Part 3a – Motor Speed Control.

There are three ways to control the action of a dc electric motor. The first is to turn the motor on and off by means of a switch. The second method is to control the direction of rotation by controlling the direction of the current through the motor. The third method is to control the speed of the motor by controlling the amount of current passing through the motor. The first two methods have been discussed in the first parts of this series of articles. The subject of this article is the control of the speed of the motor.

Before designing any circuit to control the speed of the motor by controlling the amount of current passing through the motor, it is necessary to determine the electrical characteristics for the motor to be used. The motor used in this article is the Meccano M5 motor.

A set of six M5 motors was assembled for the necessary measurements. An examination of the motors revealed that four were from one manufacturing run while the remaining two were from another. This was determined by an examination of the label affixed to the motor. The set of four motors had a label with only three lines of printing while the other two motors had a label with four lines of printing, the first three lines being identical with those on the smaller label.

Checking the labels, they were found to read:

Type: 6V/1000

Volts: 3 – 12 v DC

No Load: 140 ma.

J Max: 1.1 Amps

This name plate or label on a motor provides valuable information about the motor. However if further information is needed then measurement of the actual motor characteristics is necessary. The method employed was to make the measurements on each of the motors and then average the results. But, first since some of the motors had not be run for several years, they were connected to a source of 6 volts and run for 3 minutes to insure that they were in good condition and "limbered up" for the tests.

The resistance of the field coils was measured and the average value for the M5 motor was found to be 7.3 ohms. Then the inductance of the coils was measured and found to be 3.2 mhy at 1000 Hz. This measurement procedure required that the shaft of the motor be slowly turned until a minimum resistance reading was obtained which indicated that the brushes were making contact through the commutator to the coil.

Each motor then had its gear box set for a 3:1 ratio and in turn mounted on the test stand.

Next the shaft speed of the M5 motor was measured for a voltage supply from 2 to 12 volts. The motor circuit is shown in Fig. 35.

Figure 35. Motor test circuit.

The Variable Voltage Source was a laboratory grade variable power supply supplying 0-15 volts dc at 5 amp maximum. The voltmeter used is indicated in the diagram as a circle with a "V" in its center, and the ammeter is likewise indicated by a circle with an "A" in its center. Both meters used were moving coil, mirror back laboratory grade meters at 2% accuracy.

NOTE: Moving coil meters are best used for this type measurement rather than the Digital Multimeters because the continuous "off-on" of the current through the coil caused by the brushes on the commutator creates electrical interference for many digital instruments resulting in erroneous readings.

Each motor in turn was tested to determine the shaft speed at the applied voltage. The shaft speed was determined by using a hand held tachometer in contact with the shaft. The increase of speed with an increase in voltage was thus measured and the results were what would be expected. However, the speed increase was not absolutely proportional to the increase in voltage nor did each motor tested give the same speed output at the different voltage settings. Thus, the results were then averaged for the six motors to give a performance curve for the M5 motor.

The table of speed measurements appears below in Table 1. The average of all six motors appears in the right hand column.

Table 1. Meccano Motor M5 Speed Test.

When this data is plotted as a line graph, the speed increase for each motor can be compared:

Figure 36: Graph of individual motor speeds

A graph of the average speed vs voltage is shown in figure 37. In these two graphs, the speed is displayed on the vertical axis and the voltage on the horizontal axis.

Figure 37: Average motor speeds.

In the running of the tests, the current for the motors rose from 90 ma. to 160 ma. as the voltage was increased from 2 to 12 volts. At the two popular voltages for the motor, the current was 145 ma. for 6 volt operation, and 160 ma. for 12 volt operation.

If the motor was to be stalled and unable to turn, the current in the motor would immediately rise to a high value. The name tag on the motor indicates that the maximum current for the M5 motor is 1.1 amps. However, if the motor were to stall with a supply of 12 volts, then the current (computed with the aid of Ohm’s Law) could rise to 1.64 amps possibly causing the wire of the rotor to overheat, burn and ruin the motor. Why then, if there is no load on the motor, is the current only 160 ma. or 0.16 amps at 12 volts?

A DC Permanent Magnet Motor (the M5) is a motor and also a generator. This has been observed in several articles in the Meccano literature in which a handle has been fitted to the shaft of the motor and the resultant generator used to light a lamp, etc. When a motor is functioning without load, the unit is generating a voltage which is in opposition to the voltage applied to the motor from the external source. This opposition, sometimes called the "back EMF" reduces the amount of current flowing in the motor.

When a load is placed on a motor, it causes the motor to slow down, but the amount of current from the external source tries to keep the motor running at the desired speed. The external source of current increases in an attempt to make up for the drag – the more load placed on the motor, the more current that is required from the external source. In effect then, the current required by the motor is determined by the load placed on the motor.

This can easily be demonstrated using the circuit in Figure 35. If the motor shaft is squeezed between the fingers to produce a drag on the motor, the current being consumed by the motor will increase in proportion to the amount of drag.

It is interesting to note that in making the speed vs. voltage measurements, it was found that the minimum voltage required to cause the motor shaft to start revolving was only 0.46 volts and that it would continue to rotate until the voltage was reduced to less than 0.25 volt.

A very simple speed control for the M5 motor can be fashioned from a single series resistance. This can be an actual resistor, a variable resistor such as a rheostat, a diode, or a transistor. Figure 38 shows a suitable circuit using a rheostat for the series resistance.

Figure 38: Rheostat motor speed control.

The determination of the value of resistance for the rheostat is as much a math problem as it is a "cut and try" approach to find a value that works. To save the reader the trouble of determining the value, it has been found that for 6 volt operation, the optimum value for the rheostat is 60 ohms, but since that is not a value normally listed in the catalogues, 50 ohms is suggested. For 12 volt operation, the value is 100 ohms. Rheostats are rated in watts which reflects the amount of current that will pass through it to the motor. A rating of 25 watts is the optimum size. Smaller power ratings can be tried, but they will be subject to overheating.

While the adjustment of the resistance of the rheostat will control the speed of the motor, this is not as good a control as others which will be discussed later in this series of articles. The rheostat limits the current to the motor, and thus the amount of voltage across the motor windings – but that current is the same as the current in the rheostat. As the current increases under load, the voltage drop in the rheostat increases further reducing the current in the circuit. Thus, the speed of the motor is further affected by load.

Speed regulation in a motor is the ability of the motor to maintain a constant speed under variable load conditions. There are numerous methods available to accomplish the goal of good speed regulation. Certainly the series rheostat is not one of these.

If a method can be used to cause a "feedback signal" to adjust the value of the series resistance, then the speed of the motor could be held more constant with variable load conditions. This method is possible using solid state electronic circuits which will be included in further articles in this series.

Another technique, Pulse Width Modulation (PWM), can be used to realize a degree of regulation in the speed of the motor that is unobtainable in any other way. In PWM, instead of a constant current provided to the motor, a series of pulses of varying width are applied. This overcomes many of the problems inherent with a series resistance to control the speed of the motor

The best solution to date is to combine the feedback principle with PWM for near perfect speed control. This requires the addition of a Microcontroller. The one that will be used to illustrate the technique in a future article in this series is the Stamp II by Parallax. In preliminary tests, it is showing great promise for the programming of many functions for Meccano models as well as giving superior motor speed regulation.