Transformer too big?

Rson · Jan 31, 2020

So, I ran into a motor and transformer someone was trying to get started today.

Motor = 1HP,
Transformer = 750VA

With the transformer tapped from the line, the motor won't start. With the transformer removed, it starts perfect.

I suspect the transformer is too big. The motor has ~30 ohm at each leg and the transformer is only about 8 ohms. So when it is tapped, two legs are only reading an 8 ohm resistance.

Am I in the wrong here? I suppose it could be something else and the way it is wired.

(note - it is only a motor and transformer, nothing else on the 'machine')

V0N_hydro · Jan 31, 2020

are you feeding the motor from the transformer and it won't start?
or the motor is connected to the line, and the transformer is connected to the line, and the motor won't start if the transformer is connected at all? What loads are on the transformer? What gauge wire is the line?

In any case what does your meter say the minimum voltage is at the terminals of the motor when you are trying to start the motor?

rupej · Jan 31, 2020

It could be wired wrong, but either way that transformer is too small.

Look at the FLA on the motor.

GaryS · Jan 31, 2020

Your transformer is undersized
1 HP is 750VA no room for start up power surge
you should have at east 1000va or better yet a 1400va transfomer

lfe · Jan 31, 2020

This depends a lot on what type of load the motor moves, if it is something with constant torque as an elevator you need a much larger transformer, I suggest not less than 3000VA. Instead if the motor starts without load it will surely start with a smaller one .

But one of 750VA will always be too small, because the transformer must be chosen according to the apparent current, usually indicated in the motor plate, and not according to the active power of 736W which is what corresponds to 1HP

Rson · Jan 31, 2020

Sorry, I think I confused some people.

The motor is started directly from three phase.

The 750VA transformer is across 2 legs for control voltage. It seems abnormally large, and the motor starts when the transformer is removed and the motor is connected directly to three phase.

rupej · Jan 31, 2020

Rson said:
Sorry, I think I confused some people.

The motor is started directly from three phase.

The 750VA transformer is across 2 legs for control voltage. It seems abnormally large, and the motor starts when the transformer is removed and the motor is connected directly to three phase.

So the transformer is providing the control power? It's not too large for that- there's really no such thing. You have a problem with your control wiring or your motor starter is bad.

Rson · Jan 31, 2020

rupej said:
So the transformer is providing the control power? In that case, it's not too large. You have a problem with your control wiring or your motor starter is bad.

Ok. I'll double check it.

I was wondering if the transformer was causing too great of an unbalanced load on the three phases and not letting the motor start correctly.

If I disconnect the control transformer, the motor runs properly. But with it tapped off L1 & L3, the motor acts like it wants to start and pulls 10A, trips the breaker and never starts.

rupej · Jan 31, 2020

Can you post the schematic of how it is wired right now?

Rson · Jan 31, 2020

I don't have the exact schematic, but here is how it is wired.

I remember it because the L1 is 'tapped' at the transformer, and not the disconnect or MCB.

View attachment xf.PDF

That's it. The transformer is 750VA for no reason. It has two 120vac lamps and a timer on it. Pulls about 0.5A at 480VAC though.

Rson · Feb 1, 2020

So I verified my theory this morning (I think).

I had a 90W 480-24VDC power supply in the shop that I hooked up in place of the 750VA transformer. With the 90W supply, the motor starts with no issues. Hooking the transformer up again (without the power supply) causes the same issue of over-amps on a motor.

No transformer = motor runs
90W supply = motor runs
750VA transformer - overcurrent trip every time

AustralIan · Feb 1, 2020

Is "line" the town's power supply or another transformer or a generator?
It is possible the imbalanced load is dragging the voltage away from 480. Normally on a generator set you would have two alternators, a 480 three phase alternator and a (for example) 110V single phase alternator.

Gene Bond · Feb 1, 2020

Overload Relays and Motor Circuit Protectors accelerate the trip time based on current imbalance.

Why is the transformer on the secondary of the starter?

If it's to save an auxiliary contact from the starter, then shame on the designer for being so cheap!

rupej · Feb 1, 2020

I'm confused. Is your schematic incomplete? Because it looks like you're starting the motor with a disconnect. I realize that's OK in certain scenarios but is uncommon.

You should be able to measure voltage at the motor terminals and quickly determone what is going on. What is the voltage there? Is your CB tripping?

Rson · Feb 5, 2020

Ok, all.

I ordered a new transformer. No start.

Removed new transformer, put the power supply back on. No start.

Turns out, I had a voltage drop somewhere between the safety disconnect and the MCB. I had 500V on L1-L2, 150V on L2-L3, and 175V on L1-L3.

I removed the safety switch, wired a new one, and all is good.

Transformer too big?

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