Electric Motor Horsepower
How to calculate electric motor HP using RLA, Amps or Watts
ELECTRIC MOTOR HORSEPOWER - CONTENTS: calculate the horsepower of an electric motor. How to convert RLA or LRA / FLA into motor horsepower for typical electrical motors. Relationship of HP, Watts, and Amps.
InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.
Electric motor horsepower: this article explains how to calculate the horsepower of an electric motor if we know its wattage or current draw in amps. The article gives a simple formulat to convert electrical amps to horsepower and explains what data are actually needed to make an accurate horsepower calculation or rating.
We explain the relationship between running load amps (RLA), and full load amps (FLA) and lock rotor amps (LRA) ratings on electric motors. These electric motor horsepower formulas, definitions, and ratings are suitable for determining the horsepower of an air conditioner compressor motor, a fan motor, a well pump motor and similar equipment.
Question: relationship of RLA and electrical motor horsepower
(Aug 2, 2014) Eng.M.Mossad2010@gmail.com said:
From the part of " FROM EQUIPMENT RLA # - Air Conditioning Equipment Capacity from Equipment RLA Numbers " in the article above can we find a relation between the equipment's RLA and the HP "horse power " for the Compressor motor?
Reply: relationship of RLA, LRA and FLA motor ratings & converting to horsepower
Mossad: for a specific air conditioner or heat pump motor I'd obtain the horsepower rating first by looking at the BTUh rating on the data tag or encoded in the model number. For example, 18,000 BTUh is equivalent to 2 HP (horsepower) or 24,000 = 2.5 HP. You can find the basic rating data for an air conditioner compressor motor (or for other HVACR motors) by noting information on the data plates we describe in detail at DATA TAGS on AIR CONDITIONERS
Let's also note for other readers that RLA = running load amps - the current drawn normally during the operation of an electric motor.
RLA will always be a smaller number than LRA which is lock rotor amps - the start-up current draw. FLA = full load amps, a fixed number that will also always be higher than RLA. Refrigeration engineers generally state that if we know LRA we can estimate RLA as follows:
RLA = LRA / 5.56
(for most electric motors LRA is about 5 times greater than its RLA/FLA and some may be 7 times greater.
An accurate answer to the question of translating RLA into horsepower requires more data: the voltage leel and the number of phases of electrical current. And comparing HVAC compressor motors by amperage ratings alone may be misleading. Compressors need to be compared for the same rated conditions and same area of application, as motor designs and uses vary.
But for typical residential air conditioner systems and heat pump systems we can assume we're talking about 240V and 2-phases of power.
Since the maximum rotating power or force exerted by an electrical motor is that needed to get the motor spinning we should look at the motor's LRA or FLA rating. The RLA or running load current draw in amps will always be a smaller number than the motor's maximum. We'll give the motor horsepower as its maximum in our examples below but you can choose any amperage rating to calculate an equivalent horsepower number.
To express watts in amps or current is not reasonable since these are two different measurement scales (Watts in power and amps in coulombs per second). But if we know any two of the three basic electrical measures (amps, watts, and volts) we can calculate the third value. We can write the famous (and simplified) equation in any of the three ways using simple algebra:
AMPS = Watts / Volts OR Watts = Amps x Volts OR Volts = Watts / Amps
We can easily express Amps in a formula to obtain horsepower as follows:
AMPS = ( Horsepower x 746 ) / Voltage
Example: For a 220V electric motor with a 40-Amp LRA or FLA rating
40A = (Horsepower x 746) / V
40 = (HP x 746) / 220
(40 x 220 ) = HP x 746
(40 x 220) / 746 = Horsepower
11.8 = Horsepower
So this is a 12 horsepower motor based on its LRA or FLA rating.
Here is a different example:
At 220V a 240kBTUh A/C unit will draw around 8 amps when running. Using the same calculation as above but finding the horsepower exerted when the air conditioner motor is running at normal load (that is not at start-up and not with abnormal head pressure),
8 A = (HP x 746) / 220 which calculates out as 2.4 HP
So the horesepower being delivered by our air conditioner when running at normal load is around 2.4 HP
No FAQs have been posted for this page. Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
Ask a Question or Search InspectApedia
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
Home Inspection Education Home Study Courses - ASHI@Home Training 10-course program. Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
The Home Reference Book, a reference & inspection report product for building owners & inspectors. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
The Home Reference eBook, an electronic version for PCs, the iPad, iPhone, & Android smart phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter inspectaehrb in the order payment page "Promo/Redemption" space.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones
Wikipedia provided background information about some topics discussed at this website provided this citation is also found in the same article along with a " retrieved on" date. NOTE: because Wikipedia entries are fluid and can be amended in real time, we cite the retrieval date of Wikipedia citations and we do not assert that the information found there is necessarily authoritative.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.