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Generators & Electric Motors General Discussion

DC Motor Speed Control


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  #1  
Old 08-10-2019, 09:04:17 PM
uglyblue66 uglyblue66 is offline
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Default DC Motor Speed Control

Ok,I am not up on all this new tech. I know on my 36 volt golf carts I have 3 resistors that control the speed of the motor.
Nowadays there is a Curtis controler for this.
I don't know much about all this.
I am wanting to learn more about how I could control the rotation speed of the dc magnet motors like on my Elec-trak.
I read about PWM controllers,and I found this on ebay while
https://www.ebay.com/itm/UCTRONICS-D...6df628109b51f7

Can you folks in the know shed some light?
I want to eventually put together a electric tractor made up of parts I have laying around.
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Old 08-10-2019, 09:33:47 PM
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John Newman, Jr. John Newman, Jr. is online now
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Default Re: dc motor speed control

First rule of Chinese PWM controllers:
Don't believe the Amp rating.
Assume (maybe) half of what they claim. In my old truck the heater / AC blower motor will not operate on anything but "HIGH". I can mostly live with that, but it would be nice to turn it down sometimes. Installed a Chinese PWM controller rated for 30 Amps. Fuse in the truck panel for the blower is a 20 Amp. Never blew one. First time I used the new PWM to turn down the blower speed it melted the plastic screw terminal strip. Removed that and soldered wires directly to the circuit board. Blew the 30 Amp fuse in the PWM. Replaced that unit with one with a 60 Amp rating and it has worked as it should.
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Old 08-10-2019, 11:52:59 PM
Vanman Vanman is offline
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Default Re: dc motor speed control

There's nothing wrong with the resistor control, except for the power wasted when running at reduced speed. If the motor spends the majority of the time at full speed, I would opt for the rugged simplicity of resistor control.

Twenty years ago, in the EV conversion circles, Curtis had a bad reputation and was known as Curse It. Don't know about today.

Keith
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Old 08-11-2019, 12:17:25 PM
Zephyr7 Zephyr7 is offline
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Default Re: dc motor speed control

You’re probably used to thinking in volts and amps. To understand PWM controllers, think in Watts, specifically “watt seconds”, discrete units of energy.

If you look at the PWM waveform over a finite period of time (like one second), the percentage of time the waveform is on relative to the percentage of time the waveform is off gives the total energy delivered to the load. Higher total on time in a given period of time = more energy delivered to the load.

For a given load (like a fixed weight of a golf cart, for example), more delivered energy gives higher speed. Each pulse of the PWM waveform delivers a known amount of energy to the load.

With a resistor, energy is diverted from the load into heat, which is wasteful. A PWM controller doesn’t have that problem since it delivers energy in controllable small increments. The inductance of the motor handles the averaging function between pulses. The “average” in this case is equivalent to the DC voltage that would have been delivered by a resistor-type controller. Note that PWM controllers often give better torque than other types since the instantaneous voltage delivered is always full voltage, which builds up the field in the coils faster.

Bill
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Old 08-11-2019, 12:48:05 PM
Tracy T Tracy T is offline
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Default Re: dc motor speed control

bill if i understand what you said correctly, its just like how a electronic fuel injector works? full voltage delivered in miliseconds of pulse, the more miliseconds on the more fuel delievered only in this case we are controlling the motor speed in the same fashion? yeah, i think i got it!
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Old 08-11-2019, 02:16:05 PM
Zephyr7 Zephyr7 is offline
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Default Re: dc motor speed control

Quote:
Originally Posted by Tracy T View Post
bill if i understand what you said correctly, its just like how a electronic fuel injector works? full voltage delivered in miliseconds of pulse, the more miliseconds on the more fuel delievered only in this case we are controlling the motor speed in the same fashion? yeah, i think i got it!
A little bit similar, yeah. Most electrical things can be explained using fluid analogies so letís try this:

On pulses of the PWM waveform are like squirts of fuel in a fuel injected engine. Longer on time = more fuel injected. More fuel means bigger bang in the cylinder, more energy delivered to the load, so higher speed for a given load.

Bill
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