SPEED CONTROL FOR RECTIFIER-FED DC MOTORS
Reproduced from Machinery Lloyd and Electrical Engineering (Vol. 37, No. 5 - 27th February 1965) with kind permission of the author and the publishers – The Certificated Engineer May 1966.
?A direct current motor is a versatile power unit which can be designed for practically any speed and speed/torque characteristic required and can readily be operated at various speeds over a wide range. Using modern rectifying equipment such motors can be operated efficiently from normal ac supply mains.
An electric motor automatically adopts a running speed at which it develops torque exactly equal to the resistance torque of the load to which it is coupled, providing this is within the range of the motor. The torque T developed by a de motor is equal to k I Φ, where k is a constant for the motor, IA the armature current and Φ the magnetic field strength. In an armature of constant resistance, I is equal to k1(V-e), where k, is a constant, V the voltage applied to the armature and e the back emf developed due to the armature windings cutting the magnetic field flux. Thus, the motor torque is equal to k2, (V-e) Φ,
where k, is a constant. The speed N is equal to k3e/Φ,
where k, is a constant. Thus, the peed N is equal to ka (V/Φ – T/k2 Φ2)
The torque developed by a d.c. motor at a given speed can thus be reduced either by reducing the voltage V applied to the armature, or by reducing the field strength Φ. Reduction of the voltage V at constant field strength Φ causes increase of motor speed. To avoid overheating of the armature the armature current IA should not be allowed to exceed its rated value, unless the rate of heat dissipation from the armature is appreciably increased at high speeds, and it may be necessary to limit IA to less than its rated value at low speeds if the cooling is adversely affected. In some cases, here a high torque is required at very low speeds, a separately driven cooling fan, or an air-to-air or air-to-water heat exchanger may be used. When running a motor above its normal speed consideration must, of course, be given to the effect of the centrifugal stresses on the rotating parts, which stresses are proportional to speed2.
Speed Control Methods
The armature volt drop is equal to IA RA, where RA is the resistance of the armature windings, and is normal very small compared with V. Thus, ignoring the slight effect of armature reaction because of armature current, the field strength of a shunt or a separately excited motor is practically unaffected by the motor load so that, with a given voltage and field current, the speed of such motors is little affected by the motor load. With a given field current the speed is almost proportional to the voltage V applied to the armature, as shown in curves A to 0 n Fig. 1. Subject to the effect (if any) of changed cooling on altered speed, speed control by variation of armature voltage gives practically constant full load torque, the full load horsepower, proportional to the product of torque and speed, being practically proportional to the speed or armature voltage. This method of speed control thus gives results which are suitable for many drives and, if the variable armature supply voltage is independent of the load and is obtained with little loss, is very efficient. The motor speed may also be controlled by varying the field current to alter the field flux Φ. However, the current through shunt and self-excited field windings must, of course, be limited to rated value to avoid overheating. This method, therefore, can only be used to raise the speed by reducing the field current. As shown by curves E to H in Fig. 1, the speed variation on varying load is quite small on reduced field current but, with the armature current IA limited to its normal value, the full load torque is inversely proportional to the field flux Φ, the full load horsepower being practically constant.
The motor must be large enough to develop the torque required at the highest and all speeds which means that, on most drives which require a lower torque on reduced speed, the motor is underloaded and operates at reduced efficiency on the lower speeds. It should be noted that the field strength Φ is normally not directly proportional to the field current, due to saturation and other effects in the magnet circuit of the motor. Since the power taken by the field windings is comparatively small the field current can be reduced by quite a small series resistance with limited losses. More efficient control of field current is obtained if the field current is obtained from a rectifier which is fed with a variable ac voltage controlled by tappings on a transformer or auto-transformer or through an induction regulator, the latter providing infinite variation of field current over the whole control range.
Series and Compound Motors
A series motor inherently has a high fall of speed on load, as indicated in curve A of Fig. 2, but the speed on any load can be altered by changing the voltage applied to the motor. Subject to the effect of changed cooling on changed speed, this method of speed control gives practically constant full load torque on all speeds, the full load horsepower being practically proportional to the speed.
The speed variation on varying load in a compound motor is modified by a suitable series field winding in addition to the shunt or self-excited field winding. The series winding in a cumulative compound motor is connected to assist the shunt winding, thus increasing the starting torque obtained with a given armature current and increasing the fall of speed on load. In a differential compound motor, the series winding opposes the shunt winding to reduce the fall of speed on load, but the series winding may need to be cut out at starting. Compensating windings may be fitted in the pole shoes of large motors to compensate for the magnetic effect of the armature reaction on load. Commutating poles are usually fitted in variable speed dc motors to ensure good commutation on all speeds.
Variable Voltage Control Methods with Rectifiers
One method of obtaining a variable de voltage at the armature is to supply the armature through a suitable rectifier to which a variable ac voltage is applied. The latter may be obtained from a tapped transformer or autotransformer, the number of voltages and motor speeds then depending on the number of tappings. Alternatively, the variable ac voltage to the rectifier may be obtained from a constant ac voltage through an induction regulator, saturable choke or transductor, or a magnetic amplifier, which devices can provide an infinite variation of voltage and speed over the full control range.
Variable ac voltage control is employed with the simpler types of rectifiers such as selenium rectifiers, vacuum and gas filled thermionic diode valves, mercury arc rectifiers without control grids and semiconductor diodes; and the harmonics which are set up depend on the type of rectifier, number of phases and transformer connections. Harmonics are impulses of frequencies which are a multiple of the supply frequency and are undesirable; they can be limited by suitable design of rectifiers and transformer and, if necessary, using suitable smoothing circuits. Transformer tappings and induction regulators do not, in themselves, introduce harmonics, but this is not the case with saturable chokes, transductors and magnetic amplifiers, which control the ac voltage by varying the wave form of the ac supply to the rectifier.
Another method of providing a practically stepless de voltage to the armature of a variable speed de motor is by using a controlled rectifier to which a constant ac voltage is applied. The type of control system employed depends somewhat on the type of rectifiers used, which may be vacuum, or gas filled triode valves, ignitrons, grid-controlled mercury arc rectifiers or the modern silicon-controlled mercury are rectifiers or thyristors. These rectifiers are controlled by varying the duration of the conducting portions of the ac cycles, which method produces harmonics so that, to satisfy the supply authority, large rectifiers may need to be operated from a transformer in which the number of phases is increased.
For some purposes it is a di advantage that rectifiers cannot accept regenerative current, and special switching may be necessary for reversal and braking. However, regenerative braking can be obtained, if required, by using additional inverter rectifier circuits. Many rectifiers fed variable speed motor circuits incorporate means of compensating for the IA RA volt drop in the armature on load, and f r variation of field flux due to change of resistance of field windings on changed temperature; while many control circuits are provided with a current limiting device which protects the motor against overload and enables it to be switched on at any present speed. In many systems the change of motor speed which normally occurs on changed load is automatically limited to a very small amount by making the applied armature voltage responsive to the difference between a reference voltage, on a speed setting potentiometer, and the back emf of the armature or the output voltage of a tachometer generator which is proportional to the speed of the motor.
Transformer Tapping Control of Selenium Rectifiers and Thermionic Diodes
Fig. 3 shows a simple circuit for supplying a fractional horsepower variable speed motor through selenium rectifiers from a single-phase ac supply. The armature is fed from the rectifier bridge DA through the smoothing reactor or choke L, while the field windings are fed from the rectifier bridge DF. The motor speed can be reduced below normal, with approximately constant full load torque, by using the tapping switch B on the autotransformer T to reduce the voltage applied to the rectifier DA. The speed can also be increased above normal, with full load torque approximately inversely proportional to the speed, by using the switch S to reduce the field current by reducing the voltage applied to DF. In this case the speed is controlled purely by hand; on a given setting of the transformer tapping switch the speed variation on varying load depends largely on the design of the motor.
Fig. 4 shows a suitable circuit for a fractional horsepower shunt or compound motor fed from a single-phase supply, the motor having a speed range up to about 15 to 1. When the ac is switched on the field windings are energised from the selenium rectifier D1 fed from tappings on the autotransformer Tr, while the cathode of the full wave hot cathode gas filled valve DA is heated by the transformer winding W. After a short heating period the time delay relay A will close the contacts B to switch on the autotransformer T2 which applies voltage between the anodes and cathodes of the valve DA through the armature of the motor and the double pole two-way switch S, and through the smoothing reactor 1. The speed is controlled by varying the voltage applied to the armature by adjustment of the tapping switch PI' or by varying the field current by adjustment of the tapping switch P1, both tapping switches being controlled by a single handle. The motor can be reversed by using the switch S to reverse the armature current. In the mid position of this switch the armature is connected to the dynamic braking resistor R, in which position the voltage which is generated in the armature, while driven by its momentum and that of the driven machine, causes the momentum of moving parts to be dissipated as heat in the resistor R.
Transducer Control of Selenium Rectifier Circuit
Speed control by variation of ac voltage to a selenium rectifier OA supplying the motor armature is the method employed in the circuit shown in Fig. 5. This voltage is governed by the volt drop in the load windings A of the transductor T, which volt drop is determined by the current through the control winding C as governed by the setting of the speed setting potentiometer P, and by the current through the control winding B as governed by the feedback voltage of the armature. The field windings are separately excited through the selenium rectifier bridge DF energised by the transformer E. The relay F operates to protect the ballast resistor R in the event of violent changes of speed being demanded. This system is designed for operation on single phase supplies to supply motors up to about 2 hp with speed ranges up to about 20 to 1.
Three Phase Transductor Silicon Diode Circuit
A three-phase silicon diode semiconductor bridge Da is used to feed the armature of the simplified variable speed motor circuit given in Fig. 6, which is suitable for motors of about 15 to 200 horsepower, with range of speed control up to 40 to 1 with constant full load torque output. The voltage applied to this rectifier is governed by the volt drop in the load windings A1, A2 and A3 of the transductor, which is controlled by direct current through the control windings on the transductor. One set of control windings is fed through a two-stage voltage and power amplifier, the input voltage to the amplifier being equal to the difference between the voltage applied to the armature and that of the speed setting potentiometer. Current limitation and compensation for armature voltage drop are incorporated in the circuit, with resistor/capacitor circuits across the rectifier input to absorb voltage surges. The drive is suitable for inching operations. On normal supplies the speed is maintained within +2.5 per cent of the maximum motor speed, while a greater accuracy can be obtained by using a tachometer generator as a speed sensor.
Grid Controlled Thyratron Circuit
Fig. 7 shows a fractional horsepower motor circuit in which the full wave gas filled rectifier DF is used to supply the field windings, while the armature is fed through the thyratron CA. The instant at which conduction occurs between the cathode and anode of the thyratron during the positive half cycles of the ac is determined by voltage impulses applied between the grid and cathode of the thyratron. This grid voltage comprises a de voltage from the speed setting potentiometer on which is superimposed an ac voltage from the secondary winding SI of the transformer through the phase control circuit consisting of the capacitor C and resistor R. The overload element 0 controls the contacts P to provide overload protection. Thyratrons are used in three phase circuits to supply variable speed motors up to about 40 horsepower.
Mercury Arc Rectifier Circuits
Mercury pool cathode-controlled rectifiers, such as grid-controlled mercury arc rectifiers and ignitrons, are used to feed the armatures of larger motors. Fig. 8 shows a simplified circuit in which ignitrons are used, this being suitable for motors of about 40 to 90 hp. The ignitrori is a single anode steel cased water-cooled rectifier with mercury pool cathode. Electronic emission from the cathode to the anode commences during the positive half cycles after a cathode spot has been created on the mercury by passing a current pulse between the mercury pool and an igniter which dips into the mercury pool. The duration of the anode ·current flow' in the positive half cycles, and thus the average de voltage applied to the motor armature, is controlled by suitable phase timing of the ignition pulses in the positive half cycles of ac voltage applied between the cathode and anode, from the autotransformer through the motor armature. In this way the applied armature voltage can be controlled from zero to its maximum value. Phase shift control of the ignition instants is governed by the voltage on the speed setting potentiometer F and, in the circuit shown in Fig. 8, by feedback from the tachometer generator G driven by the motor.
One disadvantage of using phase-controlled rectifiers, in which the variable voltage is obtained by controlling the conducting periods in the ac cycle, is that the power factor is reduced at low speeds. In Fig. 8 this undesirable feature is limited to some degree by using armature voltage control below about 50 per cent speed with field control for the higher speeds, at reduced full load torque; The speed control potentiometers F and G are coordinated.
Silicon Controlled Rectifier Circuits
Whi1e rectification occurs at the p-n junction of a semiconductor diode the controllable semiconductor rectifier, or thyristor or trinistor, has a four-layer p-n-p-n structure with a control lead brought out from the centre p region. By applying a low power signal at low voltage to this lead, or control gate, the rectifier can be controlled in a somewhat similar way to a thyratron valve. It can block normal voltage or, after the signal has been applied to the gate terminal, the rectifier can conduct with a very low forward voltage, while retaining high resistance in the reverse direction. After conduction has thus been triggered off it continues until the applied voltage has fallen to a very low value, i.e., practically to the end of the positive half cycle, even after removal of the trigger voltage. The average de output voltage can thus be controlled by suitable phase timing of the instants of application of the control voltage. This system is employed to obtain a wide range of speeds from de motors ranging from one to several hundred horsepower and enables a large power to be handled by rectifiers of small dimensions, the system requiring no warmup time, and having a high efficiency and fast response to the control voltage. The armature voltage may be compared with that on a speed setting potentiometer in conjunction with compensation for the lARA volt drop in the armature, the voltage difference being used to control the thyristor gates to stabilise the speed within about ±5 per cent. The speed may be stabilised within about -+-1 per cent if the thyristors are controlled by the difference of voltage between that of the speed setting' potentiometer and the output voltage of a tachometer generator.
Fig. 9 refers to a typical half controlled three phase system which employs a set of controllable silicon rectifiers C1, C2 and C3, in conjunction with the silicon diodes D1, D2 and D3, the shunt diodeD4 provides a path for inductively produced circulating currents, while the resistor R permits sufficient load current to ensure correct operation of the rectifiers. The output current of the rectifiers is smoothed by the reactor L. The timing of the firing pulses of the controllable rectifiers is governed by the adjustment of a speed setting potentiometer, the speed being stabilised by feedback from the tachometer generator.
A back-to-back arrangement of controllable rectifiers may be employed for motor reversing, if required, or controllable rectifiers may be used to switch a resistor across the armature for dynamic braking. Fig. 10 shows a simplified circuit in which one set of rectifiers CF is used for forward rotation, and the set CR for reverse running. The speed is set by the potentiometer S and stabilised by the speed sensing circuit, which may be connected to a tachometer generator, or supplied at armature voltage with lARA volt drop compensation. When reducing the speed, regenerative braking is obtained through one of the rectifier bridges acting as an inverter, the momentum of the drive then generating de in the motor armature, which results in ac being returned to the supply mains through the main transformer T. Feedback signals from current transformers prevent both rectifier bridges conducting at the same time.
Electronically controlled rectifiers are particularly suitable for processes in which several motors require to be independently driven, with a given ratio of speeds, or where the speed must be automatically varied in accordance with the requirements of the driven plant.