SOLID STATE ADJUSTABLE SPEED
DC MOTOR CONTROL
1681 INSTRUCTION MANUAL
TABLE OF CONTENTS
3.1 Power Section & Transient Protection
3.2 Block Diagrams of DC Motor Control
3.3 Input Amplifier & Ramp Circuits
3.4 Reference Voltage
3.5 Minimum Speed Circuit
3.6 Electronic Mode Selector Switch
3.7 IR Compensation
3.8 Speed Regulator
3.9 Initial Starting Current Timer
3.10 Isolation Circuit
3.11 Firing Circuit
3.11.1 Clock (Voltage Controlled Oscillator)
3.11.2 Reference Detectors
3.11.3 Counting Circuit
3.11.4 Gate Pulse Oscillator
3.12 Fault Protection
3.12.1 Phase Sequence & Phase Loss
3.12.2 Overtemperature & Interlock
3.12.3 Current Trip
4.1 Mechanical Installation
4.2 Electrical Installation
4.3 Checks Before Initial Start-Up
4.4 Trim Adjustment Settings
- Servicing and Trouble-Shooting Procedures
- Diagrams – Tables – Application Information
- GENERAL DESCRIPTION
This manual is applicable to all integral horsepower DC motor controls of this series from 40 HP to 300 HP. The integrated circuit regulator selection of these DC motor controls are modular in construction, affording a minimum of down time in the event of a malfunction. Exceptional speed regulation and torque limiting is realized over their full operating range.
Operational features include: minimum and maximum speed adjustments, acceleration and deceleration, current limit and IR compensation adjustments. In addition, forcing and stabilization adjustments are provided for stabilizing the drive in systems with various intertials and the constants. Fault logic circuitry protects the controller from over current, phase loss, incorrect phase sequence and over temperature.
All circuitry and power components are built to withstand the vibration and handling of industrial environments.
1.1 Electrical Description
The power section of the controller is a three phase full wave bridge (six thyristors). This configuration yields full-wave six-pulse control of the motor over the entire speed range.
Thyristors, which are silicon controlled rectifiers, conduct when their gate terminal is pulsed positively with respect to the cathode. Once switched on the thyristor remains conducting until its anode voltage falls to zero. Pulsing the thyristor “on” late in the half cycle, when its anode is positive, will cause a small amount of DC power to be delivered to the motor. Pulsing the thyristor early in the half cycle will result in a large amount of power being delivered to the motor. Thyristor switching waveforms are shown in section 3.1.
A printed circuit board assembly called the firing circuit provides the pulses to gate or turn on the thyristors. Three pulses spaced 60 0 apart switch the thyristor on at the adjustable point in each half cycle they appear. When low speed is called for the pulses appear late in the half cycle. When high speed is required the pulses appear earlier in the half cycle. A full description of firing circuit operation is discus s ed in section 3. 2.
The speed regulator circuit controls the firing circuit by introducing a DC level output which is proportional to the desired speed and the error in speed due to loading.
Eight trim adjustments are located on the front of the speed regulator card. These adjustments include minimum and maximum speed adjust, current limit, acceleration and deceleration, and IR compensation adjust. In addition, forcing and stability adjustments are provided. Minimum and maximum speed adjustments trim the ends of the speed range. The acceleration and deceleration adjusts set the time it will take to reach the speed setting. The acceleration and deceleration rates are essentially linear.
The current limit adjust will provide effective torque limiting from 200/0 to 250% of the controller rating. The current limit circuit will hold armature current constant within 5% at all conditions including locked armature. Current limiting is stable under all conditions.
The IR compensation adjust will allow speed regulation of 2.5% over a 20:1 range with armature voltage feedback. The speed regulator is discussed in section 3.3.
Fault conditions which might hinder operation of the motor control are sensed by the fault circuit on the control card. This circuit will shut the motor controller down if conditions of over temperature, blown fuse, phase reversal, phase loss or extreme overcurrent are present. The cards are interlocked electrically; removing any card while the system is running will cause immediate shut down of the controller. Fault detection circuitry is discussed in section 3.4.
1.2 Mechanical Description
All assemblies are mounted on a painted steel panel with electrically conducting items mounted on insulated panels. Every assembly and part is front removable. Assemblies with heat dissipating components have been carefully designed for efficient cooling. All assembled parts use locking hardware to withstand vibration and shock from handling and shipping.
Interwiring of assemblies generally conforms to J.I.C. specifications.
Surfaces of all metal parts subject to corrosion are plated, painted or anodized depending on type of metal. Circuit boards are high quality epoxy glass with gold plated contacts and are coated with epoxy, making them highly resistant to most atmospheres and humidity conditions.
Cooling is achieved by forced air.
Horsepower: See nameplate
Power Input: See nameplate
Speed Range @ rated torque, armature feedback for 2.50/0 speed regulation 20:1
Speed Range @ rated torque, tachometer feedback for + 0.10/0 speed regulation *40:1
Adjustable from 200/0 to 2000/0 of rated armature current. Limiting with locked armature within 50/0 of initial limit point.
May be adjusted for 0 to 200/0 boost
Minimum and Maximum Speed Adjustments
Minimum speed adjustment range: 0 to 200/0 of base speed
Maximum speed adjustment range: 70 to 1150/0 of base speed
Acceleration and Deceleration Adjustments
Linear ramp acceleration and deceleration, independently adjustable from 2 to 30 seconds.
*With suitable tachometer generator
Fault and Overload Protection
System shutdown for:
Overtemperature – 80° heatsink temperature Phase loss or fuse loss
Overcurrent – Instantaneously at approximately 5 times rated peak to two minutes at 150%
Phase sequence – incorrect phase sequence
Automatic Current Limit Level – cuts back to 200% maximum if over 2000/0 more than 10 seconds
Manual – 1K to 2.5K potentiometer for control of zero to base speed
Programmed – 0 to 5 VDC.
Withstand Voltage: 1500 volts peak to ground or AC line, maximum
2.2 Mechanical Specifications
Dimensions: 24″H x 25″W xl01/4″D
All assemblies are front removable.
The motor control must be operated in a vertical position with power input connections at the top.
Where practical, all wiring conforms to J. I. C. electrical specifications.
Cooling: Forced air.
Approximate weight: 80 lbs.
2.3 Environmental Specifications
Maximum operating temperature around controller (in or out of enclosure) 50°C.
Atmosphere: All parts subject to corrosion are plated or coated. However, highly corrosive atmospheres may eventually cause corrosion to these parts.
Operation in up to 95% relative humidity (non-condensing) is permissible.
Theory of Operation
- THEORY OF OPERATION
3.1 Power Section
The power section is made up of three pairs of thyristors connected as shown in the simplified schematic of Figure A.
Each of the three phase voltages is connected through fast acting current limiting fuses to the common of the anode of one thyristor and the cathode of another . The cathodes of three thyristors are connected together to form the positive output of the supply. The anodes of the other three thyristors are connected together to form the negative output of the supply.
Without a gating signal, the thyristors block the flow of cur rent in both directions and there is no current in the motor armature. As a properly timed gate signal is applied to each thyristor gate, the device goes into conduction and acts like a rectifier, i. e. , it conducts as long as the anode potential is positive with respect to the cathode. When the polarity changes and the current falls below the holding current of the device, the thyristor regains’ its blocking state and will remain off until it receives another gating signal.
To achieve proper bridge control, the thyristors must be pulsed in a given sequence. Conduction always occurs in pairs, i.e. current flows from an AC line through a thyristor to the armature and back through a thyristor to another AC line.
With any conduction angle the current paths are as follows: From L1 through SCR 1, motor armature, back through SCR 4 to L2. 60° later conduction from L1 is through SCR1, motor armature, back through SCR6 to L3.
From L2 through SCR3, motor armature, back through SCR6 to L3. 60° later conduction from L2 is through SCR3, motor armature, back through SCR 2 to L1.
From L3 through SCR5, motor armature, back through SCR4 to L2. 60° later conduction is from L3 through SCR5, motor armature, back through SCR2 to L 1.
Since power is supplied to the armature six times per cycle, the ripple frequency at all conduction angles is 360 Hz. Figures Band C show typical armature voltage wave shapes at half and full speed conditions with high and low torque levels.
FIGURE A. 1681 DC MOTOR CONTROL.
3-PHASE FULL WAVE (6SCR) POWER CIRCUIT
TYPICAL ARMATURE VOLTAGE WAVE SHAPES
1/2 BASE SPEED
3.1.1 Transient Suppression and DY /DT Protection
The power thyristors and diodes are subject to voltage transients from both the incoming power source and the output armature circuit. As shown in figure A, a series resistor-capacitor network and an “MOV” (Metal Oxide Varistor) is connected ac ros s each of the bridge SCR’s. The “MOV” absorbs transients on the lines with the help of the line impedance. Since the effect of these devices are dependent on the impedance of the AC power source, it may be necessary in some applications to add additional inductance to the incoming lines in the form of chokes or an isolation transformer.
The “MOV” device has the characteristic that it maintains a high resistance until the RMS voltage across it exceeds a certain level (well below the voltage rating of the SCR’s) I when it sharply switches to a low resistance. It recovers rapidly after the voltage level drops. The resistor-capacitor network not only serves to by pass very fast transients spikes I it also serves as a “DV/DT” buffer. “DV/DT” is the rate of rise of voltage across the semiconductor, immediately after it stops forward conduction. The buffer circuit acts to reduce the rate I giving the device more time to recover its voltage blocking characteristic.
3.2 Block Diagrams of DC Motor Control
Simplified block diagrams of the DC motor control are shown in figure 1 and figure 2. The diagrams show that the control section is isolated from the power section in both the forward and feedback paths.
FIGURE 1. 1679/1681 DC MOTOR CONTROL.
SIMPLIFIED 8LOCK DIAGRAM
FIGURE 2. 1679/1681 DC MOTOR CONTROL.
LOGIC CIRCUIT BLOCK DIAGRAM
3.3 Input Amplifier and Ramp Circuits
The speed control potentiometer is normally connected between a five volt reference supply and the minimum speed amplifier, as shown in figure 3. The speed signal from the wiper of the control potentiometer is inverted by the input amplifier ICZ3 and fed into the ramp circuit. The ramp cirO1 it controls the motor acceleration and deceleration.
Increasing the speed setting increases the positive voltage applied to the input amplifier, causing a corresponding increase in the negative signal at the amplifier1s output. The negative signal from the input amplifier causes the output of the comparator section of ICZ8 to switch to its negative voltage limit. The negative voltage step from the comparator passes through Diode D17, and is attenuated by a voltage divider formed by the acceleration Rheostat R74 and Resistor R78. The voltage step output of the voltage divider is integrated by the integrator section of ICZ8, R 77, and Capacitor C38. The negative input step causes the output of the integrator to increase positively as a linear function of time (ramp) on a slope or rate determined by the amplitude of the input voltage step, until its positive output voltage equals the negative output voltage of the input amplifier. When the signals from the integrator and input amplifier are equal in magnitude but opposite in polarity, the comparator output will drop towards zero, and the output of the integrator will be maintained at the new speed setting.
A reduction in speed command setting reduces the input voltage to the input amplifier ICZ8, causing a corresponding reduction in the amplifierls negative output voltage. This decrease reduces the negative signal to the comparator below the positive signal coming from the integrator, resulting in a net positive signal at the comparator, causing its output to switch to the positive limit. This positive step signal passes through Diode DI8, is attenuated by the voltage divider formed by deceleration Rheostat R75 and Resistor R 78. The attenuated step signal causes the integrator to ramp in a negative direction until its output signal is equal to the new speed command.
FIGURE 3. 1679/1681 DC MOTOR CONTROL.
INPUT AMPLIFIER AND RAMP CIRCUIT
FIGURE 4A. 1679/1681 DC MOTOR CONTROL.
LOGIC CIRCUIT POWER SUPPLY REGULATOR (IC21) SIMPLIFIED SCHEMATIC
FIGURE 48. 1679/1681 DC MOTOR CONTROL.
CONNECTION OF LOGIC CIRCUIT POWER SUPPLY REGULATOR
3.4 Reference Voltage
The reference voltage for the motor speed control is obtained from a monolithic integrated circuit voltage regulator, as shown in figure 4. The integrated circuit contains a temperature compensated voltage reference of approximately 7 volts, an error amplifier, a series pass transistor, and a current limiter, as shown in figure 4A. The compensated reference is attenuated, to 5 volts by Resistors R51, Potentiometer R52, and R53, as shown in figure 4B. The attenuated reference is connected to the non-inverting input of the error amplifier. The error amplifier compares the 5 volt reference signal with the regulator output voltage. The amplified error signal controls the series pas s transistor to maintain 5 volts at the regulator’s output. However, an abnormal current flow in external Resistor R54 will turn the current limiting transistor on and reduce the signal to the base of the series pass transistor. Reducing the base signal reduces the output voltage, thereby protecting the regulator against accidental short circuits or shorted logic components.
3.5 Minimum Speed Circuit
The minimum speed adjustment is buffered by a unity gain amplifier as shown in figure 5. Its adjustment range is zero to approximately 20% of full speed.
FIGURE 5. 1679/1681 DC MOTOR CONTROL.
MINIMUM SPEED CIRCUIT
The minimum speed adjustment may alternately be used as an offset adjustment. Figure 6A shows the connection for “minimum speed” adjustment. The variable output from IC27 is added to the speed adjust potentiometer, providing a bottom limit to the speed setting.
Figure 6B shows the connection for “offset” adjustment. The variable output from IC27 is connected to the non-inverting input of input amplifier IC23. This provides an offset signal to compensate for speed corrunand devices with minimum outputs higher than zero.
3.6 Electronic Mode Selector Switch
The mode selector circuit, shown in Figure 7, is used to switch between the acceleration/deceleration mode, inch mode, and jog mode. In the normal acceleration/deceleration connection, the run enable signal is high (+5V), switch “C” is on, and the speed commands come from the speed control potentiometer through the acceleration/deceleration ramp. However, this input may also be used for a “speed follower” input signal, when the ramp is not required or desired. In the inch mode, the inch adjustment potentiometer is connected directly to the speed command amplifier through switch “B” without an acceleration/deceleration ramp. The inch mode is selected by applying a logic 1 (+5V) to the inch enable input. Selection of the jog made by applying a logic I (+5V) to the jog enable input, connects the jog adjustment potentiometer directly to the speed command amplifier via switch “A”. Usually the jog adjustment potentiometer is preset to the desired jog speed and the jog enable input is used to jog the motor.
FIGURE 6A. 1679/1681 DC MOTOR CONTROL.
MINIMUM SPEED CIRCUIT CONNECTIONS
FIGURE 68. 1679/1681 DC MOTOR CONTROL.
CONNECTION FOR OFFSET ADJUSTMENT
FIGURE 7. 1679/1681 DC MOTOR CONTROL.
ELECTRONIC MODE SELECTOR SWITCHES
The mode selector circuit has three other functions of interest. The first is a “run enable inhibit” function. If either the inch or jog enable circuit is energized, a signal appears on the base of Transistor 019. When 019 conducts, the enable terminal (13) of enable switch C of IC 26 is clamped to ground, preventing the turn on of the run circuit. The second function is a ramp reset action, from the run enable circuit. When the run enable circuit is energized, a “Logic I” (+5V) signal is sent to NAND gate IC18, causing the ramp to reset. This prevents “bumping” the drive if the drive is turned on before the deceleration ramp has reached a low value.
The third function causes each enable circuit, through Diodes D39, d40, and D41. to provide a turn-on signal to the high gain error-integrating amplifier IC28. The purpose of this circuit is to provide fast control response to turn-on commands. and to eliminate effects of amplifier offset at the operating speed.
3.7 IR Compensation
IR compensation is combined with the speed signal at the speed command amplifier. as shown in figure 7. IR compensation adjustment range is zero to approximately 20% boost.
The IR compensation circuit automatically proportions the compensating signal to armature voltage, so that the compensation is essentially constant over the entire speed range.
3.8 Speed Regulator
The speed regulator circuit is shown in figure 8. It consists of four operational amplifiers that function as a speed error amplifier, current error amplifier, regulator, and level shifter respectively.
The speed and current error amplifiers function together to supply an error signal to the speed regulator. When the motor current level is below the current limit level, the current error amplifier ICZ8 is at its positive voltage limit and the motor armature voltage or speed error amplifier is supplying the error signal to the regulator. However, if the current limit is exceeded, the current error amplifier will take control of the regulator.
The voltage and current error amplifiers have fast response times and relatively low gains. The ;regulator amplifier has a high gain but relatively slow response. This arrangement produces a motor control with a quick responding current limit while maintaining good stability and excellent speed regulation.
The level shifter amplifier inverts the zero tb minus five volt regulator signal to a plus five to zero volt signal. The plus five to zero voltage signal controls the voltage controlled oscillator (clock) frequency in the digital gating circuit.
With a command signal, from a potentiometer or external command device, applied to the “external current limit” input to the current error amplifier ICZ8, the control becomes a cur rent control amplifier. The current command signal can be brought through the acceleration/deceleration ramp circuit, or directly to the speed command circuit. No internal potentiometer is provided for this function.
FIGURE 8. 1679/1681 DC MOTOR CONTROL.
SPEED REGULATOR CIRCUIT
FIGURE 9. 1679/1681 DC MOTOR CONTROL.
INITIAL STARTING CURRENT TIMER
3.9 Initial Starting Current Timer
The maximum current limit value is higher for starting then for running (200% start and 150% run). The circuit t hat permits the higher starting current is shown in figure 9. Initially, the output of NOR gate D is high (+5V), 5V is applied to amplifier “B” and the reference voltage