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Relay question

  1. Feb 23, 2012 #1
    I've encountered a problem that I can't seem to make any logical sense of. Hopefully someone here can help!

    I have a 24VDC motor with an attached electric brake to prevent back-driving. The motor is controlled with a 5V analog joystick. The joystick has a button that needs to be pressed to activate the motor circuit. I want the same button to also release the brake on the motor.

    I've installed a relay that has a 5-24VDC control voltage (Part #9938K53 from McMaster Carr). I have the relay wired such that pressing the button applies 5V across the control circuit. This closes a switch that applies 24V to the motor brake. The other wire on the brake ties back into ground (see attached diagram).

    When I connect the brake to the relay, the brake won't disengage. I've check the brake separately by connecting it to a 24V battery, and that works. I've checked the relay without the brake attached, and the output wires show 24V on my multimeter. However, when I attach the brake, the wires shows exactly 0V on my multimeter and when I press the button it only shows 3.3mV.

    I can't seem to figure out what the problem is, and appreciate any help or advice anyone can offer.

    Attached Files:

  2. jcsd
  3. Feb 23, 2012 #2
    Your description and diagram are a little unclear. Is the brake connected to the motor electrically? If so, how.

    With motor-brake systems I have worked with, the brake was connected in series with the motor. This means the brake must have very low impedance. In your case, it doesn't sound like the brake has low impedance so could it be connected across the motor? How is the brake activated to prevent back-driving?
  4. Feb 23, 2012 #3
    The brake is basically a solenoid that grips on a shaft sticking out of the backside of the motor. There are 4 wires that come from the motor. 2 for the motor and 2 for the brake.
  5. Feb 23, 2012 #4
    My initial reaction is that you should use a DPDT relay instead. One pole for the motor, the other pole for the brake.
  6. Feb 23, 2012 #5
    I'm not sure you understand the circuit well enough to effectuate a solution. Do you know how backdriving the motor causes the brake to be applied?

    There is a good reason for connecting the brake in series with the motor. If the coil or wiring to the brake should open for any reason, the brake will automatically be applied and power removed from the motor which makes it failsafe. This is not true for a shunt connected brake. Do you know how your circuit is supposed to work?
  7. Feb 24, 2012 #6
    The motor in question is a parallel gear motor that attaches to a pinion, which mates into a rack to traverse a platform. Without the brake applied, someone can grab the platform and turn it freely. The motor will not resist this motion. To prevent this, we added a brake to the motor which will lock the shaft and pinion so the platform can't be moved.

    Pardon my ignorance, but what is a DPDT relay?

    I can't seem to figure out why I read 24V on the two wires before I connect the brake, and that voltage disappears when I connect the brake.
  8. Feb 24, 2012 #7
    It means Double Pole Double Throw. The contacts can control two independent circuits and each circuit can select two different positions. I don't understand why a DPDT relay would be needed.

    It is likely that the brake, which was intended to be connected in series with the motor, is essentially a short circuit. I urge you to try that arrangement to see how it works.
  9. Feb 24, 2012 #8
    If there is no voltage applied to the brake, it is engaged. Once 24V is applied, the brake disengages.

    In my application, I want the brake applied unless the motor is moving. Pressing the button on the joystick activates the motor controller, and is wired to the relay to release the brake at the same time.

    I have touched the wires that control the brake directly to a 24V battery, and you can hear the brake click, so I know the brake works. I can't figure out why applying 24V via a battery works, but applying 24V via the relay does not.
  10. Feb 24, 2012 #9
    First of all, you are not applying 24 V to the brake because as you said when you do connect the brake across a 24 V battery, you see zero volts across the brake. This is because the brake is a short circuit and is operated by the current through it, rather than by the voltage across it.

    If the brake is in series with the motor, when you apply voltage to the series combination, current flows through the brake and the motor. The brake releases and the motor turns. Once you remove voltage, current stops flowing and the brake applies, stopping the motor.

    While you still need the relay between the joystick switch and the motor-brake circuit, you don't need two separate circuits for the motor and brake. The brake was designed to be connected in series with the motor.

    Edit: If your joystick controls the motor controller directly, you probably do not need to use a relay at all.

    In the previous quote you mention a motor controller. If you are trying to activate the brake with a motor controller instead of a battery, the motor controller may not have enough current capability to activate the brake, especially if it is also supplying current to the motor. Many motor controllers have over-current limits and shut down if the current limit is exceeded. That is to prevent the motor from burning up should it become stalled.
    Last edited: Feb 24, 2012
  11. Feb 24, 2012 #10
    When I hook the brake up to the battery, I do see 24V across the 2 wires, and the brake disengages as expected.

    When I hook the brake up to the relay, I see no voltage across the brake and it won't disengage. If I disconnect the brake wires from the relay wires, I see 24V across the wires coming from the relay.
  12. Feb 24, 2012 #11
    Does the relay connect the brake to the battery or the motor controller?
  13. Feb 24, 2012 #12
    The relay connects the brake to the battery, and is controlled by the 5V button on the joystick.
  14. Feb 24, 2012 #13
    When the relay closes, do you have 24 V on both sides of the relay contact?
  15. Feb 24, 2012 #14
    I have one wire into the relay from the +24V side of the battery. The other wire goes to the + side of the motor brake. Then the - side of the brake goes to the - side of the battery.
  16. Feb 24, 2012 #15
    Is this what you have?
    (1) +24V --> (2) relay NO (Normally Open) contact
    (3) Other NO relay contact --> (4) + brake lead.
    (5) - brake lead --> (6) -24V.

    If it is, when the relay is activated and the relay contact closes, do you still have +24V on points (1), (2), (3) and (4)?

    Do you also have zero volts at points (5) and (6)?
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