Tachometer (RPM Feedback)

General - Installation - Troubleshooting

The force of magnetic fields interacting drives all electric motors. How the magnetic field is created on the moving rotor determines the type of control required on the stationary stator to produce the desired end result. In the case of a hollow ware glass production line, maintaining synchronization requires either multiple synchronous motors connected to the same power source (Synchrotie and Turner drives are examples of this), or some type of feedback sensor on each motor supplying information to a control algorithm. Using synchronous motors connected to different low horsepower inverters is not sufficient without some type of control scheme.

By way of example, the first multi-inverter drive supplied by Emhart required the “ring counters” of the Emerson inverters be slaved to one master. The problem with this concept was the non-standard product from Emerson and not being able to substitute a different vendor’s inverter into the drive system.

Once you select any drive using technology later than the 1970’s and 1980’s, a feedback sensor is required. It may be called a tachometer or it may be called an encoder (on the back of servo motors, but a sensor is a fact of life.

We have has chosen a digital tachometer because time is one of the easiest physical quantities to measure. Gone are the errors associated with amplifier gain settings, temperature drift, etc. The magneto-resistive sensor chosen (Red Lion #MP37-CA) or the sensor with a mounting kit for NEMA dimensioned motors has no electronic parts. It is a permanent magnet and a coil of wire, so also gone are the temperature failures associated with lack of cooling. Keep the sensor as cool as the motor and the insulation of its coil of wire will not fail. To find a local Red Lion source for your area click on the following link: www.redlion.net/Distributors.html

Installation

The Red Lion pick-up is designed to generate an AC voltage as a steel target passes its face. The amplitude of the voltage is proportional to the rate of change of magnetic flux passing through its coiled wire (exactly the same principal as a generator). Passing the steel target through the sense area more quickly will increase the rate of flux change by decreasing the time. Placing the target closer to the sensor will increase rate of flux change by increasing the amount of flux to be changed.

Setting: A 0.005-inch (0.13 mm) gap from the pole piece of the sensor to the top of the highest gear tooth will generate a minimum of 0.5 volts (AC) at 200 RPM. This will increase to approximately 5 volts (AC) at 2000 RPM. The amplifiers on the Synphase card will handle 0.25 volts or lower.

Gear Installation: Securely attach a 20 to 60-tooth 3/8-inch (10 mm) face spur gear to the motor shaft near the motor face. More than 60 teeth requires too many interrupts at 2500 RPM and less than 20 teeth gives poor resolution at less than 100 RPM. Synphase is default programmed for a 60-tooth gear. If you mount a gear with a different number of teeth, just alert the Synphase controller by changing the variable “J” in the “Customize Card Window”.

If you do not want to provide your own gear, Red Lion offers a 60-tooth spur gear as part of a mounting kit. However, their gear only has a small setscrew to secure it to the motor shaft. We recommend increasing the Red Lion mounting setscrew to ¼-20 NC (6mm) and possibly adding a second setscrew 90 degrees away to prevent loosening and spinning caused by vibration.

Sensor Installation: Position the sensor with a 0.005-inch gap to the highest tooth of this gear. Red Lion provides a kit with mounting hardware for this. Their kit is designed to flange mount between the “C”-face of a NEMA-B motor and the motor bell of a gearbox. A longer coupling between the motor output shaft and the gearbox input shaft is all that is required. The problem with this kit is that it requires you to use a NEMA-B motor instead of a DIN dimensioned one.

To find a local Red Lion source for your area click on the following link: www.redlion.net/Distributors.html

In the situation where open belts are used in a “C”-face motor instead of a gearbox, removing up to ¾ of the ring can reduce the overhang of the pulley load. Simply band saw cut away the unused part of the ring and pass the timing belt through the newly obtained free area. The only part of the ring that needs to remain is 2 boltholes and the part of the ring connecting them and having the sensor mount area. This was done in the specific example of the Feeder.

The pictures below shows one solution when the customer did not want to use a “C”-face motor and had a DIN dimensioned motor available.

Click image to enlarge
Click image to enlarge

Troubleshooting

Before doing anything else, observe the yellow LED next to the Tachometer connector on the Synphase control board. It is in series with the input LED in the tachometer opto-isolator. If it flashes with each sequential gear tooth passing under the sensor, it is a cursory, but not definitive, indication that the sensor is functioning, gapped properly, connected, and that the isolated power supplies and amplifiers on the Synphase board are working properly.

Next, look in the central part of the diagnostic window for the motor appearing to give trouble. You should already be familiar with these displays. If not, they are explained below.
  • Look at the calculated RPM.
  • Compare it to the measured RPM.
    • Use this comparison for general indication only. The displays are not accurate enough to observe synchronization. However, if the comparison shows that the motor is being controlled and the motor appears to be running at the wrong speed, the fault could be with the setup. Download the card setup in the “Card” window and confirm proper settings. Also ask machine repair if they just replaced anything on the mechanism.
  • Note the value on the Missed Pulse counter
    • Each and every tooth passing under the sensor generates a hardware interrupt. For a 60-tooth sensor gear with the motor running at 2500 RPM, the microprocessor performs a calculation every 0.000 4 seconds. If this repetitive calculation is not preformed within 175% of the allocated time, the Synphase software assumes that the pulse was not present instead of assuming the machine went to less than ½ speed in less than 1 millisecond.
  • Note the value on the Noise Pulse counter.
    • Each and every tooth passing under the sensor generates a hardware interrupt. For a 60-tooth sensor gear with the motor running at 2500 RPM, the microprocessor performs a calculation every 0.000 4 seconds. If this repetitive calculation is preformed a second time within 75% of the allocated time, the Synphase software assumes that the pulse was generated by noise instead of assuming the machine went to doubled speed in less than 1 millisecond.
  • Note the value on the Heaviest Loading register.
  • Note the value on the Largest Overhauling register.
  • Zero the registers.
  • Observe how long and by how much much the registers change value after resetting.
    • These 2 registers capture phasing excursions as well as synchronization fluctuations. After the motor has been running a while, the phasing excursion should be less than 0.020 seconds and should first be visible in the bottom 1/3 of the window. Loading and unloading of cams will cause some fluctuations and should be compared to historic values. If you are not familiar with this display, it will be cyclic with the mechanisms cycle and around 2-10 milliseconds large. Things to look for here could be a bad bearing on the mechanism, an incorrectly set inverter if just changed. An undersized motor or inverter.

Before continuing, you may want independent confirmation of what the motor/tachometer is doing. Attach the leads of a digital multi-meter to the tachometer either at the motor or at the entry termination to the control cabinet.

  • Measure the AC Volts. It should be 0.5 or more.
    • If it is less, the sensor may have too large a gap. Check the gap setting between the sensor and the pick-up gear. The setscrew could have come loose allowing the sensor to move.
    • Check the pole piece of the magnet in the center of the sensor. If chipped, it will disperse the magnetic flux lines and lead to low voltage.
    • Check for iron or steel dust covering the pole of the sensor’s permanent magnet. If the magnetic flux lines are shorted out, little or no voltage will be generated.
    • Check the wiring from the sensor to the entry point of the cabinet. The easiest way is to measure the resistance when the motor is not running. The sensor is approximately 300 ohms. A reading of zero ohms indicates a short and a reading of infinite ohms indicates an open.
  • Measure the frequency on the tachometer. There are 60 seconds in a minute. If the tachometer gear has 60 teeth, the frequency read will be the RPM of the motor. (i.e. 751 Hz is 751 RPM) Use this to confirm the RPM reported by the Synphase troubleshooting window.
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Tachometer Specifications.pdf
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