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Old 03-23-2009, 06:52 PM   #16
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Converting std 220v/60hz to Single phase 240V 3-wire


A one to one center tapped secondary transformer would work. However, the transformer with enough capacity to carry the load would be very expensive.

Probably the price of 2 or more dryers!

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Old 03-23-2009, 07:04 PM   #17
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Converting std 220v/60hz to Single phase 240V 3-wire


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Originally Posted by hayewe farm View Post
1 to 1 transformer with a center-tap secondary would work and would heat your house at the same time. 2 step-down transformers with the right legs commend would also work. However you may still have a problem with the timmer moter and dryer motor if India is 50Hz.

The US residential electricity is single phase because all three legs are in phase with each other.
Well, transformers are generally very efficient, often 98% or better, so the amount of heat generated would be quite small in a good transformer. As for the 50 Hz problem, well that wouldn't be very much of a difference. A 60 Hz induction motor will run at 5/6 the speed on 50 Hz that it would on 60 Hz. So the 80 minute timing cycle would stretch to 96 minutes.

Anyhow, I would not do any of this, because a center tapped 1:1 transformer at say 5 kVA would be quite pricey.
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Old 03-23-2009, 08:16 PM   #18
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Converting std 220v/60hz to Single phase 240V 3-wire


The impedance (resistance to AC) of the motor's coils is less at 50 Hz compared with at 60 Hz. The difference may be enough to cause the 60 Hz motor to draw excessive current at 50 Hz and burn out.
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Old 03-23-2009, 10:29 PM   #19
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Converting std 220v/60hz to Single phase 240V 3-wire


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Originally Posted by AllanJ View Post
The impedance (resistance to AC) of the motor's coils is less at 50 Hz compared with at 60 Hz. The difference may be enough to cause the 60 Hz motor to draw excessive current at 50 Hz and burn out.
I think that you are right! I remember when Canada converted from 25~ to 60~ and 25~ motors could be run on 60~, but the other way was no good!
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Old 03-23-2009, 11:06 PM   #20
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Converting std 220v/60hz to Single phase 240V 3-wire


There are a large number of industrial motors rated for 50 or 60 HZ, a lot of them with no loss of HP.

The ideal frequency of a motor (or any inductive device) is dependent on the ratio of conductor material (windings) vs. the amount of iron in the core. Higher frequency needs less iron.

This is why aircraft use 400 HZ. A 10 HP 400 HZ motor is about the size and weight of a 2 HP 60 HZ one. The same is true with generators and transformers.

A motor designed for 50 HZ will be physically larger and heavier than one designed for 60 HZ.

To answer the original problem here, a transformer could indeed be used to produce the 120 volts needed by the motor and controls. An 1 KVA model with a 240 volt primary and a 120 volt secondary would suffice. The problem with this arrangement is if one side of the 240 volt line is grounded, then the neutral of the dryer would be 120 volts above ground. Not exactly the safest possible scenario.

The transformer secondary (120 volt) side would need to be fused at 10 amps. If this fuse were to blow, power would be lost to the controls, and the dryer would shut off. The 240 volt element would be de-energized by the centrifugal switch in the motor, or the airflow switch.

Using a transformer like this would certainly violate code here in the USA, (the neutral is required to be grounded, no fuse allowed in the neutral, etc.), but I don't know about anywhere else.

Rob

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