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Topic Title: Load Current Rating For 3 Stage Bypass Vacuum Motor
Topic Summary: Load current to be clarified
Created On: 21 April 2014 10:41 AM
Status: Post and Reply
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 21 April 2014 10:41 AM
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ptracey63

Posts: 5
Joined: 05 July 2012

Hi ,
I have been asked to design a control circuit for a water suction system, similar to Scarab Wagon which is used on regular street drainage systems. The electrical system already proposed will be a 48 Vdc battery set,supplied fed from an electric vehicle, supplying 2 vacuum motors 24 volt,680W rated andwired in series.The aim of using the vacuum motors is to evacuate air from a water tank and draw water up a hose from the drain into the tank. My basic calculations for load current for the above circuit is Power /volts equals Current, in this case 680w/24 equalling 28.3 amps, and the fact that the motors are wired in series we would achieve the volt drop of 24 Vdc across each motor.
The type of motor being used is an Ametek 3 Stage Bypass Vacuum motor ,its motor termination cables are 2.5 mm,which begs the question what will be the start up current and the running current.
I have contacted the suppliers with a very limited response, hence my question.
All input would be much appreciated.

Thanks
PT
 21 April 2014 06:01 PM
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broadgage

Posts: 1357
Joined: 07 August 2007

The supplier of the motors may have assumed an actual supply voltage of 27 or 28 volts, this being the voltage actually achieved in practice in a truck etc. with the engine running and the battery on float charge.
A lot of electrical equipment for large vehicles is by convention referred to as "24 volt" but is actually designed for 27 to 28 volts.

In your proposed application, if the 48 volt battery is being charged, then the actual voltage will be about 54 to 56 volts.
If the battery is being discharged, then the actual voltage will be about 47 to 48 volts most of the time, and as low as 44 volts at the end of the discharge.

I would double check what the actual design voltage, and what the actual current will be at the actual voltage.

2.5mm sounds marginal and I would use at least one size larger.
 21 April 2014 08:20 PM
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ptracey63

Posts: 5
Joined: 05 July 2012

Many thanks for your response Broadgage. For information I have just run a test on the 680 watt 3 stage bypass vacuum motor, the supply voltage being 24V dc , I measured the load current using an inline ammeter which registered 18 amps.I have calculated that a secondary motor of the same type and rating connected in series across a 48V dc supply would draw a load of 18 amps.However , I am wondering if the load will increase at the point where water is being drawn to the tank, once all the air has been evacuated from tank and 3 metre hose.
I will definitely go with a 4mm size cable to cover all eventualities.
Thanks

PT
 21 April 2014 08:20 PM
User is offline View Users Profile Print this message



ptracey63

Posts: 5
Joined: 05 July 2012

Many thanks for your response Broadgage. For information I have just run a test on the 680 watt 3 stage bypass vacuum motor, the supply voltage being 24V dc , I measured the load current using an inline ammeter which registered 18 amps.I have calculated that a secondary motor of the same type and rating connected in series across a 48V dc supply would draw a load of 18 amps.However , I am wondering if the load will increase at the point where water is being drawn to the tank, once all the air has been evacuated from tank and 3 metre hose.
I will definitely go with a 4mm size cable to cover all eventualities.
Thanks

PT
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