One for the electrical boffs...

[ChasB]

If you run a 24V pump off a 12V supply you get half the power.

Or do you? I vaguely recall from my schooldays some calculations that showed that in fact you get only a quarter of the power (a half squared). Is that correct?

Also, what other dangers are there in running a 24V pump off a 12V supply?

 

[alan]

Assuming that resistance of the circuit is constant then you are correct, you get a quarter of the power.
P = V^2/R

 

[savageseadog]

To cover the other part of the question, no danger.

 

[andy01842]

A electric motor is inductive not resistive. So at a lower voltage the motor will run slower this means the frequency will be lower and thefore the reactance will be less and this will cause an increase in the ampage which in turn caould cause the motor to burn out!

 

[ChasB]

So the dodgy boatyard's (we meet in court in a few weeks) reassurances that it will run fine are twoddle.

And I would never have even known had it not been for close inspection of their computer printed invoice...


In additon there are several 'u' bends to prevent back-siphonage. Without sufficient power to create a strong water current through the system it will eventually silt up with waste (much like the u bend in your home sink eventually does) thus creating more strain on the motor, which then burns out...

 

[jeremyshaw]

I am no electrician, but I have a 24v pump fitted as standard to my 12v system to circulate cooling water for my freezer. I was told that this is quite common to get a reduced flow AND longer pump life. Had no probs with it in 3 years.

 

[BAtoo]

IMHO if you have a 12V system then the yard should fit a 12V pump - unless the correct pump is unobtainable & they had your express permission beforehand.

 

[ChasB]

Unfortunately that was only one of several things they did wrong. I can't say more because we'll be in court soon.

 

[gjeffery]

"A electric motor is inductive not resistive. So at a lower voltage the motor will run slower this means the frequency will be lower and thefore the reactance will be less and this will cause an increase in the ampage which in turn caould cause the motor to burn out! "

This opinion is straight over my head; I would be very interested to know the assumptions on which it is based, and any references to support it.
dc motors are complex, and it would be unfortunate if the original poster were misled.

There are three basic types of dc motors, depending on how the field is energized. Series, parallel (shunt) and independently excited motors have distinct characteristics.

The simplest type is the separately excited, including permanent magnet motors; these have a constant field flux density. The motor in question may be of this type.

The speed of such a motor depends on the armature current, and speed control is obtained by controlling the voltage applied to the armature. Within the operating range speed and armature current are proportional.

Separately excited motor
speed is proportional to voltage
torque is proportional to armature current

Series motor
speed is approx proportional to the inverse of the armature current
torque is proportional to the armature current squared


Shunt motor - mostly used for traction applications
speed is approx proportional to voltage (and almost independent of armature current)
torque is proportional to armature current

For first pass analysis, hysteresis effects are ignored.

I have known small motors to cook when a low voltage is sustained. There is insufficient torque produced to rotate the armature, the motor stalls and a single armature winding is continuously energized, but because the armature is not turning, there are no back emf or hysteresis effects due to the "frequency" with which the armature circuits are switched.

 

[andy01842]

I would basically agree with gjelfery But the speed of a motor is dependant on the voltage and the current i.e. the power and also the load. Some motors have cooling fans so if run slowly the fan is less efficient.

“I have known small motors to cook when a low voltage is sustained. There is insufficient torque produced to rotate the armature, the motor stalls and a single armature winding is continuously energized, but because the armature is not turning, there are no back emf or hysteresis effects due to the "frequency" with which the armature circuits are switched. “

If you measure the resistance of a stationary dc electric motor and apply ohm’s law
You do not get the figures that you get in practices with a running motor. The “back emf or hysteresis” is induction. A motor that is slowed due to load pulls a higher current because the back emf/ hysteresis/induction is lower. In the same way if you measure the dc resistance of many aerials they measure 0 ohms but at 160Mhz they are 50 ohms. The unit of induction is the henery.

At the time I replied to chasB I did not know their was a court case I think he needs someone much better qualified than me but as has been said a 12volt system should have a 12 volt motor.

This explains much better than me.
[url="http://www.sos.siena.edu/~rf.../~rfinn/courses/phys120/P120-11-induction.pdf [/url]

 

[david_bagshaw]

to preven syphoning the u has to be upside down, so wont collect silt.
also it is some times, though not always, vented at the top.

 

[ChasB]

The pump in question is made in 12V and 24V versions. There was no reason to fit one of the wrong voltage, nor has any reason been offered.

The boatyard did not inform me before nor after of what they'd done. I think their 'engineer' simply made a mistake and thought he was dealing with a 24V boat! But how he made that bad a mistake is beyond me. The boatyard refuses to acknowledge any error.

The fact that I was not told before nor after, and the job done is not what any reasonable person would believe is what I had asked for is suffiicient to maintain a case of breach of contract, as well as a breach of the Sale of Goods and Services Act - and even of the Misrepresentation Act! (I've been doing my homework!)

The fact that there is also the possibility that the installation is unsafe only compounds the matter.

Oh, and as it happens the pump outlet was left disconnected resulting in water being pumped into the engine compartment, and a hole in the hull left open...

This is meant to be a prestigious boatyard!

Once this case is over I shall pass this on to the Ofiice of Fair Trading. The bit is firmly between my teeth! /forums/images/graemlins/mad.gif

I hope they're reading this... /forums/images/graemlins/grin.gif