Speed control

Are you thinking of an induction motor or a brush motor? Brush motors are variable speed based on voltage whereas induction motors are locked to supply frequency. In the case of the induction motor, consider the number of poles as a multiplier.

So in a brush motor run on AC you can “chop” the top and bottom off the sinewave to alter it’s effective voltage and control the speed. Bung the input from the tacho into a feedback circuit and bob’s your proverbial.

Induction motors are a different kettle of herrings as you have to alter the frequency to alter the speed (unless you’re looking at one of the old SOLE multiwound multispeed motors…). So this is why the latest big WMAs have an inverter with it’s associated high voltages. Now you’re rectifying mains AC, then using the resultant 400v or so DC to power the motor- it’s switched to each of the three phases of the motor in rotation, and the speed of the motor is locked to the speed of this switching.

make sense?

David

Re: Speed control

dpm – Whilst it might be possible to control a ‘brush’ motor in the way that you say this is not how it is done on the vast majority of washing machines.

In the case of a series wound AC motor (using brushes) as found on most washing machines control is achieved by varying the amount of current available to the motor. This is done by employing a triac in series with the field coil and armature. The full mains voltage can be found across this circuit. Control of the triac and thus the motor current is achieved by electronic means.

A triac is effectivly two silicon controlled rectifiers (SCR’s) fabricated back to back on a single chip sharing a common trigger connection (the gate) two other connections MT1 (main terminal 1) and MT2 are across which we deploy the load, in our case the motor. The reason two scr’s are used in this manner is so that both polarities of the phase can pass through the device and thus make full power available. With no signal applied to the gate connection, no current passes through the circuit, but when a pulse signal is applied to the gate the triac will ‘fire’ and allow maximum current (dependant on the reactance / resistance of the load) to flow. Once the triac begins to pass current it will continue to do so until the voltage across it is reduced to a certain level. In the case of our AC supply this occurs as the half wave (eg + transition) passes through the 0V level, at this point the triac will switch off and cease to conduct.As the cycle commences into the – half wave the triac needs to be fired again in order to allow current to flow again. So two pulses are needed for each complete cycle.

The secret to controlling the motor speed in this manner is to vary the point at which we apply this gate pulse relative to the angle of the phase. In most cases this is achieved by the use of a dedicated motor controller IC such as the TDA1085 which was used on many speed control modules, although the earlier modules eg. Hotpoint did use discrete componants only. The timing for the gate pulse is usually derived from sampling the mains frequency, processing this sine wave into a suitable pulse signal and then by delaying this pulse to fire the triac at a suitable angle of the phase. Obviously the later the pulse, the further the phase progression and so less current is allowed to pass.

As Kenny rightly states feedback from the motor tacho is passed to this control circuit to inform the electronics how fast the motor is rotating. This signal was nearly always analogue ie. the amplitude increased relative to the increased speed of the motor, (although the TDA1085 could also utilise a digital signal). The received tacho signal is processed and its value compared to a preset value derived from an external resistor network. Each element of this resistor network corresponds to a selected motor speed / ramp and is switched into use by the timer, relative to the progression of the wash cycle ie. wash, distribute and various spin speeds. With this feedback the control circuit is able to monitor the motor speed and adjust its output to the triac in response. By adding a variable resistance into the network element motor speed control can be adjusted by the user ie. variable spin control.

Most tacho generators nowadays are digital, that is the signal that they output is in the form of pulses. The reason for this is that with the advent of timers / modules that use a microcontroller IC it is much easier to count the number of pulses in a given period than to convert an analogue signal into a form that could be usefull. (eg. analogue to digital conversion)

This type of motor speed / ramp control has also been used with induction motors. The original Bendix induction motors did not have any control at all and were a little violent sometimes when entering the spin mode. Bendix later introduced the models with the small module. These only had two selectable speeds but the main advantage was that this also enabled the current to be gradually increased (ramp) into its full spin speed, rather than a sudden switching of current to the motor.

Andy

Sorry David I did’nt notice your real name on the previous post.

The statement that I could’nt agree with was:-

dpm wrote:So in a brush motor run on AC you can “chop” the top and bottom off the sinewave to alter it’s effective voltage and control the speed

In this you refered to removing the top and bottom of the sinewave ie. decreasing the amplitude or voltage. In actual fact phase angle control is time slicing the signal and does not necessarily decrease the voltage. Consider a case where the pulse is applied at 90deg and 270deg of the cycle. On both half cycles the signal commences at full amplitude (-ve & +ve) and decreases to 0V passing current only during this period. The actual power delivered is therefore approx halved.

I noted with interest your comments on the new 3 phase motor. Jimboxxx has also commented on this in another post. I would be really interested to learn more of this if yourself or anybody has any info.

Regards Andy

Quote andy2
I noted with interest your comments on the new 3 phase motor. Jimboxxx has also commented on this in another post. I would be really interested to learn more of this if yourself or anybody has any info.

Think of the little dc brushless fan motors (basically an induction motor) where rotation is achieved by electronically pulsing the poles (usually at a fixed rate) to achieve rotation rather than 50 cycles (hertz) alternating current pulsing the poles (at a fixed rate) to achieve rotation

The 3 phase dc brushless motors are exactly the same as the small dc brushless except that the rotation speed is varied by the electronics in the controller which (unlike the small dc brushless) are external/remote from the motor

Also good as a stepper motor, and also good for electronic braking

http://www.muirheadaerospace.com/pdf/Pr … Aug04a.pdf

Re: Speed control

Cracking stuff you guys! Thanks.

Chris.