Bilge pump DC motor speed control

[aquarius]

I try to control speed of 12V DC, 5A bilge pump through my PC soft and KBIC-120 variable speed drive. There is a range of applied voltage from the drive
3 to 12V DC. There is a fuse 8 Amp between pump and drive. When it starts, the fuse burns out. I've tried to use 10 Amp fuse and after several minutes pump motor burned out. I've already tried the same way, but with regular
12V DC car fan and everything worked perfect. I understand that when the pump works with load it takes more current. How to resolve this problem?
Need advise.
Thank you

 

[Diver300]

There must be something wrong with the pump if it burns out.

A 12 V bilge pump should run continuously on 12 (or even 14) volts without any problems.

The start up current of a motor could blow cause a fuse to blow, so it can be difficult to have a fuse that can protect a motor properly.

 

[MikeMl]

Why do you need to speed-control a bilge pump? It seems to me, a float-switch, with the motor running at full speed when it has something to pump works just fine in my boat...

 

[aquarius]

It's a project. Forget about pumping water from boat.
The pump has to perform smoothly up and down. I can easily control AC,
3 ph pumps, but they are to big for my application. That's why I try to use small bilge pump.

 

[crutschow]

Did you not read the data sheet and notice that the KBIC-120 is designed to operate off 120VAC and drive 90V-130V DC motors? Thus it likely overdrove the 12V bilge pump motor, causing the blown fuses and the burn-out. That fact that it worked with the blower motor was likely just a fluke. You need a 12V adjustable power supply, either linear or, for better efficiency, switching.

 

[aquarius]

As I've mentioned before with my PC soft I can obtain from KBIC-120 any output DC voltage, from 0 to 130V DC . For the bilge pump it had been adjusted from 3 to 12VDC. I was told probably 3V is too low for the pump motor, but is OK for the fan motor.

 

[Diver300]

I hadn't realised what the KBIC-120 is. It is designed for driving DC motors from and AC voltage that is about the same magnitude as the DC motor. The KBIC-120 doesn't actually reduce the voltage, it just turns it on for some of the time and off for others, at twice the mains frequency.

The output waveform at 12 V will be far from a steady 12 V. When running a 90 V motor, the inductance of the motor winding will keep the current reasonably constant as the supply turns on and off. The supply is turning on and off 120 times a second, which is faster than you can see or your meter can respond to, but it is slow for your motor.

A 12 V motor has far less inductance so the current will be varying a lot more, and probably reducing to near zero at times. This means that the peak current will be a lot higher than it is designed for, so the heating of the windings is too much.

The heating is proportional to the square of the current, so twice the current gives 4 times the heating. Twice the current for half the time, which gives the same motor torque, gives twice the heating.

Also your 12 V motor is not designed for the high voltage peaks that the KBIC-120 gives out. That may be damaging the brushes.

As others have said, get a 12 V supply and a float switch. If you need to control the speed, get a speed controller for low voltage DC motors. They will keep the peak voltage much lower, and they will turn the supply on and off much faster, maybe 20,000 times a second, so the current in the motor won't change so much.

 

[aquarius]

Thank you for the detailed explanation. What about 12VDC car fan. It works fine from the same controller and motor is not overheated.
I was searching for low voltage DC motor speed controllers which would accept 0-5V or 0-10V controlling signal and couldn't find similar. Would you recommend something?
Thank you.

 

[Diver300]

Thank you for the detailed explanation. What about 12VDC car fan. It works fine from the same controller and motor is not overheated.

There could be more inductance. I really don't know.

I was searching for low voltage DC motor speed controllers which would accept 0-5V or 0-10V controlling signal and couldn't find similar. Would you recommend something?
Thank you.

Sabertooth 2X5 regenerative dual motor driver. Analog, R/C and serial motor control

I don't have any experience of those, but the specification says that it does what you want. You will need a 12 V supply for it.

 

[aquarius]

Thank you again for the link. Looks like what I need. Any way I have to test it first.

 

[electech52556]

This looks like an old thread, but I couldn't resist offering a solution for others stumbling upon this...

Basically, one of the best methods for controlling DC motors is to utilize PWM control. The pulse-width modulation can be obtained by using a circuit with a LM555 connected as a astable multivibrator. The square wave output from said circuit can drive a MOSFET to boost the power needed to suit your application. A MOSFET like the IRF520 should be suitable for motors requiring up to around 9 amps. Essentially, on an oscilloscope the square wave would start at 0VDC and propogate up to 12VDC with a fast rise time and return to neutral potential. The amount of time the square wave is on (12VDC), the faster the motor will turn. Obviously, the opposite is true; the more time the square wave is at 0VDC (off), the slower the motor will turn.

This circuit is so popular that there is a special chip which monitors temp and gives a PWM signal to control the speed of DC fans for cooling purposes on equipment. I can't recall the exact chip, but a quick search could track it down.

Likewise, a quick Google search of "PWM control of 12VDC motor" will reveal a ton of sites with schematics and kits to build a cheap DC motor controller based on the cheap and popular LM555.