Ac motors are obviously the most widely used prime movers for the fixed-speed requirements of industry. They are simple, dependable, and rugged; what’s not to like?
Nevertheless, electric motor technology was first built around the principle of direct current. Dc motors have been in service for close to a century, and they aren’t going away. Some applications are better suited for dc drives and the distinct characteristics they offer.
The dc drive “package” consists of the motor and the control (which is preferably called the regulator). A properly applied dc drive package is capable of results such as a wide speed range, good speed regulation, ease of control, efficient size and weight relative to mechanical variable speed, durability, and low cost. But some further analysis of the motor and regulator is necessary to appreciate the full benefit.
Dc motor basics
In practically all packaged dc drives, the motor has a common trait: it turns the shaft at a speed almost directly proportional to the voltage applied to the armature; in a “no load” situation, the voltage vs. the speed describes an almost perfect linear relation.
However, with a constant applied voltage and a gradual load increase, dc motors tend to show a speed “droop.” Such a drop in speed is common enough to physical systems. A car with a fixed engine throttle position behaves much the same as it goes from level ground to uphill. In such a case, most operators will compensate by depressing the accelerator; in a dc drive, the control will respond accordingly, and will help the motor maintain nearly constant shaft speed.
This tendency to slow down is measured as regulation, which in dc drives is generally expressed as a percentage of motor base speed. Percent regulation is formulated as:
%Reg = ((nls-fls)/fls)100
Where nls is the no load speed, and fls is the full load speed. Without regulator compensation (adjusting the voltage) as the load goes up, the full load speed will go down, as does the overall regulation.
When it comes to current, the armature amperage exhibits a nearly linear relation to output torque, regardless of speed. It can therefore be generalized that the load torque determines the required current in the armature.
The basic concepts established to this point are reiterated: Motor speed is mainly determined by the applied armature voltage, and motor torque is controlled by the armature current.
The regulator in a dc drive serves two functions. It rectifies ac supply power, and it manipulates the dc output voltage and current in response to control signals and feedback, thus regulating the speed and torque of the motor.
Rectification is basically handled by power semiconductors. The regulating function is provided by a relatively simple electronic circuit that monitors numerous inputs and sums their signals, producing a so-called “error” signal. This is then processed and transformed into precisely timed pulses (bursts of electrical energy). The pulses are applied to the power switches, thereby regulating the voltage to the dc motor.
The regulator inputs are the ac power supply, the set point input, and the feedback signals consisting of current and voltage. The ac input is turned to a dc output, a familiar function that uses solid state electronics. The set point input and the feedback will be examined more closely.
The set point input in most packaged drives involves a closely regulated fixed voltage source applied to a potentiometer. (Ten volts is a common reference.) The potentiometer can take the fixed voltage and divide it down, for example from 10 all the way to zero volts. In such an example a 10 volt input to the regulator from the speed adjustment control (potentiometer) corresponds to maximum motor speed, and zero volts pertains to zero speed. Similarly any speed between zero and maximum can be obtained by adjusting the speed control to the appropriate setting.
Speed feedback is required to “close the loop” and accurately control motor speed. In a simple control, one typical method is to monitor the armature voltage (directly related to speed) and feed it back into the regulator, which compares it with the input “set point” signal.
When armature voltage goes above the set point established by the speed potentiometer setting, an error is detected and the output voltage from the power bridge is reduced accordingly. When the armature voltage dips down, an error of opposite polarity is sensed and the output voltage from the regulator is automatically increased in an attempt to bring up the speed. This armature voltage feedback system is standard in most packaged drives, and is usually called a “voltage regulated drive.”
A more accurate method of getting speed feedback information is called “tachometer feedback.” In this arrangement the speed feedback is obtained from a tachometer mounted to the motor. Its output relates directly to the speed of the motor, and such feedback often improves a drive’s regulation. With tach feedback, the drive is called a “speed regulated drive.” Most regulators can be modified to operate under tachometer feedback.
Feedback frequently comes from a motor-mounted encoder. The encoder feeds back voltage pulses at rates relating to motor speed. These are processed digitally, and are compared to the set point; error signals are then produced to regulate the armature voltage and speed.
Current feedback is obtained by monitoring the motor armature current. As mentioned, this is a direct indication of the load torque. Current feedback signals are used for both positive and negative feedback; the positive feedback maintains speed under a torque (and current) increase by slightly raising the voltage in the armature in response to the higher armature current – the speed droop is thus taken care of. Negative feedback uses a threshold control that limits the current level, protecting power semiconductors from damage. Adjusting this function allows the maximum motor output torque to be varied. Hence this current limiting is also referred to as torque limiting.
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Adjusting from within
Besides the usual external adjustments, such as the speed potentiometer, there are internal modifications that are often applied to simple analog drives, such as minimum and maximum speed, current limiting, IR compensation, and acceleration time.
Minimum speed can be set so that the motor will not stop even when the speed potentiometer is turned all the way down. Without proper modifications, such a potentiometer setting often means the control output voltage goes to zero; but there are cases when a stopped motor is not wanted. For instance, stopping the motor by the potentiometer can be a hazard when a maintenance person mistakenly assumes it’s safe to work on the machinery. Rather, before handling, the motor should be shut down by switching off the power supply to the motor or the control. And a minimum speed is sometimes desirable as the motor is cycled up and down during its operation. Common minimum speed settings go as high as 30% of the motor base speed.
Maximum speed can limit speeds obtained by turning up the potentiometer or by raising the input signal all the way. If, for example, a dc motor is only rated for 1,750 rpm, but the control can deal out 1,900 rpm, the maximum speed needs to be fixed. The internal potentiometer can be set at a lower point, thus restricting the maximum output voltage and maximum speed. Maximum speed is typically between 50 to 110% of motor base speed.
Current limit is provided in the regulator, which conveniently monitors the current entering the motor at all times. The maximum current can be set so the motor stalls instead of exceeding its torque rating. Current limiting is advisable where a jam load might be encountered. It’s also useful where torque is essential and speed can vary, as in some material winding applications. The torque limit is set, and the speed fluctuates accordingly to maintain material tension. Factory current limit settings are normally 150% of the motor rated current; this enables the motor to generate enough torque to start and accelerate the load. Current limit adjustment capabilities typically fall between 0 to 200% of the current rating.
IR compensation is a method that adjusts for the speed droop in a motor due to armature resistance. Mentioned previously as positive feedback, IR compensation makes the regulator output voltage rise a little if output current increases. This helps stabilize the motor speed throughout a no-load to full load transition. If the motor happens to be driving a load where the torque is nearly constant, this adjustment is usually unnecessary. But often, motors drive a load with a heavily changing torque requirement, and fluctuations in speed may be unacceptable. A word of caution: Too high of an IR compensation setting results in a speed increase; as the load is applied, the motor is actually forced to run faster. With this, the voltage and current to the motor increase, which in turn speeds the motor up further. Such an improper adjustment causes an unstable “hunting” or oscillating condition.
Acceleration time adjustments moderate the acceleration rate on the drive. This can mean extending or shortening the time a motor takes to get up to set speed, and can also mean controlling the time it takes to change speeds from one setting (say 50%) to another setting (perhaps 100%).
If too rapid an acceleration time is called for, it will be overridden by the current limit. Acceleration will only occur at a rate that doesn’t conflict with the current level sent to the motor through the regulator. Also, on most small controls the acceleration time is not linear – in other words, a 50 rpm change might occur quicker at low speeds than it does as it approaches the set point speed.
Deceleration time adjustments allow loads to slow over a longer period of time. For example, if power is removed from the motor and the load stops in 3 seconds, then the decel time adjustment would allow you to increase that time and “power down” the load over a period of four, five, six or more seconds. A conventional simple dc drive will prohibit a slowdown duration below the “coast to rest” time.
Edward Cowern is a retired District Manager in New England for Baldor Electric Co., Fort Smith, Ark.