SCR based DC motor Speed Controller

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umery2k75

Joined: 19 Apr 2006
Posts: 378
Helped: 24
Location: Pakistan

25 Dec 2007 14:57 speed control of dc motor using scr

Hi,
I'm developing Motor Speed Controller for Permanent Magnet Synchronous Motor(PMSM).I don't know why people love PWM so much for controlling the speed of the DC/AC motor.Every where I see is the PWM method for speed control, mostly in H-bridge manner equiped with transistors.I have my university final year project that is SCR based DC motor(1-phase) speed control in closed loop.So I'm desiging this project and I'm controlling the speed by the varying the firing angle of SCRs.The motor drive which I'm developing is AC-DC, that is it takes the Alternating Current to get Power.This motor operates in the 1st quadrant where both Voltage and Current is positive

Any comments,suggestions and view points will be highly appreciated Smile

I bought this Permanent Magnet Synchronous Motor:
=======================================
D.C Motor 5 Horse Power
240 V, 16A, 2050 RPM
Single Phase

I am using 8051 microcontroller at this time for controlling the speed of the motor and this motor only has armature control.The speed will be set by entering the RPM through Keypad and speed would be control by Proportional Intergral controller, I will develop PI algorithm in the code, so I don't need to make extra analog circuit for this and I will have the LCD for the output of LCD and for feedback I am using bicycle dynamo for the feedback( I'm using this as RPM to Voltage converter-> the higher the wheel spins, it's corresponding voltage would be generated, so I can know with what speed the wheel is spinning).My circuit has zero cross detection circuit, I'll connect this output with interrupt of the 8051 controller, as soon as the (AC sine wave) 0 Voltage is reached, I'll come to know about the position of the sine wave and then fire the corresponding SCRs from 0-180 degrees ranging depending on speed required.

SCR based DC motor Speed Controller

This tire will also contains the braking mechanism, as soon as I apply the brake the controller would sense the increasing of load and will reduce the firing delay for SCRs to generate more power to keep the wheel spining at constant speed in NO-LOAD to FULL-LOAD region.
The initial circuit which I have designed is this, I didn't get the time to make the proper schematic, I'll replace them as soon as I can.

That's the initial design

Feedback portion

The bicycle dyamo voltage is feed into ADC by which controller can sense the speed of the spinning tire.

It's power circuitry

I am also thinking of replacing the optoisolators with the pulse transformer as a better substitute( I don't know whether I'm correct or not) and to add the snubber circuit in paralled to Silicon Control Rectifier and Diodes for protection.My friend advised me to first design this circuit by using the smaller DC motor like 12V, because there is less likelihood for destroying the SCR's while making the circuit, because of high voltage/current and back emf.Once the coding for microcontroller is done and other necessary things, just simply replace the motor and few components with higher Voltage/Current ratings.

So I bought another small DC motor 12V and some BT151 SCRs which has current rating of 10A.

SCR based DC motor Speed Controller

For 220V DC motor 5 HP, BT152 SCR will be suitable which has current rating of 15-20A as my motor need 16A.

drbizzarow

Joined: 24 May 2006
Posts: 276
Helped: 9

26 Dec 2007 7:31 scr speed controller

PWM is easy to control

Osolong

Joined: 11 Jun 2007
Posts: 9
Helped: 1
Location: San Luis Potosi, Mexico

27 Dec 2007 19:21 scr speed control

Quote:

I don't know why people love PWM so much for controlling the speed of the DC/AC motor.Every where I see is the PWM method for speed control, mostly in H-bridge manner equiped with transistors.

Well, PWM it's easier than controlling SCR's to get the DC Voltage you want. Also more efficient. And with the H Bridge you can reverse the motor and stop it. Just a thought.

Quote:

For 220V DC motor 5 HP, BT152 SCR will be suitable which has current rating of 15-20A as my motor need 16A.

Mmmhhh I'm not sure, but remember that a motor can use 3 times the nominal current at startup. Maybe you need >48amps SCR's. You should check it before you buy them 'cause they're expensive.

umery2k75

Joined: 19 Apr 2006
Posts: 378
Helped: 24
Location: Pakistan

28 Dec 2007 16:01 dc motor speed control using scr

Quote:

Mmmhhh I'm not sure, but remember that a motor can use 3 times the nominal current at startup. Maybe you need >48amps SCR's. You should check it before you buy them 'cause they're expensive.

So do I have to buy the SCRs with more rating or is there any other effective method for controlling the inrush current at startup?

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pks123

Joined: 09 Jan 2008
Posts: 1

09 Jan 2008 5:54 scr based project

for your project you need to divide your project in following parts
1. Synchronising network (For detection of Phase(+))
2. Control Network (Usually Microcontroller)
3. SCR Firing Network (Pefr a Optocoupler MCT2E ).
4. ADC Network

since our power system is on 50/60 Hz.
thus each time you need to find out the startinf of phase via a ZERO CROSSING NETWORK.

Reply for mosre confermation

Regards,
Prashant
prashant_hcet(at)yahoo.com

umery2k75

Joined: 19 Apr 2006
Posts: 378
Helped: 24
Location: Pakistan

09 Jan 2008 7:32 scr control dc motor

Very correct prashant, this is how I also had divided my circuit.I was using the 8051 but I'm thinking to replace this one with the PIC16F877A and unfortunately last night I discovered that the PIC was dead.Right now I'm heading toward the local market for some semiconductor shopping...and I'll also purchase components to make the Incircuit debugger for the PIC too.

About the optocoupler, I'm getting little confused.Some people are telling me to use pulse transformer and some about optocouplers.Well I'll also purchase few optocoupler MCT2E and judge at the end, what would be the better substitute.

Tailgunner

Joined: 08 Jun 2008
Posts: 1

08 Jun 2008 6:47 dc motor speed control schematic

I have a DC motor with the same ratings: 240 V / 16A
It has a Tacho on the back of it. Basically i'm looking for a controller (or schematic) for it. Can someone point me in the right direction?

shadil

Joined: 06 Nov 2009
Posts: 2
Location: a.p

06 Nov 2009 6:42 Re: SCR based DC motor Speed Controller

hi man i had constructed this ckt before.but this does not have any dV/dt and dI/dt protection for that reason this might cannot be applicable.the thing with [size=18]pulse T/F is not so recomended where the area is compact, reason for this is, if use it we have to provide the over voltage protection ckt for the gate(R,Cand a zener diode) and it T/F has phase inversion i.e. 180° of o/p voltage and,also it consumes more area than a opto-coupler.[/size]
"Tell whether we can use driver IC (ULN28XX series of drivers) or an darlington opto-coupler to drive the gates of SCR from the µController ."
hope the reply will as early as possible.
Dilip

umery2k75

Joined: 19 Apr 2006
Posts: 378
Helped: 24
Location: Pakistan

06 Nov 2009 7:00 Re: SCR based DC motor Speed Controller

I haven't seen ULN28xx series being used to drive the gate of the SCR. They are sink circuit blocks. They sink current, you need to come up with the idea that you need to keep the gate of the SCR as reverse biased by keeping the driver on. Once they are off, the gate transistor inside the SCR must be forward biased. In other words, yes you can use them also.

It's not correct to say, that pulse transformer gives 180' out of phase signal. You can find inphase pulse transformer also.

When you talk about dv/dt or di/dt. You cannot give protection for dv/dt, di/dt. You need to see the SCR rating for that. When you turn the SCR ON at 90' sinusoid, it gives the highest dv/dt and di/dt across the SCR. For inductive load, you can add a leakage network or you call it as a snubber, you add this protection only for the zero crossing, because you want to turn your SCR off. It would help your SCR to turn off by reducing it's latching current of anode.

shadil

Joined: 06 Nov 2009
Posts: 2
Location: a.p

12 Nov 2009 7:59 Re: SCR based DC motor Speed Controller

there are pulse T/F which does not gives 180° but their terminals should be be reversed.the direction of induced EMF in a T/F can be known by Lenz`s Law.

umery2k75

Joined: 19 Apr 2006
Posts: 378
Helped: 24
Location: Pakistan

12 Nov 2009 14:37 Re: SCR based DC motor Speed Controller

Hello shadil,
You looked very confused by your posts. The model of pulse transformer can be thought of as a differentiator circuit with gain stage output.If you want to understand the 0' in phase output or 180' out of phase output. You must know the square hysterisis of pulse transformer and behaviour of it.Apart from it, there's no degree you can in between them.What you mean by forward and reverse terminals?