNL2803 with propfer design to drive MOSfets. In fact they may be useful to allow for moving up to the 24 volts that the motor wants.
You may want to include logic that prevents the H-bridge from burning up when changing direction. Software code off includes a 'forbidden state' as each half bridge has to be on or off. If both are on, damage occurs. Some refer to this as a 'smokeless' H-bridge design.
Some kind of current sensing and shut down would likely prevent damage to the bridge. 190amps is really up there. Even a fuse or circuit breaker would be better than nothing.
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It's sunny and warm here. It is always sunny and warm here.... (unless a typhoon blows through).
Tropically, G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan
They are available from Farnell and can handle 79 amps each, but don't forget the heatsinks.
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www.fd.com.my
www.mercedes.com.my
I have done a quick drawing to see if I understand what you’re proposing…
John Bond
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To measure current, instead of inserting "another" resistor, use the Rds "ON" resistance of your mosfets.· Depending on which mosfet leg is active, will determine which leg that you read your voltage from to determine your current draw.
·
Also, use something like this for your H-Bridge topology... the one you have can create an H-Bridge short if the Control inputs are floating, and/or when the processor starts up.
H-Bridge Example 1
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Post Edited (Beau Schwabe (Parallax)) : 7/23/2008 4:03:32 PM GMT
Also - Does anyone know how one does regenerative braking?
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You should be able to PWM that circuit.
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As far as regenerative braking goes.. the reverse biased diodes on the mosfets automatically take care of excess voltage in free-wheel-mode and direct it back into the power supply.· Only in a free-wheel coasting mode do you have any regenerative effects using this already built-in method.
·
Shunt braking (shorting the motor) can be done with a bridge rectifier and a single mosfet transistor.· The AC terminals of the bridge rectifier go to the motor leads, while the negative terminal goes to the Source on an NMOS transistor and the positive terminal goes to the Drain on an NMOS transistor...· By varying the amount of PWM you apply·to the·NMOS transistor, you can control the amount of brake that's applied.
There are no regenerative characteristics of Shunt braking, unless instead you do a power conversion of some kind sacrificing Amps for Voltage to a level that can be re-applied into the power system.
·
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
I did some quick work this weekend with a smaller cordless drill motor... Extrapolating the numbers seems to show that their claims on the DeWalt Drill are about right.
The 12V drill ran at 12 volt with small load (additional gearbox) and 0.6A. (these motors seem to be power hungry at no load). At startup, the Amps were right off the dial (10 Amps?)
The motor was also quite powerful
Torque Approximately 0.025Nm @ 400Rpm and 6.2V @ 3.2Amp. So 20 Watt electrical = 10Watt mechanical after drill gearbox or 50%(Remember this is well outside it's optimum design envelope!!! I would expect efficiency to increase to about 80% at design Volts/Amps/RPM)
The motor also enjoyed being driven PWM at about 20% overVoltage and a small duty cycle (15%). Using 20% Overvoltage should give 40% more power for brief spurts if needed.
I am comfortable designing round their specs derated by 30% but my requirement is only 50% (allowing for efficiency of the additional gearbox of 75%) so I should be OK...
@ Beau
Thanks again for your valuable advice.
OH - Sorry about the delay in responding but someone stole a 110Kv power cable pylon so we've been without electricity. Local power pylons are made out of a low grade stainless steel which fetches a good price on the scrap metals market. Hey - this is Africa... (I am starting to find the incredible violence and crime VERY depressing!)
John Bond
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· - Can handle 400 Amps for 30 seconds and 200 Amps continuous and up to 48 Volts.
· - Has good protection against “Shoot through” either due to a logic/wiring error or due to the MOSFETs own capacitance.
· - Has a built in Charge Pump to drive the top MOSFET drivers.
· - Has protection against almost any form of voltage spike.
· - Is as bullet-proof as a US battleship.
· - Has been built and tested by hundreds of robot builders.
The only failure I can find is if the voltage drops below about 8 volts, the system oscilates and this can distroy the electrolytic caps.
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But hey, I’m not out to convince you, you hold onto your views on cordless drills and I’ll keep mine…
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I made up 25 boards, but haven't done much with it of late. If anyone cuts a wire, the whole system is immediately aware and different kinds of sensors can be used with different microprocessors at each node. It works with BasicStamps or anything via SPI.
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It's sunny and warm here. It is always sunny and warm here.... (unless a typhoon blows through).
Tropically, G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan
Electronic Security is a leading industry in South Africa. You could sell your device here. Most of the perimeter alarms are wireless. Short distance radar is also used quite a bit. The modern inexpensive vehicle tracking system ·was developed in South Africa and the leading·company internationally is South African. It is now fitted to all sorts of things like large plasma screen TVs used for advertising, ·computers, shipping containers full of merchandise and even solar panels.
Taking down power lines is quite an art. About a dozen people get fried each year practicing their art. They tie a long stout nylon cable some way up the pylon and attach it to a light truck. They undo the 48 bolts on the legs of the pylon and pull it over with the truck. So many of them get killed that the local press doesn’t report the deaths unless they are particularly freak. The last one was where they tried it just after rain and the nylon cable was wet. The photos were gruesome; the human body bursts open at those voltages. Here is an amusing newspaper·headline…
Cable-theft epidemic wreaks havoc in SA
South Africa has no copper mines, but copper exports to China are booming: the result of a cable-theft epidemic that regularly plunges whole suburbs into darkness, strands thousands of train passengers and is wreaking havoc with the national economy.
http://www.mg.co.za/article/2007-06-20-cabletheft-epidemic-wreaks-havoc-in-sa
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These controllers work really well. I wrote most of the assembly docs for this board and the original MOB controller. The original doc was just a parts list and is all I had to start with. These were one of the first projects where I had to learn how to install some surface mount parts. I used a pair of these on a Middleweight BattleBot years ago and they've held up well.
I know that several people have built the board with only a single FET for each leg if that is all they needed for a smaller motor.
Robert
Do you know any alternative supplier for the PCB? Robot Power want US$29 for each board and I'll need a couple.
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That link seems to no longer exist.
for what it is worth . a few 100 amps is not to high . 2kA is high . Also IR makes some sweeet 180A rated TO 220 fets that are Logic level . they work like a charm. Peter
Sometimes things change over 5 1/2 years.
But the TO220 package cannot handle 180A! (Yes I know datasheets often give
the silicon values rather than bond-wire and package limits, marketing spin)
For high power I'd not consider using a logic level MOSFET as I'd have a decent
MOSFET driver chip holding the gate stiff against the gate-drain Miller capacitance,
and those drivers all work at 10 to 12V. You get better immunity from ground-bounce/
ringing on the source lead inductance too if the gate threshold is higher.
2kA
however that Low Rds On was soo sick! . cool as ice .
as far as hold on . I was at many KHz of PWM so the gate was allwas being refreshed so often the back leakage was never a issue .
Mr Bond perhaps a modded drill speed con and then a relay for the swap might be OK at the reverse speeds you need .
So to the person that revived this thread, I suggest you look a newer solutions than what John Bond had in mind. Also, there is a lot ready made. I recently saw an Arduino H-bridge shield rated for 2/3 HP and reasonably priced. It may not be worthwhile building one from scratch. It may be more worthwhile to adapt that Arduino ready-made to Propeller inputs -- not a hard thing to do.
IGBT Modules at Digi-Key
If one of those claims 600A, they really mean it. I'd recommend the second-cheapest FET module on the list, APTM10HM19. Rated at 100V, 70A continuous, 300A peak.