# How does a SCR controls the speed of motor

1. Mar 29, 2015

### freshbox

can you do it by increasing the series resistance?

2. Mar 29, 2015

### sophiecentaur

Yes. For a 'small' motor, using a variable series resistor can work quite well. Scalextric sets always used to do it that way, with a chunky-wound rheostat inside the handle of the controller. But, even relatively small motors can require quite a lot of heat dissipation if you use a purely resistive voltage adjuster / regulator.
It all depends on what sort of power you motor is rated at. More details please.

3. Mar 30, 2015

### meBigGuy

SCR is a silicon controlled rectifier. It is an ON/OFF affair, not a variable resistor.

It controls motor speed by the phase of it firing. If it only fires for a short portion of the waveform, the energy to the motor is small. so it goes slower.

http://www.seekic.com/circuit_diagram/Control_Circuit/SCR_MOTOR_SPEED_CONTROL.html shows an example. A simple search will turn up waveforms and circuit examples. Always try that before asking questions here (to better prepare a sensible question).

4. Mar 30, 2015

### rollingstein

The other way is to use a VFD and feed a different freq. to the motor right? I assume this is an AC motor.

5. Mar 30, 2015

### freshbox

What is the criteria to meet for a sensible question? I have my own doubts that's why I'm asking.Please let me know so the next time I won't post a stupid question here.

6. Mar 30, 2015

Hello Fresh - I do not think anyone was calling it a stupid question - but your title mentions an SCR and your post asks about resistance. -- There are many ways to control the many types of motors -- so we really do not know what you are asking about.

7. Mar 30, 2015

### Averagesupernova

SCRs and TRIACs generally are used to control the speed of universal motors. These motors are series wound and would be used on AC when SCRs and TRIACs are involved. A VFD would only be used on 3 phase induction motors. A VFD would be useless on a universal motor.

8. Mar 30, 2015

### sophiecentaur

It's very hard to ask a question that 'everyone' will see as being sensible. Your question was quite reasonable - except that you didn't mention the sort of power involved. That is very relevant for making a suitable choice of method. Like I said, a fat variable resistor is fine and cheap for very low powers.
The main reason that an SCR is not suitable for controlling DC is that it can be switched ON but stays on till the volts drop to zero. So an AC supply is necessary (see the documentation that's been quoted). But there are plenty of devices available these days that will turn on and off quickly and have a very low ON resistance, which means that they do not dissipate a lot of heat during the switching process.
Do NOT stop asking questions, btw!!

9. Mar 30, 2015

### freshbox

I really appreciate sophiecentaur, rollingstein and Windadct for helping and asking for more details. I didn't know it can be used to control many types of motors because what I read from my notes is for Synchro. I am actually confused between increasing the resistance or just by controlling the voltage supply.

10. Mar 30, 2015

### freshbox

sophiecentaur

11. Mar 30, 2015

In a sense you are controlling the voltage - in the resistance case you are really making a Voltage divider between the Rheo and the Motor... so for a larger motor - with higher current, you see losses in the Rheo / Resistor proportional to I^2.. so not really efficient. If you have an AC supply to control a DC motor - you can use the SCR on the AC side and then rectify - this will allow adjustment of the DC voltage... effective for higher currents, or a PWM DC chopper to control (chop) the DC supply voltage down to the desired Motor Voltage. Each method has its own benefits and drawbacks.

12. Mar 30, 2015

### rollingstein

Naive tangential question: What about an IGBT? Can it be switched on and off by cycling the gate current at the right freq. without needing an AC to drive it to OFF.

Does that produce modulated power similar to what phase control with an SCR does for AC?

13. Mar 30, 2015

### sophiecentaur

There are quite a number of devices for achieving what is in effect pulse width modulated power. If I wanted to achieve something like you want, I would first establish the required characteristics - Current, Volts, Possible Over-volts, operating frequency etc. etc.. Then I would look in catalogues / specs to find a suitable device (or cheat and look on the net for some ready designed circuits. I think power mosfets are pretty good in many cases.

I read the OP as if it related to a specific application. I may have been wrong and it may have been after some sort of review of methods in general. The OP hasn't told us yet.

14. Mar 30, 2015

### rollingstein

Thanks @sophiecentaur. You are right about the OP's application.

My question didn't pertain to the OP's problem. My question was tangential. Sort of academic I guess. I was merely imagining the DC analog of a partially chopped sine wave for power modulation applications.

15. Mar 30, 2015

### sophiecentaur

That would be PWM, then i.e. %age of the cycle that the device is turned on for. Frequency is of secondary importance, depending on what you need or can achieve (filtering and switching device) Given the choice, you'd probably use a frequency much higher than the Mains. You can do that in dozens of different ways.