scr motor controler - Page 1
Forum section: electronics / projects

author: House91320
author_rank: Regular Contributor
timestamp: November 27, 2010, 09:37:14 pm
content: Could i use a scr to control a dc motor

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 27, 2010, 09:57:55 pm
content: It's very common to use a SCR as a controlled rectifier to operate DC motors.

author: tecman
author_rank: Frequent Contributor
timestamp: November 28, 2010, 12:52:41 am
content: To control a DC motor is OKay, but if the supply is DC you will have a problem. Once you fire an SCR, it will not turn off until the controlled current through it goes to zero. If the supply is AC, its easy. Each 1/2 cycle the voltage goes to zero and the SCR shuts off. If you have DC, the voltage never crosses zero and the SCR will remain on. The standard solution is to use a second SCR and a tapped inductor. The second SCR momentarily diverts the current, shutting off the first.If you are using DC as a supply, better to use an IGBT.Paul

author: House91320
author_rank: Regular Contributor
timestamp: November 28, 2010, 02:57:35 am
content: the reason i want to use a scr is because i am controlling a golf cart and that motor stalls a 300a

author: Time
author_rank: Frequent Contributor
timestamp: November 28, 2010, 03:24:30 am
content: A golf cart is powered by a bank of batteries which is DC. I guess I am confused with how you would use a latch device in this application. Maybe something like a GTO thyristor is more up your alley?http://en.wikipedia.org/wiki/Gate_turn-off_thyristor

author: TechGuy
author_rank: Regular Contributor
timestamp: November 28, 2010, 04:44:15 am
content: Better off using high current low voltage MOSFETs. I assume since it s golf cart motor the voltage is low (12/24/36) You can purchase 600A 40V Mosfets such as IXTX600N04T2 which has an Rds-On resistance of 1.5 milliohms. Ideally a MOSFET is better than IGBT\GTO because it will be more efficient at low voltage. IGBTs\GTO usually have a significant voltage drop typically aroung 2.1V (Vce) with a 12V system, the voltage drop will reduce voltage to about 9.9Volts. You can also parallel multiple MOSFETs to spread the load and reduce Rds-ON. If you run a parameterize search on Digikey, Mouser, etc you should find a significant number of MOSFETs that meet you design needs.

author: Zero999
author_rank: Super Contributor
timestamp: November 28, 2010, 08:35:32 am
content: I agree, use MOSFETs but you'll need to use a large gate drive circuit because the input capacitance will be huge.

author: qno
author_rank: Frequent Contributor
timestamp: November 28, 2010, 07:28:23 pm
content: Check out some controller manufacturers.Usually a golf cart has an encoder in the motor to detect and control speed.There are some safety issues here.You should prevent the cart run off without a driver.Also going down a hill should maintain the selected speed.Also prevent the controller from blowing up when the motor acts as a generator going down hill.I think it is easier to buy a controller off the shelf (COTS).

author: Zero999
author_rank: Super Contributor
timestamp: November 28, 2010, 08:49:14 pm
content: Yes, safety is important. Don't forget that semiconductors have a nasty habit of failing closed circuit and the contacts on switches can weld together. At the very least, there should be an emergency kill switch which disconnects the battery when pressed or to have the switch turn off every time the accelerator is released. An emergency stop is less likely to fail but there's a risk it could move when the power is on and there's no driver. An accelerator switch is more prone to failure but it's not possible for it to move when there's no driver. Either way there should be a physical set of switch contacts in series with the motor, in addition to the semiconductors.I'm not sure that maintaining a constant speed when going down hill is important, no vehicle I've ever driven does that and the only time I think it would be important would be for cruise control which I doubt would be a feature of a golf cart.