# Amp reading truths



## zen (Jun 15, 2009)

We were checking on a 3 phase 20 amp blower motor , fed with # 10 awg thwn. They are fed from a mcc that has thermal protection. When I check amps it goes as high as 100 amps @ start up. 

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## Vintage Sounds (Oct 23, 2009)

Ok, and what's the problem? Motor starting current can be 6-8x the running current.


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## zen (Jun 15, 2009)

It trips the mcc breaker usually on day where its in heavy use. This blower is used to dry cars so it comes on by a time relay. When a car gets to a certain point it slowly starts then when the car has passed it slowly winds down til the next car. The mcc and the motor would be others equipment. Only our conductors. I am trying to understand what causes it to trip when it cycles all day and also how accurate is an amp test based on testers

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## Big John (May 23, 2010)

There's a possibility it's even higher than that. Many multimeters don't have a sampling time fast enough to capture peak starting current.

The breaker that trips, what type is it? Is it thermal magnetic? 
How often does the blower start? 
When you say the blower "starts slowly" is it actually ramping up through a starter or is it going across the line and just drawing high current until it gets up to speed?


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## 3xdad (Jan 25, 2011)

Sheesh, with this setup, the carwash might lower costs by just eighty sixing the relay.


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## NC Plc (Mar 24, 2014)

That's the inrush current and it is probably going higher than what you are reading. You should be able to get the inrush current figures from the manufacturer of the motor.


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## zen (Jun 15, 2009)

Thermal nagnetic. Starts with starter I guess vfd.. the blower starts based on the time a car goes through. Sometimes back to back starts sometimes minutes apart I probably cant give you enough correct info. I was there with ny boss just watching and learning. With it not being our equip he documented what he checked and left it for the equipment co.

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## varmit (Apr 19, 2009)

This is a common problem with car wash "kits". If this fan is on a VFD, it should stay running continuously if cars are in line and delay for several minutes after the last car is dried before shutting off completely. In between dry cycles the VFD/ motor should ramp down to a slow speed- but stay running.

If this motor is on an across the line starter and is stopped and started every minute or two, neither the motor, starter or breaker will last very long. The reason: Starting a motor repeatedly with very little run time between starts causes the motor and all of the circuit components to overheat dramatically due to the repeated high inrush currents.

Most all motors have a maximum starts per hour rating. From your info I would guess that this is around a 7 1/2 HP (6 KW) motor. It probably should not be started more than six to eight times an hour.


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## zen (Jun 15, 2009)

Mr Varmit. I dont know motors and yes it has a vfd , and while im watching it cycle I thought basically what you said. While its winding down it begins to restart and to me that looks like wrong. My boss did suggest what you said about run time is better on all the equipment and our wires to continue running until the sensor shows no cars a period of time. You said it should / could be set to ramp down for inbetween cycles. . I like that idea. Youre a knowledge able man and I think you all for letting me learn in a day what could have taken a lot longer

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## JRaef (Mar 23, 2009)

varmit said:


> This is a common problem with car wash "kits". If this fan is on a VFD, it should stay running continuously if cars are in line and delay for several minutes after the last car is dried before shutting off completely. In between dry cycles the VFD/ motor should ramp down to a slow speed- but stay running.
> 
> If this motor is on an across the line starter and is stopped and started every minute or two, neither the motor, starter or breaker will last very long. The reason: Starting a motor repeatedly with very little run time between starts causes the motor and all of the circuit components to overheat dramatically due to the repeated high inrush currents.
> 
> Most all motors have a maximum starts per hour rating. From your info I would guess that this is around a 7 1/2 HP (6 KW) motor. It probably should not be started more than six to eight times an hour.


To his point on Starts-per-hour... see Page 11 of this document

Just because you hear the blower sounding as though it is accelerating slowly does not mean it has a VFD or even a Soft Starter. The type of squirrel cage blowers used in car washes often have a "spin-up" time that sounds longer than normal, but the start is still done across-the-line. 

With a cheap Thermal-Mag breaker that does not have any andjustments for the magnetic trips, it's quite common to see nuisance tripping. Some factory settings are 400%, some are 6-800%, some are more. There is no exact standard on a T-M breaker, as long as it is not over 10X the breaker rating. That's why on motor starters they like to use MCP (mag-only) breakers, they give a wide adjustment range because the NEC will allow a lot more for AC motors, depending on circumstances. Check your breakers to see if the mag-trips are adjustable. If they are, read all the rules in 430.52 on adjustments and turn them up as high as you can in your circumstance. If not, and there is no VFD or Soft Starter, condiser adding one or the other. As varmit said, a common use in car washes is to have a VFD that just lowers the speed between cars, then turns off only if there is no new car coming in for a few minutes. The blowers sometimes have heaters in them, especially in colder areas, so turning them off and back on can also damage the heater elements.


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## micromind (Aug 11, 2007)

If the motor is started across the lines, instead of a full-fledged soft-start, I've used Allen Bradley STC (bulletin 154) soft-starts. (JRaef, help me out here, it's sort of a distant memory and I may have the numbers wrong......lol)

This type of starter is installed downstream of the regular across-the-lines starter. When the starter closes, this device brings the motor up slower than blasting it with full voltage. It resets itself when the starter opens. 

I don't know if it can catch a spinning motor or not. 

And no, I don't work for Allen Bradley, I simply use their stuff whenever it's my decision, and I know their product lines. I'm pretty sure other manufacturers make the same item. 

Rob


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## Big John (May 23, 2010)

Based on the current it sounds like a DOL starter. I wouldn't expect a VFD to pull 100A when ramping up. I was just trying to clarify that "slow start" comment.


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## zen (Jun 15, 2009)

I may be mistaken. I thought I was told it was a vfd. It starts slowly and stops slowly. I dont know much about motors and mcc cabinets. This is the first job with them for me. We ran the wire and made the connections and so when a motor trips we get called first to see whats wrong. I dont believe its our conductors yet they call us every time . Eventually the equipment co. will solve this problem but I know the more I know today the more I can apply on the next project

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## Vintage Sounds (Oct 23, 2009)

Is it possible that if the contactor is closing into an already-spinning motor it could be doing so at a time where the motor rotation and the supply frequency are out of sync, causing a current spike sort of like in an open-transition wye-delta starter?


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