Though the overall function is the same as analog drive, none of the signals can directly be measured. Also none of the pots can be adjusted. All adjustments are done on the control in the form of various parameters. The same tach can be used for positional and speed feedback. In digital drive, there will be one bus carrying all the commands from the control. This bus can be anything like a two high speed serial bus or optical bus or some kind of custom bus of the drive maker. The feedback from the Digital pulse coder or Tach can go to the drive. In some of the fanuc designs it will directly go to the control.

1) If a drive is suspected just a simple switch which addresses the drive can be changed and find out if the problem switches to another axis. ( In Mitsubishi)

2) Some drives do have parameters stored in the drive itself. When a new drive is put in these have to be loaded; or else the drive will not function right. In Siemens Vickers and 611D controls these parameters are downloaded to the drive.

3) In most cases the gain and balance etc are adjusted in the control.

4) VGN is velocity gain which acts as gain in analog drive

5) PGN is positional gain. This comes in to play when there is a positional problem.

6) The best way to handle this kind of situation is to have a backup of parameters. It helps to know the default parameter from the book.

7) If it is humming, there is a parameter to suppress a particular frequency. This will help if the motor is not really being overloaded.

8) The key is to always look at the load current. If it is within limits then the machine is OK. In most cases like Fanuc and Mitsubishi it can be looked at the CNC. In some drives like Vacon, ACtech there will be panels to look this up on the drive itself.

AC motor is cheaper and needs less maintenance. It can be single phase (2 wires) or three phase (3 or 4 wires). It has a solid rotor.

Resistance check: Check the resistance across armature, and is should be in single digit Also check the resistance between the ground and one terminal and armature (ground resistance). It should theoretically be fully open (Infinity). But it will be Meg ohms. This check gives a quick idea of whether there is problem. But a Meggar check is the foolproof check for ground resistance, since it generates about 400V AC. When doing a Meggar check there is a chance to damage the circuit to which the motor is connected. Always disconnect the motor from rest of the circuit to have a good reading. There is a good chance there is more than one motor of the same kind in the machine. It is always a good idea to compare it with another good motor. If necessary the motor could be switched, to see whether the problem switches. Remove the wires from the drive if it is connected to one, while measuring.

Current Check: Check the current using clamp on meter and see if it is abnormal compared to the nameplate specification of the motor. It is not abnormal for a motor to draw up to 8% of its rated current, at standstill. Also check for almost equal current on both directions of movement (Axis which moves against gravity will have unequal current).

Heat check: If the motor is warm it is OK. If it too warm or not tolerable by hand-touch, then there is a problem.

Cable check: The cable, which carries current to the armature, should have almost zero drop. Check for any short of the cable to the ground.

What not to do

Ac motors are easy to fix by any local company. But Ask a few questions before you send

1)  Do they balance the rotor? If it is grinding spindle motor, take extra care in sending it out.

2) Most of the places do not check for phasing of the motor to the tach or pulse coder.  Invariably there will be a runaway condition. If it is old Siemens motor with 3 phase speed feedback, send it to Siemens only. This phasing adjustment needs special equipment.

3) Do not beat the motor. In many cases the feedback is unit is at the back.  The glass parts will certainly break.

When a servo alarm occurs in the control, check the drive and trouble shoot from that alarm. When a drive failure occurs, normally OEM wires up in such a way only the power is shut off to the power section and not the control section. So the alarm should still be latched irrespective of what kind of drive it is. Some drive may have a led and some might have a seven-segment display. Take this number and troubleshoot with this information. This can also be looked at the control in diagnostics 200, 201 or 204, which essentially tell what kind of alarm it is.
Any Drive has two sections. One is power and the other is control. Anything to do with current related alarm is in power section. Anytime it is a current related alarm use a clamp on current meter on the wires and keep a watch of the load. Also servo-monitoring screen in the control can be used depending on the control
AC alpha drives with separate power supplies (PSM), convert the AC to DC and supplies the DC power to the drive as input. The drive in-turn inverts DC to AC and powers the motor. There are occasions one might get both a power supply alarm as well as a drive alarm. In that case the power supply has to be detached from the drive and checked if the power supply is OK. There are some machines, which has more than one power supply. In this case, there is front cover, which covers the bus bar, can be removed and made sure which power supply powers the drive
In one kind of Alpha drive (SVU/SVUC) the PSU is built in the drive. Watch the 7 segment to identify the alarm.

Put the machine in the handle mode. Setup a gauge, which can measure 1/1000th movement, against the axis that needs to be checked. If the machine has a scale then it has to be deactivated electrically ( this will take a few parameter changes - Should use only OEM recommended parameters).  The following are the parameters for Fanuc 16*B control. 1240 Reference Point , 1320 Overtraval Limit Plus, 1321 Overtravel Limit Minus , 1815 Bit #1 OPT , 1820 CMR, 1821 Reference, 2024, 2084 SDMR, 2085 SDMR

Otherwise watch the pulley or motor for the amount of jump when moved in opposite direction
1) Backlash Check- Move the axis into the gauge about 10 thousand and move it back. The difference is the backlash. Backlash is the lost movement of an axis when the direction of movement is reversed. Scale is used to avoid backlash, because it is placed to the body of the machine. This can be adjusted by parameters up to a certain extent. Backlash can come from the ball screw, thrust bearing, loose nut. Backlash could be different at different places of the table. Usually the center part of the table will have the most wear. If the backlash cannot be adjusted with parameters and if it changes every time check the tightness of the gib. When you run the indicator on a straight line if it does not stay straight check the gib.
2) Lost motion Check - Setup the gauge as mentioned above and try to push and pull the axis and see whether it comes back to position. The amount it is not coming back is the lost motion. This originates from the same sources as mentioned above. The same way, try to move the axis sideways. If there is looseness, try adjusting the gib.
3) Repeatability checks - Setup the gauge as mentioned above and zero the gauge out. Try moving the axis one-inch away and back. Try moving it 2, 4, 6, 12 inches away and back. If the difference is same every time, it is mechanical. If it is different every time and the difference increases proportionately, than it is electrical.
4) Hand check - With no brakes or motor try moving the ball screw by hand to see if it is free and not binding. Though this is rare, it is always a good idea in some form to check and make sure there is no binding.

To Know:

a) If the Preload is not right on the thrust bearing, or the nut is loose the end, the machine will run one way rough than the other.

b) With the scale on, the backlash will be suppressed. Buf if you watch the pulley, you can tell when you change the jogging direction .0001 the pulley will move a lot more than it would in the same direction. IF the axis moves smoothly with only motor feedback and gets rough with Scale feedback could also mean a mechanical problem. The control is trying to adjust for the big backlash,

c) In older machines like Siemens 3 or 8 or 810 the scales cannot be eleminated. Only in the machine with the Pulse coder and has digital communication from CNC to drives can use the scale elemination functionality. Fanuc 16,18 a, b, i series and Siemens 840 series on, it can be done.

This model of servo amplifier was introduced after C series amplifiers. Some of these can be substituted in place of C series.

This model appears yellow in color like C series. But the face plate will have alpha on it. This model is called  as SVU/SVUC. The model number would appear like A06B-6089-H****/A06B-6090-H****  This model of amplifier has two sections.

A) Power section - 3 Phase AC power is fed in. The three phase bridge rectifier converts AC into DC voltage for the bus.

B) Control section

1) There is electronics, which converts the incmoing digital command voltage signal from the CNC into pulse width modulated signal. This PWM signal fires the inverter to send appropriate amount of AC volatage to the motors.

2) Speed loop - It also has electronics; which will take the feedback from the motor Pulse coder signals and control the speed at which the motor moves.

3) Position Loop - It has electronics which will convert the speed feedback and convert into usable positional feedback and supplies it to the CNC. This circuit is optionally used depending on the switch position of S1.

4) Error detection Circuit - It detects any error and displays in the from of 8 segment display. It also communicates the error to the control and the control will show this information as alarms.

If this drive is suspected the entire drive needs to be changed.  If the status indicator is just a horizontal line (Dash), then the ampifier is waiting for CNC to give a ON command. Irrespective of what control is used on the machine,

• If the drive has a Zero on the display, it is good to go.
• If it has a DASH, it is waiting for enable from the control.
• If it has no display, check the incoming voltages.
• If it has any other number then troubleshoot by that number from the alarm section.

There are two inputs to the amplifier. One is a single phase for control section. This is responsible for the all the controlling electronics and the display.There is a power section of 3 phase which is responsible for the motor voltage/current supply.

There a single amplifier and dual amplifier. When working on the dual amplifier, L and M are two axes. L amplifier uses JF1, JS1B, JV1B, UL, VL, WL. M amplifier uses JF2, JS2B, JV2B, UM, VM, LM.  Once an alarm is identified to that axis, prespective cables and terminals should be used for checking.

Switch Settings: Ther are 4 switches. The opt one is 4 and the bottom one 1. Right side is ON and the Left side is OFF. Switch 1 on means Type B interface. JF1 is for Pulse coder connection JS1B JS2B are used for communicating to the control. Switch 1 OFF means Type A interface. JV1B and JV2B interface. SW2 ON means SVUC and OFF means SVU. Switch 3 and 4 are for Regen Braking.

Battery is connected to JA4. If the control uses TYPE B and it has absolute encoder for positioning, then this battery needs to be replaced periodically. THis needs to be done when the machine is ON.

Troubleshooting C amp

There are 4 types of power going in. L1, L2, L3 should have 3 phase AC going in about 220V. Terminals 100A and 100B should have 1 phase 120V. L1c and L2 c should have 120V ac too. At this stage MC1 and MC2 both should have 24V dc. IF not his drive this bad. If any of the Ac is not present something external is causing it. If all AC and DC is present, bring the drive up and watch the 8 Segment display, the specific drive should get 0 on the display, or else the drive is bad. If it does come up, the cable or the control board on the control are the suspects for alarm DRIVE NOT READY.

a) Check whether there is input 3 phase or 1 phase.

b) Check for the fuses in the drive on the power section.

c) Some machines use a separate 1-phase ac supply to the control section. Make sure they are ok

d) Check all the voltages like 5V, +12V, -12V are ok on the drive.

e) Check for enabling signals, if they are used.

f) Check and see whether there is some DC voltage (0 to 10V) coming in when it is commanded to move.

g) Do the Tach check mentioned in Tach section.

h) Do the motor check mentioned in motor section.

Analog Drives - If there is an alarm look up the alarm to see if it is due to overload, Following error, powersupply or something else. Use the symtom section if you do not have any alarm.  Basic adjustments for a drive on the drive are as below.

i) Balance adjustment: Make sure the motor does not move when Command signal is zero (Put a jumper across the command voltage, after removing the wires coming from control). If it moves adjust the balance potentiometer to stop the movement.

j) Gain adjustment: If the movement is rough, or the motor overshoots, or if the motor vibrates; reduce the gain, by adjusting the potentiometer on the drive

k) If another axis has a similar drive, it can be swapped to see whether the problem switches to the other axis. If it has an absolute encoder home position could be lost

Following error amount, Following error time, Rapid jog, Max Jog, Max feerate,Acceleartion, KV are some of the parameters used for adjustment on the control side.

Digital Drive:

Look for any lights or display of alarm number. In most cases there are no adjustments in the drive. Follow the alarm number or symptoms to troubleshoot.

Following error amount, Following error time, Rapid jog, Max Jog, Max feerate, VGN, PGN, Int Gain, Acceleartion, KV are some of the parameters used for adjustment. Check the servo drive manual for tune up. All these adjustments may have to be done on the control.