Hi, I have a question for you Moparts electrical and HVAC gurus out there. I had an HVac service tech today tell me that he wants to replace the motor in a rooftop HV AC unit because it is not drawing enough amps. Is a 1.5 horsepower, 3-phase, 208 volt motor running the fan. He says it measured two amps on one leg and the rated amperage of the motor is 6 amps. So I'm wondering if the motor is larger than the fan requires, would it draw less amps? Also when measuring amperage of 3-phase, do you add the amps measured at each leg together to get the total nameplate amperage? Thanks, Joel

No
Each leg should not increase nameplate amps
You dont add them.
What prompted the original call ?
Did he have the door open while measuring amps
Was there a load on the motor
Is it the original motor to the unit ?
Is there duct attached to unit
More info necessary

get a new service tech !!

More info needed.

Assuming this a propellor type upflow fan, are the fins of the coil dirty?

On most common fans if the air flow is more and more restricted
amps will drop, not increase like on a piston water pump.

If blocked airflow is corrected and airflow now doubles,
Amps will not just double too,
but will increase slightly less than 2 x2 x2=8
( rises by the power of 2.5 or so)

Ask the tech to measure the fans airflow before changing the motor.
If the tech is well trained
he should have already checked the 3 phase motor leads for
continuity, voltage balance, current balance, and “megged” the leads to ground.

This is a 10-year old carrier rooftop packaged HV AC unit that heats and cools the dining rooms of a restaurant. it has an electronic programmable thermostat on the wall of the dining room, and runs on 208 volt 3-phase power, with gas heat, and a 1.5 horsepower squirrel cage belt driven fan.. The restaurant closed about 3 months ago, and the new tenants are getting ready to open a new restaurant next week. The building is in Minnesota. the tenant called me a couple weeks ago and said that heat wasn't working. I told them to call an HV AC company and gave them a name and number. They called me Tuesday night this week and said that they still didn't have heat, that they hadn't been able to get anybody to come out and look at it and they had called three different places. We had gone through a cold snap where it was 10 or 20 below and they didn't have heat, and now a sprinkler head for the fire protection system was dripping because it probably froze. So I called up a different company and they came out after hours Tuesday night and called me up from the restaurant and said that there is a safety gas valve lockout that is part of the fire prevention system and it was shut off. So he pressed the reset button as well as check into unit for clean filters and a good belt and the unit started up and was making heat. He charged me $365 and left. The next morning, yesterday, the tenant called and said it was cold in there and that the heater wasn't working. I went over there and met with them, and it was cold in there, but they had a salamander kerosene heater running in there. they told me that they had had two different contractors out to look at it, and neither one could fix it, but they both charged them money. So now we have a bit of an inconsistency in the story, because over the phone they said nobody would come out. I had the tenant show me this gas lockout safety thing. It was an electric enclosure on the kitchen wall with a reset button, and it was connected to another box that had a relay in it that was buzzing. I asked him if it needed to be reset again. He said when it's working right it buzzes and it was buzzing now, so it had to be good i pressed the reset button and it it felt like a magnetic switch that was already engaged so it didn't make any difference, it was still buzzing, the button didn't feel like it actually did anything. He showed me what he had been doing in the restaurant as far as remodeling and cleaning to get it ready for reopening, and within a half an hour the rooftop heater came on by itself. I had the tenant showed me the thermostat and talked to him about if he knew how to operate it. He assured me that he did and that it was calling for heat, and had been calling for heat even when the restaurant was cold. Then i left the site and I called the company again and they sent the same guy out. And after an hour or so he called me and said that he thinks the unit is locking out on high limit, due to poor airflow. He did not say, and I did not think to ask, if he had looked at the circuit board on the unit to read the flashing light codes. I thought about that later, and I would think if he had done that he would have told me about it. He said he measured the amps at the motor and got 2 amps and the nameplate full load amps was six. So he wanted to put in a new motor, new capacitors, and a new belt for $1,750. I asked him if the unit locked out on high limit how did it started self up again hours later? He said that he guessed that the tenants had reset it somehow. at this point I was a little skeptical, and didn't want to spend $1,700 based on a guess. He said that he wasn't sure if it was the motor and the capacitors that were the problem, or just the capacitors, but it was probably the motor. I asked him what the capacitors were, start capacitors, or run capacitors. He said they were start capacitors. I asked him what size motor it was because I was thinking the price tag was pretty high as well. He told me you wasn't sure he thought 6 horsepower. I asked him to check it for me so he went back up on the roof and looked at it and called me back and said it was 1.5 horse Power. I told him I was reluctant to spend that much money based on a guess and asked if he could guarantee that this would solve the problem. at this point he suggested that maybe I would want to start with replacing the capacitors only because it would cost less. I asked him if they were start capacitors, and the motor was starting and running, how would that affect the RPM of the motor once it was up and running? He said good point. At this point I was really losing confidence in him. I told him I was going to get a second quote and to pack up and leave and I would get back to him. He charged me $95 for the call. I'm thinking this morning I'm going to tell the tenant to put new batteries in the thermostat, to download Emmanuel from the thermostat from the internet., study it didn't make a hundred percent certain that this thing is always calling for heat and that it is not shutting off due to some program. And I'm wondering if this motor failure Siri holds water. after the tenant verifies the thermostat is working properly, I'll probably go over on the roof and look and see if there's a circuit board on this unit and if there's any diagnostics lights lit up.
looking for suggestions, I don't want to throw away a bunch of money on a new motor that it doesn't need. Once it's up and running, and the restaurant is open, it is 100% the tenant's responsibility. But I'm on the hook for it until then. Thanks

Starting capacitors are for single phase motors.
I doubt that these are power factor correction capacitors like a three phase unit possibly could have.

The roof mounted fan motor might not even be needed for gas heat?

You are in a tight spot with your local HVAC guys very busy,
but you have to somehow hire a competent one.

I agree.

The Tech is feeding us GIGO.

The motor doesn't seem like its a major contributor to your situation.

Only issue with it sounds like might mainly be if its losing a phase intermittently, ie an electrical supply problem.

Did he check the voltage? Does it have 2 amps on all 3 legs? The 6amps is the full load rating per leg and it might not be fully loaded in your application.

Too many variables to trust them guessing

I read your long post a 2nd time because I am stuck waiting.

A few more thoughts:

If the “over-fire limit switch” is kicking it off
because the gas burner heated air is too hot (over 155 F typical)
the airflow generated by the
downstairs belt driven
( squirrel cage type maybe) fan
is too low.

This low airflow could be caused by.....

(A) too many air registers around the restaurant are manually closed

(B) in the squirrel cage fan the numerous blades are clogged with dirt/lint/grease.
These blades are little airplane wings and their tops and bottoms must be bare metal shapes, not “crud mounds”
This is a tedious but important cleaning chore

(C) a replacement air filter may have gotten loose,
blown into the unit guts,
and blocked off most of the AC evaporator coil fin slots
or heating tubes,
interior duct elbow,
etc

Flame sensors are frequent trouble causers of burner shutdowns.
The flame sensing designs that use the
“Ion Direct Current micro-amps”
between the ignitor and electrical ground are notorious for being unreliable.

Relays are not supposed to buzz

Yes. Current draw will vary based on load. You will have a high load during surge on start-up then the load will drop and level off once it's up to speed. If the motor is overkill for the application it will never draw max running current(except during the start-up surge) as listed on the tag.

CHECK THE HEAT EXCHANGER BUNDLE

We have 24 of the Carrier gas-fired package units on the roof of the school. After 8-10 winters and countless heat cycles the weld-seams of the heat exchanger tubes will rust and split open, causing the flame to start to rollout and the rollout switch to disable the burner. The blower fan will continue to operate blowing cold air, and the thermostat will continue to call for heat, but the draft fan will lock out so no heat. The safety gas valve they found closed may be programmed to close when the rollout switch trips as a precaution. There should be an LED "flash code" you can use to confirm why it tripped out. Or fire up the unit with the side panel off and watch the burners--if the exchanger is cracked the flame will dance in an unstable way and "roll out" will be pretty obvious. If your unit is 10 years old and it has never been changed it is likely due. If you don't see any actual roll out and the flame looks good, the rollout switch itself could be bad--cheap part and super-simple to replace.

Changing the exchanger bundle out is about a 2-hour job for two competent guys--we do them regularly, so we have gotten pretty fast. The size varies depending on the size of your unit--the larger ones can be cumbersome for 1 guy to handle. Our units are 2001 models and are generally very reliable and pretty simple. If something goes wrong in the wintertime with them, 80% of the time it is the heat exchanger reaching the end of it's life.

Thanks for all the help, guys! I think I need a better tech who will be more capable of assuring me that his diagnosis is accurate. I was thinking that if you guys told me that a 2 amp draw on the motor for sure indicated that it was shot, I would go pick one up a Graingers and drop it in myself. But it doesn't sound like that is the case.

A motor is a torque matching device. It will draw whatever load it requires to keep the motor speed up. It gets tricky with fans because of affinity laws - lower speeds/flow reduce load by a lot.

As mentioned you need to know the voltage going to the motor and if the the load is balanced on all 3 phases. The caps make no sense and I can almost guarantee are not your problem. I doubt you have a motor problem.

The fan motor can be taking low load because of low flow - the flow could be restricted by closed off vents. If you throttle the air flow by closing or restricting air vents the load goes down by the square of the flow.

Holy long post Joel
Lol - my Siri would have stalled by after two sentences ..

We set it for "fan only" and the fan ran indefinitely without a problem. I have a different teck lined up for monday morning.

I like to see the amps even on all three legs and if its under its way better than being over

Ok Joel it's been a week did you get it fixed?

Mike

Yes it is fixed! I actually just found out the details on it this morning. After we ruled out the thermostat with new batteries and double checking the programming, I had a different company send a service tech out Monday morning. he confirmed that it had been going out on high limit. He checked the pressure in the duct and it was not what he wanted it to be. they thoroughly checked out all of the duct for any obstructions, and there was none. he checked the amp draw of the motor and just like the other guy he found it at 2 amps versus the nameplate of six. So he adjusted the pulley to speed up the fan. He adjusted until the motor was drawing 6 amps, and the unit has been working fine ever since.

so should I just pay the other guys Bill and keep my mouth shut, or should I call his boss and tell him that he was kind of on the right track but was going to sell me a $1,700 motor that I didn't need and I'd like an adjustment on the bill?

Thanks for all of your help everybody.

It begs the question if the fan speed was wrong all along, why a problem now all of a sudden? The fan itself is clean, no crud build up on the blades?

Kevin

Belt wear