Anyone have a way to control the startup current on a 180vdc motor? Its the thing I can't figure out

Discussion in 'General Electronics Chat' started by BigAlscorpio, Mar 26, 2016.

  1. BigAlscorpio

    Thread Starter Member

    Feb 5, 2015
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    Hi to all,

    I have a treadmill motor from a mate that I want to use on my lathe with speed control. It is 180vdc brushed perm mangnet (2 wires only) and 2.5hp continuous rated. I can do the control of speed ok using a pic micro and a pot driving PWM to a FET Driver and then 600v 40A FET without issues and can probably cobble together some kind of feedback for speed stability under load. BTW I'm in the UK and power here is 220vac.

    My problem is that the startup current from static on the motor is hundreds of Amps, seeing as the windings are only about 5ohms. The motor is happy to run on unsmoothed full bridge rectified dc and the PWM works as it should but only with a 10 or so ohm resistor in series or the fet would die immediately on startup.

    I know that many industrial motors have multiple series resistors that are switched in as needed mainly by mechanical means but I aslo know that on the original treadmill boards this is not how it works. I can't find out how the treadmill boards manage to limit the startup current and there seems nothing on the web about it. Anyone able to help?

    Regards, Al
     
  2. mcasale

    Member

    Jul 18, 2011
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    The DC motor is rated for 180V and the windings are 5 Ohms. This gives a rated DC current of 22.5 amps. Correct?

    The line voltage (220VRMS), when rectified gives about 311VDC. So a total of 15 Ohms in series gives a maximum DC current around 20 amps. Makes sense so far.

    The "startup current" is limited by the L/R time constant of the windings.

    If you can drive the winding with current, instead of voltage, like a chopper drive (PWM) it'll be a lot more efficient. I know Allegro has some H-bridge drivers that can drive these kinds of currents. They require external power MOSFETs, and some tricky PCB layouts, but they work.

    Do you need to reverse the direction of the DC motor?

    Have you considered this alternative?
     
  3. BigAlscorpio

    Thread Starter Member

    Feb 5, 2015
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    Hi mcasale,

    I never considered H bridges as the motor will only ever go one way. I have FETs which will easily handle the current once the motor has started spinning but the startup or stall current is prohibitive, once the back emf is present then the current would be managable but I can't get that far without the series resistor. Once the motor is spinning the resistor is no longer a help but a hindrance meaning I can never get full torque and the resistor also needs to be massive to dissipate the running power, the largest I have is 10w and it gets very hot in only a few seconds so it needs to be switched out somehow. That is the only thing I can think of but it does mean that I would maybe need to use another FET with the resistor in series with drain and after a short time of starting with the resistor then switch to a FET that is directly connected to the motor for full power? The PWM would be strange to work out mathematically but maybe I could ignore calculations altogether and just go for trial and error for the switchover times?

    Further thoughts?

    Al
     
  4. tcmtech

    Well-Known Member

    Nov 4, 2013
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    The simplest way to control the speed on a line level voltage DC motor liek that is to use a common incandescent light dimmer in series with a full wave bridge rectifier.

    Dead simple and very easy to control the starting current and speeds that way.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The only problem with Triac or SCR bridge type controllers in a lathe app is they tend to be noisy at lower rpm's and not that smooth, the PWM is what most T.M.'s use now, you just have to remember to zero the speed control before starting, T.M. controllers have a built in protection that the speed has to be set at zero before it will start. Especially because they also have a large flywheel as well as a person possibly standing on the belt.
    The Pic will handle the zero start by programming a ramp up to desired speed.
    Incidentally using it for a spindle, make sure you remove the flywheel if fitted.
    KB make drives suitable for these motors see ebay listings.
    Do NOT use series resistors!
    I have also built them based on a H bridge PWM picmicro driven.
    Max.
     
    Last edited: Mar 26, 2016
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    :confused: I make the theoretical max current about 1.4*230V/5Ω = 65A. That should be manageable with a half-decent FET.
     
  7. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Soft start of your PWM can also be used. The motor's inductance limits the current with short pulses.

    John
     
  8. BigAlscorpio

    Thread Starter Member

    Feb 5, 2015
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    Hi Guys,

    I have a large IGBT - IGBT G80N60RUFD (80A 600V) which I think I can substitute for the FET with few changes?

    I already have a kind of soft start implemented in code where the PWM ramps up from 0 to whatever the control pot is set at in about two seconds or so. Is that suitable?

    Al
     
  9. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    I used a dedicated chip (TPIC2101) with a 12-V treadmill motor. As I recall, the soft-start ramp was 2 seconds. Never had a problem with that part.

    John
     
  10. BigAlscorpio

    Thread Starter Member

    Feb 5, 2015
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    Hi John, I am using a dedicated FET driver but mine is the MAX627.

    I think I will go ahead and try one of the big 80A IGBTs in and see what happens.

    Anything I should know beforehand?

    Al
     
  11. jpanhalt

    AAC Fanatic!

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    I have not used an IGBT. The only problem I had with that drive (1/2H, not full H) was that it could start with the mosfet's in an unknown condition or on (my mosfet driver was an LT1158) . That was cured by adding a turn-off resistor.

    John
     
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I recently did an empirical test on a 2HP 120vdc 18amp max T.M. motor, I used a 0-180v variable supply to gradually increase the voltage/rpm from 0 to maximum.
    No load and no flywheel fitted and the motor connected directly to the DC supply.
    At approx. 10v the current was 2amps, as the voltage was gradually increased the current remained at 2amps right up to the full voltage at maximum rpm.
    This obviously showed the effect of the BEMF voltage generated by the motor.
    At this point the current will only increase when a load is applied.
    For the PWM H bridge I used IRF9540 Mosfets.
    Max.
     
    Last edited: Mar 27, 2016
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The way to measure the current at power on is to find the armature resistance which should be measured by a voltage applied to a locked rotor and a known low fixed voltage applied and measure the current, a simple resistance check is not accurate enough.
    Max.
     
  14. BigAlscorpio

    Thread Starter Member

    Feb 5, 2015
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    Dammit! Just blew the 80N60! just the first second of switchon was enough to blow it, no smoke or heat but its now an expensive jumper.

    Looks like maybe I will have to try measuring the actual current as Max said. Can you recommend a voltage to test at? Would a 12v Pb battery do or do i need 5v or whatever?

    Al
     
  15. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Did you use protection diodes ?
     
    jpanhalt likes this.
  16. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Or protection "mosfets". In my case, high-current mosfets were smaller and cheaper than diodes; although, they performed the same function.

    John

    @R!f@@ Are you really GMT +5? Have you moved from the Maldives?
     
  17. ramancini8

    Member

    Jul 18, 2012
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    Use a thermistor of some type to limit the start up current, and will decrease in value after it warms up. This is done in BDC washing machine motors.
     
  18. BigAlscorpio

    Thread Starter Member

    Feb 5, 2015
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    Hi Guys,
    Yes I used a fast schottky across Motor pos to Collector on IGBT. I must have miscalculated the current somehow. Can there be a massive difference measuring the motor resistance and working it out by the locked shaft and power method?

    Maybe the motor is a rogue that just wants to rebel. What gets me is how FETs can die so fast and never show a sign of damage!

    What next?

    Al
     
  19. BigAlscorpio

    Thread Starter Member

    Feb 5, 2015
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    Hi Ramancini8, think our posts crossed so I only just saw yours. I never used a Thermistor before so I did a little research on them.

    I can't seem to find any that change resistance fast enough for this job, probably less than a second would suit me but all the ones I found were in the tens and hundreds of seconds and the math that accompanies them is quite complex too, I am no mathlete. ;)

    If you know of any that are fast acting enough I would gladly try this approach, or any other ideas flying around out there.

    Al
     
  20. R!f@@

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    Apr 2, 2009
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    @jpanhalt
    Nope...why do you ask ?
    Our time difference has always been UTC+05:00.
     
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