[or the current limit circuit is at +5V corresponding to 200% of rated current.

When the current starts to flow, transistor Q17 turns on. The output of NOR gate “A” switches to a logic high, NOR gate “C” switches low, transistor Q18 turns off and capacitor C49 charges through R148 and D32. The capacitor voltage is inverted by amplifier “A” and compared with a fixed level at the comparator. The capacitor charges until its voltage level causes the comparator to switch positive, after approximately 5 seconds. The positive voltage from the comparator causes NOR “D” to switch to a logic zero (0V). A logic zero from NOR “D” attenuates the voltage applied to amplifier “B”. This reduces the reference voltage for the current limiter to approximately 3.3V corresponding to 150% of rated current.

When current stops flowing, transistor Ql7 turns off, NOR “A II and NOR “B” switch low and capacitor C49 starts to discharge through R1SO and D33. After approximately three minutes of discharge, the comparator switches low, the NOR gate “D” switch high and the current limiter’s reference voltage returns to 5V, or 200% of rating.

3.10  Isolation Circuit

The armature current and voltage signals are isolated from the control circuit as shown in figure 10. The current and voltage signals are modulated or chopped by an electronic chopper, transformer coupled, and demodulated to produce isolated signals.

The chopper consists of an operational amplifier and an electronic switch. The operational amplifier has a gain of plus one when the switch is off and minus one when the switch is on, as shown in figures 11 and 12. Switching between plus and minus gains modulates the signal to produce an AC signal (et in figure 12) at the transformer. The modulated signal is transformer coupled to the output amplifier. The output amplifier is synchronously switched between a plus one and minus one gain to demodulate the signal. The output amplifier is in effect a synchronous rectifier. The current signal isolator output is also brought to terminal TB2, to provide a signal proportional to armature current, but isolated from armature voltage.

3.11  Firing Circuit

The function of the firing circuit is to sequentially turn on each thyristor at the proper time. This is accomplished by detecting a reference point with the applied voltage cycle and initiating a firing pulse train after the proper delay time.

It can be seen from figure 13, that the “on” time of the thyristor is increased by decreasing the delay time and vice-versa. It follows that the output power can be varied by changing the delay time. The delay time is obtained from a fixed length electronic counter, counting pulses from an electronic clock. The clock is a voltage controlled oscillator. When the clock is at a low frequency, more time is required for the counter to reach its preset count, hence there is a long delay. If the clock frequency is high, the counter needs less time to reach the required count.

FIGURE 10. 1679/1681 DC MOTOR CONTROL.
BLOCK DIAGRAM OF ISOLATION CIRCUIT

FIGURE 11. 1679/1681 DC MOTOR CONTROL.
INTRODUCTION TO ISOLATION CIRCUIT

FIGURE 12. 1679/1681 DC MOTOR CONTROL.
ISOLATION CIRCUIT, MODULATOR-DEMODULATOR

FIGURE 13. 1679/1681 DC MOTOR CONTROL.
FUNCTION OF FIRING CIRCUIT

When the counter has obtained the required coWlt, meaning the required delay has passed, and assmning no fault conditions exist, an electronic gate opens, allowing firing pulses to turn on the thyristor. The firing pulses are removed after 1200, or at the next crossing of the timing reference point, whichever occurs first. The coWlter is always reset at the reference point crossing. Figure 14 is a simplified block diagram of a three SCR, three diode motor control. Figure 15 shows a six SCR motor control. Each SCR has its own reference detector and counter. The coWlters work off a common clock signal to assure virtually identical delay times for each SCR firing. Independent referencing detectors allow the angle between phases of the line to vary (phase shift) without effecting the firing angle.

3.11.1  Clock (Voltage Controlled Oscillator)

A simplified block diagram of the clock or voltage controlled oscillator is shown in figure 16. The Capacitor C charges at an exponential rate which is set by Resistors R_A, R_B, and Capacitor C. When the ‘capacitor voltage reaches the control voltage, the threshold comparator triggers the flip-flop so that its output is set high. This sets the clock’s output low and turns on the discharge transistor, allowing the capacitor to discharge through Resistor R_B. When the capacitor discharges to half the control voltage, the trigger comparator triggers the flip flop so that its output is set low. This turns the capacitor discharge transistor “off” and drives the clock output high. The capacitor starts to charge again and the cycle repeats.

The capacitor voltage oscillates between the control voltage and half the control voltage. Increasing the control voltage will increase the magnitude of the oscillation and the time required to complete a cycle. Hence, increasing the control voltage will decrease the frequency of oscillation and vice-versa.

FIGURE 14. 1679/1681 DC MOTOR CONTROL.
SIMPLIFIED 8LOCK DIAGRAM OF A 3 SCR, 3 PHASE DIGITAL FIRING CIRCUIT

FIGURE 15. 1679/1681 DC MOTOR CONTROL.
SIMPLIFIED 8LOCK DIAGRAM OF A 6 SCR, 3 PHASE DIGITAL FIRING CIRCUIT

3.11.2  Reference Detectors

Each reference detector locates a reference point on the input voltage cycle and generates a timing pulse. The reference point is very close to the negative zero crossing, since this allows full variation of output power with minimum variation of delay time.

The circuits that develop the reference or reset pulses are shown in figure: 17 and figure 18. The circuit of figure 17 simply conditions the line voltage, while the circuits in figure 18 develops the actual pulses.

Each phase of the three phase line voltage is transformer coupled into a low pass filter which removes transients and noise. The output of each filter is amplified and clipped to produce square waves. The square waves convey zero crossing information to the reference pulse circuits of figure 18 and the phase sequence circuit, figure 22.

The three square waves are connected to the twelve monostable multivibrators which produce six reference pulses displaced 600 from one another as shown in figure 18. Each reference pulse resets one of the six electronic counting circuits. Note: a three SCR. three diode motor control requires only six multivibrators and
three reference pulses displaced 120°.

FIGURE 16. 1679/1681 DC MOTOR CONTROL.
VOLTAGE CONTROLLED OSCILLATOR (VCO)

FIGURE 17. 1679/1681 DC MOTOR CONTROL.
AC LINE FILTER & CLIPPER

FIGURE 18. 1679/1681 DC MOTOR CONTROL.
DEVELOPMENT OF RESET PULSES IN TRIGGER CIRCUIT

3.11.3  Counting Circuit

A typical counting circuit is shown in figure 20, with its timing diagram in figure 19. The. zero crossing of the line voltage resets the counter, to start the timing cycle. The first part of the timing cycle corresponds to the SCR off time and lasts for 1000 clock pulses, on a 60Hz system (1200 for 50Hz systems). After counting a 1000 (1200) clock pulses, the counter output (QlB of counter “B”) switches to a logic “1” (+5V), and allows the firing pulse produced by the gate pulse oscillator to pas s through the NAND and NOR gates of figure 20. The firing pulse out of the NOR gate is amplified by the two transistors, and transformer-coupled to the SCR gate with which it is associated.

The number of gate pulses and the manner by which they are removed depend on the output level of the motor control. For low output levels, small conduction angles, the SCR receives gate pulses until the counter is reset by the next zero crossing. However, at high output levels, the gate pulses are inhibited by the NOR gate after 120 electrical degrees, since the SCR is commutated off at this point and further gate pulses would simply cause gate heating.

3.11.4  Gate Pulse Oscillator

The SCR firing pulses are generated by the gate pulse oscillator, sh own in figure 21. NAN? gate “A” and “B” form an oscillator which is synchronized by the NOR gate “A” and “B” at the turn on of each SCR. The synchronization circuit is designed to produce a wide initial firing pulse, which increases accuracy of the firing angle, and adds to the DI/DT capability of the SCR.

FIGURE 19. 1679/1681 DC MOTOR CONTROL.
COUNTER :TIMING DIAGRAM

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FIGURE 20. 1679/1681 DC MOTOR CONTROL.
COUNTER AND GATE PULSE CIRCUIT

Gate pulses can only pass through NAND “c” if the phase sequence and phase 1088 circuits determine that the correct power is present. Likewise, NOR “c” only allows pulse passage when the control is enabled and no fault condition is detected.

3.12  Fault Protection Circuits

The fault protection circuitry senses several different circuit conditions and inhibits thyristor gating in the event these conditions become harmful to the system.
The following is a list of conditions which will inhibit operation:

1. hnproper phasing of input voltage
2. Loss of an input phase voltage
3. Excessive load current
4. Over-temperature of heat sink as sembly
5. Opening of interlock circuit

3.12.1  Phase Sequence and Phase Loss

Correct phase sequence of the incoming power lines is necessary for proper thyristor firing. Therefore, the fault logic circuits contain a phase sequence detector to prevent incorrect operation.

If the incoming phase sequence is not ABC, the phase detector will inhibit thyristor firing and no power will be supplied to the motor. Incorrect phasing is corrected by reversing any two input power lines.

The phase detector is shown in figure ZZ. Squared-off phase voltage signals are fed to it from the line filter circuit (figure 17). When the phase sequence is correct (ABC), the positive and negative phase signals directly coupled to each NAND gate and the capacitor coupled positive leading edges and negative trailing edges of the preceeding phase signal are opposite polarity. The output of the NAND gates therefore remain high, allowing gate pulses to flow to the SCR IS. However, if the sequence is incorrect (ACB), the directly coupled input signals and the leading and trailing edges of the capacitor coupled input signals are in the same polarity, causing the NAND gate to pulse low. These low pulses discharge the capacitor C20, which inhibits the gate pulse oscillator.

Loss of an input phase also results in direct and capacitor coupled pulses to be in phase, causing the NAND gate to pulse low. The pulses keep capacitor C20 discharged, thereby inhibiting the gate pulse oscillator.

FIGURE 21. 1679/1681 DC MOTOR CONTROL
GATE PULSE OSCILLATOR

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FIGURE 22. 1679/1681 DC MOTOR CONTROL
PHASE SEQUENCE CIRCUIT

3.12.2  Over- Temperature and Interlock

Thermal overload protection protects the thyristors from operation beyond their temperature limit. Without this protection, high ambient temperatures or high continuous power overloading could produce excessive thyristor temperatures. A bi-metal temperature switch mounted on the heat sinks accomplishes this protection.

The over-temperature protection circuit is also shown in figure 23. If an over-temperature condition occurs, one or more of the thermal switches will open. Opening a thermal switch will set the latch to a logic “1”, remove the gate pulses and turn on the fault trip LED. This inhibits the controller until it is reset.

NOTE:  The controller will not reset until bi-metal switch has cooled enough to allow the contacts to reclose.

External interlock devices (normally closed) may be connected into this circuit between terminals ITBI and ITB2, as shown on the installation drawing.

Opening the external interlock shuts the controller off in the same manner as the thermal overloads.

After the fault condition is corrected, the reset command will reset the latch, enable the gate pulse oscillator, and resets the analog control circuits. The control circuits will not restart the motor until the reset command is removed.

3.12.3  Current Trip

An excessive peak current will cause the output of the current trip amplifier to switch negative. This sets the latch in figure 23 to a logic 1, removes the gates pulses and turns on the fault trip LED. Depressing the reset will clear the latch and turn off the LED.

FIGURE 23. 1679/1681 DC MOTOR CONTROL.
TRIP CIRCUIT

Starr-Up Procedures

4. STAR T UP PROCEDURES

The following steps and procedures should be followed closely to insure proper operation of the motor controller.

4.1  Mechanical Installation

Since in most cases the motor control is previously installed in an enclosure, only a quick mechanical check is necessary. Check mounting hardware holding the controller base to the enclosure.

Open the front panel and check that all circuit boards are in place. Be sure all circuit cards are firmly in place. The location of the controller apparatus should be a reasonably clean, dry environment. Ambient temperature around the system should be such that the temperature inside the controller enclosure does not exceed 50°C.

4.2  Electrical Installation

Before programmed inputs are made to the motor controller, an operational check is suggested, to confirm that the drive is functioning properly.

Refer to the Installation Drawing:

Sheet 1 of the Installation Drawing shows the connections for a standard drive, with a single speed adjust potentiometer to control motor speed from a minimum speed, as determined by the minimum speed control setting, to a maximum speed, as determined by the minimum speed control setting. Connections arc shown for either armature voltage feedback regulation, or tachometer generator speed regulation. The tachometer generator feedback circuit is designed for a 5 volt DC signal at maximum generator speed. If the generator output exceeds 5 volts, a voltage divider network is needed to reduce the voltage to the 5 volt level. The voltage divider, its connection, and the formula for calculating the resistors is shown on the Installation Drawing. An AC tach generator may be used if its output is rectified and filtered. Ripple voltage should not exceed one percent at any point in the speed range. However, the ripple filter may cause response or stability problems.

Sheet 2 of the Installation Drawing shows typical connections for other uses of the control. For jogging or inching, the speed adjustment potentiometers are shown connected to apply the speed signal into one of the direct inputs, without going through the ramp circuit. This is to produce crisp action when the jog or inch button is pressed. The jog connection is shown for about a zero to 50 percent adjustment range. For a lower maximum jog speed, increase the value of the fixed series resistor and decrease the value of the jog potentiometer the same amount.

For “offset” control, to compensate for an instrument or other speed follower signal that has a residual output at minimum (such as 5 to 50 milliamperes), the minimum speed amplifier of the control is connected as shown. This connects the  minimum speed amplifier to the non-inverting input of the input amplifier.

A typical connection is shown for connecting two controls with one as the lead, or master control, and the other as the follower, or slave. The speed signal into the follower control is connected to a direct input instead of through the ramp, to insure tracking of the drives during acceleration or deceleration. The connection shown does not provide for individual minimum speed adjustment, however. The minimum speed adjustment in the lead drive is common to both drives. The minimum speed circuit cannot be connected in both drives because the higher signal will over ride the lower one. Diode isolation cannot be us ed because of the low signal level. If individual minimum speed adjustment is needed for accurate system tracking, ‘a master reference supply with isolated 0 – 5 volt speed signals should be used. A less costly but less accurate alternative is the use of two ganged speed adjustment potentiometers, controlled by a single knob.

4.3  Checks Before Initial Start-Up

Ohmmeter Checks (DC Motor Disconnected)

1. Check all power terminals and control terminals for grounds, using the X10 scale on the ohmmeter.
2. Check between AC power terminals L1, L2, L3, on the Xl ohmmeter scale. Readings should be approximately 340 ohms. Check between plus and minus armature terminals on the X10 scale. On the 1681 control, reading should be meg ohms with meter in either polarity. On the 1679 control, reading should be megohms with plus of meter on plus armature terminal, very low resistance with opposity polarity.
3. Check all motor leads (including tachometer and motor thermostat leads) for grounds, and for insulation integrity between the shunt field and armature circuit. It is advisable to use a megger for the motor insulation tests.
   Check motor field resistance, and compare to nameplate data.

Voltage Checks (SEE WARNING, Section V)

1. Connect jumper on regulator card for armature voltage feedback.
2. If available, connect a light “dummy” load to armature terminals. A number of 200 watt light bulbs in series will do (three for 240 volts DC, six for 500 volts DC).
3. Turn’ speed adjust pot to minimum.
4. Apply AC power. Check AC line volts at terminals L1, L2, L3.
5. Phase sequence LED indicator on regulator board should light. If it does not, SHUT OFF AC POWER, interchange any two AC power leads, reapply AC power.
6. Push drive start or run button.
7. Check armature output volts. With minimum speed adjustment on regulator board full CCW, should be close to zero. With minimum speed pot turned full CW should read approximately 20 percent of rated armature voltage. Reset minimum speed full CCW.
8. Slowly turn speed adjust pot turns maximum. DC armature volts should increase to approximate rated value. Turn maximum speed adjustment on regulator board through its full range. Armature voltage should vary from approximately 100 percent rated volts to 120 percent rated volts. Return speed adjust pot to minimum.
9. Check for correct DC motor field volts. In 1679 control, 300 volts or 150 volts DC fixed field supply can be obtained from the control, per connection indicated on the Installation Drawing. When the 1681 control is used, the field supply must be obtained from another source.
10. If any of these checks indicate an abnormal condition, refer to section v – Trouble Shooting Procedure for additional check steps.
11. When the control is checked out, Shut Off AC Power. Remove dummy load and connect motor leads. Adjust jumper on regulator board for correct feedback mode.

4.4  Trim Adjustment Settings

Adjustment of the trim pots located on the regulator card may be necessary of for the needs of your application.

Adjust trim pots as follows:

 Trouble-Shooting Procedures

5. TROUBLE-SHOOTING PROCEDURES

The following trouble-shooting guide (see page 40 ) should be referred to when operating difficulties occur.

The guide is broken down into three sections:

Symptom – description of problem
Probable Difficulty – description of what may be wrong
Check or Replace – action to be taken

This guide is also supplemented with a power module bench test. Ommeter readings are given as a check for faulty semiconductors.

The steps shown in the trouble-shooting guide should be followed before substitution of any modules.

TROUBLE SHOOTING
TYPE 1679 AND 1681 DC CONTROLS

In the event of malfunction, check the obvious but frequently
overlooked causes of trouble:

1. Loose or broken connections. All terminals should be tight.
2. Control devices and interlocks external to the control.
3. Excessive1y high or low incoming AC voltage. Standard tolerance is + 10%.
4. Atmospheric contamination can cause insulating films to develop on relay and push button contacts, expecia11y those in low energy circuits.
5. Noisy power lines. Incoming supply may contain “high energy spikes” which can cause erratic operation.
6. All scope or meter readings called for in the trouble-shooting chart are measured with respect to common, at terminal TB1-14.

SYMPTOM	PROBABLE CAUSE	CHECK	REPAIR OR REPLACE
Phase sequence L.E.D. does not lite with power applied	Incorrect phase sequence	Interchange two incoming power lines.
	No power to control	Measure input at L1, L2, L3
	Blown power or control fuses	All fuses	Fuses
	Jumpers missing or not connected on Filter Board	Filter board	Install jumpers
	Filter board faulty	Check output of filter board with scope at TB2-1, TB2-2, TB2-3 *	Filter Board
	Control board faulty	With scope, check for 5.0 Volts from anodes of D7, D8, D9	Control Board
Control blows line fuses with power applied	SCR’s and power diodes	Check SCR’s and Diodes, MOV’s	SCR’s or Diodes, MOV’s
	Faulty Motor	Motor field and armature (megger if breakdown is suspected after disconnecting from o	Motor
Phase sequence L.E.D. lit no output from control with speed adjust full C.W.	No supply voltages	+5.0 volts from TB1-1	Control or filter board
		+12.0 volts from TB2-4 (filter board)
		-12.0 volts from TB2-5 (filter board)
	No reference voltage	+5.0 volts from TB1-13	External speed adjust
	Current limit set full C.C.W.	Adjust current limit pot for 3.0 volts from TB1-12	Control Board
	No run enable signal	+5.0 volts from TB1-7 external enable contacts	External enable contacts or control board
	No ramped output	+5.0 volts from TB1-10	Control Board
	Open Jumper	Connection diagram	Jumper
	“M” contacts faulty power or aux.	Contacts	“M” Contactor or “run” relay.
	Gating & Isol. Board	Check pulse train with scope *	Gating & Isol. Board
Phase sequence L.E.D. lit, fault L.E.D. lit, no output from control	Open interlock or heatsink thermostat open	+5.0 volts from TB1-2	Interlock or Thermostat
	Overcurrent trip (I.O.T.)	Excessive load condition, short circuit in motor or motor leads intermittent short in motor, SCR breakover, megger SCRs (out of circuit).	Check machine & processes Check motor. Replace SCRs with low, megger readings.
Motor runs at base speed or higher with low reference voltage applied.	Jumper missing or in wrong mode in control board.	Jumper A to T for arm. feedback. Jumper T to T for tach feedback.	Reconnect or install jumper
	Defective tachometer or coupling	Tach output	Tachometer or coupling
	Defective feedback isolator	+2.25 volts from terminal A on control bd. at 500 volts output	Isolation & gating board
	Defective control board	TP-10 control bd. for +4.0 volts	Control Board
Control starts but will not reach max. speed with speed adjust full C.W.	Max. speed set too low	Adjust max. speed	Control Board
	Current limit set too low	Adjust current limit	Control Board
	Reference voltage	+5.0 from TB1-13	Control board, Power Supplies (Filter board)
	Jumper in wrong mode on iso. & gating board	Jumper 1 to 2 for 500 volts Jumper 2 to 3 for 240 volts	Jumper for correct arm. voltage
Control will not regulate speed with load	Current limit set too low	Adjust current limit setting +3.0 volts TB1-12	Control Board
	Incorrect ammeter shunt	Shunt	Install correct shunt
	Current isolator defective	Load current TB2-1 for 1.25 volts @ 100% load.	Isolation & gating board
	Line voltage drooping with load	Line voltage when load is applied, voltage should not droop more than 10%	Check AC supply for adeqacy and/or loose connections.
	Overload condition	Load current	Correct cause of overload
Control Unstable	Stability or forcing not properly adjusted	Control Board	Adjust “stability” and “forcing”. Replace board
	Ammeter shunt signal leads reversed	Check lead polarity	Connect leads correctly
	Defective tachometer or coupling	Tachometer and coupling	Tachometer or coupling
	Motor or load binding up	Load	Correct load condition
	Speed amplifier	Control bd., IC25, IC27	Control board
	Voltage isolator defective	Voltage isolator 2.25 volts from terminal A on control board @ rated arm. voltage	Isolation & gating board
	Reference voltage unstable	TB1-13 for stable volt. level	Control bd. or external po
	Current limit set too low	Current limit setting (speed wil rise as soon as it is turned to higher current set.	Correct current limit setting
	Wrong ammeter shunt	Ammeter shunt	Install correct shunt
	Control bd. defective	Control bd. (substitue spare board)	Replace control board
		* See schematic for waveforms.

 

POWER MODULE BENCH TEST

The purpose of this test is to determine whether the power module being tested has a defective thyristor, or thermal sensing switch.

Remove the module from the base panel and lay on its side. At this point it will be helpful to refer to the “Exposed View of the DC Motor Controller” at end of manual. The upper portion of the picture shows in detail the parts of the SCR/SCR power module.

Use a Simpson type 270 Multimeter or equivalent and set the meter to ohms Rx 1 or RxIO.

Thyristor Check

Connect (+) of ohmmeter to SCR cathode test jack, (-) of ohmmeter to SCR anode test jack. Meter should read open. Reverse the ohmmeter leads. Ohmmeter should again read open.

Now place (+) ohmmeter lead on gate test jack, (-) lead on anode test jack. Ohmmeter should read open. Reversing leads should again give an open reading. Connect (+) of ohmmeter to gate test jack, (-) of ohmmeter to cathode test jack. Ohmmeter should read between Ion and 300n. Reverse ohmmeter leads. Meter should now read between 20n and soon.

Thermal Sensor Check

Connect leads of ohmmeter to the two terminal tabs located on the thermal sensor. The ohmmeter should read zero ohms.