Motor Control with Meccano

Part 2a – Direction of Rotation



The M5 motor once supplied by Meccano is a permanent magnet motor, designed for a voltage between 6 and 12 volts.
There is a reversing switch on the rear of the motor case which in effect reverses the polarity of the voltage applied to the motor.
The direction of rotation for a permanent magnet motor is determined by the polarity of the voltage applied to the motor.
The direction of rotation for the M5 motor is determined by two factors, the polarity of the voltage applied to the two jacks and the setting of the reversing switch.
If the handle of the reversing switch is tilted toward the positive jack, then the rotation of the motor shaft as seen from the front of the motor is clockwise.
For this discussion, the clockwise direction will be called the “forward direction”.
This is shown in Figure 21.
Note that the positive wire (red) is connected to the left hand jack and the negative wire (green) is connected to the right hand jack.
The handle of the reversing switch is tilted toward the left hand jack



Fig 21. Rear of M5 motor. Set up for forward direction.

If the direction switch is changed to its other position (handle tilted toward the negative jack), the shaft will then turn counter-clockwise (reverse direction). This is all well and good if the object is to control the direction of rotation of the motor from the motor itself.
There are however instances when it is desirable to control the direction of rotation from some remote position. Meccano provided for this in the Battery Box (#620) supplied with the Electronics Kit.
The small switch on either side of the box controls the polarity of the voltage supplied by the box. This slide switch has three positions, On – Off – On.
However, the two “on” positions are of opposite polarity.
Thus, if the model’s source of power is the Battery Box, then the “Off – On” and the direction of the motor can be controlled from this location.
This article is concerned with the switching circuit exemplified by the one in the Battery Box.
The circuit is a classic one in electronics, “Polarity Reversal Switching”.
At a minimum, this requires a DPDT (Double Pole Double Throw) switch which can be any convenient switch type or even relay contacts.


Fig 22 Polarity Reversing Switch.

There are some conditions when a SPDT switch can be used, however this circuit requires two power sources of opposite polarity.
If batteries are used for a source of power, this just adds an additional expense.


Fig 23 Polarity reversing using two sources of power.

A relay provides an elegant method for changing the direction of rotation of the motor.
However, since a DPDT relay is desired and only a SPDT relay (#606) was provided with the Electronics Kit, one must either acquire a DPDT relay or if using exclusive Meccano parts, adapt the Meccano relay to the job of polarity reversal.

One trick is to connect two Meccano relays with their coils in parallel.
The total number of contacts then available provides the action of a DPDT relay.

Fig 24. A Meccano DPDT Relay.

Schematic Symbols:

Perhaps, at this point some coverage should be given to the schematic symbols which are being used in the diagrams.
The schematic symbol for a cell, and the symbol for a battery are shown below in Fig 25. Note that the symbol for a battery is composed of a number of cells connected in series.
In some instances, the voltage of the battery can be determined by the number of cells used in the symbol.
However in high voltage (over 6 volts) batteries, this becomes somewhat awkward and so only two or three cells are shown in the symbol with the voltage rating displayed beside the symbol.

Fig 25 Symbols for cell and battery.

The symbol for a switch is an arm with as many contact points as necessary to determine the type of switch.
Several switch symbols are shown in Fig 26.

Fig. 26 Switch symbols

The symbol for the relay is shown below. The coiled line represents the coil of the relay and the straight lines beside the coil represent the magnetic core of the relay.
The contacts for the relay appear as switch contacts aligned so that the arm or movable contact is directly above the magnetic core.
In some instances, a dashed line extends from the core to the arms to indicate that they are all activated at the same time by the magnetic circuit of the relay.
This, however is not always necessary and is only done for relays which have 3 or more poles.
Relay contacts are called “springs” and are identified as common, normally open (n.o.), and normally closed (n.c.).
An examination of the springs of a relay show the similarity of these elements with the Wiper Arms (#531, #532, and # 533) which were part of the Electrikit Set.

Fig. 27 Relay Symbols

Relay Control Circuits:

There are several basic relay control circuits used in technology. However, only three of these are of particular usage in the making of Meccano Models.
The first is the Direct Control.  In this circuit, a switch is used to complete the circuit to the relay coil, thus energizing the relay causing it to activate the switching elements.


Fig. 28 Direct control of a relay. Note: The circuits shown in this discussion of control all use the Meccano Relay #606, either single or two in parallel when a DPDT relay action is needed. The supply voltage is 12 volts. The relay in direct control can be energized using a switch or may use a semiconductor amplifier to increase the sensitivity of the relay. This will be discussed when the use of amplifiers for the speed control of the motor is discussed later in these articles. With direct control, the relay is energized when the controlling switch is closed and will remain energized as long as that switch is closed. Shunt Control: Shunt Control circuit has a novel feature of deenergizing the relay coil by using a n.o. switch contact. When operating, closing switch S2 will effectively short the relay coil. The series resistor R1 serves as a load to prevent S2 from shorting the supply voltage. The value of R1 is determined by the value of the supply voltage and the resistance of the relay coil. To continue with the Meccano relay as an example; it has an average coil resistance of 680 ohms. It’s pull-in voltage is 6.1 volts. The recommended supply voltage is 12 volts. Thus the resistance of R1 should be such that it allows the voltage across the relay coil to be at least 6.1 volts, and should have a sufficient resistance to prevent excessive current drain from the source. A value of 330 ohms for R1 provides 7.4 volts across the relay coil, enough to insure pull-in. When the switch S2 is closed to short out the relay coil, the drain on the source is only 13.9 milliamps, a nominal value. Incidentally, a 14 volt lamp as supplied with the Electrikit can be used as a shunt resistor. It has an added feature of lighting up when the switch S2 is closed. Switch S1 in the diagram below is used to turn the relay off and on using direct control.



Fig. 29 Shunt control of a relay.

Lock-up Control
There is a relay circuit that can be used to control a motor that can be operated from push buttons using one as the “on” control. When pressed, this push button (S1) energizes the relay coil and in turn closes the lock-up contacts which being in parallel with the “on” pushbutton, lock up the relay so that it remains energized even when S1 is no longer pressed.

The”off” push button is used to break this lock-up circuit which in effect releases the relay. Switch S1 is a single “make contact” (n.o.) while S2 is a single “break contact” (n.c.).

Fig. 30 Lock-Up control of a relay.

This circuit is similar to that used on power equipment in the shop which has On-Off push buttons for control. It is not necessary to use push buttons for the switches. Relay contacts from other relays can also be used, in fact there are a number of circuits which feature several relays using a variety of relay control methods.


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