Electric clutches and brakes sometimes fail for lack of proper electrical-suppression circuitry. But it’s not difficult to design a suppressor
The past decade has seen a growing trend toward powering electromagnetic clutches and brakes with programmable controller outputs. Clutch and brake manufacturers make power supplies and controls for their products that are designed to integrate cleanly into many applications.
But some applications need only a programmable controller to do the job almost as well as the OEM hardware. It is the “almost” in that statement that is the area of concern. Before proceeding, let me provide a quick review of electromagnetic clutch and brake operation. Electromagnetic clutches and brakes work on dc power. Power supplies convert ac power to dc power through a rectification circuit. Typically these supplies provide an on-off function: either full power or no power.
The most common supplies produce an output of 24 or 90 Vdc. Clutch and brake controls not only provide the ac-to-dc conversion, but may also provide a way to change the output voltage applied to the clutch or brake. Applying less than full voltage to an electromagnetic unit creates less than full engagement force within the clutch or brake, with lower torque transmitted through the device. For a given load, lower torque means longer engagement times which may be desirable. For example, slower acceleration can help keep loads upright or prevent a spill induced by sudden starts and stops.
Power supplies or controls built by electric clutch/brake manufacturers contain a suppression circuit. This circuit provides two functions. The first is to protect the electronics in the system from the inductive spikes generated when coil power abruptly shuts off. Its second function is to make the clutch or brake respond more quickly to the controlling voltage.
When current flow is abruptly cut off in an inductive circuit, the collapsing magnetic field of the inductive coil generates a high voltage spike. This voltage spike (VS), sometimes incorrectly called a back electromotive force, can reach 7 to 10× the value of the applied voltage. A coil that operates on a 90-V supply could easily generate a VS of over 600 V. Over time, a voltage spike that high will break down insulation and damage electrical components. A simple suppression circuit that limits the VS to the supply voltage consists merely of a single diode, sometimes referred to as a flyback diode, placed in opposing polarity across the winding of the brake or clutch.
However, the addition of a flyback diode maintains a current flow through the clutch for a short period after removal of the applied voltage as the magnetic field of the winding collapses. This extends the time the coil remains “energized” after power is removed, delaying the response of the clutch or brake to the removal of power.
The energy stored in the magnetic field of the coil must be dissipated quickly if the device is to respond quickly. A resistance inserted in series with the coil and flyback diode can dissipate the energy, but at the cost of raising the VS.
Another option is the addition of a Zener diode to the suppression circuit. The Zener diode acts as a variable resistor, changing its conductance value to limit the value of the VS. Overall, the Zener shortens the decay time needed to dissipate the magnetism within the clutch or brake. The lack of a proper suppression circuit extends the release time for the unit leading to possible damage and a shorter life as for example in cycling applications, which may see inaccurate starts and stops as well as aggressive wear of the clutch or brake.
Programmable controllers, by the nature of their versatility, do not contain this suppression circuitry. The solution for this is quite simple. When a clutch/brake is used with a programmable controller, designers should include the suppression circuitry as needed in the wiring harness of the machine.
The process of choosing the proper flyback diode for the suppression circuit is quite simple. First, determine the current the clutch or brake demands. For example, the Warner ERS-42 electrically released brake needs 0.239 A at 90 V to release its total maximum holding torque of 7 lb-ft. The specifications of the flyback diode should, at a minimum, double those values: a continuous forward current (If) of at least 0.478 A with a peak inverse voltage (PIV) rating of 180 V. A common diode used in this application is the 1N400x Series. The 1N4004 has an If of 1 A, with a PIV of 400 V, or four times the demand of the brake.
Low-voltage brakes usually need more current to operate. The release current for the 24-V version of the ERS-42 is 0.973 A. So it needs a diode that can handle at least 1.946-A If. In that case, switch to the 1N540x Series — specifically the 1N5404 with a PIV of 400 V and an If of 3 A. Note that the 1N5401 would also work in this application, though its PIV rating is only 100 V. That is still over 4× the 24-V applied to the brake. The cost of the 1N5401 and 1N5404 diodes are typically the same and the 1N5404 can serve in place of the 1N4004 if necessary, even if its physical size is slightly larger. A higher PIV rating does not affect circuit operation in this application.
The choice of Zener diode for the suppression circuit does take a little bit more work. Zener diodes remain in an open-circuit or high-resistance mode until the voltage applied to them equals or exceeds their Zener voltage (VZ). So a 90-V device should use a Zener diode with a VZ slightly over 90 V.
When selecting the current rating of the Zener diode, one must use the maximum Zener surge current (IZSM). The IZSM rating is the maximum current the diode can withstand in Zener mode for a given amount of time. The table shows that the ERS- 42 brake takes approximately 20 msec to engage when power is lost. So the Zener must withstand an IZSM value of 0.239 A for at least 20 msec. The 1N53xx Series of Zener diodes seems to fill this need.
The 1N5378 has a VZ of 100 V and its IZSM can handle 2.7 A for 8.3 msec. This IZSM is more than adequate to handle 0.239 A for 20 msec. If a 24-V device is used, then a proper Zener diode choice would be the 1N5368. The Zener and flyback diodes are wired in series cathode-to-cathode, and then across the winding. The anode side of the Zener diode connects to the positive voltage terminal of the coil.